Defect of the Week: LPG and the Gas Locker

On the evening of 29 June 1999 the sail training yacht Lord Trenchard, a Nicholson 55, lay alongside TS Royalist at Town Quay in Poole. At supper the gas ran out, and the mate changed over to the second propane cylinder in the ready-use locker. Later that night the crew had trouble lighting the cooker. They opened the locker, noticed a slight smell of gas and opened the cylinder valve fully. The cooker lit, and everyone went to bed. At about ten past seven the next morning the skipper put the kettle on and tried to start the generator in the hold. On the second attempt, the gas that had gathered in the hold overnight exploded. He lost his left leg above the knee.¹

The Marine Accident Investigation Branch traced the leak to the connection on that second cylinder, which had not been properly tightened when the cylinder was fitted earlier in the month. The locker had a drain overboard, and the investigators judged that a leak inside it should have done no harm. The locker itself let them down. Holes for the sliding bolts that secured the cylinders, and for the lid's latches, passed straight through its walls into the hold, and the seals around the gas pipe and a moulded recess were probably not gas-tight. The gas alarm sensor in the hold was reportedly mounted some way above the lowest part of the space, and was probably not working.¹

The report's central point is the one this entry is built on. The connection at the cylinder is the one joint in a gas system that people break as a matter of routine, so the installation has to stay safe when that joint leaks. The locker is what keeps it safe, and the survey grades it as a barrier. Heating season is a good time to look hard: cookers, water heaters and gas heaters work longest in winter, with the hatches shut.

The Locker Is the First Barrier

The Boat Safety Scheme's checking procedures are the most detailed public benchmark for a locker.² The sides must rise at least to the top of the cylinder valves, or of the regulator, changeover or any other high-pressure part where that sits higher. Below that line the bottom, sides and seams must be free of any hole, crack, rust-through or failed weld, and no hatch is allowed there. Every pipe that leaves the locker below the line passes through a bulkhead fitting, a gland, or sealant that fills the gap completely, and the same rule covers the cable to a solenoid valve. Each is pushed gently by hand, and none may move in its seal. A damaged locker is made good in its own material, with glass and resin or welded plate, since its integrity may not rest on glue or sealant, and a moulded plastic locker is replaced.²

The drain opening sits no more than 30mm above the lowest point of the locker, and the drain line falls continuously to an outlet on the outside of the hull. Here the BSS figures are inland minimums: an outlet above the normal laden waterline, and a bore from 12mm for up to 18kg of gas to 19mm for 38kg or more.² A yacht heels, and her waterline moves with her. For a vessel coded under the MCA's Sport or Pleasure Vessel Code, the drain is at least 19mm, ends 75mm or more above the waterline at rest, and sits with the vent 500mm or more from any opening into the vessel. That is a sound benchmark for any boat at sea, provided the outlet stays well clear of the water when she heels. Existing vessels coming from MGN 280 or the Yellow or Red Code may keep their current installation until it reaches the end of its life; the clause does not name the Blue Code, so check with the Certifying Authority.³

Most of this needs only a torch and a little time. Empty the locker of anything that is not gas equipment. Look at the inside, and at the outside of the locker from below decks wherever it can be seen. Push on each pipe and cable where it leaves. With the owner's consent, pour a little water into the locker and watch it arrive at the outlet, as the BSS procedures suggest to examiners. Note whether the drain passes through a valve or seacock at the hull. A valve can be shut, and a shut drain turns the locker into a tank, so trade advice favours an open skin fitting with no valve.⁴ Check that the regulator, pigtails and any changeover all sit inside the locker, that every connected cylinder stands upright and is secured so it cannot drag on its hose, and that the locker does not open into an engine or battery space. Note where the spare cylinders live, too. The BSS asks for every cylinder, full, part full or empty, to sit in a drained locker, a ventilated housing, or in the open where a leak would flow overboard;² the Code allows only the open deck or a vented, drained locker.³

Hoses, Regulators and the Dates on Them

Under the BSS, any hose on the high-pressure side, between cylinder and regulator, must be a ready-made assembly no longer than a metre, marked to BS EN 16436 Class 3 or BS 3212 Type 2, or a steel hose to BS EN ISO 10380. Where two cylinders feed one system, each connection needs a non-return valve. On the low-pressure side, hose is allowed only for the final connection to an appliance or a regulator, again no longer than a metre and visible along its length, except on the all-hose systems built to ISO 10239 that are common on imported boats. Spring clips are not accepted on gas hose, and neither are regulators that can be adjusted by hand.² The regulator must suit the gas in the cylinders: propane and butane regulators work at different pressures and are not interchangeable,⁴ unless it is a 30 mbar regulator made for both.²

The BSS checks a hose's marking and condition, and leaves its age alone. The hose itself carries a date, and the trade's long-standing advice is to renew it five years from that date, or sooner if it shows wear.⁴ For regulators, the BSS's own guidance to owners recommends replacement after ten years.² Record both dates. Where a date has worn away, the record says so, and the age is unknown. On a gimballed cooker, swing it through its full arc and watch the hose, which flexes every time the boat rolls, and check the cooker's lock.

