Hull, Steel & Aluminium Structures

STRUCTURAL MATERIAL SELECTION — NAVAL ARCHITECT'S PERSPECTIVE

The material specification for a steel or aluminium yacht begins long before the first plate is cut.

The Naval Architect does not simply select one grade of steel for the complete vessel. Material selection forms part of the overall structural design and depends on the yacht's dimensions, displacement, operating profile, design speed, structural arrangement, Classification Society requirements, environmental conditions and intended service.

A 50-metre displacement yacht operating principally in the Mediterranean may require a very different structural specification from an explorer yacht designed for prolonged operation in Arctic or Antarctic waters.

Design Starts With the Yacht's Mission

Before determining plate thicknesses and material grades, the Naval Architect must understand how the yacht will be used.

The design basis may include:

• length overall and Rule length

• beam

• draught

• displacement

• hull form

• design speed

• operating range

• machinery arrangement

• fuel and tank arrangement

• superstructure configuration

• helicopter or heavy deck loads

• tender and crane loads

• Classification Society

• Flag requirements

• unrestricted or restricted navigation

• expected minimum operating temperature

• ice or Polar operation where applicable

Global & Local Structural Loads

The hull behaves as a complete structural beam while also being subjected to local loads.

The Naval Architect therefore considers both global and local structural demands.

Global loads may include:

• longitudinal hull-girder bending

• hogging and sagging

• shear forces

• torsional loading

• wave-induced loads

Local loads may include:

• hydrostatic pressure

• slamming

• bow impact

• machinery loads

• engine and generator foundations

• stabiliser foundations

• crane foundations

• mast foundations

• tank boundaries

• shell doors

• tender garages

• helicopter-deck loads where fitted

Material grade, plate thickness, stiffener size and structural arrangement are therefore designed together rather than considered independently.

EXPLORER & POLAR YACHTS

A yacht intended for operation in Arctic or Antarctic waters introduces additional structural considerations.

Low temperature changes the material-selection problem. The designer must consider not only structural strength but also the ability of the material and welded structure to maintain adequate toughness at the yacht's intended service temperature.

An explorer yacht intended for Polar operation may therefore use higher-toughness grades such as D, E or F grades in selected structural areas, together with increased structural scantlings and additional ice strengthening where required by the applicable Class notation.

Ice-Exposed Structure

Particular attention may be required in areas such as:

• forward shell plating

• bow region

• ice belt

• stem structure

• forward framing

• bottom structure

• appendage foundations

• rudder and steering structure

• areas exposed to ice impact or ice pressure

The solution is not simply to specify 'thicker Grade A steel'.

The Class-approved design must consider structural loading, material toughness, plate thickness, framing, welding, fatigue and the yacht's design service temperature as a complete system.

Low-Temperature Toughness

Steel can retain adequate strength while becoming less tolerant of crack propagation at low temperature.

For this reason, Class-approved marine steels are specified with different impact-toughness requirements.

The Naval Architect therefore selects the material grade according to the structural location and temperature requirements rather than assuming that the same steel used for a conventional warm-water yacht is automatically suitable for Polar service.

STEEL HULL & ALUMINIUM SUPERSTRUCTURE

Many large yachts combine a steel hull with an aluminium superstructure.

One major reason is weight distribution. Reducing structural weight high in the yacht can reduce the vertical centre of gravity and assist the Naval Architect in achieving the required stability, motion and loading characteristics.

5083 Marine Aluminium

5083 aluminium alloy in appropriate marine tempers such as H116 or H321 is widely used for marine plate.

Typical applications include:

• aluminium hull plating

• superstructure plating

• deck plating

• large fabricated marine structures

The alloy combines good weldability, useful structural strength and good resistance to the marine environment when correctly specified and fabricated.

6000-Series Structural Extrusions

Aluminium alloys such as 6082-T6 and, depending on the approved specification, 6061-T6 may be used for structural extrusions.

Typical applications can include:

• stiffeners

• beams

• frames

• extruded profiles

• superstructure framing

• window and door structural framing

The designer must take account of the mechanical properties of the base material and the reduction in strength that can occur in the heat-affected zone adjacent to welded joints.

Steel-to-Aluminium Transition

Steel and aluminium hull structures cannot simply be fusion welded directly together using a conventional welding process.

Where an aluminium superstructure is connected to a steel hull, an approved transition-joint system may be used.

These transition products provide compatible steel and aluminium faces so that the steel side can be welded to the steel structure and the aluminium side can be welded to the aluminium structure.

The design must also address galvanic isolation, water trapping, coating protection and the long-term condition of the transition area.

NEW-BUILD FABRICATION — WELDER'S PERSPECTIVE

From the welder's perspective, the material grade is not a choice made at the welding station.

The Naval Architect and approved structural drawings define the material. The shipyard welding department then establishes the approved welding procedures required to fabricate that structure.

A qualified welder works within the limits of the approved Welding Procedure Specification and their individual welding qualification.

Before Welding Starts

The welder and fabrication team should be able to confirm:

• correct drawing

• correct drawing revision

• material grade

• material thickness

• material certificate and traceability where required

• heat or batch identification

• approved Welding Procedure Specification

• correct welding process

• approved consumable

• joint preparation

• root gap and fit-up

• welding position

• preheat requirement where applicable

• interpass-temperature limits

• welding sequence

• distortion-control requirements

• inspection and NDT requirements

WPS — Welding Procedure Specification

The WPS tells the welder how the approved joint is to be produced.

Depending on the process and material, it may define:

• parent material group

• thickness range

• joint type

• welding process

• consumable classification

• electrode or wire diameter

• current and voltage range

• polarity

• travel speed

• heat input

• shielding gas

• preheat

• interpass temperature

• welding position

• number and sequence of passes

The welder should not change the approved procedure simply because another technique appears faster or easier.

Carbon & Higher-Strength Steel

When welding hull structural steel, the welder must control joint preparation, consumables, hydrogen, heat input, preheat where required, interpass temperature, welding sequence and distortion.

Higher-strength steel may require tighter procedural control than normal-strength steel. The fact that two steels look identical does not mean that they can automatically be welded using the same procedure.

