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Old 29 August 2011, 15:30   #11
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15 - yes but as I say I bought the high 5 17" now and await collection. If Over propped is slip of this extent expected do you know?
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Old 29 August 2011, 15:30   #12
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Originally Posted by captnjack View Post
Are you expecting zero slip or something???

I am running a 13" prop which gives me 5900rpm WOT lightly loaded, about 30knots. Theoretically that's 6400ft/min (5900rpm x 13" /12) which would be ~63knots. Since I am actually doing about 30knots there's 47% slip.

I would drop to the 17" pitch or possibly less with that many divers.

For that length, a substantial deadrise(?) + that big of a load, I think you are quite underpowered with only 150hp.
is that not 5900rpmX(13/26)x13/12=3196ft/min
the gearbox being 13teeth on the input gear 26 on the output gear????
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Old 29 August 2011, 15:31   #13
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No doggy, Jack doesn't have any teeth
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Old 29 August 2011, 15:35   #14
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sure 1.2 ratio I think. doesn't magically make the slip disappear though its still pretty substantial.

still think you are underpowered
Yes Jack, it brings YOUR slip down to 9.5% which as mentioned above is pretty near expected.
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Old 29 August 2011, 15:35   #15
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No doggy, Jack doesn't have any teeth
Surely that would be 100% gearbox slip

just found this on prop manufacturers site.

prop slip will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent.
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Old 29 August 2011, 15:36   #16
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Yeah doggy I know but not as funny though eh?
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Old 30 August 2011, 05:41   #17
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In your earlier thread about planeing technique you posted this in reply to one of my posts

"The little thing in front of the prop is the skeg! 25 inch transom and 20 inch leg doesn't quite work does it? You mentioned before that I am experiencing 46% slip or thereabouts. The boat is a standard DR750, the prop is a standard 3 bladed lazer in good nick - surely that only leaves leg length no?"

I see what you mean about the L v XL issue, but that doesn't seem to have solved the problem and it is a big boat so maybe it just isn't going to do what you want. My question is what do you mean by the skeg? Do you mean the gearbox housing of the outboard itself or a fin of some kind mounted to the hull? If this is on the hull and directly infront of the outboard leg it would create all the symptoms you describe, no bite, difficult to get on the plane and low top speed due to disrupting the flow of water over the prop. Even a small obstruction can cause big problems.

Any chance of a picture to show what you mean if it is not the leg?

Apologies if this sounds pedantic but I put your figures into a slip calculator the get the 46% and those slip levels are awful. It must be possible to improve. I don't think the High 5 17" will be the answer if top speed is important, good hole shot but not much top end.

I'm no expert, but Ken at the Propgods forum is probably the best in the business. See if he can give you any advice.
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Old 30 August 2011, 18:08   #18
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Hi Oscar and thanx for coming back in on this. By skeg I mean the base of the keel band. The very lowest strengthening strip along the hull. Having now extended the leg by 5 inches the cav plate is now level with this as it should be (I believe). So there is definitely no obstruction in front of the prop now. Still I get this horrible figure on the slip calc!

Regarding doing what I want - I have little expectation other than to bring the efficiency of the rig to within possibilities and that's somewhere close to the ten percent slip you quoted really. I suppose it is the wrong prop for the job and little more after all this?

If I had a 15 inch prop and ten percent slip I could expect performance in the mid thirties I suppose. I'm just amazed myself that with lowering the prop I haven't actually achieved anything really. I know there's more drag now but surely it should be better than it is now no?

I shall try to borrow a 15 inch prop and see how I get on........
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Old 31 August 2011, 04:18   #19
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Best of luck and I hope it works out for you.

The reason I have some basic knowledge is that I get about 18% slip which to my mind is still too high so I have looked at ways to reduce it. I have raised my motor one hole (which helped me get another 200 rpm) and would like to go another but that would mean a jack plate or redrilling the transom which I dont want to do.

Have a look at this link regarding prop height.
http://www.veradoclub.com/smf/index.php?topic=137.0

My thoughts are that if your Anti Cav plate is level with the bottom of your hull it is too low but it does vary from hull to hull. The further back (set back) your prop is the higher it can go.

Any chance of a picture of this strengthening strip, I wonder if that is causing turbulence just in front of the prop.

And finally do have a look at the Prop Gods forum. It is USA based but the advice on prop selection type, diameter and pitch is the best around

Good luck again
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