Bildge or engine fans ?

[Jmc1764]

I have two 3" inline fans that ventilate the engine compartment, one blows air in the other sucks it out. They run ok and are reasonably quiet but the air movement is rather poor, you can barely feel the air blowing out the outlet . Are the snail type fans with the turbine better than the inline fans with their propellers ? any views or experiences ?

 

[jwfrary]

Test is your engine room temp! Most manufacturers you need to be a maximum of 16degrees above ambient.

The snail fans, can be pretty powerful, we are using the largest units jabsco supply and the draw a fair amount, but also like sticking your head out of the window on the motorway

 

[clyst]

Are you sure your discharge trunking isn't restricted or crushed ?

 

[William_H]

Fans in design come as axial type as in household fan or computer fan, or centrifugal fan as in vacuum cleaner or car heater fan. (you call it a snail type)Axial fans tend to not move much air until they run at full speed. (commensurate with diameter) while centrifugal type will work well at lower speeds. Possibly your axial fans are not getting full voltage hence not full speed.
Now just to waffle on fans are an essential part of a jet engine. You are needing a compression ratio around 15 to one. On big jet engines this is achieved with 10 or 12 stages of axial fan with guide vanes between each stage. Typical tip speed 1000kmph However on smaller jet engines the best option is still 2 stages of centrifugal fan (compressor). probably because to get the 1000kmph tip speed would be too high RPM in say a 25cm diameter fan..
I note a typical old electrolux vacuum cleaner has 2 stage centrifugal fan.
So while axial fan should do the job a centrifugal fas might be more positive. ol'will

 

[harvey38]

I purchased two 4" Seago centrifugal bilge fans from Ebay @ £35 each new, quiet and very efficient. They draw air in through the engine vents and are ducted outside via separate dedicated vents, I leave them on for 15 minutes after shutdown to draw fresh air into the engine room to help cool the engines.

 

[Spirit (of Glenans)]

I purchased two 4" Seago centrifugal bilge fans from Ebay @ £35 each new, quiet and very efficient. They draw air in through the engine vents and are ducted outside via separate dedicated vents, I leave them on for 15 minutes after shutdown to draw fresh air into the engine room to help cool the engines.

In the colder months of the season I crack open the engine cover in the aft cabin to warm it up before going to bed!
(Note to self: Must look at the viability/practicality of fitting a diesel heater?)

 

[pvb]

Possibly your axial fans are not getting full voltage hence not full speed.

Just to give another view on this, my boat has a 4" axial fan in the engine compartment, sucking warm air from the top and blowing it out of a vent on the transom. There's another vent on the transom with a 4" duct to allow cool outside air to enter at the bottom of the engine compartment. It works very well. Now, here's the thing - the fan is a 24v version, supplied with 12v.