Hull crackrepair.

A crack in the shell plating is quickly welded up. Whether it stays closed depends on three questions: where does it come from, how is it gouged out and tested, and is a weld enough or does plate have to be renewed? For all three there is a yardstick: the IACS Shipbuilding and Repair Quality Standard, Recommendation No. 47, Part B¹.

Rec. 47 is an IACS recommendation, not a rule; the current status is Rev. 10 with Corrigendum 1 of October 2025². Its applicability is agreed with the classification society in each case, and for critical, highly stressed areas class may require a more stringent standard¹. This article summarises what Rec. 47 provides and adds the causes described in IACS guidance. It is written for superintendents, yards and anyone who has to assess a crack in a hull.

01Why does hull steel crack?

Mostly where stresses concentrate. IACS groups structural defects into material wastage, fractures and deformations⁴. Fractures that develop under repeated stresses below the yield stress are fatigue fractures, caused for example by wave loads or by vibration from main engine and propeller⁴.

According to the IACS guidelines for bulk carriers, fractures are mostly found where stresses concentrate: at weld defects and where lifting fittings were not properly removed. Typical spots include bracket toes, sniped ends of stiffeners and knuckles⁴. Contact damage from quays, ice or underwater objects is another cause; the guidance warns that even when the shell looks only slightly damaged from outside, the internal members are often heavily damaged⁴. Grooving corrosion along welds can also create stress concentrations⁴.

02What needs to be settled before the repair?

Class first. Under IACS, repairs to hull structure covered by classification are in general carried out under the supervision of the class surveyor¹.

Then the cause. Rec. 47 is clear: restoring the structure to the original standard may not be a durable repair where the damage comes from insufficient strength or inadequate detail design¹. For fatigue cracks from wave loading, IACS guidance recommends considering an improved structural detail⁴.

And the material. Replacement material is in general to be of the same grade as the original approved material, and higher tensile steel is not to be replaced by steel of lesser strength unless specially approved by class¹. Hull steel grades are defined in IACS UR W11⁵:

The letter "H" may be put in front of or behind the grade mark, so AH36 and HA36 are the same grade⁵. Rec. 47 Part B generally does not apply to special ship types such as gas tankers or to structures made of stainless or other special steels¹.

03How is a crack repaired to IACS Rec. 47?

It is gouged out completely, tested and rewelded with low-hydrogen consumables. What Part B, section 6.8 provides^1,3:

• Groove: V-shaped with an opening angle of 45 to 60 degrees and a root radius of 5 mm. Through-plate cracks are prepared as for newbuilding.
• Extent: as standard for cracks in plating up to 400 mm long; the limit is 500 mm for a straight, unbranched crack. The groove extends 50 mm past the end of the crack.
• Groove inspection: 100 % magnetic particle or penetrant testing before welding.
• Welding: always with low-hydrogen consumables; step-back technique for cracks longer than 300 mm.
• Weld termination: slope of 1:3. Where a crack ends at an edge, the weld ends on a run-off tab.
• Testing afterwards: 100 % surface crack detection, plus ultrasonic or radiographic testing for butt joints.

04When is an insert plate needed?

When the damage is too large for a weld or the plate is wasted. The damaged area is then cropped and renewed. The IACS guidelines for bulk carriers say that where part of the structure has deteriorated to the permissible minimum thickness, the affected area is in general to be cropped and renewed⁴.

For insert plates Rec. 47 gives these values^1,3:

• Size: standard at least 300 × 300 mm with a corner radius of 5 × plate thickness; circular inserts at least 200 mm in diameter. The limit is 200 × 200 mm with a radius of at least 100 mm.
• Material: same grade as the original or higher.
• Edge preparation and alignment: as for new construction.
• Weld sequence: 1, 2, 3, 4. For primary members welds 1 and 2 run transverse to the main stress direction.

05Can you weld on a doubler plate?

Only as a temporary repair. Under Rec. 47, local doublers in the main hull structure are normally only allowed as temporary repairs, except as original compensation for openings. Where the existing plating is less than 5 mm thick, a permanent repair by insert is required¹.

For a temporary doubler Rec. 47 specifies at least 300 × 300 mm, rounded corners with a radius of at least 50 mm, a doubler thickness not more than the original plate and more than a third of it, and circumferential and slot welds of 0.6 × doubler thickness^1,3. The IACS bulk carrier guidelines add that doubler plates must not be used to compensate for wasted plate⁴.

An article by the International Institute of Marine Surveying puts it plainly: overplating does not constitute a repair, it merely hides the defect⁷. Where class accepts a temporary repair, it can attach a condition of class with a time limit.

06Does it need preheating?

That depends on steel, process and restraint. Rec. 47 says the need for preheating is determined by the chemical composition of the materials, the welding process and procedure and the degree of joint restraint. Below 0 °C ambient temperature a minimum preheat of 50 °C applies, and the weld zone must always be dry¹.

For higher strength steels Rec. 47 includes a table of recommended minimum preheat temperatures by carbon equivalent and combined thickness. For re-welding or repair of welds these values are to be increased by 25 °C¹. The exact temperature belongs in the WPS.

The reason is hydrogen. According to TWI, hydrogen cracks need three things at once: hydrogen from the welding process, a hard, susceptible microstructure and tensile stress. Preheating slows cooling, lets some hydrogen diffuse away and reduces hardness⁶. Rec. 47 also provides that welding is sheltered from rain, snow and wind¹.

07How is the repair tested?

With non-destructive methods, each with its own standard:

NDT personnel must be qualified to the rules of the society or a recognised scheme¹, for example to ISO 9712¹⁴. For NDT methods and acceptance standards, IACS UR W33 may be used as reference, although its scope is newbuilding¹.

Where thickness measurements are part of a class survey, for example because of wastage, IACS generally requires a firm approved by the society¹⁵.

08How does Nauticon handle a crack repair?

1. Record. Location on the hull, visible length, photos. For contact damage, look at the internal members as well.
2. Agree. With owner and class: type of repair, surveyor attendance, material.
3. Prepare. Hot work only with a fire watch, the hot work permit as required by owner or yard. Access, staging, lighting, weather protection, preheat where needed.
4. Gouge and test. Groove to past the crack end, then magnetic particle or penetrant testing.
5. Weld. To the WPS, by welders qualified to EN ISO 9606, under welding coordination to EN ISO 14731.
6. Test and accept. NDT to the agreed extent, acceptance with the surveyor, feedback to you.

09Summary

A crack in the hull points to a cause. Rec. 47 gives clear figures for the repair: groove 50 mm past the crack end, 100 % testing of the groove, low-hydrogen consumables, inserts with generous radii. Doublers are a temporary fix. Where the structural detail itself is the problem, the change should be discussed with class.