I have a Rinker v153 with a 75 hp Chrysler outboard. The two 5 gallon gas tanks fit under the splash well and there is a vinyl flap attached to the entire length of the stern to close off the area. The area also contains the battery. I was wondering if it is common to have a blower in this area to prevent possible explosions? With the flap up the area is completely open. The flap is usual up when I start the boat because I need to get in there to pump the bulb once the gas tank is attached. What is typical for outboards? I know inboards use blowers all of the time.
I respectfully disagree. By having a vinyl flap you are creating a "compartment" which can contain gasoline vapors. At the very least I would install two of the vents Cal mentioned. This would give the compartment natural ventilation. I installed a permanently mounted tank under the front deck of the Glastron. Even tough the open area compared to compartment size met the criteria for "open to atmosphere", I still installed a blower. Ask any suicide bomber, you only get to blow yourself up once!
What the Coast Guard and ABYC rules say is that if the compartment is "open to the atmosphere" then ventilation is not required. Open to the atmosphere is defined as 15 sq inches of open area for every cubic foot of volume of the compartment. Also ventilation is not required if there is nothing in the compartment that can be a source of ignition. Batteries are not considered a source of ignition. Things such as electric motors, switches, relays, are sources of ignition unless they are specified as ignition protected, meaning they won't ignite fuel vapors. So, generally outboard boats meet those requirements and don't need a ventilation blower.
However, if you are still uncomfortable, simply install two cowlings, one facing forward, one facing aft. It really doesn't matter which points which way. Studies have shown that the flow of air in and out of a compartment with two openings depends on a lot of things, the way the wind is blowing, forward motion of the boat, the temperature differential outside the boat and inside the compartment. The direction of air flow can change from day to night. And if you want to put in a blower, make sure it is ignition protected. It would be horrible if the blower you put in to prevent explosions was the cause of the explosion.
Thanks to everyone for responding to this post. I was surprised to see so many comments and also the difference in opinion on this issue. The post that references the coast guard rules and regulations was great to see.
My '59 Thompson Sea Lancer has the vent scoops from the dealer/factory. I have 2 12 gallon tanks under the back seat which is enclosed, but there is open area on both sides of the splash well. With the battery and the power trim pump under there also they probably aren't necessary but it doesn't hurt.
Aren’t they typically facing opposite directions? Every outboard boat I had since 70s did, but maybe because they had more restrictions on both sides of seat internally. Maybe when skirted with openings on each side, perhaps they forced air into both sides in from the deck and out both sides of the skirt. I thought it was better to face one aft so that fumes exit out of the hull faster . Having 2 face forward like in the picture would seem to force fumes into the open hull before diluting and exiting the passenger hull area. Just wondering why both in the picture faced forward.
Actually it doesn't matter. In the mid 80's the USCG, ABYC, and NMMA did a study of natural ventilation. The findings were it doesn't matter which way the vents are facing because there are a lot of variables. If the boat is moving against the wind then the one facing forward is the intake and the one facing aft is the exhaust. But if you're going downwind then the opposite applies. If the boat is standing still, the prevailing wind determines which is the intake and exhaust. If no wind, then the temperature determines which is intake and exhaust due to the temperature differential outside and inside the boat. And, if you have a blower running then the one the blower is on is the exhaust. Before that most boatbuilders put one facing forward and one facing aft. Now they do whatever looks good. On my boat I have two facing forward and one big vent facing aft that combines a natural vent and a blower exhaust.
The recommended configuration is, the intake is not that long bringing air into the top part of the compartment, and the exhaust extends to the lower part of the bilge, not so far it will be in the water (if you have water in the bilge) but so that any vapors will be sucked out and away. Usually 3 to 4 inches above the bottom of the bilge and as directly as possible under the engine. As was said, gas vapors are heavier than air and collect in the lowest part of the engine compartment. You want to get those out. Ideally though, if everything is working right you won't have any gasoline vapors to get rid of.