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Old 11 January 2010, 11:43   #31
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[it's a black art the this prop game dontcha know.. [/QUOTE]

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

Billy
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Old 11 January 2010, 12:27   #32
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Quote:
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[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..
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Old 11 January 2010, 17:58   #33
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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.....)
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Old 11 January 2010, 18:33   #34
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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.
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Old 12 January 2010, 08:46   #35
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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.
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Old 12 January 2010, 08:49   #36
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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
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Old 12 January 2010, 08:58   #37
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...if I get the motor I'm after,
What would that be?
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Old 12 January 2010, 09:01   #38
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Quote:
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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
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Old 12 January 2010, 09:07   #39
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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!
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Old 12 January 2010, 10:20   #40
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Fair play, that would be mental on an XR20
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