Engine bay fan?

[Cardo]

Our little boat has been fitted with an engine bay fan since she was new back in 1979. I've discovered from reading the original paperwork that it was an optional extra specced by the first owner.
The fan is one of those blower jobbies that sucks air out of the engine bay and out through a vent on the topsides. However, it only comes on when I manually switch it on.

Question: What's its purpose? Should I turn it on when we're motoring? Should it ideally come on above certain temperatures? Should I fit some sort of thermostat?

 

[Colvic Watson]

Turn it on for 5 minutes before you start, you vent any combustible gasses e.g. domestic gas or fuel fumes. No point when you're running the engine because the air intake sucks in way more than a blower blows. Use it again for 5 minutes after shut down to remove hot smelly smells

 

[sarabande]

running the fan before an engine start woul make sense particularly if you had a heavier than air gas leak AND the fan intake is near the bilge AND is the non-sparking type (otherwise KABOOM).

If it is 'just' an ordinary extraction fan, then what it does best is remove hot air from the engine compartment, which in turn drags (probably) cooler clean air in from outside which helps the engine run more effectively. If the engine room is fed from adjacent cabin(s) then a fan helps keep them fresh.


Worth checking to see if it a special extraction fan.

 

[RupertW]

running the fan before an engine start woul make sense particularly if you had a heavier than air gas leak AND the fan intake is near the bilge AND is the non-sparking type (otherwise KABOOM).

If it is 'just' an ordinary extraction fan, then what it does best is remove hot air from the engine compartment, which in turn drags (probably) cooler clean air in from outside which helps the engine run more effectively. If the engine room is fed from adjacent cabin(s) then a fan helps keep them fresh.


Worth checking to see if it a special extraction fan.

We have twin fans which go on whenever the ignition is on and for about 10mins after engine stop. The main advantage I think is to stop the engine compartment getting hot and overwarming itself then the cabin.

 

[Cardo]

Worth checking to see if it a special extraction fan.

What's a "special" extraction fan?

The fan's intake is at the top of the engine bay. I'm guessing it's to get rid of hot air to cool things down. But then, should it run all the time? (I've never actually bothered to use it) or should I add a thermostat?

I was checking out a friend's engine bay (ooh, err missus) and he had a similar setup, which he claimed came on automatically when the temperature was high.

 

[TimBennet]

Running an extraction fan when the engine is running is pointless. Even a 1 litre engine running at 2000 revs will be sucking about 225 gallons of air per minute. A little fan will just slow the air being pulled in through the fan vent.

So either plumb the fan to vent the bilge as mentioned above, or run it after the engine has shut down to more efficiently rid the engine compartment of heat.

 

[Colvic Watson]

Running an extraction fan when the engine is running is pointless. Even a 1 litre engine running at 2000 revs will be sucking about 225 gallons of air per minute. A little fan will just slow the air being pulled in through the fan vent.

So either plumb the fan to vent the bilge as mentioned above, or run it after the engine has shut down to more efficiently rid the engine compartment of heat.

Wot he said. It will cool an engine room about as well as putting an ice cube in. Once the engine has stopped it's good to run it for about 10 minutes to freshen up the air in there a bit and because our engine room is open to the rear bilge I use to freshen it up after we haven't been to the boat for a while.

 

[Marsupial]

It should run all the time, its there to scavenge the hot air from the engine compartment and ensure a plentiful supply of fresh cool air to the engine inlet. Very often the fans are fitted with a thermostat. All modern installations have them and yes the engine will run without one but it runs better with one. We specified them back the 1970's for our marinised engines but often the boat builder knew best . . . . it seems your first owner knew what he was on about. Running the fan after the engine is stopped is not necessary for the engine but it will clear fumes from the engine compartment, funny how the added bonus becomes the primary function!

 

[TimBennet]

It should run all the time, its there to scavenge the hot air from the engine compartment and ensure a plentiful supply of fresh cool air to the engine inlet. Very often the fans are fitted with a thermostat. All modern installations have them and yes the engine will run without one but it runs better with one. We specified them back the 1970's for our marinised engines but often the boat builder knew best . . . . it seems your first owner knew what he was on about. Running the fan after the engine is stopped is not necessary for the engine but it will clear fumes from the engine compartment, funny how the added bonus becomes the primary function!

Sorry, that's complete rubbish. An engine room extraction fan will never compete with the air sucking requirement of a diesel engine that's drawing air from the engine space.. You could try blowing air into an engine space, but this slight positive pressure would exacerbate the problems of smells / heat / fumes getting into the rest of the vessel. It is therefore bad practice.

The only way to reduce air intake temperatures is to feed the engine intakes directly from outside the engine space (or water cool the intakes). Then if you remove this massive suction effect of the engine, you could use an extractor fan for the heat venting. This would be the arrangement on much larger vessels.