Stainless Steel

Stainless steel fabrication requires particular attention to cleanliness and contamination control.

Carbon-steel grinding dust, tooling or embedded iron contamination can compromise the corrosion resistance of the finished stainless-steel component.

Dedicated tools, appropriate consumables, controlled preparation and suitable shielding or backing-gas arrangements may therefore be required.

Aluminium

Aluminium welding requires a different fabrication discipline from steel.

Surface cleanliness, oxide removal, shielding-gas control, correct filler selection, heat input and distortion control are particularly important.

Dedicated tooling should be used to reduce cross-contamination from carbon steel.

The welder must also understand that welding can change the mechanical properties in the heat-affected zone, particularly in heat-treated aluminium alloys.

Copper Alloys & Bronze

'Bronze' describes a family of copper alloys rather than a single weldable material.

Marine components may include nickel-aluminium bronze and other copper alloys in applications such as propellers, valves, bushes, seawater components and underwater machinery.

Before any weld repair, the exact alloy and approved repair procedure should be established.

A welder qualified for structural steel is not automatically qualified to perform an approved repair on aluminium, stainless steel or a copper-alloy component.

REFIT STRUCTURAL REPAIR — WELDER'S PERSPECTIVE

Structural welding during a refit can be considerably more complex than fabrication during a new build.

In new construction, the yard normally begins with identified materials, approved drawings, controlled plate preparation and known fabrication history.

During a refit, the welder may be working on structure that has been in service for decades and may have been modified or repaired several times previously.

Do Not Assume the Existing Material

Before welding an existing structure, the repair team should establish what material is actually present.

This may require:

• original construction drawings

• material certificates

• previous repair records

• Classification Society records

• material identification

• laboratory analysis where necessary

• hardness testing where appropriate

• test coupons or additional technical investigation for unusual material

An old plate should not automatically be assumed to have the same composition or weldability as the replacement marine steel being fitted beside it.

Establish the Full Extent of the Defect

Before cutting begins, the repair boundary should be established from sound material rather than simply following the visible edge of corrosion.

The assessment may include:

• visual inspection

• ultrasonic thickness measurement

• dye penetrant testing

• magnetic-particle testing

• ultrasonic testing

• radiography where appropriate

• inspection of adjacent frames and stiffeners

• examination of previous weld repairs

Control the Structure Before Cutting

Removing part of the hull changes the load path through the surrounding structure.

Temporary support, bracing or an approved cutting sequence may therefore be necessary before defective plating, frames or foundations are removed.

The Project Manager, repair engineer, Class surveyor and yard should understand the proposed repair sequence before significant structural material is cut away.

Old Steel to New Steel

One of the refit welder's principal challenges is producing a controlled new weld into existing structure.

The existing plate may contain:

• corrosion

• pitting

• contamination

• old coatings

• residual stresses

• previous weld repairs

• heat-affected material

• local thinning

• laminations or defects

The repair boundary should therefore extend to material that is suitable for the approved welding procedure.

New Build Versus Refit

A new-build welder normally creates the structure from controlled materials according to an approved production sequence.

A refit welder must first understand the condition and history of the structure that already exists.

The fundamental welding standards remain, but the refit introduces additional uncertainty.

The welder must often manage:

• restricted access

• existing distortion

• contaminated material

• mixed material history

• unknown previous repairs

• difficult welding positions

• adjacent machinery and outfitting

• fire risk

• damage to paint and insulation

• heat transfer into occupied spaces

• preservation of alignment

• restoration of coatings after completion

For this reason, successful refit welding depends as much on preparation, investigation and repair planning as it does on the final weld itself.

HOT-WORK SAFETY DURING YACHT REFIT

Structural welding during a yacht refit introduces risks that do not exist to the same extent in a controlled fabrication workshop.

The yacht may contain insulation, cables, pipework, fuel systems, joinery, furnishings, coatings and machinery immediately behind or below the area being welded. Heat and sparks can also travel through openings into spaces that are not visible from the welding position.

For this reason, welding, cutting, grinding and other spark-producing operations should be controlled through the shipyard's hot-work permit system and associated QHSE procedures.

Before Hot Work Begins

The Project Manager should coordinate confirmation that:

• the Captain or authorised yacht representative has been informed

• the responsible yard department has authorised the work

• a valid hot-work permit has been issued

• the welder is appropriately qualified for the material, process and work being undertaken

• the applicable Welding Procedure Specification is available where required

• the welding area has been inspected

• adjacent spaces have also been inspected

• combustible materials have been removed or adequately protected

• cables and pipework have been identified and protected

• fuel, gas and other hazardous services have been isolated where necessary

• suitable ventilation has been established

• suitable firefighting equipment is immediately available

• a competent fire watch has been assigned

• emergency communication is available

• scaffolding and working platforms are safe and suitable

• access and escape routes remain clear

HOT-WORK AREA ISOLATION

Where welding is carried out on an exposed deck, superstructure or within an occupied yacht, the work area may require physical isolation from the surrounding refit activity.

A dedicated hot-work enclosure or protective tent may be erected around the work zone using suitable flame-resistant materials.

The enclosure should prevent welding sparks, grinding debris and hot metal from reaching surrounding equipment while still allowing adequate ventilation and safe access.

Ordinary plastic sheeting or other combustible materials should not be used where they could be exposed to welding heat or sparks.

Scaffolding & Working Platforms

Where the work cannot be safely reached from the existing structure, suitable scaffolding or working platforms should be provided.

The arrangement should consider:

• safe access for the welder

• safe working position

• fall protection

• welding cable routing

• gas-hose routing

• extraction or ventilation equipment

• fire-watch access

• movement of replacement plates

• lifting arrangements

• emergency escape

Fire Watch

A dedicated fire watch should be positioned wherever heat, sparks or molten metal could ignite surrounding material.

Depending on the configuration of the yacht, more than one fire watch may be necessary. A welder working on one side of a deck or bulkhead can create a fire hazard in the compartment on the opposite side.

The fire watch may be a suitably trained yard employee or, where permitted by the shipyard's QHSE procedures, a competent member of the yacht's crew.

