Gas is the quiet danger aboard. Nobody worries about the cooker the way they worry about the weather, and that is exactly why gas occasionally kills boaters who would never have set off in a Force 8. A leak gives no drama and very little warning. It simply pools, invisibly, in the lowest part of the boat, and waits for a spark. Cruising in France adds two wrinkles to the usual story: you are away from your home supply, and you will eventually have to think about how you refill. Here is how I keep the system honest on a long season abroad.
The one fact that explains the whole risk
LPG, whether butane or propane, is heavier than air. Propane is roughly 1.5 times the density of air. When it leaks it does not waft out of a hatch, it sinks, and it collects in the lowest point of the hull, the bilge. A boat is the worst possible shape for this: a deep, enclosed sump under the accommodation, often with the engine and its electrics nearby. A leaking bottle stowed in the wrong place can fill the bilge and then the cabin with an explosive mixture while you sleep.
Everything sensible about marine gas design follows from that single fact. The locker drains overboard, not inboard. The bottle lives above the waterline. The pipe runs are continuous. You are managing a gas that wants to find the one place you least want it.
The gas locker and its drain
A proper gas locker is sealed from the interior of the boat and drains overboard from its lowest point, above the waterline, through a drain that is never blocked. That drain is the whole safety case. If butane or propane leaks from a bottle or a connection, it should fall straight out of the locker and over the side, not seep into the cabin.
Things to check on your own boat before a long cruise:
- The locker drain is clear and exits above the waterline. Pour a little water in and watch it run out.
- The locker is genuinely sealed from the cabin and the bilge. No cable holes, no gaps around the lid that lead inboard.
- Bottles are secured upright so they cannot fall and stress the connection in a seaway.
- The locker is for gas only, not a handy place to stuff fenders and warps that can foul the regulator or the drain.
Regulators, pressure and why bottles are not interchangeable
This catches out cruisers who swap between countries. Butane and propane run at different pressures: butane at around 28 millibar, propane at around 37 millibar, and their regulators are not interchangeable. Fit the wrong regulator and the appliances run badly or dangerously. Calor, the main UK supplier, recommends regulators are replaced every ten years regardless of how they look, because the diaphragm perishes with age whether or not it has failed yet. Note the date on yours; a regulator from the last century is a liability.
When you move to French bottles you change the bottle fitting and very possibly the regulator and pigtail too. Do not bodge it with adapters of dubious origin. The whole subject of supply abroad is worth its own read, and I have set out the options in Calor versus French gas bottles, including which connectors you need and where to fill up once your British bottles run dry.
The bubble test: cheap insurance
The single best habit for a cruising boat is a bubble tester fitted in the line between the regulator and the pipe into the boat. It is cheap, it takes a minute, and it turns leak detection from guesswork into a yes-or-no answer.
To use it: turn off all the appliances, press the red button for about a minute, and watch. If no bubbles appear, the system is tight. If bubbles rise, you have a leak somewhere downstream and the gas stays off until you find it. I run the test at the start of every season and again after any work on the system, and once a month or so on a long cruise as a matter of routine. It has caught two slow leaks for me over the years, both at a worn appliance connection, neither of which I would have smelled in time.
If you do not have a bubble tester, a brush of soapy water on every joint with the gas on and the appliances off will show a leak as growing bubbles. It is cruder but it works. What does not work is sniffing and hoping.
Hoses, pipes and the boring stuff that matters
The rubber hose between bottle and rigid pipe has a finite life. Marine LPG hose carries a date and should be replaced on schedule, typically every few years, sooner if it shows cracking at the bends. Rigid copper or properly rated pipe runs through the boat with no joints in inaccessible places. Every appliance has a shut-off valve so you can isolate it. The bottle valve gets turned off at the bottle whenever the boat is unattended and overnight, not just left on for convenience.
A gas system that has been neglected for a decade is one of the most common faults a surveyor flags, and for good reason. If you are buying a boat to cruise France, the gas installation deserves the same scrutiny as the hull; the careful eye described in 10 tips for a used sailboat hull inspection applies just as much to the plumbing you cannot see.
Cabin heaters, the extra hazard
A gas heater warming the cabin is also burning oxygen and producing combustion products inside the living space, which raises a second, separate danger: carbon monoxide. Flueless gas heaters are particularly risky in a closed cabin. This matters most to the canal and winter crowd, and I have written it up in full in carbon monoxide and the canal-boat cabin. The short version: ventilation is not optional, and a CO alarm is not a luxury.
Smelling a leak, and what to do about it
Pure LPG is odourless, so suppliers add a stenching agent (the rotten-egg smell) precisely so a leak can be detected by nose. If you smell that aboard, treat it as the emergency it is. Turn off the gas at the bottle. Do not operate any electrical switch, not the cabin lights, not the bilge pump, not the engine start, because a spark is the one thing that turns a leak into an explosion. Open the boat up and ventilate. Because the gas has sunk into the bilge, you may need to physically clear it; some boats carry a gas-tight bilge or a means of pumping the heavy vapour out, but on most yachts the honest answer is open everything, get air moving, and wait until you are certain it has cleared before you light anything or flick a switch.
The reason this matters so much on a boat, rather than in a house, is that there is nowhere for the gas to go but down into the one enclosed space you cannot easily see into. A kitchen leak in a house disperses; a galley leak on a yacht concentrates. That asymmetry is why marine gas discipline is stricter than the equivalent at home, and why the bubble test and the locker drain are not optional extras.
A pre-cruise gas checklist
- Regulator within its ten-year life, correct for your gas, dated and noted.
- Locker drain clear and above the waterline, locker sealed from the cabin.
- Bubble tester fitted and used; or soapy-water test on every joint, no leaks.
- Hose in date, no cracking, properly clipped, no chafe points.
- Appliance shut-off valves working; bottle valve off overnight and when away.
- A plan for refilling in France before the British bottle runs dry.
- A CO alarm fitted if you have any gas appliance burning inside the cabin.
Gas safety is not glamorous and it does not make a good sea story, which is precisely the problem. The boats that have a gas incident are rarely the ones that thought about it. Fit a bubble tester, keep the locker drain clear, respect the difference between butane and propane, and sort your French refill before you are running on fumes. Ten minutes of dull discipline buys you a season you never have to think about it.
