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)

[Powerabout]

when the 70/75 has a nitro gearbox ( small one) its a bit restrictive to the exhaust so the holes offer an alternate path

[pdt]

so is this better if using an over hub propeller just for the holeshot or better all round for the over hub and thru-hub props?
its definitely making sense to un-restrict the exhaust as much as possible.
its also a cool look I think with the large holes.
is there any down side to doing this though ?

[pdt]

I can now see it being better to breath and run better yes, but would it also make it better to have NO exhaust running over the blades of a over hub propeller ?
I know that the over hubs are designed for surface piercing so do work with a certain amount of air mixing with water, but if the exhaust gases are
now non existant is this going to give even better results for this type of prop. or is there still a certain amount of through the gearbox exhaust ?

[Popa Sam]

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)

Just a FYI. The 1989 and 1990 EP nationals were won with a motor similar to the one pictured. The winning motor did not have the exhaust holes as shown, others in the race did have them.

[Powerabout]

Just a FYI. The 1989 and 1990 EP nationals were won with a motor similar to the one pictured. The winning motor did not have the exhaust holes as shown, others in the race did have them.
so the winner didnt sound as good as the rest? lol

[pdt]

so the winner didnt sound as good as the rest? lol

ha ha brilliant ha ha
winner didn't sound and good :cool:

[pdt]

opened up the gearbox to have a look in the area to drill the holes for exhaust vent/relief.
Was surprised as ive never even bothered looking down there and taking much notice before, its just a big empty.
I see its a nice flat area with no webs to worry about so I will be drilling those out as real soon.
had a look at the water pump while there and managed to have a bolt break in the gearbox !! now I am f-------d, need to get that out now.(separate topic)

When drilling the holes is there a good size to do them ? ive seen some with 2 holes and a couple with an elongated hole !!!! is one better than the other or
just as good ?
looking forward to having the holes now, sound of a 2 stroke is nice even when loud, and if it makes the engine go a touch better than great.
i am always prepared to try anything to make it run a bit better, and this is definitely an easy job to do, just the type I like.


.

[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 ?

[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.