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Old 18-01-2010, 09:08 PM   #1
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Question Something I've never really understood....

... about props.

At the Risk of confusing everyone and slightly embarrassing myself, here goes...

Is there a difference in performance (albeit probably totally minimal) between props that have what i can only really describe as a 'sleeve' around the hub, where the prop-shaft goes in, for example the laser II, tempest, trophy, etc. and props such as the chopper, cleaver, etc. which don't have this 'sleeve'?

My reasoning for the question is that the props without the 'sleeve', which are the same size, for the sake of and example 23in(pitch is it?) technically have a slightly larger surface area with which to push the water, although saying this, is this section of the blade more parallel to the direction of travel, and therefore only really pushing the water sideways?

Thinking about it again, the 'sleeve' on the props such as a tempest pretty much lines up with the 'torpedo' on the gearbox, so that water may flow better to the blades, compared to having a slight step from this 'torpedo', to where the foot of the blade meets the middle of the prop, and would this mean the water is more disturbed, and therefore the prop may work less efficiently?

It's nothing massively important, but i thought it would be intriguing to hear some thoughts and views from some people who may actually know what they're on about!
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Old 18-01-2010, 09:11 PM   #2
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for anyone who's confused this is the difference between props i'm on about.....
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Old 19-01-2010, 12:18 PM   #3
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from what I understand its all about the exhaust and aeration around the prop.

Over hub aerates the water around the prop.

Through hub it dont (as the exhaust exits through the hub - hence "cleaner water").

The fact that many racing props (choppers and cleavers) are overhub would suggest that this is a benefit to high speed apps. Causes slip though at take off and hence can affect hole shot.

From what I gather, aeration can / will give a higher top speed but cannot explain how or why?
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Old 19-01-2010, 12:23 PM   #4
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interestingly, merc have introduced a custom venting system (basically a hub plug) so that you can release exhaust gas over the hub on a through hub prop!

"Performance Vent System (PVS)
PVS, a patented Mercury ventilation system, allows the boater to custom tune the venting of the propeller blades for maximum planing performance. On acceleration, exhaust is drawn out of the vent hole located behind each blade. When the next propeller blade strikes this aerated water, less force is required to push through this water allowing the engine RPM to rise more rapidly. Water flows over the vent holes once the boat is on plane sending exhaust through the exhaust passage. Varying the size of the exhaust holes engine RPM can be controlled, outboards perform better with venting and stern drives typically require less venting if any at all."

from merc prop site
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Old 19-01-2010, 02:23 PM   #5
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Ahhh i see thanks... i think i know what the PVS thing is aswell, think i've seen the holes on our prop

I don't really know a great deal about engines, but on an engine with an offshore leg, like an XR2 or a 2.5 is the exhaust not over the cavitation plate, above the prop, or is there some exhaust venting also through the gearbox?
Because correct me if I'm wrong, but as far as i can personally remember, i have very rarely, if ever seen anyone with an XR2 of 2.5 using a prop with the through flow hub.

Obviously i understand that these are race engines and it would therefore be appropriate to use a race prop (and as a racer i have only really seen these using a race setup), but if what your saying about the aeration of the water helping top end (and i'm not knocking your theory at all i'm sure you know much more than i do!) then unless of course there is some exhaust venting through the gearbox, there is only the fact that water will be disturbed as it channels off of the gearbox torpedo that may create a similar effect and to aerate the water at the prop.

unless of course i'm thinking too much about the theory and in reality the difference is completely minimal, which is quite likely!
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Old 19-01-2010, 02:25 PM   #6
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that's all about venting to get the motor 'into the power' for getting on plane. a high speed boat, with very tall props might bog the motor a bit, letting it deliberately slip, will get the motor singing.

The same applies for a lot of turbo diesel installations, to get you past the turbo lag issue. without venting, some boats simply cannot start the boost cycle. Numb Nuts's SR9 is a perfect example of this.

Buzzi did it by diverting exhaust gas down a pipe to force ventillation to get the revs/boost up, Sabre used a torque converter to get over the same issue, a very different way.
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Old 19-01-2010, 03:01 PM   #7
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Black Magic!!

If you want to learn anything about props, i'd borrow as many as you can and try them all.
You'll find they can dramatically affect the way your boat performs, even slight differences in prop can have a big effect.
Beg,borrow and steal as many as you can to find the right one that works for you.
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Old 19-01-2010, 03:54 PM   #8
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Thanks Everyone
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