The person assigned to the fire watch should have no conflicting duty that prevents continuous observation of the hot-work risk.

Suitable firefighting equipment should be immediately available and appropriate to the hazards present in the area.

The fire watch should continue for the period required by the yard hot-work procedure after welding has stopped and should include inspection for heat transfer, smouldering material or fire in concealed or adjacent spaces.

WELDING IN CONFINED & ENCLOSED SPACES

Hot work inside tanks, bilges, cofferdams, void spaces and other restricted areas requires additional controls.

A confined-space entry should not be treated simply as an ordinary welding job with additional ventilation.

The space should first be assessed under the shipyard's confined-space procedure and, where required, tested and released as safe for entry and hot work by the authorised competent person.

Typical Controls

• confined-space entry permit

• hot-work permit

• atmosphere testing before entry

• continued or periodic atmosphere monitoring where required

• oxygen concentration verification

• flammable-gas testing

• toxic-gas or vapour monitoring where relevant

• forced mechanical ventilation

• local fume extraction where practicable

• clear access and escape route

• competent standby person

• rescue arrangements

• communication with personnel outside the space

• suitable lighting

• appropriate respiratory protection where required

• removal or control of combustible material

• fire watch

Oxygen should never be used as a substitute for ventilation.

Gas cylinders should remain outside confined spaces unless the applicable approved procedure specifically permits another arrangement.

Torches and hoses should not be left unattended inside a confined space when the operation stops.

HOT WORK NEAR CABLES, PIPES & EXISTING SYSTEMS

Before cutting or welding a deck, bulkhead or hull structure, the Project Manager and welding team should understand what exists on the opposite side of the plate and within the surrounding structure.

A visually clear welding area does not necessarily mean that the structure is clear internally.

The assessment should identify:

• electrical cables

• fibre-optic cables

• hydraulic lines

• fuel lines

• lubricating-oil lines

• seawater pipes

• fire-main pipes

• bilge pipes

• sanitary pipework

• refrigerant lines

• insulation

• combustible joinery

• tanks and void spaces

Where required, services should be isolated, drained, removed, relocated or protected before hot work begins.

Particular care is required when a structural repair affects Class-related or essential-service piping.

If cutting or modifying such piping becomes necessary, the PM should stop and establish whether the proposed modification requires Class review or approval before proceeding.

Bilge & Essential-Service Pipework

Structural repairs in bilges frequently encounter existing pipework.

The easiest physical solution is not necessarily an acceptable Class repair.

Cutting an existing pipe and later reinstating it using short inserted pieces, sleeves, straps, clamps or other improvised arrangements can create additional joints, corrosion locations and potential leakage points.

Where the affected pipe forms part of a bilge, seawater, fire, fuel or other essential system, the proposed alteration should be referred to the appropriate technical authority and Class where required before cutting begins.

Where practical, rerouting, controlled removal of a complete spool or another approved permanent repair may be preferable to introducing an unnecessary temporary-style repair into an essential system.

CLASS-CONTROLLED STRUCTURAL REPAIR

When welding affects Classed hull structure, decks or superstructure, the repair should be approached as a controlled technical process rather than simply assigning a welder and beginning to cut.

1 — Inform the Yacht

The Project Manager informs the Captain or authorised yacht representative of the defect and proposed structural work.

2 — Welding Inspection

An appropriately qualified welder or welding supervisor inspects the repair area and determines the practical fabrication requirements.

The proposed repair should consider:

• extent of defective material

• access

• existing structure

• replacement material

• plate thickness

• joint configuration

• cutting sequence

• fit-up

• welding process

• welding sequence

• temporary support

• distortion control

• required inspection and NDT

3 — Repair Proposal & Welding Documentation

A repair drawing or sketch should identify the material to be removed and the proposed replacement structure.

The associated welding documentation may include:

• Welding Procedure Specification

• welding-process identification

• welder qualification certificates

• material specification

• material certificates

• joint preparation

• welding sequence

• inspection and NDT requirements

4 — Commercial Approval

The Project Manager prepares the associated shipyard quotation and presents the technical repair proposal and commercial cost to the Captain or authorised yacht representative.

5 — Class Review

Where Class approval or survey attendance is required, the repair proposal, welding documentation and supporting information are submitted to the Classification Society.

The Class surveyor may then review the proposed repair with the Captain, Project Manager, yard and welding personnel before authorising the next stage.

CLASS HOLD POINTS DURING REPAIR

The Class surveyor may establish inspection hold points during the repair.

Hold Point 1 — After Crop-Out

The welder removes the initially agreed defective material and stops work for inspection.

Once the structure is exposed, additional corrosion or damage may become visible that could not be identified during the original survey.

The surveyor may require the crop boundary to be extended into sound material.

Where a revised boundary is marked directly onto the structure by the surveyor or agreed during the inspection, the final repair documentation should record the revised extent.

Hold Point 2 — Fit-Up

The replacement plate is prepared and positioned within the opening.

Temporary alignment devices such as strongbacks, dogs or other approved fit-up attachments may be tack welded to maintain plate position and alignment.

Before final welding, the surveyor may inspect:

• plate material and thickness

• crop geometry

• edge preparation

• root gap

• alignment

• fit-up

• temporary support

Hold Point 3 — Welding

Once the fit-up has been accepted, welding proceeds in accordance with the approved WPS and agreed welding sequence.

The sequence should control heat input, shrinkage and distortion while achieving the required penetration and weld profile.

Hold Point 4 — Final Examination

Following completion and suitable cooling, the repair is visually inspected and the required NDT or tightness testing is undertaken.

COMMON MARINE WELDING PROCESSES

The welding process should be selected by the welding specialist and approved through the applicable WPS rather than chosen by the Project Manager.

SMAW — Shielded Metal Arc Welding

Often referred to as stick welding or electrode welding. In French yard terminology this may be described informally as welding with a 'baguette'.

SMAW is highly portable and remains useful for repair work, restricted access and outdoor operations.

GMAW — MIG / MAG

Gas Metal Arc Welding uses continuously fed wire with shielding gas.

It can provide high productivity and controlled weld quality but requires suitable protection from wind and contamination of the shielding-gas envelope.

