Port Credit Marine Surveys
& Yacht Delivery

Gasoline engine compartment ventilation safety !

The majority of twin engine gasoline powered boats and a very high percentage of single engine boats (sailboats included) I survey have improperly and unsafely ventilated gasoline engine compartments. Aside from the obvious explosion potential of gasoline vapour laden air is the insidious potential for CO poisoning which closely mimics the symptoms of seasickness.

Why you should be concerned ….. Two reasons ….. 1.) Carbon monoxide is poison. 2.) Gasoline fumes can explode.
There is no bigger safety leap you can make that is so easy and so cheap to do right.

This is so simple and so inexpensive to do right that I find it hard to believe so many boat builders still get it wrong. I am not talking about older boats here. Check out the new models in the showroom and you'll be hard pressed to find one done right. Is your family worth $20.00 worth of flexible duct work from your local hardware store ?

While this issue is critical on gasoline fueled boats, CO from diesel can also be an issue. Diesel does produce much less CO than gasoline but you must remember that CO poisoning is cumulative and will build up over hours or days and may take up to two weeks to leave your bloodstream. CO from a diesel is unlikely to kill you but the cumulative effect on a long trip can make you ill. For more on CO take a look at Carbon Monoxide Poisoning.

Note :
Gasoline fumes are heavier than air and will collect in the bilge. CO is near buoyancy neutral and will drift with the air.

The following recommendations are based on ABYC® " Ventilation of Boats Using Gasoline" Standard H-2, with a little common sense thrown in. The diagram at below right shows a properly ventilated engine / fuel compartment.

All three powered exhaust ducts draw air from under their respective engines.

Gasoline vapours are heavier than air and will seek the lowest level. The space directly below the engines may not be the very lowest level but are generally bordered by tall stringers which contain the fumes to these areas. The whole exercise here is aimed at drawing gasoline or CO fume laden air from these areas and directing it overboard.

2. ABYC® requires one blower for "each gasoline engine used for propulsion". I think they screwed up here and forgot about the generator. Did they think generators couldn't leak gas ? I always include genny blowers in my recommendations.

Once again, stringers or other structural frame members can trap heavier than air vapours so it's essential that each area have its own blower.

3. All powered exhaust ducts should exit on the same side of the boat with fresh air intakes on the opposite side. Neither intakes nor outputs should be on the stern of boats with accomodation spaces.

If intakes and outputs are on the same side you may end up re-circulating the same gasoline vapours. Many boats are produced with plenums/louvers on each side then intake and outputs connected to the same plenum on one side ... sheer laziness on the production floor.

More often than not I see intakes and output connected to the same plenum which is a surefire way of re-circulating vapours rather than ventilating your engine compartment.

ABYC® standards also dictate that intake/outputs shall not be on stern. With intakes and outputs at the stern, The "station wagon effect" may push fumes back in through the intakes or directly into the cockpit..
Some believe that it is not necessary to run the blowers at cruising speed and turn them off as soon as they are under way. I prefer to see blowers run whenever the engines or generator are running but they most definitely should be run when the vessel is moving at slow speeds and at anchor if the generator is running.

A few other ABYC® requirements…. Exhaust ducts must terminate in the lower 1/3 of the bilge but above the normal accumulation of bilge water. Exhaust duct ends must be no closer than 24" to intake openings. Air intakes and exhaust outlets shall be no closer than 15" from gasoline fill and tank vent fittings. Ventilation outlets must remain outside of weather enclosures. You will get better cross ventilation if the intake ducts are high in the engine compartment. All duct ends must be secured. ABYC and NMMA also prohibit ventilation intakes or outputs at the stern (on boats with accomodation spaces) due to the "station wagon effect" and the fact that people tend to hang around the swim platform with the generator running.

Of course none of this makes any difference if you don't use your blowers. Canadian law requires that blowers be run for a minimum of 4 minutes before starting your engines. Keep an eye out this season and watch most people flick the blower switch and immediately fire up their engines. The insurance investigation takes place shortly thereafter.
One of many at the Toronto Boat Show and is an affront to all boat buyers. Note all the intake and output holes across the stern (round holes below the seat back) and right under the "NMMA Certified" sign, a clear violation of common sense, ABYC and NMMA standards which the manufacturer claims to meet. Then they have the temerity to put a CO warning label (shown below) on the transom !

.........................CO can cause brain damage ! Maybe this builder
.................................spent too much time on his own boats.
Above is one of my favourites. Close up your canvas and block your fresh air intakes and blower ouputs (just above the transom doors on both sides), Blower output is forced into a sealed cockpit. Waddayathinks gonna get him first, asphyxiation or explosion ?
Above are two brand new "trawlers" with intakes and outputs in the cockpits. Sooner or later the owners are going to install canvas enclosures. Where are exhaust fumes going to get trapped ?
It ain't rocket science.
At right - Another Regal with intake & output transom ventilation (those three slots just above the name) within inches of each other but they are not alone.

The builders are well aware of ABYC® Standards and NMMA (National Marine Manufacturers Association) is their own organization which purports to build to ABYC® standards. Why can't they get it right?

Why don't you call and ask them before you hand over your deposit cheque ?
Above - this exhasut duct is flush against a deck, it can't suck anything. Above - Intake and output ducks adjacent to each other. A recipe for recirculating the same air and both were connected to the same plenum.
Above - This blower draws air from under the transmission but any gasoline fumes will collect forward of the drip pan bulkhead under the engine will remian untouched. Above - For about $4.00, a length of duct could be added to this blower leading under the engine (on the other side of the stringer).
Above - This fella thought it was a good idea to add a louver to vent his gasoline engine compartment into his saloon. Above - Air intake butted agains outboard side of stringer, Air intake on wrong side of stringer and both lead to the same plenum. With CO poisoning and seasickness having virtually the same symptoms how do you know which one you are experiencing. If this is your boat I suggest you find out !
Wallace Gouk AMS®
SAMS® Accredited Marine Surveyor, 2002
ABYC® 2003
ABYC® Standards Certified, 2009
ABYC® Certified Corrosion Anayst, 2015
Transport Canada Licensed Master, 2002
Transport Canada Tonnage Measurer, 2004
BoatUS® Approved Marine Surveyor, 2003