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Old 19 May 2010, 19:47   #1
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Prop "lift"

Thought I would start a new thread rather than hijack the cupping thread.

I've never understood how one sort of prop can give more "lift" than another. Whether it is three or four blade, more or less cupped etc etc, surely at the most basic level there is one lot of metal going up on one side of the prop, and another lot of metal going down, so the net effect is going to be zero?

Evidently this is not the case from many things you read, so what actually makes the difference and why?
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Old 20 May 2010, 04:12   #2
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My thoughts on this are that it's not the prop alone that gives the lift, it's the interaction of the prop & boat with the water.

The only 2 things that wil llift the stern of a planing boat are either a hydriofoil of some descrption (aka doel fins, trim tabs etc) and the water pressure under the hull. Add a couple of knots, and there's your lift increased as the water pressure (effectivley dynamic head) goes up. That's how chine walking starts - you travel so fast that you lift a deep V too far out the water and it starts to overbalance) The prop affects that by the thrust it creates.
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Old 20 May 2010, 05:01   #3
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Quote:
Originally Posted by 9D280 View Post
My thoughts on this are that it's not the prop alone that gives the lift, it's the interaction of the prop & boat with the water.
Not so, the force coming off a prop is in the shape of a cone, the nearer the surface the more force in the lower half of the cone thus pitching the boat stern up bow down. Cupping will reduce the size of the cone therefor reducing the pitching effect and producing "bow lift".
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Old 20 May 2010, 06:52   #4
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Taking that to an extreme, a flat blade would lift the stern out the water as the cone would be enormous. (and before all you hyderodynamic experts come along and tear that to shreds I'm using an extreme to illustrate a point).

Once the water leaves the blade it's got no way of pushing the boat anywhere. Think of the pressure distribution your car tyres cause on the ground as it drives along the road. It's the reason that above a certain angle, road embankment sides need engineered retaining walls instead of the natural slope of a "pile of earth" which can only stay put up to a certain angle.

Your cupped prop does produce a slightly more directional thrust, which by the laws of trigonometry means more of the available power goes straight backwards = more thrust = a bit more speed = a bit more lift as discussed above.........

Once the fluid is clear of the rear, it will have negligible effect on the body that is passing through it, as there's nothing to pass any forces to.

As for bow lift:
http://rib.net/forum/attachment.php?...1&d=1263303967
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Old 20 May 2010, 08:01   #5
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i suppose its a bit like the paddle wheel effect ,similar to when a boat is alongside a dock and the slow rotation of the prop will move its stern it to/away from a dock side ,but when at speed will then have the effect of giving a bit of upward lift instead of sideways .
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Old 20 May 2010, 08:18   #6
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I assume you're taking big ships here?

at a dockside you have a wall and a relatively close by bottom of the dock so the water moving round the prop will be asymetrically fed / released form the area, and will likely be bouncing off other ships / dock walls. Also if it's a big slow moving prop you'll be a good few orders of reynolds number different from an outboard whanging along at 30 odd knots. And just how accurate is the ruder angle indicator that says "dead ahead"?

Once again it's down to the hull / prop / speed interactions.........
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Old 20 May 2010, 08:49   #7
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Can we just clarify this is a discusion about the effect of different props on the same hull at the same speed. , not different hulls, trim angles,engines, power inputs, road embankments or sky hooks.
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Old 20 May 2010, 09:13   #8
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Can we just clarify this is a discusion about the effect of different props on the same hull at the same speed.
Change the prop you will get a different engine RPM, so will producing more (or less) power / torque at a given RPM, thus altering your speed, therfore the pressure on the underside of the hull.........

A boat is a system with numerous variables traveling along a non uniform interface between two fluids. The speed a rib travels through said fluids means different reynolds numbers are applicable to varous bits of the hull / engine / superstructure, and will change considerably as speed increases / decreases. This isn't a single variable system - changing your prop will instantly change at least 2 other variables whether you like it or not, both of which will in turn alter third level variables, which then come back to affect the first lot until everything balances out.

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, not different hulls, trim angles,engines, power inputs, road embankments or sky hooks.
The hull, trim angle, engine, power input are all part of this big balancing act that is a planing rib.

I was using the road embankment as an example of how pressure waves travel. I'll leave someone else to explain the sky hook, although I believe most marinas have one you could hire for about 50/hr.
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Old 20 May 2010, 09:26   #9
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With the best will in the world I feel you have spent too long behind a book.
Stephen asked what is on the face of it a simple question, now we all know that there is no such thing if you take into acount all the variables, what I believe he was looking for was a simple answer in general terms.
So the simple question I put to you is
Do you know how cupping effects what people erroniously call bow lift?
If yes, then how. If no then........................
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Old 20 May 2010, 10:19   #10
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With the best will in the world I feel you have spent too long behind a book.
Lets just say some of us have to work with fluids & do system level analyses 5 days a week....... if I had time to read books, you'd have had formulae quoted.

Quote:
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Stephen asked what is on the face of it a simple question,
Quote:
Originally Posted by BogMonster View Post
Evidently this is not the case from many things you read, so what actually makes the difference and why?
but that's the point - it's not a simple answer.......


Quote:
Originally Posted by kitten View Post
So the simple question I put to you is
Do you know how cupping effects what people erroniously call bow lift?
If yes, then how.
See post 2, of for a more wordy attempt, post 4.........
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