Flame Proof Vent

I am wisely advised to have a sealed bulkhead between the gas engine bay and the gas tank compartment.

However the tank compartment needs draining aft to the bilge which is the engine bay (inboard outboard pair of engines) The floor cover above the gas tank is exposed to the elements.

How would I construct flame proofed tubes to drain down through the tank bulkhead to the bilge pumps?

 

My tanks are in the same space as my gasoline engine, vent overboard and fill above deck, everything is a clean installation and up to Coast Guard Specs. Fairly common set up in this country, may be different in your neck of the woods.

 

Up to Coast guard specs, must be OK then.
In the UK we don't have many gas engines and there are not your stringent regs. for all boats.
I posted a query yesteday about venting the engine carbs through the tank compartment and rasorinc said:

'If the fuel tank and engine are open to each other I would close that off as a fire stop and vent each space independently. Is it a wood boat or fiberglass? You need to allow for water to flow to the transom and you can vent top side say port side and exhaust starboard side using power vents. What is your engine source? Sparks and heat, need to be seperated from fuel.'

It is a GRP planing hull; aft I/O engines with forward tank.
I was very concerned from the above the advice, thinking an explosive engine flame would travel through drain holes to the gas tank compartment and ignite any possible tank fumes - then the tank . . .

 

As I said my tanks vent and fill externally. The engine compartment is sealed from the vent and fill. The weak link is the plumbing from the tank to the carb and then any issues you may have with the carb itself. Why not post some pics? If your lines are done well I'm guessing things would be ok but a picture is worth a thousand words.

 

You could cover the drain holes with flame arresting copper wire mesh, but because the location is near the bottom there will be serious corrosion.
Another solution is a labyrinth made from a stack of folded aluminium plates.

The makers of my boat used a separate manual bilge pump for the whole area in front of the bulkhead.

 

As far as flame arresting the drain holes, I think stray fumes/leaks that would be a problem would settle and so the fumes would find the ignition source through any screen. Vent the tank overboard; ventilate the engine compartment well to blow/suck any stray fumes out while underway and by force before starting.

 

I would also suggest the copper wire mesh.

Got myself in a sticky situation a few years back, bought a bunch of copper wire cloth...
http://www.bwire.com/

The corrosion wasn't as bad as I expected afterwards, but there was definitely some there.

 

What do you mean by venting the engine carbs? You should have a bilge blower to take the fumes out. When the engine is running it will suck the fumes. At idle it is better to keep the blower running. There should be no source of sparks or flames in a marine application, therefore, no need to separate the engine from the tank. I am a surveyor, and very familiar with petrol installations. There are few regulations in the UK or expertise because so few are installed.

 

I second Gonzo.. I am a marine surveyor as well. The blower should be in the engine compartment to remove the fumes. As far as construction of the drain tubes, a great reference to what you are referring to is in the ABYC Technical Standards Manual (See sections H-2, H-24, and A-14). I know you're in the UK, however these U.S. Standard Recommendations are a great tool to use and is what most boat manufacturers guideline their boats by. See www dot abycinc dot org for more information.

 

Just a random idea, but what about one of those solar powered vents they sell now days? It's not a ton of air movement, but if it were continuous it should keep the atmosphere below LEL.

I'm not so sure about removing sources of ignition. The reality is that if you have a flammable atmosphere it WILL find a source of ignition sooner or later. Better to prevent the fuel accumulation than to worry as much about the ignition sources.

Something else to think about...

If you read the NTSB / TSB accident reports on fires at sea the most common source of ignition seems to be hot exhaust manifolds. Most common fuel source is diesel supply piping that has been damaged by excessive vibration.

 

No, it would not be safe or legal. Also, if you boat at night they would not work.

 

Gonzo is making sense here but there is a reason for CDK's comment: "The makers of my boat used a separate bilge pump for the whole area in front of the bulkhead." - It's because one shouldn't let water flow between compartments. I saw a boat sink once by the stern from water leaking through a broken taping job in the chain locker. Of course, a bad chop, the poorly tabbed seam, and clogged bilge pump came together to take this boat down but why supply the sea with another way to get you - to save money on a bilge pump?

 

Trailer boats are routinely stored with the drain plug off. Fuel and engine compartments must be separated from the living areas and any other area with open flames or sparks-galley, etc. In these cases, there usually is a long pipe to drain the forward compartments aft when the boat is stored. Planing hulls float low at the bow when stopped and low at the stern when running. At least two pumps are necessary.