GTAW — TIG

Gas Tungsten Arc Welding provides excellent control and is frequently used for stainless steel, aluminium, copper alloys and high-quality detail work.

It is generally slower than high-deposition production processes but provides precise heat and weld-pool control.

FCAW — Flux-Cored Arc Welding

Flux-cored welding is widely used for structural fabrication and can provide high deposition rates appropriate to substantial steel repairs when covered by the approved procedure.

MULTI-PASS STRUCTURAL WELDING

A thick structural butt joint will normally not be completed with a single weld pass.

Depending on the approved joint preparation and WPS, several passes may be required to completely fill the joint while controlling penetration, heat input and distortion.

Typical Weld Sequence

• root pass — establishes penetration at the root of the joint

• hot pass where specified — reinforces and cleans the root region

• fill passes — progressively fill the prepared joint

• cap pass — completes the external weld profile

The number, sequence and direction of passes are defined by the welding procedure and joint design.

For large plate inserts, the overall sequence around the replacement plate is also important because uncontrolled welding can produce significant shrinkage and distortion.

ONE-SIDED FULL-PENETRATION WELDING

During yacht refit it is not always possible to gain access to the reverse side of a structural plate.

Where approved by the welding procedure and Classification requirements, a full-penetration butt weld may be produced from one side using an approved temporary backing system.

Ceramic Backing

One common method uses a temporary ceramic backing strip positioned behind the weld joint.

The backing supports the molten root weld and allows the welder to form the required penetration from the accessible side of the plate.

After completion, the temporary backing is removed where the arrangement permits and the finished joint is examined in accordance with the approved procedure.

Other approved backing systems may use suitable temporary copper backing or specially qualified backing materials.

The use of backing should form part of the approved Welding Procedure Specification and should not be improvised by the welder simply because the reverse side cannot be reached.

Important Distinction — Back Gouging

Back gouging is a different technique.

Where both sides of a butt joint are accessible, the first side may be welded and the root subsequently gouged or ground from the opposite side to sound weld metal before completing the second-side weld.

Where physical access exists to only one side, an approved one-sided welding procedure with temporary backing may be required instead.

POST-WELD INSPECTION & NDT

Completion of the weld does not automatically complete the structural repair.

The finished joint should first receive visual examination and then the additional non-destructive examination required by the approved repair plan or Classification Society.

Typical Examination Methods

• VT — Visual Testing

• MT — Magnetic Particle Testing for surface and near-surface defects in suitable ferromagnetic steels

• PT — Penetrant Testing for surface-breaking defects, including suitable stainless steel and aluminium applications

• UT — Ultrasonic Testing for internal weld examination

• RT — Radiographic Testing where specified

• PAUT — Phased Array Ultrasonic Testing where approved and appropriate

Watertight & Tank Boundaries

Where the repair forms part of a watertight or tank boundary, an additional tightness test may be required.

Depending on the structure and approved test programme this may include a hose test, vacuum-box test, air test, hydrostatic test or another approved method.

The required examination and acceptance criteria should be established before the repair begins rather than decided only after welding is complete.

CASE STUDY — WHEN THE PROJECT MANAGER MUST INTERVENE

During a yacht refit, corrosion was discovered in the steel decking forming a U-shaped transom banquet area.

The original construction incorporated drainage points at the corners of the structure so that water could evacuate rather than remain trapped within the deck arrangement.

The corroded steel required crop-and-renew repairs, which meant that the existing drain connections also had to be removed and subsequently reinstated.

Initial Welding Attempt

A welder was assigned to the repair.

Before the work progressed, the Project Manager asked the welder to explain how the drain connection would be removed and subsequently reinstated.

The proposed method did not provide sufficient confidence that the material combination and joining method had been properly identified.

The welder made an initial attempt at the joint. The result was unsatisfactory and the affected area began to deteriorate rather than producing a controlled weld.

The Project Manager instructed the welder to stop.

Despite this instruction, further attempts were made and the damaged area progressively increased.

The Work Was Stopped

At this stage the Project Manager stopped the welding operation completely.

Continuing to deposit weld metal onto an unidentified and already unsuccessful joint would not resolve the underlying problem.

A failed weld should not simply be covered repeatedly with additional weld material. Where a weld repair is required, defective material normally needs to be removed back to sound material, the joint correctly prepared and the repair carried out using an appropriate approved welding or joining procedure.

Material Identification

The next step was to establish what materials were actually being joined.

A simple magnet check immediately demonstrated that the drainage component did not behave like the surrounding carbon-steel deck.

This was a useful initial material-identification check, but it was not sufficient on its own to positively identify the material.

The Project Manager therefore obtained the original construction drawings for the transom banquet area.

The drawings confirmed the construction:

Copper pipework interfacing with a steel deck structure.

The difficulty was therefore not simply poor welding technique. Two different metals were involved and the connection required a suitable dissimilar-metal joining procedure.

What Had Gone Wrong?

Several project-control failures had occurred before the actual welding failure became visible.

• The welder had not requested or reviewed the relevant construction drawing

• The materials forming the existing joint had not been positively identified

• The proposed joining procedure had not been established before work began

• The suitability of the welder's qualification for the specific material combination had not been demonstrated

• The first unsuccessful attempt was not treated as a reason to stop and reassess the repair

• Further welding increased the extent of the damaged area

The correct response was not to tell the welder how to perform the weld.

The correct response was to stop the work because the technical basis for continuing had not been demonstrated.

Corrective Approach

Once the material combination had been established, the repair could be treated as a controlled technical problem.

The next steps were to:

• stop further welding

• establish the extent of damaged material

• review the original construction drawings

• positively identify the materials involved

• inform the Captain

• involve the Classification Society where required

• establish the approved repair and joining method

• appoint personnel qualified for the required materials and process

• remove defective material back to an acceptable condition

• prepare the joint correctly

• carry out the repair using the approved procedure

• inspect and test the completed repair

• reinstate corrosion protection and surrounding finishes

• document the completed repair

THE TECHNICALLY COMPETENT PROJECT MANAGER

The Project Manager does not instruct a qualified welder how to weld.

