Yes,you need to have a blower.It's a requirement on I/O powered boats.Even if it wasn't required,it would be foolish not to have one.
I run an I/O,on my current boat,and i always run the blower for about three of four minutes.
Accidents can happen,in fuel systems,and gas vapors in bilges can make for one hell of an amusement ride,that won't return to the park once you turn the key and ignite them.
My procedure is to run the blower,while we're getting the rods and tackle ready.After a minute or two,I stick my big nose near the vent that exhausts the air,and smell for any gas odors,if there's no smell,I'll fire up the engine.
Any vents should be big enough to provide,enough fresh air in,to replace the air that's exhausted out by the blower.As Snookered mentioned,it's a good idea to have foward and rear facing vents to help force air in and out while underway.
The blower should be mounted as low as possible in the bilge,as gas vapors settle,but it has to also stay dry.Hook up the outlet side of the blower to a flexible vent hose,and attach it to a rear facing vent to exhaust the vapors.
Explosions from gasoline bilge vapors in boats are not uncommon,there are several of them every year.
Last year i was launching my boat,and as my buddy was backing it down the ramp,the rear vent was about even with my face.
All of a sudden i got a very strong smell of gasoline,from the vent.We ended up taking the boat back to the yard on the trailer,and when i opened the motor box cover,i found about a gallon of gasoline in the bilge.
I take extremely good care of my mechanicals,but we found a tiny split in the fill hose way up out of site in the hull.
I had filled up at a gas station just before we were going to launch.
Needless to say,we didn't use the boat that day,we drained the gas out and washed out the bilge with detergent and lots of water.Installed a new fill hose,and all is ok.
I would definitely install and use a blower,it's a wise decision.
This is a picture of the bilge in my project Tsunami 23.You can see the blower in the front left corner on the engine bed.I have the silver exhaust hose on the blower, going to the starboard rear vent,and another vent hose from the port side to bring fresh air in.There are other intake vent hoses,that don't show in the picture.
There are also port and starboard vents on the hull facing forward,that are about even with the front seats. They force fresh air into the hull.