But the OP has a extractor fan on a simple small engine installation venting the top of his engine space. The only function of this could be to extract heat from said space after the engine has shut down if he thinks that would make a difference to something. Only he can decide. Most of the year I'm glad of the engine warmth!

 

[Marsupial]

Sorry, that's complete rubbish. An engine room extraction fan will never compete with the air sucking requirement of a diesel engine that's drawing air from the engine space.. You could try blowing air into an engine space, but this slight positive pressure would exacerbate the problems of smells / heat / fumes getting into the rest of the vessel. It is therefore bad practice.

The only way to reduce air intake temperatures is to feed the engine intakes directly from outside the engine space (or water cool the intakes). Then if you remove this massive suction effect of the engine, you could use an extractor fan for the heat venting. This would be the arrangement on much larger vessels.

But the OP has a extractor fan on a simple small engine installation venting the top of his engine space. The only function of this could be to extract heat from said space after the engine has shut down if he thinks that would make a difference to something. Only he can decide. Most of the year I'm glad of the engine warmth!

And you have specified and installed how many engines?

Do you have any idea how to size an engine space ventilation fan? If you did you might want to discuss how great a temp rise can be tolerated or what density and volume of air we are trying to move instead of dismissing best practise as rubbish. Find me ONE engine manufacturer. Who does not specify engine space ventilation requirements and the need for mechanical ventilation if the space is enclosed. Find me one alternator. Manufacturer who does not require clean fresh air below 120deg for their equipment to work properly. Pleasure boats are not Destroyers, we don't pressurise the engine space but we do ventilate it PROPERLY.

 

[jerrytug]

The best way for a little fan to help the engine run cooler and deliver more power is to have it blowing IN TO the engine compartment.
However you wouldn't notice any difference, because the suction of the engine is hundreds of times more powerful.
Yours is just for getting rid of hot air or smelly air, before or after you run the engine. Also, as someone said, if it is not explosion proof, it is a brilliant source of ignition...

 

[TimBennet]

And you have specified and installed how many engines?

It's not how many you've installed, int's how many have you installed properly.

An engine is a massive air pump. The design requirement is about getting enough air into the engine.

Here's the instructions that come with a Vetus engine room fan "We do not recommend using these extraction ventilators to provide air to the main engine(s). Every marine engine is able to draw sufficient air by itself, provided of course, that the ventilation openings to the engine room are adequate. If an extraction ventilator is placed in the air duct, the electric motor may overheat, as the suction power of the engine will cause the fan to over-rotate.
The purpose of these VETUS ventilators, is to extract the heat from the engine room when the engines are stopped, or, when petrol engines are installed, to extract any possible petrol vapours prior to starting the engine(s).

 

[Marsupial]

OK here goes, the VETUS fan is not a recommended ventilation fan for providing reliable cooling and combustion air, commendably Vetus admit it's not durable. The vetus manual says engines are pumps but must have adequate ventilation - a bit vague but their install manual says different and describes best practise. A brief foray into CAT, VOLVO and YANMAR installation manuals reveals that engine space ventilation must be provided and sized to ensure engine bays do not exceed 50 deg C and they expect a mechanical ventilator, in fact if you do the sums there is no other way of achieving that standard. The American ABYC guide also sets out the methods and temp limits on engine bays and methods for venting.

CAT publish the formulae for sizing the fan the last part of which is "+ combustion air" so there's a clue that the pumping action of the engine is not sufficient to "clear the air".

A brief foray into other engineer sites and forums reveals that the majority of premature engine and ancillary failures are attributed to inadequate engine space ventilation which accords with my field experience putting this right after a bloke down the pub said he could install it. But please don't take my word it, just don't properly ventilate your engine and let it fail sooner that it would have had you bothered to read the manual.

Lastly I only know one way of installing an engine - the right way I've specified boats, hovercraft, gen sets, pumps, diggers tractors and the rules don't change just the challenge complying with them.

I wont reply to anymore cheap shots ruin your engines . . . .

 

[prof pat pending]

What's a "special" extraction fan?

The fan's intake is at the top of the engine bay. I'm guessing it's to get rid of hot air to cool things down. But then, should it run all the time? (I've never actually bothered to use it) or should I add a thermostat?

I was checking out a friend's engine bay (ooh, err missus) and he had a similar setup, which he claimed came on automatically when the temperature was high.

Does it look like this?

Do you have a petrol engine?

 

[TimBennet]

I wont reply to anymore cheap shots ruin your engines . . . .

Thanks for the abuse. This thread is about small, inboard diesels. It's the PBO forum. The specific inquiry was about an engine box with a small extraction fan mounted to its upper end. In all probability it has a water cooled exhaust system.

