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Why boats sink at the berth
Short answer
Many boats go down alongside, at night, with nobody aboard and no witness at all. What decides the file afterwards is not how much water came in but whether it came in suddenly, and that argument is won at the skin fittings.
Many boats that sink do it alongside rather than offshore, usually at night and with nobody aboard. That is the hardest case to deal with afterwards, because there is no witness, no sequence of events anyone can describe and often no visible cause, only a boat on the bottom of a marina berth and a question about how the water got in. Marinas on the Solent, on the Broads and along the Dutch canals see it every winter, most often in the weeks when owners visit once a fortnight and the pontoon is empty in between.
The dividing line that matters in a hull wording is sudden against gradual, not a lot of water against a little. A grounding, a collision, a hose that bursts under pressure or a skin fitting that shears are events with a moment attached to them. A seacock that has wept for years, a perished stern gland, a hose clip that has slowly given up are not events at all, they are wear, and wear sits outside these wordings by design. The awkward part is that the two meet inside the same component: a ten year old hose goes hard on the inside, then splits in a single second, and the surveyor's report has to decide which of those two facts caused the loss.
Everything useful an owner can do sits in the short list of places where it starts: seacocks and their hoses, the stern gland, log and echo sounder transducers, cockpit drains and the heads inlet. Double clip every hose below the waterline, work each seacock once a season and note the date, tie a soft wood plug beside each skin fitting instead of burying the set in a locker, and keep the bilge dry so that new water is obvious. Close the seacocks on leaving, which is the one habit that reliably prevents the berth case. Afterwards, ask the marina office for the camera recordings on the same day, in writing, because many systems overwrite within a week.
How to stop a leak on a boat: the next few minutes
When water is coming in, the order matters. Start every pump, give the crew clear jobs, and look for the source while there is still time to see it. Close every seacock you can reach, which stops a large share of leaks at once. If the water is still rising, call for help early on the VHF, because a pan pan that turns out to be unnecessary is far better than a mayday sent too late.
Once the source is found, stop it with what is at hand: a soft wood plug driven into a broken skin fitting or hose, rags and a cushion pressed against a crack and held in place, underwater epoxy putty for smaller holes. For damage to the hull from outside, a collision mat or a tarpaulin pulled over the hole from outside uses the water pressure to seal it. Reduce speed and, on a sailing boat, change tack if that lifts the damage out of the water.
Finding the leak: taste the water and follow it
The quickest test is taste. Fresh water usually comes from the tanks, the pipework, the calorifier or rain, salt water from below the waterline or through the deck. Then follow the water upwards: dry the bilge in sections, use kitchen paper to find where new water appears, and check whether the leak happens at rest, under engine or under sail when the boat heels.
Leaks that occur when the engine runs often come from the stern gland, the raw water pump or the cooling circuit. Leaks under sail can point to the keel bolts, the chainplates or a hull deck joint. Keep a note of what you find, with photographs, because a leak that appears and disappears is much easier for a yard or surveyor to trace with that record.
Seacock inspection and replacement: materials and wear
Seacocks are made of bronze, DZR brass or reinforced plastic. Ordinary brass is not suitable below the waterline, because it loses its zinc in seawater, turns pink and becomes brittle without much warning. Check the material of each seacock and through hull fitting, and replace any made of the wrong metal. Look at the handle movement, the backing pad, the hull around the fitting and the hoses connected to it.
There is no fixed age at which a seacock must be replaced; the condition decides. Have them inspected whenever the boat is out of the water, work each one regularly so it does not seize, and replace any that are stiff, weeping or of doubtful material. A surveyor can tap test a fitting and look at it from both sides, which is more reliable than a glance from inside.
Bilge pump: capacity, float switch and high water alarm
The capacity printed on a bilge pump is usually measured with no lift and full voltage. Installed in a boat, with a hose rising to the outlet and a battery that is no longer full, it delivers noticeably less. Fit at least one electric automatic pump with a reliable float switch and a manual pump that can be worked from the cockpit, and check both at every visit to the boat.
Float switches stick, fill with debris or fail, so an independent high water alarm, ideally one that sends a message to a phone, is the most useful addition for a boat left alone. Check the outlet as well: a pump discharging through a hull fitting close to the waterline can siphon water back in when the boat heels or sits low, unless a vented loop or a non return valve prevents it.
Worth remembering
Sudden
is an event with a moment attached
Gradual
is wear, and wear sits outside
Both end with the same boat on the bottom. Only one of them ends with a file that goes anywhere.
Questions
What owners ask us about this
My boat sank at the pontoon, what decides the claim?
The cause rather than the place. If an event can be identified, a grounding, a burst hose, a failed skin fitting, the file has a starting point. If all that remains is a fitting that leaked for years, the wording will treat it as wear.
What about a seacock left open?
That is gross negligence, and wordings differ sharply here: some reduce the settlement, some decline it outright, and a few say nothing at all, which is the worst version of the three. It is a line worth reading before the season starts.
Which fittings are worth checking first?
Seacocks and their hoses, the stern gland, transducers, cockpit drains and the heads inlet. Hoses below the waterline want two clips each, and every seacock should be worked once a season with the date written down.
What evidence is worth collecting straight away?
Photographs before anything is moved, the marina camera recordings requested in writing on the same day, dated invoices for hoses and seacocks replaced earlier, and the name of whoever was last aboard.
What should I carry on board to stop a leak?
Soft wood plugs tied next to each skin fitting, underwater epoxy putty, self amalgamating tape, spare hose clips and hose, rags, and a collision mat or tarpaulin for damage to the hull.
How often should a seacock be replaced?
There is no fixed interval. Have it inspected whenever the boat is ashore and replace it if it is stiff, weeping or made of unsuitable material such as ordinary brass.
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