improve lift and repitching of prop

[jwalker]

Originally Posted by 9D280

....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.

[IBWET]

Originally Posted by two stroke mick

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....

[wavelength]

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!)

[9D280]

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.....

[jwalker]

See the video first, it's all important re bow lift.

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.

[jwalker]

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

Originally Posted by two stroke mick

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?

[wavelength]

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

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

[9D280]

Originally Posted by jwalker

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......

Originally Posted by jwalker

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.......

[jwalker]

Originally Posted by 9D280

Will pop back later this eve.......

Okidoke, then you can tell me how a prop generates it's 'thrust'.

[IBWET]

Originally Posted by jwalker

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..

[jambo]

[it's a black art the this prop game dontcha know.. [/QUOTE]

Is that why your rib is called raven!!??

Billy

[IBWET]

Originally Posted by jambo

[it's a black art the this prop game dontcha know..

Is that why your rib is called raven!!??

Billy [/QUOTE]

Nope, but it fits..

[9D280]

Originally Posted by jwalker

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.

This is a joke, right?

What I got after a long boring wait was a sales video for an auto pitching prop set to something that sounds a bit like the soundtrack to a 70s luuuuurve film. As far as the current prop discussion goes it's irrelevant. other than to clearly demonstrate low pitch = good hole shot, but crap top speed & vice versa.

Yep, the prop does what it says on the tin, but bow lift? No f***ing wonder - 150Hp on a boat that can't weigh much more than 400Kg with a prop at half the pitch it would normally have for fast running? That's not bow lift, that's vertical take- off, perfectly explainable by a simple beam balance round the CoG of the boat! I could get my Humber to do that if I put the anchor at the stern & stuck a 7" prop on it!

I think we're just going to have to agree to disagree on this one......

On the plus side, the vid does show quite well where your hull sits when on the plane, and how a subtle adjustment of weight up forward could make a huge difference to the angle of the hull relative to the water. (jumping back to Malthouse's post.....)

[jwalker]

Originally Posted by 9D280

I think we're just going to have to agree to disagree on this one......

It would seem so. Never mind, with a bit of luck you might one day have the opportunity to experience a bow lifting prop and then you'll understand.

[9D280]

This is how your prop blade tweaking lifts the bow:

At a very simple level, you alter the pitch / shape, you alter the thrust. The see saw becomes unbalanced in favour of the engine thrust, so the bow rises.

Bottom line is for TSM, it's going to be easier & cheaper to move heavy stuff around. Once the balance is sorted, can tweak the prop for optimal performance.

[Paul E]

I've been reading up a lot about propellers recently... I'm hoping to run a surface piercer on my XR20 if I get the motor I'm after, and prop choice is vital!

Everything I've read suggests that Chopper type props give good bow lift, and Cleaver types good stern lift. That's why most high speed single engine monohulls around 20ft run choppers, and cats run cleavers.

Of course that's a very sweeping statement and there's a massive amount of variables, which all contribute to make props genuine witchcraft

[Downhilldai]

Originally Posted by Paul E

...if I get the motor I'm after,

What would that be?

[PeterR]

Originally Posted by wavelength

if the original poster can't get the bow down enough I would suggest he should be looking at transom angle rather than faff about with a prop. We transferred crew from one of our Humbers on the south coast to a tornado occupied by ex clients of ours a couple of years ago. Running into a head sea we were trimmed down with a decent amount of power on and doing fine whereas the novices onboard the other boat were struggling, so we took them onboard and transferred two of our experienced commercial skippers onto their boat. When we got back to the slipway our two were ****** off with the inability of the outboard to get in to a sufficient angle to hold the bow down. If this is the problem on the boat on question then a bit of shimming by inserting a shaped wooden wedge under the mounting bracket could solve the problem. Also a bit of weight up front in the shape of a second fuel tank would help. Trimming up with a following sea is easy enough as you have loadsa power on-trimming down in a head sea is a tad more difficult as you havent usually got the same amount of power on.

They used to wedge the transom bracket many years ago with the seariders

[Paul E]

Originally Posted by Downhilldai

What would that be?

225 promax Merc with an offshore leg, sportmaster gearbox and low water pickups. It would beyond perfect for what I want, I just need to get back in contact with the guy and pursuade him to let it go for the right price!

[Downhilldai]

Fair play, that would be mental on an XR20