Big engines, especially with dry exhausts will require forced ventilation to maintain satisfactory ambient temperatures and boats with engine rooms will need it to maintain working conditions for engine room crew, and in my original reply I said that separating the feed of air to the engines from ventilation of the engine space in this sort of big boat installation, stops the two systems working against each other.

But with small, wet exhaust, yacht installations the key is to have adequate air available to the engine. With older aft cockpit boats this was rarely a problem. With centre cockpit, aft cabin boats, the engine can be more confined, especially as the engine insulation becomes more efficient. Therefore passively feeding air becomes more difficult and forced feeding can be used (Jeanneau etc). But it risks the chance of forcing hot air and possibly fumes into the accommodation space and is not ideal practice. But running a small, extraction fan from an engine box will never achieve anything and risks, as Vetus say the fan motor burning out.

 

[MASH]

Every marine engine is able to draw sufficient air by itself

Hmm. Not so sure about this. The 42ft Picket Boats I trained on at Dartmouth had engine room blowers that had to be used above a certain rev limit(?). Certainly if they were driven past this limit without the blowers on the engines flagged, and revived miraculously when the erring Middie was delivered a memory-jogging blow to the head and the relevant switches were made.

As to cooling, I often wished for an engine bay ventilation (extractor) fan in my boat in the Med to take the donk's heat away but this would have required some substantial big-bore plumbing and to have run for hours with all the noise that this would entail.

300lb of Yanmar at the best part of 90'c does not cool off in 10 minutes - or anything like it.

Forced draft blowers and engine bay extractor fans are two completely different things.

 

[TimBennet]

Hmm. Not so sure about this. The 42ft Picket Boats I trained on at Dartmouth had engine room blowers that had to be used above a certain rev limit(?).

Forced air feed in that case is because the engine rooms didn't have sufficient passive air intake capacity. If you put those engines out in a field you wouldn't need the blowers at any revs. If the engines need more air for combustion, then they will be turbo or supercharged - the blowers overcome some shortcoming in the engine room design, either by omission or a constraint imposed by other considerations in the boat's design.

Maintaining the temperature of the intake air, controlling the working environment of the engine room, and evacuating the heat as engines cool, are all separate issues. As you say, 'forced draft blowers and engine bay extractor fans are two completely different things'.

 

[William_H]

Thanks for the abuse. This thread is about small, inboard diesels. It's the PBO forum. The specific inquiry was about an engine box with a small extraction fan mounted to its upper end. In all probability it has a water cooled exhaust system.

I am not sure where Tim gets his idea that this is about small diesel engines. If the OPs engine is petrol then it is most likely an extractor fan as already said to get rid of fumes in the bilge. It is quite likely that the ducting from the fan to the bilge has been lost. This extractor can be a vital safety feature to remove combustible gases that might builds up especially if the petrol engine is hard to start and is flooded. The fan is run prior to engine start and switched off once the engine starts.
If indeed it is a small diesel then it surely is an attempt to cool a shut down engine and possibly stop heat drifting into cabin. good luck olewill

 

[Heckler]

And you have specified and installed how many engines?

Do you have any idea how to size an engine space ventilation fan? If you did you might want to discuss how great a temp rise can be tolerated or what density and volume of air we are trying to move instead of dismissing best practise as rubbish. Find me ONE engine manufacturer. Who does not specify engine space ventilation requirements and the need for mechanical ventilation if the space is enclosed. Find me one alternator. Manufacturer who does not require clean fresh air below 120deg for their equipment to work properly. Pleasure boats are not Destroyers, we don't pressurise the engine space but we do ventilate it PROPERLY.

Plus one! Both of my Benes have a fan, running all the time. Blowing hot air out from the top of the engine. There are 3 vents in to the engine/sugar scoop. One out from the blower, one in to near the engine intake and one general to air the sugar scoop. The idea is to get rid of the hot air from the top of the engine compt. One of the biggest manufactures must know what it is doing!
Stu

 

[sarabande]

What's a "special" extraction fan?

The fan's intake is at the top of the engine bay. I'm guessing it's to get rid of hot air to cool things down.

Unless the engine installation has been specifically designed to rely on air being blown INTO the bay (which on a saily boat is highly unlikely) the fan will be an extract one.

As you have the intake at the top of the bay, it has been designed as a heat extraction one, and will not be need to be an explosion proof one, unless you feel very risk averse.

If the intake were at the bottom of the bay, then there is more of a chance that hydrocarbon gases MIGHT be sucked up past the fan; in which case you do need a non-sparking fan.


As indicated in other posts, run it for a few minutes after stopping the engine to get rid of heat and perhaps some fumes.