Say How Tightness Was Established

A sniff in the locker and a look at the joints speak for the joints that could be seen. Whether the whole system holds gas needs a test, and under the BSS every LPG system must have a means for one: a test point for a pressure gauge, or a bubble leak detector in the locker.² Either way, the joints between the cylinder and the bubble tester, or the supply valve for a gauge test, are checked first with leak detection fluid, since neither test reaches them at cylinder pressure. They include the cylinder connection, the joint that failed on Lord Trenchard.

Two details are worth knowing. The first concerns the bubble tester. Before its silence means anything, it has to show bubbles: with a burner lit and the test button held, bubbles should stream through the fluid. If none appear, the tester is dry or faulty, and the BSS procedures fail it as a means of testing. Only then, with every burner off, is the button held while the steady bubbles are counted. On a post-2008 Alde 4071 the BSS passes three or fewer in two minutes for butane and four or fewer for propane, and does not run the test on butane below 10°C. On an older Alde, or another make such as Gaslox, it allows none in sixty seconds.²

The second concerns who may test. On hire boats, passenger boats, residential boats, houseboats and floating businesses, the Gas Safety (Installation and Use) Regulations apply, and even removing the screw from a test point counts as gas work for a Gas Safe registered engineer.² On a liveaboard narrowboat, then, a surveyor who is not registered establishes tightness with the boat's own bubble tester, or by watching a registered engineer's gauge test. On a private boat, a gauge test is for a surveyor competent and equipped to run one, with the owner's consent. Nothing else is disconnected or adjusted, no test is run where gas can be smelt, and the gas goes off at the cylinder afterwards.

A BSS certificate shows that the boat passed its examination, tightness test included, on the day it was examined. That is useful history. The record for today names the method used and its result, or says that tightness was not verified and recommends a test by a Gas Safe registered engineer with the LPG boats competence.

Appliances, Alarms and the Air Below

Under the BSS, every burner and pilot light on an appliance installed since 3 January 2000 needs a flame failure device, which shuts the gas off when the flame goes out. Older appliances need one on any catalytic heater, continuously burning flame or pilot light.⁵ Light each burner and let it run until the device holds. Turn the tap off, wait for the valve to drop out, then turn the tap on again without pressing it in: no gas should flow. With every burner lit at full, each flame should burn clean and blue, which the BSS also checks.² Look at the fixed ventilation, since a vent stuffed with a cloth against winter draughts is a finding, and at the flue of any flued heater or water heater.

A private boat needs a readily accessible main shut-off, which the BSS normally takes to be the cylinder valve. On a yacht, record whether that valve can be reached quickly from the galley; under cockpit cushions and a liferaft it may not be. A solenoid valve switched from the galley is a convenience many owners choose, and the MAIB strongly recommended a gas detector after Lord Trenchard,¹ but the BSS requires neither.² A solenoid sits downstream of the cylinder joint, so it never takes the locker's place. The investigators also found that the crew's checks at the alarm panel had only ever tested the panel, and recommended a functional test with gas at the sensor itself.¹ Record each sensor's height, the date on it, and whether it was tested with gas.

A coded vessel asks more. The Code requires a bubble leak detector, flame failure devices on every burner, no catalytic heaters, and gas detection in any space containing or adjoining a gas appliance, or into which gas may leak or accumulate. The sensor heads sit at the lowest practicable point, and the alarm sounds at no more than 0.5 per cent gas in air, about a quarter of the lower explosive limit.³

Gas that escapes finds whatever sparks in a low space: a generator's starter motor, a fridge compressor, a bilge pump switch. The Code requires any electrical equipment inside a gas locker to be certified for an explosive atmosphere.³ Our battery and shore power entry sets out the electrical side of the same record.

Gas appliances also burn the cabin's air. Since 1 April 2019 the BSS has required carbon monoxide alarms on boats with accommodation, certified to BS EN 50291 and within the maker's replacement date, and it recommends alarms marked BS EN 50291-2, which are tested for conditions on boats.⁶ A private yacht at sea has no such rule, a coded vessel does, and the alarm earns a line in the record either way.³

The Narrowboat Locker

A narrowboat's locker is usually steel, in the bow, with the drain a hole through the hull side close to the locker floor. On some boats canal water can reach the locker through that drain. The BSS then wants a second outlet above the water, and advises owners to watch the locker for corrosion, since a rusted floor lets gas into the boat and water too.² Lift any mat or false floor. Cylinders often stand on the very patch of steel most likely to have rusted, and the BSS tells its examiners not to disconnect them: where the floor cannot be seen, the check is recorded as not verified.² A surveyor's record can follow the same discipline.

Inland gas usually runs the length of the boat, to a cooker, a water heater and sometimes a fridge or a heater, and much of the pipe sits behind linings. Record the length that could be seen. The narrowboat primer covers the wider inland survey, and our post on the BSS examination and the pre-purchase survey sets out where the two meet.

