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.