The numbers on through-hull flooding are the kind that change how you look at the seacocks under your sink. A failed fitting of 25mm bore, sitting about 0.9 metres below the waterline, lets water in at roughly 129 litres a minute. That is around 7,500 litres an hour. No bilge pump on a cruising yacht keeps up with that, and no human bailing with a bucket comes close. A boat with that kind of hole in her can go from dry to swamped in well under an hour. Through-hull failure is not a leak you nurse home; it is an emergency you stop at source or you lose the boat.
I learned the maths the hard way, not from a sinking but from a survey. A diver found my engine raw-water seacock seized half-open and the bronze gone pink with dezincification, the classic sign that the zinc has leached out and the metal has the strength of a digestive biscuit. One firm tug from a tangled line and it could have snapped off below the waterline. That fitting is now replaced, and I think about it every time I leave the boat.
Where the water gets in
The phrase "through-hull failure" covers a short list of usual suspects, and knowing them tells you where to look first when the bilge alarm goes:
- A seacock that snaps off, usually corroded bronze or cracked plastic, leaving an open hole the size of the fitting.
- A hose that splits or jumps off its barb, often the engine intake or the heads, which floods just as fast as the seacock would.
- A failed stern gland or P-bracket letting water past the propeller shaft.
- A cracked skin fitting after a grounding, a lightning strike, or simple age.
- The log or depth transducer pushed out of its housing, which leaves a startlingly large hole.
The common thread is that every one of these is below the waterline and under pressure, so the flow is constant and the boat is filling whether you are looking at it or not.
Stopping it: the only thing that matters first
When water is coming in, finding and stopping the source beats everything, including the radio. A boat that is still floating buys you all the time in the world to make a tidy call. A boat going down does not.
- Carry a soft wooden bung tied to or near every seacock. A tapered softwood plug hammered into a broken skin fitting will slow or stop the flow long enough to think. This is the single cheapest piece of safety kit on the boat.
- Know where your seacocks are and that they actually turn. Exercise them at the start of every season. A seacock you cannot close in calm conditions is no use when the water is rising.
- For a split hose, get the seacock shut behind it. For a hole you cannot reach, a soft pad, a cushion, a sail bag, anything jammed against it from inside and braced, slows the flow.
- Get the pumps and the people working in parallel. Electric bilge pump on, manual pump manned, a bucket if it comes to that. You will not out-pump a 129-litre-a-minute hole, but you can buy minutes while you find and plug it.
The detail on choosing and rigging pumps that move real volume is worth a read on its own, and the guide to emergency dewatering and bilge pumps goes into the capacities that actually matter rather than the optimistic figures on the box. The wider tactics of fighting an inflow, slowing the boat, heeling to lift a damaged area clear, are covered in the piece on taking on water as an emergency.
When to call, and to whom
If you have the flood under control, you still tell the coastguard, because a situation that is stable now can turn fast and you want them already aware. If you cannot control it, that is a MAYDAY. France keeps a continuous listening watch on VHF channel 16 and DSC channel 70 through its network of CROSS centres, and a DSC distress alert sends your position automatically, which is exactly what you want when both hands are busy with a bung.
Knowing the words in advance is everything here, because a flooding boat is no place to learn radio procedure. The distress and safety call procedure in France sets out MAYDAY and PAN PAN, and the practical side of contacting the French coastguard on VHF tells you who answers and where. If the situation is serious but not yet a MAYDAY, the volunteer lifeboat service may launch; the realities, including calling the lifeboat in France and the SNSM cost, are worth knowing before you need them. The SNSM is no small operation: in 2024 its crews rescued more than 11,000 people from 205 stations around the French coast.
Getting hauled out: where and how on the French coast
Once you have plugged the hole and reached harbour, you usually need to come out of the water to fix it properly, and France is well set up for this. Most significant harbours from the Channel to the Med have a yard with a travel-lift or a slipway, and a flooding emergency moves you up the queue in a way a routine antifoul never would.
A few things smooth it as a visitor:
- Phone the capitainerie or the yard before you arrive if you can, so a lift is rigged and a hardstanding berth is clear. An emergency haul-out is usually accommodated quickly even in peak season.
- Have your boat's beam, draught and displacement to hand. The yard needs them to set the slings and decide whether their lift takes you. Many town-marina travel-lifts handle yachts up to around 20 to 35 tonnes; the very big lifts at refit yards like La Ciotat or La Rochelle go far beyond that.
- As a foreign owner you can use French yards freely, but the paperwork, VAT on the work and the practicalities of dealing with a yard in another language are their own subject. The guide to French boatyards and hauling out as a foreign owner is the one to read once you are safely alongside.
It is also worth thinking about the haul-out as a chance, not just a repair. Once the boat is out, you can see and exercise every below-waterline fitting at once, not only the one that failed. A diver-led inspection afloat tells you a lot, but nothing beats standing under the hull with a spanner and a torch. Many owners discover a second tired seacock the day they fix the first, and dealing with both while the slings are already round the boat is far cheaper than two separate lifts. If you are wintering or refitting in France anyway, folding the through-hull job into a planned lay-up makes obvious sense.
Replacing a failed seacock is not a big job for a competent yard once the boat is out of the water. Bronze fittings, proper backing, hose clips doubled, and a season's habit of exercising every valve, and you are back to a boat that does not keep you awake at anchor.
The honest takeaway is that through-hull failures kill boats not because the holes are big but because they are out of sight and under pressure, and people waste their first precious minutes on the radio instead of the bung. Plug first, pump in parallel, call once you can speak calmly, then point the bow at the nearest French yard. Do it in that order and a fitting that should have ended your season ends up as a story you tell over a beer in the marina bar.