The same principle applies to electricians, hydraulic technicians, engineers, coating specialists and other professional subcontractors.

The specialist remains responsible for performing their trade correctly within the applicable technical procedures, qualifications and standards.

However, this does not mean that the Project Manager can simply accept whatever a subcontractor proposes.

A yacht Project Manager requires sufficient technical competence to understand the work being undertaken, recognise when the proposed method does not appear appropriate, ask the correct questions and stop work when the technical or safety basis has not been demonstrated.

STRUCTURAL PLATE REPAIR — CROP & RENEW

Once corrosion, cracking or structural damage has been identified and the repair has been agreed with the appropriate technical authority, the defective structure can be prepared for crop-and-renew.

A crop-and-renew repair should not simply follow the visible edge of corrosion. The objective is to remove defective material back to sound structure and install replacement material of the correct grade, thickness and structural configuration.

Establishing the Repair Boundary

Before cutting begins, the extent of deterioration should be established as accurately as practicable.

This may require:

• visual inspection

• ultrasonic thickness measurement

• hammer or sounding inspection where appropriate

• magnetic-particle testing

• penetrant testing

• ultrasonic examination

• inspection from both sides of the structure where accessible

• review of internal frames and stiffeners

• examination of previous repairs

The proposed crop line should extend into material that is suitable for the approved repair.

Where Class attendance is required, the surveyor may ask for the initial crop to be completed and then inspect the exposed structure before confirming the final repair boundary.

Do Not Assume the Damage Stops at the Plate

Corrosion visible on hull or deck plating may also affect the structure behind it.

The Project Manager should therefore consider:

• frames

• longitudinals

• floors

• brackets

• webs

• bulkheads

• foundations

• drainage arrangements

• welded attachments

Removing the external plate may reveal deterioration that was impossible to identify during the original inspection.

REPLACEMENT MATERIAL

Replacement material should correspond with the approved structural repair specification.

The Project Manager should ensure that the material being delivered to the repair area can be traced to the required grade and thickness.

The repair documentation should identify where applicable:

• material grade

• plate thickness

• material certificate

• heat or batch reference

• plate identification

• Classification approval status where required

A piece of steel that appears dimensionally correct should not automatically be accepted as suitable structural replacement material.

Plate Orientation & Preparation

The replacement plate should be prepared to match the required hull or deck geometry before final welding.

Depending on the repair this may involve:

• rolling

• forming

• bending

• edge preparation

• bevel preparation

• local trimming

• checking fairness against the surrounding structure

The objective is to achieve the correct fit before welding rather than attempting to force a poorly fitted plate into position using excessive welding heat.

PLATE FIT-UP & TEMPORARY ALIGNMENT

Before final welding begins, the replacement plate should be positioned and aligned with the surrounding structure.

Correct fit-up is fundamental to achieving a satisfactory structural weld.

The welder should verify:

• plate position

• alignment

• root gap

• edge preparation

• fairness with surrounding plating

• frame and stiffener alignment

• accessibility for the selected welding process

Strongbacks, Dogs & Temporary Attachments

Temporary steel attachments may be tack welded around the repair to hold the replacement plate in the required position during fit-up.

Depending on shipyard terminology these may be referred to as strongbacks, dogs, bridges or temporary alignment pieces.

They can be used to control:

• plate alignment

• edge position

• local fairness

• movement during tack welding

• distortion during the early welding sequence

Temporary attachments should be removed using an approved method after they have served their purpose.

The attachment areas should then be dressed and examined as required to ensure that removal has not introduced cracks, gouges or other defects into the parent material.

WELDING SEQUENCE & DISTORTION CONTROL

Welding introduces concentrated heat into the structure. As the weld cools, the metal contracts.

If this shrinkage is not controlled, a structurally acceptable plate can still finish with unacceptable distortion, unfairness or misalignment.

The larger the insert and the thinner the surrounding structure, the more important the planned welding sequence becomes.

Typical Distortion-Control Measures

Depending on the approved welding procedure and repair geometry, the welding specialist may use:

• controlled tack-welding sequence

• balanced welding

• welding from opposite sides where access permits

• staggered welding sequence

• back-step welding

• skip welding where appropriate to the procedure

• temporary strongbacks

• controlled restraint

• planned weld-pass sequence

• heat-input control

• interpass-temperature control

The Project Manager should understand the purpose of the sequence but should not independently instruct the welder to alter an approved welding procedure.

Competence Without Taking Over the Trade

A competent Project Manager should be able to ask:

• What material are you working on?

• How have you confirmed the material?

• Which drawing are you working from?

• Is this the latest approved revision?

• What procedure will you use?

• Is there an approved WPS or equivalent technical procedure?

• Are you qualified for this material and process?

• Does Class need to approve or witness the repair?

• Does Flag have any relevant requirement?

• What will be removed before the repair begins?

• How will the surrounding structure be protected?

• What inspection hold points are required?

• How will the finished work be inspected?

• What NDT or functional testing will be required?

• What happens if the repair reveals additional damage?

WELDING SEQUENCE & DISTORTION CONTROL

Welding introduces concentrated heat into the structure. As the weld cools, the metal contracts.

If this shrinkage is not controlled, a structurally acceptable plate can still finish with unacceptable distortion, unfairness or misalignment.

The larger the insert and the thinner the surrounding structure, the more important the planned welding sequence becomes.

Typical Distortion-Control Measures

Depending on the approved welding procedure and repair geometry, the welding specialist may use:

• controlled tack-welding sequence

• balanced welding

• welding from opposite sides where access permits

• staggered welding sequence

• back-step welding

• skip welding where appropriate to the procedure

• temporary strongbacks

• controlled restraint

• planned weld-pass sequence

• heat-input control

• interpass-temperature control

The Project Manager should understand the purpose of the sequence but should not independently instruct the welder to alter an approved welding procedure.

EXAMPLE — RECTANGULAR PLATE INSERT

Consider a rectangular section of corroded deck plating that must be replaced.