Classification: A, B or C

The test is the one that runs through the Category A, B and C system: does the defect endanger the vessel or the people aboard if she is used? For the locker and the high-pressure side, it comes down to one question. If the joint at the cylinder leaked tonight, where would the gas go?

A leak, for grading, means gas smelt below decks, any bubble at a joint under leak fluid, a bubble tester count above the BSS pass figure, or any discernible drop on a gauge. At 27 or more bubbles in two minutes on butane, or 32 on propane, the BSS treats the boat as hazardous.²

Category A. A leak. A locker that is holed, rusted through, open to the boat, or unsealed at a pipe or cable below the top of the valves, or one that opens into an engine or battery space. A drain that is missing, blocked, shut, led into the boat, or ending below the waterline with no outlet above it. A regulator, pigtail or changeover mounted inside the boat. A cylinder, full or empty, stowed where a leak could reach the interior, or a connected cylinder lying down or unsecured. A hose that is cracked, perished or chafed. A flame failure device that does not shut the gas off, or a heater, water heater, fridge or pilot light with none. The gas stays off at the cylinder until each is put right, and a Gas Safe registered engineer with the LPG boats competence is the right person for leaks and installation faults. A missing carbon monoxide alarm is also A on a boat under the BSS, or wherever people sleep aboard with a heater, stove, water heater or fridge burning fuel, and an alarm that fails its test or is past its date counts as missing. Fit one before anyone next sleeps aboard.

Category B. A hose with no readable date, or more than five years past it. A regulator over ten years old or of unknown age. No test point or bubble tester, or a tester that stays dry on its fluid check. Spring clips, an adjustable regulator, or a high-pressure hose longer than a metre. A drain narrower than the boat's use calls for, or one that runs through a valve found open. Pipe unsupported, or unprotected where it passes through a bulkhead. Hob, oven or grill burners without flame failure devices on a cooker installed before 3 January 2000. A gas alarm out of date or untested, or its sensor mounted high. A side-opening locker door with a worn seal, where the BSS requires one. No carbon monoxide alarm on a private seagoing boat whose only fuel-burning appliance is an attended cooker. Defined action within a set period, and before anyone lives aboard through the winter.

Category C. Missing labels on shut-off valves, a worn seal on a top-opening lid, loose items or debris in the locker, missing caps on spare cylinder valves. Clear the locker on the day; monitor and maintain the rest.

As with the seacocks entry, the temptation is to grade down because the cooker lit and the kettle boiled. The cooker on Lord Trenchard lit that evening too. A system can light every burner with a slack joint in the locker and a locker open to the hold, so grade the barriers as found.

Documenting It

Inadequate: "Gas installation inspected, appears satisfactory."

Professional: "Category A. LPG locker, cockpit, port side, GRP moulding, top-opening lid. Two 6kg propane cylinders, upright and strapped, on a changeover with non-return valves; regulator, pigtails and changeover all inside the locker. Two unsealed screw holes for the cylinder strap bracket pass through the locker wall, below the top of the cylinder valves, into the cockpit locker beneath, which is open to the bilge. Recommend the holes are filled and glassed and the bracket refixed without piercing the locker; gas to stay off at the cylinders until then. Drain 19mm from the locker floor, falling continuously to a transom outlet approximately 150mm above the waterline at rest, with no valve; water poured in with the owner's consent arrived at the outlet. Joints from the cylinder valves to the bubble tester, including both pigtails and the changeover, checked with leak detection fluid: no bubbles. Pigtails marked BS 3212/2, dated 2019 and 2024; recommend the 2019 hose is renewed (Category B). Regulator 37 mbar propane, date stamp illegible, age unknown; recommend renewal (Category B). Bubble tester (post-2008 Alde 4071) bubbled on its fluid check; no steady bubbles in two minutes with all burners off. Two-burner hob and oven: each flame failure device shut off on test; cooker hose dated 2024, sound through the full gimbal swing. No gas alarm fitted; one is recommended. Carbon monoxide alarm in the saloon, marked BS EN 50291-2, replace-by date 2029, sounded on test. Pipework behind the galley lining not seen. Gas turned off at the cylinders on completion. Photographs attached."

The second record names the barrier and its breach, the drain's path and how it was proved, the joint at the cylinder, the dates, the tightness method and its result, and what was not seen. Our post on what makes a survey report defensible explains why the "not seen" clauses carry as much weight as the findings.

Every yacht, motor boat and narrowboat template in Marine Inspect carries a Gas Installation (LPG) section, with items for the locker and its drain, the regulator and its age, the pipework and hoses, the flame failure device on each appliance, the isolation valves, and gas detection. The surveyor's findings, grade and photographs go against each item, by voice or by hand, and carry into the report. Both sample reports include a graded gas section: the Calypso Dawn yacht sample and the Meadow Lark narrowboat sample.

Read next in the series: Defect of the Week: Cutless Bearings, Defect of the Week: Battery and Shore Power, and Defect of the Week: Through-Hull Seacocks.