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Exhaust when using over hubs props
https://hpbc.ca/viewtopic.php?f=19&t=3100		 Page 1 of 2
Author:  phillnjack [ October 4th, 2015, 6:08 pm ]
Post subject:  Exhaust when using over hubs props
Ok I have a question for the guru's who run with over hub props.
when using over hub props Is it best to have exhaust coming out above cav plate or
thru the hub as normal ??????
for example when using a over hub semi-cleaver like the omc SRX type props.
I ask this as I was told to jack my engine up high, I jacked it up on a static plate so the propshaft was just 1 inch above bottom of boat.
when I tried this I trimmed the engine under as far as possible got terrible holeshot.
so then tried it with engine level trim and same result, and it just took so long to
get on plane.
If I use the same prop without a jack plate the holeshot is not brilliant but not too bad.
I am trying this prop on a 1995 Evinrude 3 cylinder 60hp 56 cu in oval port, the prop is
13 3/4 x 23 and sharp, with no nicks and in good condition.
I only get a max of 5500 rpms with this with engine down low and just 41mph, ive got a
bigger set of carbs on the way to take it up to 70hp to give me more power and more
revs at top end.
Once on plane the prop seems to bite pretty good at mid range, but slip with this prop is
far too high at around 18% !!!
The engine when raised was not lacking power on the holeshot, it just did not bite at all with propshaft just 1 inch below bottom of boat.
the setback with the jack plate is 4 1/2 inches.
question is
is this because there is too much exhaust reaching the propeller ?
or is there another problem that I am not thinking about ?
(with a thru hub 13 x 19 pitch it jumps on plane )
any help or advice on this would be good.
..
Author:  phillnjack [ October 5th, 2015, 9:38 am ]
Post subject:  Re: Exhaust when using over hubs props
Oh come on people
I thought this was supposed to be a forum for questions and answers.
surely some of you know a bit about over hub props and exhaust being thru-hub or over cav plate etc.
Author:  Tomcat [ October 5th, 2015, 10:53 am ]
Post subject:  Re: Exhaust when using over hubs props
Think you'll find that most on HPBC run larger displacement motors on larger hulls than yours.
You might have more luck posting on http://www.boatracingfacts.com/forums/index.php forum.
More guys running small hulls with a variety of setups there.
Author:  phillnjack [ October 5th, 2015, 12:42 pm ]
Post subject:  Re: Exhaust when using over hubs props
ahh so all the t750 t50 people don't count !!!
all those with 56cu in or less don't count ?
having larger engines would not and should not make the slightest difference to my question.
so forgetting the size of hull and size of engine,

what exhaust is best for a over hub semi - cleaver propeller ?

now that should get some answers.

and on boat racing f***s ,
there is far too much fiction from people with 100mp fishing punts.
Author:  OneCycle [ October 5th, 2015, 12:56 pm ]
Post subject:  Re: Exhaust when using over hubs props
depends if you need the Prop to cavitate to get the rpms into the HP to get the boat on plane ? if you need prop slip then send your Exhaust thorough the prop if you dont then send it above the cav plate.
Author:  Culprit [ October 5th, 2015, 2:24 pm ]
Post subject:  Re: Exhaust when using over hubs props
your motor was likely too high. It is not going to holeshot like a ski boat with an over hub prop. Part of the idea with an over hub is to create that slip at take off, typically the pitch is too high to accelerate very well if the prop was hooked up right off idle. Most, if not all t boats run propshafts below the pad. The exhaust flow has little affect at top end, i think you have some testing to do before you worry about where the exhaust is exiting. try the motor 2 inches below as a starting point. If the slip is still very high go lower.
Author:  Carson [ October 5th, 2015, 4:25 pm ]
Post subject:  Re: Exhaust when using over hubs props
With the motor jacked up that high i hope you have a few of the top water intake holes plugged or a lwp.
Author:  phillnjack [ October 5th, 2015, 5:25 pm ]
Post subject:  Re: Exhaust when using over hubs props
I don't think im getting the message across here to well.

Here is a pic of the engine on my jack plates, this is where I first tried this.
No lwp as still getting plenty of water pressure (same as down low)

from this setting I then tried it up higher with no difference at all, just terribly long time
to eventually get on plane.

do you think this is too high ?????

.Image

Image

.
Author:  dubber [ October 5th, 2015, 5:42 pm ]
Post subject:  Re: Exhaust when using over hubs props
One of your initial observations about jacking the motor up resulting in poor holeshot is not uncommon. This is why some will run a hydraulic jack plate to get the best of both worlds. What are you actually trying to achieve???? Acceleration? top end speed? Somewhere in between? I agree about the comment about not worrying with where the exhaust is coming out at this stage as it sounds like you have some playing with the setup to do and figure out what you want.
Author:  phillnjack [ October 5th, 2015, 6:34 pm ]
Post subject:  Re: Exhaust when using over hubs props
I do obviously understand that I wont get blistering pull away and amazing top end with any one prop with my engine.
I want maximum top end with this prop, with the ability to get on plane within a decent amount of time,
if I could get the boat on plane within about 10 seconds and get good top end I would be happy with this.
For the quick pull away days I have other props to use as recreational and towing props..

this is not just a poor holeshot, its diabolically bad. I can take way over 200 yards and still not be on plane.
its like I am getting terrible prop slip, that's why I asked about the exhaust exit..
is it too much air getting to the prop from the exhaust ?

Most engines I have seen that use these type of props have drilled holes above the cav plate.
when the motor is mounted straight on the transom and prop centre line is 7 inch below the bottom of
transom the prop bites big time and really pulls hard on the engine and gets up on plane quick.
I was told by raising the engine up the prop would infact give a quicker holeshot, well that is 100% the
opposite to what I have had happen.

maybe I will have to try lowering it 2 inches and start over, but then will it be high enough for top end ?
I cant use hydraulic jack plate as they are far too heavy for the back end of my boat.

what I want to eventually achieve is a quick boat for what it is, I don't ever expect it to be a raceboat that
gets to 60-70mph.
50mph is my dream target, but 46mph is a bit more realistic .
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