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Old 08 January 2010, 12:04   #21
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Originally Posted by 9D280 View Post
....I'm not saying it doesn't exist, I'm saying that it's negligible compared to the other things that can be adjusted,
Hehe, Oh ye of little faith!

If I'm understanding your scepticism correctly it's a difficulty in appreciation of the forces a propeller can create by what seems a fairy minor change in blade shape. Well, these little blades are capable of propelling a heavy boat at high speed just by spinning in the water so why do you doubt there effect relative to boat mass?
I'll give you a link to a wee video. It now is on a minor website because the propeller is out of production by the original manufacturer, for info it was an issue with fire damage to the blade manufacturing plant rather than a poor product issue, anyway the video promotes an auto pitching prop so much of it demonstrates that benefit but the prop also is a bow lifting prop with a blade shape which will also allow the blades to be surfaced and run in that mode. You will notice the blades are not greatly different in shape to what is commonly used on stainless props and they are nothing like the blades of specialist surface piercing props.
Anyway, it demonstrates clearly the ability of a prop to apply sufficient lift without undue angle of trim which is power wasting and can introduce instability at speed.

Link

Scroll down the page and click the link, 'Click here for Torqueshift video'. I think you'll find it interesting even if you don't believe it.
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Old 09 January 2010, 10:18   #22
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Originally Posted by two stroke mick View Post
Are there props which inmprove lift in the stern i.e. help keep the bow down

Regarding the repiotching can a prop be re pitched 0.25, 0.5 0.75 & 1" increments?

TSM
My understanding is (an you'll have to imagine this)the cone shaped pressure wave being shoved backward by the prop creates the equal and opposite reaction that move the boat forward.

The shape of the cone is the critical aspect in relation to your question; as the cone travels backwards from the prop it increases in diameter, when the surface section of the cone breaks the surface pressure is released while the bottom section of the cone is always in the water this has the affect of lifting the stern..

The more cupping on the blade edge extends the time before the cone exits the water reducing stern lift.

If you look at chopper props , they have very little cupping, this increases the angle of the cone, the pressure is therefore decreased at the surface which promotes stern lift....
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Old 09 January 2010, 11:56   #23
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oooh the things one learns on here, all good stuff and interesting.
However as we are only talking a 60hp outboard here and he wants the bow keeping down may I respectfully suggest a high tech, bow dropping, bag o' sand up in the sharp end will do the trick at a marginally lesser cost and will be much easier to adjust.
I'm sailing out of Cumbria at the moment and we still have loadsa sandbags laying about here and free for the taking after the recent "showers" we experienced before the snow falls and sub zeros!
( the ***** eberspacher has just gone on the blink -got the fan heaters plugged in to the 240v now and a few hundred horsepower running to keep the charge up!)
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Old 09 January 2010, 16:04   #24
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JW,

Unfortuantely I'm on a wind- up connection tonight, so the video isn't going to happen this side of next christmas. I read the words, bus still see nothing there that would hint of any sideways thrust or lift / drop from the prop. What they appear to be saying is nothing partuicularly new, and the vari pitch is a cool idea, but if I;m reading it right, the changes they were talking about in the text are related to the forces on the blades rather than any weird overall thing on the whole prop?

IBWET,
Yeah, I can see the cone theory, but once the water leaves the blade it's going to have absolurtely no effect other than how far your "whale tail" apppears behind you. As you say, it's the prop that transfers the "equal & opposite reaction" to the boat. The more of the thrust that goes straight back then the more forward thrust you got. The shape of the cone affects the thrust, and the shape of the blade will affet that (and therefore via forward thrust & the "balancing act", the bow rises / falls as I described before).



I'll sketch a diagram to demonstrate what I've been trying to describe, but you'll need to wait 'till monday before I can upload it. Yes, I do agree the prop blades will affect bow height, BUT I still think it's down to the thrust rather than any weird stuff at the stern. I'll be interested to see the vid too, but I'm still with Wavelength / Jambo on this one.....
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Old 09 January 2010, 16:45   #25
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See the video first, it's all important re bow lift.

Quote:
the changes they were talking about in the text are related to the forces on the blades rather than any weird overall thing on the whole prop?
Of course it's about forces on the prop blades and I don't recall mentioning anything about weird.

I've heard that cone theory but I'm gonna be even more contentious, I think it's pish. The whole objective of the propeller it not to move water.

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Old 09 January 2010, 16:53   #26
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Originally Posted by Wavelength
may I respectfully suggest a high tech, bow dropping, bag o' sand up in the sharp end will do the trick
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Originally Posted by two stroke mick View Post
The reason I asked the question regrading a prop that will create lift was due to the RI article regarding the RS4 and the "Pimp my Rib" article. In this article it was either stated or implied that a prop was obtained from steel developments which provied lift .

TSM
Mr Wavelength, did you miss this?
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Old 09 January 2010, 18:04   #27
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Mr Wavelength, did you miss this?
nope just think he's barking up the wrong tree for a cure to his problem and he is gonna be spending money for nowt by going that way instead of being realistic
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Are there props which improve lift in the stern i.e. help keep the bow down
dont think it will help at all in his power range However as said its all good stuff and I appreciate the various concepts discussed along the way and I am learning from those that know far more than me thanx
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Old 11 January 2010, 09:38   #28
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See the video first, it's all important re bow lift.
Well, the firewall here has taken the hump with it. I'll start downloading as soon as I get back......

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Originally Posted by jwalker View Post
The whole objective of the propeller it not to move water.
Didn't a chap called Einsein write a theory that essentially says it is, depending where you are sat at the time? Bottom line is you are trying to impart as much thrust as possible. Whether you stay put & the water moves or vice versa is irrelevant to the prop!



Will pop back later this eve.......
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Old 11 January 2010, 10:31   #29
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Will pop back later this eve.......
Okidoke, then you can tell me how a prop generates it's 'thrust'.
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Old 11 January 2010, 11:08   #30
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Okidoke, then you can tell me how a prop generates it's 'thrust'.
The rotation of the blade against the immovable object create a force (which excluding the slip) moves the boat forward.

Re the cone theory; it's not mine, however my experience with various designs of prop has led me to believe it's the most likely.....


As the immovable object is still immovable at the bottom of the cone the force is still being exerted in an upward direction,as the cone at the 'top' is now in air the force is reduced.....

to me it's reasonable, but i'm willing to hear about a more likely theory...it's a black art the this prop game dontcha know..
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