Blow or Suck

[maxtorque]

I've got a sealine S240 with a diesel. The factory fitted fan in the engine compartment sucks the hot ambient engine air out through the fins on the starboard quarter.
I have been told by a very experienced mechanic that the motor should be blowing air into the engine, the opposite to what it's doing now.
He says that because it's a diesel, it doesn't need to extract fumes, as much as supply the engine with colder air for the intake.
He commented that a few manufacturers do this with diesels because of their mentality towards petrol engine compartments.

Some advice gratefully received please?

WT

 

[tcm]

If you only have one blower, I think he's right to say it should blow air in. Suck air out will mean that the gasping diesel has to find air from inward leakages.

However, if you have more than one, best is to have an in and an out to make a nice flow of air, and be able to change which side blows and which side sucks, so you can change accordingly to blow outwards into a beam sea.

 

[PGD]

I've got one of these left from the refit from petrol to diesel - might see if I can switch around what it does, mine outlet to the stern so it might draw in fumes --- Hummm

peter

 

[BarryH]

I can see the point in that, but has the boat got gas onboard. It tends to sink to the bottom. So a suck joby with a bit of ducting bown to the bottom will get it out. If the ducting goes down under the engine then the air will be pulled down over the air intakes any way.

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[ccscott49]

Sorry chaps, all engine room fans should suck, in case of fire. Thats the regulations, but theres no electric fan in the world, which could blow enough air into an engine room, to feed a hungry diesel engine, try feeling the air being sucked into the engine when it's revving it's nuts off, mind your hands, it'll suck them in! When your engines do not get there air from the engine room, as in a proffesional boat instalation, the fans always suck the air out of the engine room, to effectively remove fumes and stop them being blown into accomodation areas, through any small air leaks into the engine spaces.

 

[andyball]

If there's somewhere for the air to get in (another set of fins), I don't think it'd make much difference to performance if it blows or sucks, but fumes would be less if it sucks.

If cold intake air is important(it helps), they'd rig up an inlet from outside straight to the air cleaner,(with water drain) praps with a temp. controlled valve to draw the air from inside when it was cold. err, just like most cars do<g>

 

[Dave_Snelson]

Hi Wayne - welcome aboard.

The suck bit is definately more for gasoline motors. In fact, there is always the instruction to "vent the engine bay" for 30 seconds before starting. Clearly, inducing a vacuum in the engine bay is not conducive to supplying any engine with vital air and I would suggest that any decent designer would allow for this in the engine bay design. BUT (AND THIS IS IMPORTANT!).....explosive fumes can exist with diesel if something were to go wrong, like a craked injector line / leaky fuel joint that then leaks fuel onto a hot part of the motor. You will then definately need a fan that sucks out into the open, in order to vent the vapours and avoid explosion.

Cheers.

Don't change the boats inherent design - just check adequate air intake.

Madoc Yacht Club
www.madocyachtclub.co.uk

 

[tcm]

Re: resite the gas?

It would be loads loads safer if the gas is sited outside the engine room, if poss, imho. On sailing boats they're in a sepreate cabinet often at the back of the boat so er they really blow your bum off if they go bang at sea...but at least don't get triggered by hot engines. In the open air, like under a seat but so that they don't slip about is good.

 

[BarryH]

Re: resite the gas?

Umm, didn't make my self clear. It should have read "If there was a gas leak it would sink to the bottom of the boat". I know what I'm talking about anyway! Its the stringing the words togther bit that allways stumps me!

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[ccscott49]

Gas should be in a totally seperate area of the boat, in a sealed box, open to atmosphere at the top, but with a drain over the side at the bottom, not into the cockpit or onto the deck, where it might find it's way below, but over the side, also where it wont be sucked into the engine room or anywhere else on the boat. Gas is really nasty stuff, it will explode on mass, witness the Piper "A", that was liquified gas.

