Outboard engine air consumption

machurley22

machurley22

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My 8hp 2T Yamaha twin lives in a well but can't be run without the lazarette hatch being open, reflecting the buzz forward into the cockpit, despite also having two large openings (about 1.5 sq.ft.) into the well from the cockpit.

The engine seems to suck all it's air into the cowling through a small rectangular hole, about 4 sq.in., and I was idly toying with the idea of connecting a hose to this and allowing the engine to sook directly from the cockpit. The extra convolutions of this extended path would presumably make it slightly harder for the engine to breathe.

Would this be significant? Might a small blower (computer fan) help? Is this just a cack idea?
 
Hi Dave,
not daft at all, as my Jeanneau runs with a bilge blower pump through a 4 inch wire convolute pipe., and also has a similar exit hose. Dual purpose really as the engine a0 gets enough air, and b) the hot air is taken away, whih I suspect may be more of the problem with your setup, especially if it runs for 10 mins or 15 mins before starting to fail. Try running it with the cowling removed to see if it makes a difference.
Check also that the air filter is flowing freely.
I suspect you would not ned the exit air pump eventually, as the current air would exit where it can. hey it might even be a slightly turbo set up as the air pressure increases inside the well box. /forums/images/graemlins/grin.gif
 
IIRC, high performance 2-strokes tend to require a fairly straight intake run, but I am thinking of Yamaha RD's & LC's, about the same sort of cubic capacity as your outboard. A computer fan would make no difference I think. What does it rev to with the cover closed (I'm assuming perhaps wrongly, that you have a rev counter)? I wouldn't expect it to be a resonance issue, although 2-strokes are I understand quite susceptable to exhaust back pressure, could it be down to - no that doesn't make sense. Does the hatch have clearance between it and the engine cowling? Does anyone else have any suggestions??? /forums/images/graemlins/confused.gif
 
The usual problem with outboards in wells is that they are likely to choke on their own exhaust fumes which are partly at any rate vented from a relief port or ports on the leg. A connection from there to discharge these overboard may enable you to run with the hatch closed although at slow speed or when not moving exhaust fumes from the main exhaust outlet will still find their way up into the motor well.

A small fan to ventilate the compartment may be all that is need to clear the fumes but you wont want them blown or sucked into the cockpit.

As for the cross section of the air inlet you will probably find the inlet to the air intake silencer is quite small, smaller even that the inlet to the cover.
.
 
i had this problem on a hurley 22 with a yamaha 9.9 (made it go well !)
To sort out the cutting out due to exhaust fumes building up we built a removable collar to go around the leg where the water and exhaust came out, this was then piped out through the stern. the outboard run at full speed with all covers on just taking its air where it could from around the loose covers etc.
 
Some great replies, thanks.

Hell, I didn't even know I had an air filter Jim and will certainly check that. It might be simpler just to arrange for a way of venting the exhaust from the relief valve away from the engine air intake. Thanks Vic, I hadn't thought of that either.
 
Yes, pretty unlikey for a small outboard to strangle itself due to lack of air. The exhaust fume scenario is much more likely.
Petrol engine run a air/fuel ratio of something like 13:1 so if it uses 4 litres of fuel an hour it would only require about 50 litres of air. Engines suck their air in, so provide their own supply. If it really was short of air it would initially run a bit rich and then down on power. You would notice, I think.
 
< petrol engines run on air/fuel ratio of something like 13 to 1>
For the sake of pedantry, IIRC, this is a weight ratio, not volume. The air consumption will be the swept volume of the cylinder(s) times an efficiency factor dependant on the throttle opening, ( times 1/2 if it's a 4 stroke ).
 
I welcome your pedantry, which has cleared up that point. /forums/images/graemlins/grin.gif

I reckon the rest still holds true. If you connect a bit of pipe to the inlet it'll suck enough air in for it's own needs.
 
[ QUOTE ]
Hell, I didn't even know I had an air filter

[/ QUOTE ] I missed that. It will be pretty unusual if it has an air filter. Outboards are not generally operated much on dusty roads so don't normally need one.
 
We have a Jeanneau Tonic with 8HP Mercury in starboard well. It has a small (8 or 10mm copper pipe) out of the leg which vents through the transom. The side of the locker is fairly open as well. I am not aware of problems running the outboard with the locker lid down under normal circumstances.

There might be a problem with prolonged running with boat moored on a still day but that only happens if I messing with the engine.

I don't know if it is a standard fitment or whether someone has drilled and tapped the leg to fit a pipe fitting.
 
I doubt if it was an original fitting. It would be a relatively easy mod as the inside of the leg is pretty much an empty space but it would be necessary to ensure that one did not damage one of the things that are in there, water pipe for example. Essentially one would be enlarging the existing relief port so one would be able to feel through that for obstructions. In most cases I suspect the nearest item would be the inner exhaust tube.

Usually/often there is a small vented cover over the relief port that would be discarded as part of the mod.
 
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