A simplified repair sequence could be:

1. Identify and mark the proposed crop boundary.

2. Confirm services, cables, pipework and structure behind the plate.

3. Install temporary support where required.

4. Cut out the defective plate.

5. Stop for inspection of the exposed structure where required by Class.

6. Extend the crop boundary if additional corrosion is discovered.

7. Prepare the surrounding parent material.

8. Form and prepare the replacement plate.

9. Position the plate using temporary alignment attachments.

10. Verify root gaps, alignment and edge preparation.

11. Obtain the required fit-up inspection or Class acceptance.

12. Tack weld according to the approved sequence.

13. Complete root, fill and cap passes according to the WPS.

14. Remove temporary attachments and dress the affected areas.

15. Carry out visual examination and required NDT.

16. Carry out tightness testing where the repaired structure forms a watertight boundary.

17. Restore corrosion protection, insulation, outfitting and surrounding finishes.

18. Close the repair documentation.

STRUCTURAL REPAIR RECORD

A significant structural repair should leave behind a traceable technical record.

The Project Manager should compile or reference, where applicable:

• original defect photographs

• thickness measurements

• NDT reports

• repair drawing or sketch

• final crop boundary

• replacement material grade

• material certificate

• WPS reference

• welder qualification

• Class correspondence

• Class inspection records

• photographs during crop-out

• photographs of exposed internal structure

• fit-up photographs

• completed weld photographs

• final NDT report

• tightness-test record where applicable

• coating reinstatement

• final acceptance and close-out

This documentation becomes part of the yacht's technical history and can provide important evidence during future Class surveys, refits, insurance matters, valuation and eventual sale.

GALVANIC CORROSION & DISSIMILAR METALS

A yacht frequently contains several different metals within the same structure and systems: carbon steel, stainless steel, aluminium, copper alloys, bronze and other specialist materials.

These materials cannot always be connected together without considering the possibility of galvanic corrosion.

Galvanic corrosion can occur when two electrically dissimilar metals are in electrical contact while exposed to an electrolyte such as seawater.

One material becomes more anodic and preferentially corrodes, while the more noble material is comparatively protected.

The Three Conditions

For galvanic corrosion to develop, three conditions are normally present:

• two electrochemically different metals

• electrical contact between the metals

• an electrolyte such as seawater or contaminated moisture

Removing or controlling one of these conditions can interrupt the galvanic cell.

Why This Matters on a Yacht

Dissimilar-metal interfaces are common throughout yacht construction and refit.

Examples include:

• aluminium superstructure connected to steel structure

• stainless-steel fasteners installed into aluminium

• stainless-steel deck hardware mounted onto aluminium plating

• bronze or copper-alloy valves connected to steel or aluminium pipework

• copper pipework passing through steel structure

• stainless-steel brackets attached to carbon steel

• propeller and shaft installations

• sacrificial-anode systems

• underwater fittings and transducers

A detail that appears mechanically strong can still create a serious long-term corrosion problem if the electrical and environmental interface has not been correctly considered.

GALVANIC CORROSION & DISSIMILAR METALS

Yachts commonly contain steel, aluminium, stainless steel, bronze, copper alloys and other metals within the same structure and machinery systems.

When dissimilar metals are electrically connected in the presence of an electrolyte such as seawater, a galvanic cell can be created. The less noble metal may then corrode preferentially.

For the Project Manager, the important lesson is that a mechanically strong connection between two metals is not automatically a satisfactory marine installation.

The Three Conditions Required

Galvanic corrosion normally requires:

• two electrochemically different metals

• electrical continuity between them

• an electrolyte such as seawater or contaminated moisture

If one of these conditions is effectively removed, the galvanic circuit can be interrupted.

Typical Yacht Interfaces

Areas requiring particular attention may include:

• aluminium superstructure connected to steel structure

• stainless-steel fittings installed into aluminium

• stainless-steel fasteners through aluminium plating

• bronze or nickel-aluminium-bronze underwater fittings

• propellers and shafts

• copper or copper-alloy pipework near steel structure

• sea valves and through-hull fittings

• sacrificial anodes

• bonding systems

• ladders, rails and deck hardware

• aluminium structures attached to carbon-steel foundations

GALVANIC AREA EFFECT

The relative exposed surface areas of the two metals can significantly influence the severity of galvanic attack.

A particularly unfavourable arrangement can occur where a relatively small area of active metal is electrically connected to a much larger area of more noble metal.

The corrosion current can become concentrated into the smaller anodic area, producing rapid local deterioration.

Example — Stainless Steel Fastener in Aluminium

A stainless-steel bolt installed directly through an aluminium structure may appear to be a simple fastening detail.

If seawater or moisture reaches the interface and there is no suitable isolation or protective system, the surrounding aluminium can become the preferentially corroding material.

The damage may initially remain hidden underneath the washer, fitting, sealant or coating.

Over time the aluminium around the fastening can pit, expand with corrosion products and lose structural section.

Project Manager's Question

The Project Manager should therefore not ask only:

"Is the fitting securely bolted in place?"

The PM should also consider:

"Are these materials compatible, and has the interface been correctly isolated and protected?"

CONTROLLING DISSIMILAR-METAL INTERFACES

The correct control method depends on the materials, structural function, location and approved vessel design.

Typical measures may include:

• approved electrical isolation

• insulating bushes

• insulating washers

• isolation gaskets

• suitable bedding compounds

• approved sealants

• protective coating systems

• controlled drainage

• prevention of trapped seawater

• approved transition joints

• corrosion-control systems

• sacrificial anodes where part of the approved design

• impressed-current systems where fitted

The Project Manager should not independently redesign the yacht's corrosion-protection system. The PM should ensure that repairs and modifications remain consistent with the approved technical arrangement.

STEEL-TO-ALUMINIUM STRUCTURAL CONNECTIONS

A steel hull with an aluminium superstructure creates one of the most important dissimilar-metal interfaces on many large yachts.

The aluminium superstructure cannot simply be conventionally fusion welded directly onto the carbon-steel hull.

An approved transition arrangement may therefore be incorporated between the two structures.

Transition Joints

A structural transition joint can provide a steel face on one side and an aluminium face on the other.

The steel side is welded to the steel structure and the aluminium side is welded to the aluminium structure using the appropriate approved procedures.

