smokin' in reverse

[mangomuffins]

I've a friend with a 1973 34' Erwin Center cockpit ketch that has, he thinks, a three cylinder Yanmar (hp unknown). When he puts it in reverse at high rpm (something like 4000 rpm), smoke pours from the exhaust. This is only time the engine smokes. Any ideas??


cheers,
mm

[AnchorageGuy]

4,000 RPMs in reverse? That may be his problem.

[mangomuffins]

pretty quick reply...thanks.
That's what I thought so not really sure about the rpms. Basically I thought it was a load issue.

[AnchorageGuy]

It may very well be. Most boats are not necessarily propped well for reverse and a clear sign of a prop problem is smoking.

[Chief Engineer]

prop prop prop

[mangomuffins]

so, can someone explain how this would actually cause the engine to smoke? I seem to have a feeling of how this could happen but am unable to articulate it clearly.
How about this?
drag or resistance keeps the engine from turning the normal revs for a given amount of fuel supplied... friend continues to apply power (fuel) resulting in excess fuel to the cylinders which doesn't get combusted fully thus... smoke?
I'm an idiot about many things; one being diesel engines

thanks

[Evan]

Sort of. I am actually very surprised he can get that high RPMs in reverse. Or is that just a figure of speech?

Normal propellers (let's assume his is a fixed propeller) are designed for a certain speed. In reverse, they work less efficiently, and of course the usual boat speed is close to zero. In most boats, the engine is a bit overloaded in reverse and sometimes it smokes.

Tell him to go a bit easier on the throttle in reverse.

Originally Posted by mangomuffins

How about this?
drag or resistance keeps the engine from turning the normal revs for a given amount of fuel supplied... friend continues to apply power (fuel) resulting in excess fuel to the cylinders which doesn't get combusted fully thus... smoke?
thanks

That's a very clear articulation of the way things happen.

I would be a bit concerned if the engine was REALLY turning anything like 4000 RPM. I don't know of any small Yanmar that should be running that fast. If it is, the injection pump and governer may need some serious adjusting.

If the smokeing only hapens in reverse, and the engine turns close to specification RPM when running in forward w/o smoke, then it's not really a big deal (unless your friend drives around backward a lot!) Full throttle reverse should be a relatively rare event in a normal boat's life.

As people have already ponted out, props frequently don't load the engine right in reverse even if they work well in forward.

[Jentine]

What color is the smoke? Does the engine smoke all the time when put into reverse or when backing out of the slip?
If the load is sufficient to blow black smoke in reverse it is will surely blow black smoke in forward. I would bet that the smoke is always there but to different degrees.
Over pitched is over pitched.

[Chief Engineer]

Heavy boat+prop+high rpm=smoke

Just think of those diesel dumptrucks as they leave a traffic light....they smoke

try easing the Throttle up in reverse.