exhuast out the water ?

[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 ?

[champ20B]

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 ?

First, I want to refrase "thru-prop" to "thru-hub" in regards to the exhaust outlet as it is more definite. I've heard it called one or the other over the years. I used the forementioned "thru-prop" term in my previous post, but I'll use "thru-hub" from now on.
I think I've seen some v-6 engines from mercury that have a "race only" above water set-up. I think some are based on the 2.5 liter. There were others maybe. They are special race motors though. I think I understand what you want to do with your engine. I thought about doing that for my 1997 25HP johnson I had on a small speedster once. I have seen conventional non-vented cleaver race props used like this on thru-hub exhaust type gear cases before. I suppose it could work, but it is not a very popular choice though. There is the likelyhood of cavitation that could burn up a prop. If you re-route the exhaust though, it will work I'm sure. But it wont increase performance at all. I did try this on an old mercury Mark 25. This motor has a gearcase that is shaped just like a thru-hub exhaust case but is actually above water exhaust. It used a prop that just had a large hub. Anyway, I put an old Stannus two blade cleaver on it and ran it on an old B/UTILITY runabout. I got 42.6 MPH out of it without any trouble. It seemed to work fine. But honestly, the hub wont affect performance "alone". It is the design of the blades along with proper set-up that will really count. The two types of props are every bit equal. One thing to consider though, about a thru-hub exhaust, is that during high speed, exhaust is pulled/dragged out from the middle of the prop that helps with exhaust scavenging. I notice that later thru-hub mercs tend to run stronger than the old above water types in regards to fully submerged propeller operation. When jacked up, they're about even. I believe if you are running a thru-hub gearcase, you would be much better off with a good vented "thru-hub exhaust race prop" with a nose cone added on to the gearcase. Thats even if you set the prop shaft 1/2" below the water as well. You then won't have to re-route exhaust at all. But that is just me. You may find a better edge as every set-up is a bit unique in boats and engines.

[pdt]

that all sounds pretty good to me, I was wondering about cavitation and aeration on a solid hub prop with a thru hub exhaust.
that is why I was thinking of drilling the gearcase and venting the xhaust above the cav plate when using a solid hub prop, and putting plastic bungs back in the holes when using a conventional thru hub exhaust prop.
I can see that the two types of prop would be similar in top end performance as well, but if the solid hub prop is getting the exhaust chucked over the blades this must cause terrible aeration
and must cause slippage and lower pull away ?
or does the air/water mix actually help the surface piecing type prop as its designed to be run high and mixed with air when in surface mode ??????

I will definitely have to test these theory's out for myself and see what results I get, im looking for any benefit at all. 1mph is fine if it works,if not then ive wasted my time (again).
I notice that all the race motors from the omc stable came with the drilled exhaust vents above the plate, like the sst60 etc, that's what made me ask about this type of thing.


But through hub exhaust is not dragged out the prop like you might think.