The transition region then requires careful protection from:

• seawater ingress

• trapped moisture

• coating breakdown

• mechanical damage

• galvanic interaction

During refit, the Project Manager should pay particular attention to any structural work close to an existing steel-to-aluminium transition because uncontrolled grinding, welding, water ingress or coating removal can damage an otherwise sound interface.

COPPER, BRONZE & STEEL INTERFACES

Copper and copper alloys are commonly found in yacht systems, but their relationship with surrounding steel or aluminium structure must be understood before repair work begins.

A copper pipe passing through or terminating at a steel deck is not simply a piece of pipe attached to a plate.

The designer may have incorporated a particular joint detail, transition, brazed connection, specialist weld, sleeve, isolation system or corrosion-protection arrangement.

Removing that detail without first understanding the original design can create both a joining problem and a corrosion problem.

Before Cutting a Mixed-Metal Detail

The Project Manager should establish:

• what each material actually is

• how the original materials were joined

• whether the connection is structural or only a system penetration

• whether electrical isolation is incorporated

• whether the joint forms part of a watertight boundary

• whether Class approval is required

• which repair procedure is applicable

• which welder or specialist is qualified for the required process

• how corrosion protection will be reinstated afterwards

STAINLESS STEEL — NOT AUTOMATICALLY CORROSION-PROOF

Stainless steel is extensively used on yachts because of its appearance and resistance to corrosion, but it should not be regarded as immune to the marine environment.

Stainless steel depends on a protective passive oxide layer at its surface.

Where oxygen availability is restricted and chlorides are present, localised corrosion can develop.

Areas Requiring Attention

• beneath washers

• underneath deck fittings

• inside poorly drained joints

• behind sealant

• threaded connections

• stagnant seawater areas

• crevices

• contaminated surfaces

This is one reason why a visually attractive stainless fitting installed against aluminium or steel can conceal significant corrosion underneath it.

Carbon-Steel Contamination

Stainless steel should also be protected from contamination by carbon-steel grinding dust, tools and fabrication debris.

Embedded carbon-steel particles can subsequently rust and compromise the quality and appearance of the stainless surface.

Dedicated fabrication tools and appropriate post-weld cleaning or passivation procedures may therefore be required according to the specification.

SACRIFICIAL ANODES & ELECTRICAL CORROSION CONTROL

Sacrificial anodes form part of the corrosion-protection strategy on many yachts.

They are deliberately manufactured from a material intended to corrode preferentially and protect connected underwater metallic structure.

The Project Manager should ensure during refit that:

• the correct anode type is installed

• the correct quantity is installed

• the specified locations are maintained

• electrical contact is satisfactory where required

• anode contact faces are correctly prepared

• anodes are not accidentally painted

• replacement fasteners are suitable

• the original corrosion-protection design is not casually altered

Galvanic Corrosion Versus Stray-Current Corrosion

Galvanic corrosion and stray-current corrosion are related to electrical activity but are not the same problem.

Galvanic corrosion results from the natural electrical potential between dissimilar metals in an electrolyte.

Stray-current corrosion can result from unintended electrical current entering or leaving underwater metallic structure.

A defect in shore power, onboard electrical equipment, bonding systems or neighbouring installations can therefore create very aggressive local corrosion.

Where unexpectedly rapid underwater-metal deterioration is discovered, the Project Manager should avoid assuming that the problem is simply poor-quality metal or failed antifouling. The vessel's corrosion-protection and electrical systems may require specialist investigation.

STEEL & ALUMINIUM INTERFACES

The connection between a steel hull and aluminium superstructure requires particular control because the two metals are significantly different electrochemically.

The design should therefore provide both a structurally approved connection and effective long-term corrosion protection.

Transition Joints

An approved steel-to-aluminium transition joint may be used where the aluminium superstructure is permanently connected to the steel structure.

The steel face of the transition joint is welded to the steel structure, while the aluminium face is welded to the aluminium structure using the appropriate approved procedures.

The Project Manager should ensure that the transition area is treated as a critical structural and corrosion-control detail.

Protection of the Interface

Particular attention should be given to:

• coating continuity

• sealing of exposed edges

• prevention of water traps

• drainage

• condition of transition material

• mechanical damage to protective coatings

• contamination during later refit work

Repairs near a steel-to-aluminium transition should not be treated as ordinary local welding without reviewing the original structural detail and approved repair requirements.

STAINLESS STEEL IN ALUMINIUM STRUCTURE

Stainless steel is widely used for yacht fittings because of its corrosion resistance and appearance, but direct installation into aluminium can create a severe galvanic-corrosion risk if the metals are not properly isolated.

Typical examples include:

• stainless-steel bolts through aluminium decks

• handrail foundations

• cleats

• hinges

• door hardware

• deck fittings

• equipment foundations

Isolation

Depending on the approved installation, isolation measures may include:

• insulating washers

• insulating bushes

• non-conductive gaskets

• approved jointing compounds

• suitable coating systems

• sealed fastener holes

The objective is to prevent direct electrical contact and prevent seawater or moisture from remaining within the joint.

A small stainless-steel component connected to a much larger aluminium structure can still produce serious local corrosion around the aluminium interface if protection fails.

COPPER ALLOYS & STEEL

Copper and copper-alloy components may be found in seawater systems, drainage systems, valves, heat exchangers and other marine equipment.

Where copper alloys interface with steel structure, the Project Manager should consider both the joining procedure and the long-term corrosion behaviour of the installation.

A connection that appears mechanically sound may still require:

• positive material identification

• an approved dissimilar-metal joining procedure

• electrical isolation where appropriate

• sealing against seawater ingress

• suitable coating protection

• drainage to prevent retained moisture

• inspection of the surrounding steel for accelerated corrosion

Lesson from Refit Work

If a repair reveals an unexpected copper, bronze or other non-ferrous component within steel structure, the correct response is not to continue welding until something appears to hold.

The materials should first be identified and the original drawing, approved construction detail or suitable repair procedure established.

SACRIFICIAL ANODES & CATHODIC PROTECTION

Sacrificial anodes are intentionally designed to corrode preferentially in order to protect more valuable underwater metallic structure.

