I turn them on anytime when not on plane and engines are running. I crack the hatches when off plane, 600 SCI get cranky when hatch is left closed too long at idle.

It's 6.30 am here and I would normally be sound asleep but the boat was cleared through customs last night (after a small payment of 'tea money' was agreed upon) and is due at the marina around 8.00 am so I'm wide awake!

Good to read all the replies, a few good arguements on both sides. Think I'll alternate each day out, run with blowers on one day then blowers off the next......:)

RR

I have always been taught to turn those fans OFF when fueling so they won't spark the fumes. What say you?

I was always told to run them to evacuate the fumes before the saturation level could get high enough to ignite.
think about it. If there's fumes high enough to combust sitting over night for some reason. And you turn your blowers on to get the fumes out before starting. Why would it be a problem running them while refueling?

I can almost answer my own question......
On the dash of my 'new to me' boat, right near the blower switch, it says "Never run bilge blowers while on the plane" (or something like that) I'll check the exact wording tomorrow. It's factory Donzi printing so I'm happy to run with it.

RR

Plus there is a lot of draw on the system. My volt meter drops about .75 volts when turned on.
Best of luck with your new boat!

The Coast Guard suggest running the bilge blowers after filling up due to the chance of a blower actually pulling fumes into the bilge as gas fumes are vented out of the gas tank vent. As the tank is filled with fluid (gas), the air has to escape out of the vent. If the bilge blower is blowing air, it is also sucking air from somewhere, and if that somewhere is close to the gas tank vent, it could spell trouble.

Depending on all kinds of factors, gasoline needs to be somewhere in the 1.5% to 8% of volume by air to ignite. Under this it is too lean, over this it is too rich. A spark will not ignite if outside of those limits. Sucking in gas vapor through the bilge blower can get to that lower flammability index making for a bad situation. Once you are done filling up, the incoming gas is no longer pushing out a high amount of fumes under pressure, so the bilge blowers are good to go.

An example of being too rich would be if a boat fills up with gas, and for some reason or another several gallons of gas ends up in the bilge. The boater turns on the bilge blower, unties the boat and is on their way. The boat gets out a ways and "boom". What happened is that all of the gas in the bilge put the air over the upper flammability index, until the bilge blower (or the passive vents) replaced that rich air with clean air down to the point that the air/fuel mixture was in that window between the lower and upper flammability index. Any spark at that point could cause problems (another reason to only use sealed marine parts instead of car parts).

Anyway, thats my take on it..............

Keep in mind also that while the engine or engines are running they are essentially air pumps and are continuously pulling large volumes of air out of the engine compartment and exhausting them out the tail pipes. With out taking fuel that is atomized in the mixture into account and a volumetric efficiency loss in moving the air a 540 naturally aspirated CI engine at 2000 rpm should move approx. 312.5 CFM thus: 540 CI x 2000RMP/1728(cubic inches in a sq. ft.) / 2(intake charge per RPM)= 312.5 CFM A boosted application would result in > CFM numbers. Overall this does not take into consideration the boats ventilation set up IE: what is potentially moved in and vented out of the compartment via the engine compartment ventilation design. A twin engine boat would end up approx. doubling the volume. To answer the blower question: run them after fueling and before any start up IE: shut down period and always do a visual and sniff test first.

im no electrical engineer, but I thought that a circuit only sparks when connecting or disconnecting? And it's at the switch.

Could be wrong who knows.

I always opened the hatch and looked/sniffed around before I started the engines EVERY TIME (while running the blowers). Never on plane, there's so much air being exchanged while on plane I don't see how it could be a problem unless your tank is leaking...then you're in trouble regardless of the blowers.