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Old 19 May 2010, 18:47   #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, 03:12   #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, 04:01   #3
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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.
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, 05:52   #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, 07:01   #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, 07:18   #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, 07:49   #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, 08:13   #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, 08:26   #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, 09:19   #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.

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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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Old 20 May 2010, 09:34   #11
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Lets just say some of us have to work with fluids & do system level analyses 5 days a week
Doing what?
I is just a humble boat builder, who has only been at it for 25years
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Old 20 May 2010, 10:42   #12
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I is just a humble boat builder, who has only been at it for 25years
So why the h*ll are we having this discussion??? If you build them, you'll know how it all interacts on so many levels!

Subtle example: I found a repair 6" in front of my transom that didn't show up under the antifouling, It took me 6 months from when I got the boat to figure out why it developed an unnerving rock steady list to Starboard above 20 knots, and only then after fiddling with trim tabs, weight distribution etc. I only found it when I started to remove the antifoul . The diference in "height" between P & S sides was no more than 1.5mm over an area of about the palm of my hand. That subtle lump created enough lift that even having 2 blokes sat on the port toob it wouldn't level off.

Backing off by less than 1 knot was enough to reduce it to negligible list and would sit perfectly upright. A cupped prop will likely cause that kind of speed change for a fixed RPM everything being identical.

I look at impellers & their interactions with the rest of the system they're sat in.




The one word answer to Stephen's original question is "physics"
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Old 20 May 2010, 10:47   #13
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Once the water leaves the blade it's got no way of pushing the boat anywhere.


Not according to Mr Newton!
Newton’s laws of motion, in particular the third law indicate that every action has an equal and opposite reaction, which in simple terms mean the amount of water thrown out the back of a boat is equal to the amount of forward thrust produced.

Stand on a skateboard and throw a medicine ball, you will move in the opposite direction to the ball even though you are no longer attached to it
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Old 20 May 2010, 11:14   #14
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Yep, which is how the prop pushes ou forward, the direct reaction betwen prop & water is realyl only in the few mm on the pressure side of the blade. after that it's water against more water, which unless it has sufficient pressure to transmit a "continuous pulse" back to the hull has no effect on lift.

It's the dynamic pressure under the hull that does that.


As for the ball, you have imparted a proportion of the energy of the throw (related to your relative masses) into your half of the system (conservation of momentum), at which point Newton's 3rd is replaced by his 2nd which says somethig like "and will continue in a straight line at a constant speed unless acted on by other forces". Those forces being the friction of the weheels & to a lesser extent the drag through the air........



If you'll excuse me gentlemen, I have a long weekend with a boat which desperately needs my attention.
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Old 20 May 2010, 14:17   #15
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Whatever way you look at it different design prop,s of the same pitch and diameter will effect the way your boat handles, this is why people like kitten/cookee will spend money and time finding a prop that matches the boat and not just the pitch. It may not fit your theory,s but in the real world of boating it makes a difference not huge at low speed but when a boat is on the limit the hull may need bow or stern lift to make the difference of being able to go for more speed rather than lifting or getting very wet.
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Old 20 May 2010, 14:24   #16
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Old 20 May 2010, 16:31   #17
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Fookinell what have I started

Yes the original question was why, if you take a boggo 3 blade prop off and fit one of the four blade ones sold as "giving it some extra lift" to same hull same engine same load of tubby people in rubber, does it make any difference?

The answer is obviously not that simple
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Old 20 May 2010, 17:04   #18
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Fookinell what have I started
Anyone know how to drill a hole?
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Old 20 May 2010, 18:43   #19
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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?
I know Mr 9D280 doesn't agree with this but my understanding is that the top of the prop is so near the surface that it does little work so there is an out of balance force on the prop because the top and bottom are not providing the same driving force. The force on the blade can be considered to act at a right angle to the blade. Therefore, the geometry of the blade and particularly its rake will have an effect on the angle and centre of pressure. Because, in general, our props are raked rearward, I don't think it is possible to have stern lift, just less pull downward at the stern. Likewise, I don't think there is bow lift only more pull down at the stern. Of course, these forces reacting against the hull will cause the bow to lift or fall.
I also don't think a propeller works primarily by pushing water rearward although, to get the boat moving it certainly will but the effect will quickly reduce as the boat speeds up.
When swithering about bow and stern lift, if you consider the prop to have three working sides, the vertical ones being in balance then the bottom to be the controlling characteristic, then you won't be far wrong.
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Old 21 May 2010, 10:46   #20
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Props are witchcraft, that's the only simple explanation! A mate of mine was running a cleaver on a cat, then borrowed a spinelli cleaver that was exactly the same diameter/pitch, and gained almost 20mph.

I'm currently looking for a prop with more bowlift to replace the 21" laser II on a 19' boston whaler with 200 opti, as it needs a lot of trim to get near 50knots, and it should be capable of over that I reckon
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