Their effectiveness depends on correct material selection, position, electrical continuity and sufficient exposed surface area.

Project Manager Checks

During a dry-dock period, the PM should coordinate verification of:

• correct anode material

• correct quantity

• correct position

• secure installation

• electrical contact where required

• condition of bonding connections

• absence of paint over active anode surfaces

• replacement of excessively consumed anodes

Painting over an active sacrificial anode can significantly reduce or prevent its intended protective function.

Similarly, installing the wrong anode material simply because it physically fits is not an acceptable substitution.

DISSIMILAR-METAL REPAIR — PM CONTROL

When a repair involves more than one metallic material, the Project Manager should pause before treating the work as a routine fabrication task.

The PM should establish:

• What are the actual materials?

• How have the materials been identified?

• What does the original drawing specify?

• Is there an approved connection detail?

• Is welding appropriate?

• Is brazing, transition material or another joining method required?

• Is the welder qualified for the material combination and process?

• Does the WPS cover the proposed joint?

• Does Class need to review or approve the repair?

• How will galvanic corrosion be controlled?

• How will moisture be excluded?

• How will coatings be reinstated?

• How will the completed joint be inspected and tested?

Why Technical Knowledge Matters

A Project Manager who controls only programme and cost may identify that a subcontractor is late or over budget.

A technically competent Project Manager may recognise the problem before the incorrect work is completed.

This can prevent rework, structural damage, Class rejection, safety problems, programme delay and significant additional cost.

Technical competence therefore does not remove responsibility from the specialist. It enables the Project Manager to challenge, verify and coordinate specialist work intelligently.

PROJECT MANAGER COMPETENCY TEST

Scenario 1 — Replacement Steel

A subcontractor has prepared a replacement plate for a corroded section of hull structure. The dimensions and thickness are correct, but no material certificate or confirmation of steel grade is available.

Question:

What should the Project Manager do?

A. Accept the plate because the thickness is correct
B. Accept it if the welder says it is marine steel
C. Verify the specified material grade and required traceability before the plate is installed
D. Install the plate and identify the steel afterwards

Scenario 2 — Hot Work in a Restricted Space

A steel repair is required inside a deep bilge area. Access is restricted and electrical cables, pipework and insulation are located close to the proposed cutting area.

Question:

What should happen before hot work begins?

A. Begin cutting and move services only if they become a problem
B. Establish the hot-work and confined-space controls, identify and protect surrounding services, provide ventilation and assign the required fire watch
C. Ask the welder to work quickly to reduce heat exposure
D. Cover the cables with ordinary plastic sheeting

Scenario 3 — Additional Corrosion After Crop-Out

Class has accepted an initial repair boundary. After the defective plate is removed, further corrosion becomes visible behind the original crop line.

Question:

What should the Project Manager do?

A. Install the replacement plate according to the original dimensions
B. Grind the additional corrosion and continue
C. Hold the repair and allow the affected structure and repair boundary to be reassessed before continuing
D. Cover the remaining corrosion with additional weld metal

Scenario 4 — Unsuccessful Dissimilar-Metal Weld

A welder attempts to reinstate a drain connection through a steel deck. The weld does not behave as expected and the affected opening becomes larger after repeated attempts. The welder has not reviewed the original construction drawing.

Question:

What is the correct Project Manager response?

A. Tell the welder which welding process to use and continue
B. Allow further attempts until a satisfactory weld is achieved
C. Stop the work, identify the materials and original construction detail, then establish the appropriate approved repair procedure
D. Fill the opening with weld metal and grind it flush

Scenario 5 — Stainless Fitting on Aluminium

A stainless-steel deck fitting is being installed directly onto an aluminium structure. The installation is mechanically strong, but no isolation arrangement is shown and seawater can reach the joint.

Question:

What should concern the Project Manager?

A. Only whether the bolts are tight enough
B. Potential galvanic corrosion and whether the approved installation provides suitable isolation and sealing
C. Only the appearance of the stainless steel
D. Nothing, because stainless steel does not corrode

MODULE COMPLETION

Before completing this module, the Project Manager should be able to:

• Understand why different structural materials and grades are used in different areas of a yacht

• Recognise the distinction between normal-strength and higher-strength marine steels

• Understand why low-temperature and Polar operation can require increased toughness and additional structural consideration

• Recognise the common use of marine aluminium in hull and superstructure construction

• Understand the importance of approved steel-to-aluminium transition arrangements

• Verify replacement material grade, thickness and traceability before structural installation

• Understand the difference between new-build fabrication and structural repair during refit

• Recognise that an approved welding procedure and suitably qualified welder may be required

• Understand the Project Manager's role without attempting to replace the welding specialist

• Coordinate hot-work permits, ventilation, fire watch and protection of surrounding systems

• Recognise the additional controls required for welding in confined or enclosed spaces

• Identify cables, pipework, insulation and other hidden services before cutting or welding begins

• Understand when proposed modifications to essential pipework should be referred for technical or Class review

• Coordinate Class-controlled structural repairs and inspection hold points where required

• Understand crop-and-renew repair principles

• Recognise that additional corrosion may be revealed after the initial crop-out

• Coordinate replacement-plate fit-up, alignment and inspection before final welding

• Understand the purpose of temporary strongbacks, dogs and other fit-up attachments

• Recognise the importance of welding sequence, heat input and distortion control

• Understand the basic purpose of root, fill and cap passes in multi-pass welding

• Recognise the use of approved one-sided full-penetration welding with temporary backing where reverse-side access is unavailable

• Understand the difference between one-sided welding with backing and back gouging

• Coordinate visual inspection, NDT and tightness testing after structural repair

• Recognise the risks associated with dissimilar-metal interfaces

• Understand the basic principle of galvanic corrosion

• Recognise that stainless steel, aluminium, copper and bronze interfaces may require specific isolation or joining arrangements

• Stop work where materials, procedures or qualifications have not been properly established

• Obtain and review original drawings when an existing construction detail is unclear

• Understand that technical competence allows the PM to challenge and control specialist work without instructing the specialist how to perform their trade

• Maintain a complete structural-repair record including drawings, material certificates, welding records, Class correspondence, NDT and photographs