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.

[88workcar]

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.

I agree 100%

[pdt]

on the pic I posted of the white engine with the holes above the cav plate, what are the six small holes for ?
I cant see what they are there for ?

[champ20B]

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.

.

"Above water exhaust". The real benefit to having an exhaust seperate(above the prop), is to incorperate a slimmer, more streamlined racing foot with less displacement/less drag. The idea for the underwater/thru-prop design was about quiet operation in recreational use. It would be hard to build a thru-prop exhaust gearcase that is very small in diameter, as there has to be a sufficient area for the exhaust to exit. Furthermore, the idea of thru-prop exhaust would be for nothing if the prop-shaft were run up to the water surface. There would be noise anyway. The seperate exhaust tower/racing type gearcase engines can be run very high up for minimum drag. Over the years, underwater(thru-prop) units have been utilized in certain classes of racing though. There are cone-nosed/race-like versions of these that are made as well. However, I don't think you will see the two types raced against each other much, if at all. Thru-prop race types are more of a utility/racer class of today. Unlike the shortened, "above water exhaust" race engines, the high performance thru-prop models can be used recreationally.

[pdt]

well I can see what your saying to a point about race type gearcases.
but what engines in the 40 to 100hp bracket have no underwater exhaust ? I don't know of any within the last few decades..

The thing I need to know is a thru prop exhaust of any benefit when using an over hub prop ?
i.e does the exhaust help a bigger pitch prop spin up quicker, like it does when venting a prop.
or is it better for the over hub prop to have just water without exhaust gases to get it gripping better for a faster pull away ?
the prop in question would be a surface piercing prop 13 3/4 x 23 pitch on omc triple. (omc silver streak/SRX)

im definitely gonna give it a try with a pair of 1 inch holes each side of the gearbox above the plate, and get a couple of
rubber bungs to try the difference in same conditions with in just a couple of minutes of each trial and do some
zero to 30mph times and top speed trials.

I keep thinking that at top speed although 1/2 the exhaust I will be out the water (due to being jacked with propshaft 1 inch under bottom of boat),
some of the exhaust could still be in the water causing aeration/bubbles instead of pure clean water for the prop to grip.

all gonna be trial and error and probably have disappointing results, but got to be worth a try.
just wondering if 1 inch holes are big enough or do I need to go a touch bigger ?
also from factory tests I presume the exhaust box is sort of tuned to the engine a bit and will the freely escaping exhaust cause any
other propblems due to having almost zero back pressure ?