 

[tcm]

ooh ,i will have to have a look at mine. They are reversible, but 4x quite massive 240v jobs which make a monster racket and take about 8 amps each so must be about 2kw.

Will it go faster with blowing in? Or backwards teehee?

 

[ccscott49]

I didn't notice, but didn't your engines draw air from outside the boat? If they didn't, it doesn't matter, because as long as the engine room has big enough breathers, the engines will do a fine job of pumping their own air! Any fans for the engine room will help, not with supplying air to the engines, but supplying cooler air to the engine room, which is very, very important for horses......power that is! There is a calculation for every degree C, loss of power etc. but forgotten it. /forums/images/icons/smile.gif

 

[Forbsie]

Piper 'A' - my first offshore job.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/forbsie>My Project Pics</A>

 

[ccscott49]

Was on the piper-B a while ago, still a shitheap!

 

[BarryH]

Arrrgggghhhh. I KNOW. Right you've asked for it, you get the posticon thing

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[ccscott49]

Barry, come on I wouldn't blow you up, you know that! Well I might! /forums/images/icons/smile.gif

 

[tcm]

ah agreed they wd pump their own air BUT if they drew it in long pipes then there be pumping losses due to some other bernouili thing thati have forgotten too.

The filters draw straight from engineroom. I am gonna try whamming around with the engines blowing air in and see if it goes a bit faster. Then, i will set two of them in and two of them out. They seem to be able to knock the temp down to almost ambient in a few minutes.

There's a automatic grille shutting thing to stop effect of blower altoghter in case of fire, spose best thing really, to starve if of oxygen completely, cos if a good fire then the blowers wd stop and the fire use the hole to breathe?

 

[jfm]

Re: what are fans for

Not sure about this, I mean, I'm not quite sure what the fans do. As mentioned elsewhere, TCM's boat frexample, has several massive engine room fans running off 240v. (Incidentally, as regards fire, they have electrified louvres that slam shut if the fire sensors think there's a fire)

But that engine room has 2xmassive diesels, cant remeber how big, but say 15litres each. At 2000rpm, the air being sucked in by these monsters and pushed out the exhaust pipe must be roughly 2x 15 litres x 2000 x 0.5 (becos 4 stroke) every minute. That's 30,000 litres a minute. That's a cube of air measuring 3.1 metres each side, each minute.

Now, I'm not sure if that's a lot or a little but I would think the engine could happily pump that itself and some poxy (or monster) fans aren't going to make much difference. So I'm wondering what the fans are really for in the first place?

They're not there to cool the engine room. Even if you suck out 30,000 litres a minute of warm air and replace it with ambient air, the amount of heat energy you are removing is sod all compared with the heat energy in 4 tons of hot engine. Maybe indeed the fans are to suck out horrid fumes, in which case they should suck not blow?

 

[tcm]

Re: inlet air temp control?

With the fans off, the dial thig whams up to 40+ degrees, withem off, it's down to 30. I think it is to reduce inlet air temp, so increse inlet air density and so on...


Bu is this all cobblers? Cos 10degres difference in centigrade only shows a difference in Kelvin means inlet temp is um 305 K insted of 315 K, a poxy 3 %. But maybe 1 knot? I need a long flat bit of water to test all this out, and clipboard and lab coat too..

 

[BarryH]

Re: what are fans for

The idiots guide that came with the engine from V**** on my boatsays that the "blowers should be run for 3-4 mins before starting to vent the engine compartment and kept running if the engine is running at slow speed, ie river speeds. Now ok I've got a petrol so I need to vent any fumes that may be lurking in the engine compartment. But the vent shapes themselves also play a part in moving the air. The vents on my boat create a bit of a vacum when its moving forward at speed. Much the same as when a car goes fast it gets the spray on the back window in the rain. Surely the blower is there purely for venting the engine area when the engines are not running or running at slow speed just to get cooler air into the things. Or am I totally wrong again!!

OK, to hell with it. Unbolt it and we'll use it as an anchor!