Skin fittings and other potential hull flooding points.

In my quite extensive experience of boats sinking (30+ years recovering sunken boats and investigating causation for insurance companies, and 45 years as a principal in the marine trade) I would say that significantly more incidents of sinking or inundation occur due to faulty or poorly designed skin fittings or other flooding points (technically known as “down flooding points”) related to the overall hull rather than due to deficient hull shell plating.

Unfortunately, an inspection of significant skin fittings / shell perforations, related internal plumbing and other potential flooding points is often not part of a hull condition survey regime, but should be in my opinion. Some six different independent surveyors have conducted “hull” surveys on my own ship over the years and only two (both British) inspected the skin fittings carefully. No TRIWV or ES-TRIN surveyor has made a careful inspection of the skin fittings.

The dimensions mentioned below are essentially my own figures, guided by empirical best practices, experience and, where relevant, the requirements of the most normal periodic inspection bodies for our types of craft. Possession of the relevant certificates should not be taken as proof of condition. The Boat Safety Scheme in the UK takes no interest in the design or condition of skin fittings or flooding points in private pleasure boats, indeed BSS does not address hull condition in any way. ES-TRIN covers the subject exceedingly briefly in 3.03.6, simply stating with no detail that The water inlets and discharges, and the pipework connected to these, shall be such that no unintentional ingress of water into the vessel is possible.” No figures or technical standards are cited. The Recreational Craft Directive references a minimum (down) flooding height above the water line of 400mm, but is relatively silent on other skin fittings connected to internal plumbing systems, and is of course applicable at point of sale only, with no ongoing inspection requirement.

“Significant skin fittings” (my phrase) are those that are at or beneath the water line, or within say 250mm above the normal static water line, presuming a pleasure boat of reasonably constant draft. Above 250mm from the water line suitably plumbed fittings are regarded as safe under normal operating conditions for a pleasure boat based largely on inland waterways. Open flood points (e.g. air vents, scuppers etc.) should have a safe freeboard (clearance above the normal water line) of at least 400mm.

The following comments are in no particular order, but reflect real, recorded failures within my own experience, not hypothetical problems. Examples, often numerous and repetitive examples, exist in my incident report archives.

  1. Brass flanged fittings can de-zincify, losing their strength. Brass and bronze flanged fittings can suffer simple physical impact damage, and are particularly weak at the threads, where they can shear off, leaving a large hole in the hull. Most insurers consider these fittings unsuitable for use in heavy displacement steel boats inland. They might be acceptable if protected from physical impact by guard irons fore and aft of the fittings.

  1. Welded steel pipe spigots can corrode, and/or the welds to the shell plating can corrode and fail. Often these pipes are only welded externally and weld penetration may be poor. The pipe wall thickness is usually less than for the shell plating since yards often use what can be found on the shop floor. Such fittings, if more than 20 years old, or if showing heavy corrosion, should be condemned and replaced, ideally using thick walled tubing (8mm +). It is usually not possible to confirm the thickness of these fittings reliably by ultrasonic measurement, but I have had them break off in my hands or when whacked with a small hammer.

  2. Bolted skin fittings such as Blakes pattern sea cocks can fail at the bolts. The bolts could be brass, not bronze, or stainless steel. The latter may have suffered crevice corrosion. If old, sample bolts should be removed for inspection and/or new bolts fitted.

  3. Stand pipes are often of very thin walled tube and seldom get painted internally with a bottle brush. These too can be difficult to measure accurately due to the convex/concave surfaces presented, but again I have had them break off in my hands due to excess corrosion. Routine replacement is often the most prudent course, ideally with thick walled tubing.

  4. Sea cocks, particularly standard quality gate valves, are often not efficient (i.e. do not actually seal!). Ball valves can suffer frost damage, particularly if at the water line and/or if orientated such that water is retained in the ball when the valve is closed. The water freezes, expands linearly in the bore through the ball and fractures the relatively weak cast brass valve casing. Frequently sea cocks are simply not accessible. They are there to be turned off when the boat is left unattended, so they need to be readily accessible. The presence of sea cocks does not negate the need to ensure that internal plumbing is secure up to a suitable safe freeboard above the water line.

