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Old 02-08-2012, 11:52 AM   #1
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Through exhaust prop without the exhaust

Has anyone ever experimented with through or over exhaust props where the exhaust comes trough the transom and the transom exhaust port is blanked off ?

In theory I would have thought that a depression would be caused on the rear face of the gear box hub or prop causing drag at speed. I was thinking of having an inch tube on the exhaust blanking plate running to a skin fitting, allowing the rear face of the prop to draw in air stopping any negative pressure and drag. I was also thinking that it would allow the prop to spin up on acceleration.

Has anyone ever tried anything similar?
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Old 02-08-2012, 01:29 PM   #2
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Most racing turbo diesels have something like that to allow the prop to spin up - we have a 4" pipe down to the prop as the "Bustle" covers over the area of the surface drive and stops the props from spinning up even when trimmed up.

We don't have through hub exhausts but it all works in a similar way and I would think it would be better than blanking it off.
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Old 02-08-2012, 04:01 PM   #3
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Thanks Cookee, can't see as it will do any harm.
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Old 03-08-2012, 11:32 PM   #4
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Propeller hub vortex flow

As I have not really found an answer to what effect a blanked of exhaust would have on through exhaust drive and propeller, so I did a Google search and found it is a very technical subject of which a lot of it was beyond me. However it would appear that propeller hub vortex flow can cause up to 5% drag or loss of efficiency. If anyone is interested in this subject a link can be found below to a paper that was written titled “Simulation of propeller hub vortex flow”

http://www.marinepropulsors.com/proc...tex%20Flow.pdf

Abstract

"A propeller hub vortex has strong low pressure core in its center. The low pressure of the core causes thrust deduction by pulling the propeller boss cap end. To predict the deduction is important when we design a ship. We have applied numerical simulation method to predict it. Besides, we have measured the propeller thrust in case of non hub vortex condition and hub vortex condition. It is considered that the difference of these thrusts was caused by the hub vortex. The results of the computation and the experiment were compared, and a good agreement was confirmed. The thrust deduction caused by the hub vortex was estimated to be about 5% by both computation and experiment".

For anyone who is racing or has a leisure boat that has a drive that was designed to run through prop exhaust but the exhaust exits straight through the transom, it may be possible to gain a small increase in speed or reduction in fuel consumption by adding a vent to atmosphere on the exhaust port of the drive to reduce propeller hub drag.
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Old 07-08-2012, 11:45 AM   #5
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If the propeller hub is fully immersed when running at speed, then I think it would be a good idea to vent the centre of the hub to atmosphere.
If you are running very large props then you might need additional ventilation get the prop revs up at low boat speed.
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Old 07-08-2012, 12:03 PM   #6
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IIRC, people usually just remove the exhaust "sleeve" between the drive and transom plate and this allows it to breath adequately.
Certainly my old formula was rigged like that, pretty sure Leviathan is.
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Old 07-08-2012, 10:00 PM   #7
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Quote:
Originally Posted by Matt View Post
IIRC, people usually just remove the exhaust "sleeve" between the drive and transom plate and this allows it to breath adequately.
Certainly my old formula was rigged like that, pretty sure Leviathan is.
Thanks Matt, that's a realy good idea you just saved me a load of fabrication.

Silly me, I didn't think of that
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