Fan to vent engine space - necessary?

[Sovereign]

Hi,
IMy wife and I have just taken delivery of a 'new' boat - having 'retired from sailing' last Spring!! It is a 40 year old Legend 29 - traditional build to the 'Burnes' layout.
I have discovered that it has fitted an extractor fan - automatically siwtched on/off with the engine - the purpose of which appears to be to extract air/fumes via a 4" diameter pipe from the engine space and vent it overboard. (Or it might if the pipe wasn't squashed nearly flat!) The engine is a relatively new Beta 3-cylinder.
The engine space as on all boats of that era is not enclosed but open aft into the whole under-deck area, and as the exhaust should all be contained in the exhaust hose I cannot see much point in it sitting there taking up locker space / using electricity / adding weight. We have certainly not had any such thing on our previous boats (Bowman 26 and Sovereign 32) and are here to tell the tale!
Neither of the previous 2 owners can shed any light on this... can anyone else, please?

 

[theoldsalt]

Sovereign,

Is there a possibility that the boat originally had a petrol engine fitted and the extractor fan was originally fitted to extract petrol fumes from the engine compartment before starting to reduce the possibility of a spark causing an explosion.

 

[rob2]

My thoughts exactly. Probably a relic from a petrol installation. May not be worth maintaining, but could reduce any tendency to a diesel niff in the saloon.

Rob.

 

[Frankie-H]

Don't forget the possibility of explosive battery fumes. It is very good practice to vent your engine room and leave the fan runing for a minute or so before kicking over you starter motor.

 

[LadyInBed]

Mine has an extractor fan (manually switched). I don't use it at the moment but I probably will when I get to warmer climes.

 

[Cardo]

Mine has an extractor fan (manually switched). I don't use it at the moment but I probably will when I get to warmer climes.

Our Southerly 105 has a manually switched extractor fan in the engine bay. I presumed it was mostly for the warmer climes.

 

[theoldsalt]

LadyInBed

Unlikely an "extractor" fan will help in hot climes.

The subject of fans in engine compartments has been discussed many times, especially whether they blow in to assist the air supply to engine combustion / help cool the engine compartment in hot climes or blow out.

My preference is that, if fitted, they should blow in for the above reasons, as "extractor" fans may starve an engine of air unless the engine compartment is otherwise well ventilated and do nothing for cooling.

 

[Momac]

There is a switched engine bay extractor fan in our diesel motor boat (built 2002). Instructions are to run the fan for 4 minutes before starting the engine.

I think newer boats may have the full time fan arrangement, with engine running. And I read somewhere about a new inboard petrol engined boat where the fan must be run for a few minutes before the electrics allow the starter motor to operate.

Gases may well be heavier than air, which may explain ducting taken to a low level in the engine compartment.

I agree an extractor fan is a more obvious need with a petrol engine. But I did hear of someone entering a diesel boats engine bay after experiencing some overheating issue. He became severely affected by fumes. Whether a fan would have helped - who knows.

Engine bay extractor fans are definately for extraction of gasses, nothing to do with cooling and no way it would starve the engine of air.

 

[Ex-SolentBoy]

Our new boat has fresh air inlets for the engine and an automatic outlet fan that comes on if the engine compartment is too hot, and before startup to remove an gases that shouldn't be there.

I would leave it in place.

 

[lenten]

sovereign----is it a wooden boat?---if so i might have a theory----regards lenten

 

[Laurie]

Our "new" Feeling 286, has the diesel engine (Yanmar 2GM20) amidships under the saloon table, housed in a cabinet thereunder, albeit with plentiful air access for intake. However it is also fitted with an EXTRACTOR fan & vent ducting. The manual is explicit in that this aids cooling/prevention of the heat sink effect after the engine has been run for a while & stopped. It suggests leaving the fan running for several minutes after engine has been stopped.

 

[Dave_Seager]

an extractor fan - automatically siwtched on/off with the engine

If you do decide to keep the fan then I suggest that you rewire it so that is can be switched on independently of the engine. As discussed already, these fans are normally used to remove petrol fumes from the engine bay for several minutes before starting or to cool the engine bay after the engine is stopped.

When the engine is running, the air consumed by the engine should provide a reasonable airflow to the engine room.

Personally, I would not bother with a fan on a diesel engine.

 

[Colvic Watson]

Very useful for extracting the warm diesel smell that is inevitable.

 

[oldbilbo]

The recent 'Crash Test' series run by Yachting Monthly has shown vividly the potential implications of an unsuspected gas leak. I'm advised that 'heavier than air' petrol fumes also may accumulate in a deep bilge, until an explosive concentration is reached.

Having an elderly boat with a deep bilge, I have acquired an engine bay/bilge extractor fan, for I don't want to be exhibited at the rear of the YBW stand at ExCel next January!

 

[Sovereign]

Thanks

Thanks everyone for your replies - very helpful.
Here is some clarification for some your musings: It is an all-GRP construction - there has not been a petrol engine in the boats history - we are not heading for warmer climes. (And for the information of one of you - the Sovereign 32 was GRP built by Uphams - all the 32's were GRP except for the 'plug' which was kept by Ian Anderson for his personal use.)
I think I will alter the wiring to manually switch it, then make an assesment re the hot-diesel smell over the first weeks of the season.
Thanks again.

 

[davidej]

I have a Bene with twin fans that come on with the engine.

I am pretty sure they blow air in not out but Marsupial suggests it should be one of each. One duct is directed to the alternator (to cool?) and the other near the engine intake