  5. Internal plumbing is often in unsuitable materials (weak hose, brittle domestic quality PVC waste pipe, push fit fittings etc.) with potential for physical damage or degradation. All plumbing that is submersed or close to the water line, thus potentially submersed (for example if the boat lists over), or that is arranged such that a syphon could develop, should be installed using spiral wound reinforced suction hose secured over proper metal hose tail fittings using twin stainless steel jubilee clips.

  6. Redundant fittings are often not properly closed off. Why have a hole in the boat that is not essential? Internal blanking plugs are not really the answer. Welded plates should be fitted externally.

  7. Bilge pump outlets should be carefully identified. Many owners do not appreciate that the most common form of bilge pump, a submersible impeller pump, has no valves. If the outlet becomes submersed then water can syphon back into the boat. Similar things can happen with shower drain pumps if the outlets are too low. There is a case for saying that all pumped outlets (bilge pumps, shower pumps, washing machines, dishwashers etc.) should emerge overboard at least 400mm above the normal static waterline

  8. Dutch bilge pumping systems often incorporate a single mechanical pump and a valved manifold controlling several bilge suctions and a deck wash connection. Incorrect operation of the valves can result in an open path between the deck wash inlet and a bilge suction!

  9. Special attention needs to be paid to sea toilet plumbing and anti-syphon provisions..

  10. Mud boxes and raw water strainers installed below the water line should be frost resistant.

  11. Weed hatches should have a trunking extending at least 150mm above normal static water line, but preferably more. Covers/lids should be secured suitably. If quick release clamping devices are used (applying pressure centrally to the cover plate) then the covers should be sufficiently strong to resist warping under pressure. The sealing gasket should be made from a single piece of neoprene rubber or similar, not several strips of neoprene tape jointed at the corners. The trunking thickness should be measured at survey.

  12. Rudder stock trunks should extend at least 150mm above the water line and have suitable sealing arrangements as appropriate. A stuffing gland or other positive seal should be fitted if the trunk extends less than say 250mm above the water line. The trunk should be measured.

  13. Bow thruster tubes should be assessed, particularly adjacent to the propeller(s) where severe galvanic corrosion can occur if the propeller is bronze, and where cavitation erosion can occur if the propeller is of any material.

  14. Spud pile trunks and other such structures are all wetted surfaces in at least part and need to be inspected.

  15. Transducers, usually plastic these days and easily damaged, should be installed in recessed boxes or in thick walled stand pipes.









  1. Gas lockers and other “internal” but integral structures are often forgotten. Numerous boats have sunk because the gas locker base is close to the water line. Water enters through the gas locker drain holes and causes corrosion within the lower areas of the locker, and the locker inner walls may only have been built from thin steel originally, despite being in actuality significant wetted hull shell surfaces. Often these areas are the last to get maintained. Gas lockers are the favourite, but also box coolers, box cooler vent pipes, other sea chests, basically all surfaces that get wetted by canal water, should be regarded as hull shell surfaces and should be carefully checked.


  1. Self-draining deck scuppers should have a safe freeboard to flooding level of at least 400mm. For narrow boats or wide beams this could be the forward door threshold if the well deck is sealed otherwise. Any aperture in the hull shell that could permit direct unregulated water ingress into the hull must be at least 400mm above the water line, but preferably should not be in the hull at any point below main weather deck level.


It must be noted that the ability of any surveyor to inspect all relevant areas does depend upon there being suitable access to inspect, either from the exterior, or the interior, or both as appropriate, and the client is supposed to provide that access. A surveyor is entitled to exclude from his report areas that are not presented in a readily accessible state. However, your surveyor should make as thorough an inspection as is possible of all parts of the vessel that are wetted by the water it is supposed to float on, and he should tell you if there are areas that he cannot reach.

Just in case the worst happens, you should always check all bilge pumps frequently to make sure that they actually work and that they are not blocked. Automatic switches should also be checked regularly, since these are often the first components to fail.


Balliol Fowden.

Fellow & Full Member,    The Yacht Designers & Surveyors Association.

Copyright © Balliol Fowden 2019




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