exhuast out the water ?

[pdt]

I don't know the correct term for this, but i have seen lots of race engines with the exhaust coming out of the back of the
mid-section instead of through the propeller.
is there a great benefit to this ?
is there a downside apart from noise ?
and does it make a difference to a over the hub propeller or is this not advisable to do with such props.

.

[Coastal 60 LO]

I personally don't think they offer any power increase on most motors. Some will disagree. Sometimes opening up the exhaust like that will require you to re-jet your carbs because it can cause a lean condition. Also, by cutting the mid you weaken it if done incorrectly. Some people have lost lower units due to the mid breaking from exhaust cutouts. One of the best things about opening up the exhaust like that is that for some it makes the races more interesting. Races would be boring without the noise. I prefer to drill the lower unit for some added noise. Drilling the lower unit above the antiventilation plate seems to hold up better. JMHO.

[pdt]

would that be like on some where I have seen large round holes above the cavitaion plate ?

and would this help a over hub prop grip better if less exhaust ?
or does this type of prop need the exhaust to help it spin up to revs etc.

I ask as ive often wondered what the effects of all this modifying does to the engines,
I can see you point about it weakening the mid section if not don't very well. never thought about that ...mmm very scary

[Emustard]

Hey I run the exhaust above the water and below on a 20hp evinrude running a 18pitch cleaver over hub and it makes no difference on acceleration. yet I don't have the exhaust completely cut off from the lower unit just have a silent choice for it to make it above or below the water. So with the exhaust above the water it runs about 100 more RPMs so almost 1Mph.

[pdt]

I wondered what the difference on the hole shot would be due to knowing that just venting a prop allowing exhaust escape gets a prop spinning up faster.
Now with an over hub prop all the exhaust is being allowed to go directly to the prop, does it make a difference and not allow such a fast spin up due to
NO exhaust going over the prop blades at all.

I understand that once its on plane the difference would probably be minimal, but I suppose 1mph is a lot if racing.

like the idea of being able to switch when you want to open or closed exhaust, very ingenious.

[Emustard]

yea it is a really nice feature to be able to switch on the fly, I have never been able to completely block the exhaust form going to the prop, so would not be able to answer that question for you all I can say is my thru hub will get me off plane a lot quicker than my over hub, because of the exhaust making it slip but once I get off plane and hit about 25-30mph she hooks right up and I run about 12% slip, but with my thru hub a run about 8-10% slip. only difference is I run the thru hub a lot higher out of the water so that's probably where I get the more slip.

[pdt]

I have only heard a couple of engines with the exhaust out the water, they sound pretty awesome.
Its like an advantage before you even move if it sounds good lol.

Its nice to be quiet at times as well so the switch on the fly for the exhaust sounds great idea, is this a thing you can buy or did you make this yourself ?
and do you have any pictures of this at all ?

pdt

[pdt]

so the exhaust is like an expansion chamber when the over the top exhaust is used ?
that makes sense, like a sort of turbo dragging the gasses out on each stroke rather than having to push them out due to being a specially tuned length etc

[fs5]

I got heaps better hole shot with a couple holes drilled in the gearbox above the cav plate.especialy with the over hub mini choppers.
my hull is a tunnel though.

[pdt]

is this the type of thing you mean by drilling holes above cavitation plate? the two big holes next to six smaller ones
is this just an empty exhaust cavity then , and this lets the chopper prop bite pure water instead of an exhaust mix ?

( By the way this is NOT my engine its a pic I have used I think from this forum, nice looking old engine from the seventies)