• Background

    Information Centre


The following account details how an almost certain sinking was narrowly averted by detection of hidden corrosion within critical stainless steel clamping bolts in a number of non-metallic seacocks.

A non-metallic seacock is an attractive proposition for owners of metal boats as there is no possibility of galvanic (dissimilar metal) corrosion in an item which is critical to the integrity of the boat. The normal pattern of construction is a ball valve with a body formed in two halves from fibre-reinforced polypropylene, or other engineering plastic, with a Delrin (engineering nylon) ball. The two halves are machined internally to accept the Delrin ball, which turns easily in a low-friction, plastic-to-plastic contact surface. Each half of the body incorporates a thick flange on the extremity and, when assembled, the two halves are sandwiched together with the ball on the inside. The assembled unit is clamped together with four stainless steel bolts and nuts. With the non-metallic (plastic) material kept in compression by the bolts, the assembled unit is very strong. It is also corrosion-free on the inside, low friction, and requires no lubrication. It is almost impossible for scale or grit from the seawater to get into the contact area between ball and valve body and the valves do not seize or stick. They are essentially maintenance-free.

The assembly is drilled and threaded top and bottom to incorporate inlet and outlet attachments. On a metal boat the normal arrangement is to have a tubular metal spigot welded to the hull, threaded at its inner end; the seacock is then screwed onto the spigot. A plastic hose nipple is then normally screwed into the threaded hole on the inboard face of the seacock to accept plastic piping and hose clips.

The integrity of the assembly obviously depends on the clamping action of the 4 bolts keeping the two halves together. If the bolts become loose the unit will leak and should the bolts fail, the unit will separate with catastrophic results.

In practice, if the unit is mounted in the vertical plane (as is normal), and if seawater leaks or drips from the hose connection, it will accumulate on the horizontal upper flange, which will always be wet. This environment can lead to stress corrosion cracking of the stainless steel in the area between the head of the clamping bolts and the Marlon surface of the flange which is wet, salty and oxygen-free. The result of this corrosion mechanism is for the heads to eventually drop off the bolts, thus destroying the clamping force which keeps the unit together. In the limit, the two halves of the unit will separate and the valve will fall apart.

In the case of my own boat, Al Shaheen, this is exactly what almost happened. Al Shaheen is an aluminium alloy hull launched in 2001. She is fitted with 8 such seacocks in 1½ “, 1” and ¾” sizes, all fitted in the vertical plane. These are sold in the UK as “Patay” seacocks and were supplied by Pump International in Falmouth. In most cases, at some stage, there had been some slight leakage of seawater from the pipework above the unit which had deposited on the upper flange and caused corrosion of the clamping bolts. In 2008 (7 years after launch) an inspection of the clamping bolts revealed that at least half were badly rusted and in 6 of the total 32 bolts (20%), the head fell off when touched. One valve was in a very dangerous condition with 3 of the 4 bolt heads rusted through. All 32 bolts have now been replaced with new and bedded in place with lanoline under the heads to exclude seawater from the bolt head/Marlon interface.

This was a particularly annoying and distressing discovery as, in general, Al Shaheen is a well maintained boat and subject to continuous monitoring for corrosion, as is necessary with aluminium construction. All the seacocks are included in the routine inspection programme, as one would expect with items of a critical safety nature. The fact that the corrosion of the clamping bolts had not been detected visually was partly due to it occurring in the crevice between the top of the Marlon flange and the underside of the bolt heads where it was hidden from view.


1. The corrosion mechanism described above represents a serious threat to the integrity of this type of seacock.
2. Clamping bolts should be inspected at least annually. If evidence of corrosion is detected around the bolt heads, those bolts affected should be withdrawn and replaced. Single bolts on any valve may be withdrawn and replaced whilst afloat, if necessary, provided that the remaining 3 are known to be intact and tight. If there is any doubt about the condition of the other bolts, a safer procedure would be to have the vessel hauled out and work carried out ashore.
3. This type of seacock is an excellent choice for metal boats but the clamping bolts must be maintained in good condition. Bedding the bolts in lanoline seems to prevent seawater ingress into the crevice under the bolt heads which is the area most susceptible to corrosion, being oxygen-free.

J F Franklin

s/v “Al Shaheen”
10 November 2011


On 14 January 2014, at approximately 1230, the 14m wind farm support catamaran, ECC Topaz, caught fire off the coast of Lowestoft. The fire is thought to have originated in a compartment in the starboard hull directly under the wheelhouse. The skipper became aware of the fire when he saw smoke coming out of the heating air outlet vents. Within seconds, the fire had broken through into the wheelhouse and spread rapidly throughout the vessel’s GRP structure.

Although the vessel could carry a maximum of 14 people, there were only three crew members on board on the day of the accident. Due to its rapid spread through the vessel, the crew were not able to extinguish the fire and were forced to abandon the vessel to a liferaft, from which they were airlifted to safety. The burnt out wreck of the vessel sank at 1420.

A few days after the accident, the MAIB received information from the owners of similar workboats stating that charring to the underside of the wooden deck through which an uninsulated section of a diesel fired air heater exhaust pipe was routed had been noted (Figure 2). Further investigation identified that it also was possible that ECC Topaz’s exhaust pipe might have deteriorated, allowing hot exhaust gases to impinge directly onto the wooden deck above or flammable substances stored in the heater compartment. These included drums of diesel and lubricating oils, sacks of rags and paper rolls.

Safety Lessons

Uninsulated exhausts from air heaters can reach temperatures well above the auto ignition temperature of many flammable materials including plywood, wood, paper and cotton. Therefore, any contact between either exhaust gases or uninsulated exhaust pipes and these materials has the potential to start a fire.

To ensure that your vessel is not at risk from this hazard:
- Check that all the exhaust systems on your vessel are adequately lagged.
- Inspect the exhaust pipes frequently for signs of deterioration and replace them if required.
- Follow the inspection and maintenance schedule for your vessel’s air heaters as required by the manufacturer.
- Do not store flammable material in the heater compartment.

This investigation report is posted on the Marine Accident Investigation Branch website:

For all enquiries:
Marine Accident Investigation Branch
Mountbatten House, Grosvenor Square, Southampton, SO15 2JU
Tel: 023 8039 5500, Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Marine Accident Investigation Branch

Client Login
Request Callback
Make a Payment