I use radiator cooling
12 volt fans. They cost about seventy dollars each. They are axial compressors and although a bit noisy, they blow a hell of a lot more air INTO the compartment--and the exit intakes I mount low in the bilge and take them to outlets at the stern using ordinary splash-proof ventilators. You can have as many outlets as you like--but with two engines at least two would be advisable.
The reason I like to blow a lot of air INTO the engine compartments, since when the engines are running the power used is not a problem, is that there is no risk of a spark in an explosive atmosphere from the fan itself as there would be if sucking any fume-filled air out, and if fuel
does become spilled, the more air flow taking it away the better.
I am told that petrol, mixed in the right proportions with oxygen, has about eight times the energy of nitroglycerine..
If the engines are not being used the bilges still need to be ventilated, a small snail-shell or axial turbine blower will draw about five amps or less and shift a lot of air in a day. They are supposed to be spark-proof, but I still believe in pushing air into these spaces and displacing the foul air out through ducted piping to deck
outlets.. A solar
panel is enough to keep this and a small back-up battery
for night time use charged, if the boat
is to be left unattended.
Then you also need a gas detector, with one element sampling the out-going air. If there is a developing source of fumes--you need to know about it.