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Old 05 May 2009, 18:23   #11
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Thanks for all the replies...

It only pitch's on take off... when its flat or chop its fine, but once I start getting some air it happens... its sometimes quite violant. It launches left and lands left. I can correct it by flick to the right on the crest, but thats a bit tiresome. I've tried all trim levels.

I'm not sure what prop is on it (Mercury 3 blade stainless came on it, not factory).. but I will check.

Not sure of the offset either, I'll check that too. Is there any specific spot to measure from, or just off a chime ?

Who's Manning ?

By the sounds of it I'll have to offset it more... I wouldn't want to mess with the transom myself, how much do you think it might cost to get it done ? Does it effect the transom's over all performance much ?

Thanks again
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Old 05 May 2009, 18:38   #12
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Also... is there an off set calculation or is it rule of thumb ?
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Old 05 May 2009, 18:39   #13
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I would be very doubtful of the offset being incorrect. While I don't like to admit it, old man Manning knows what he's up to and I don't expect he'd have fitted the motor (if it was he who fitted it) in the wrong place.

I have exactly the same boat & motor, which I set-up with 30mm offset and the cav plate 40mm above the planing pad. The boat runs level with none of the characteristics you're experiencing. It does however have the distinct benefit of an oversize helm, which would counteract by far, any of these finer balance issues.

Have you checked for wear in the steering shaft bushes and/or the tilt tube?
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Old 05 May 2009, 18:51   #14
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Quote:
Originally Posted by 1978SeaRider View Post
Also... is there an off set calculation or is it rule of thumb ?
Some say 1" of offset for every 100hp on a boat of this size, but it will vary from boat to boat.
You can establish the centreline of the transom by measuring between opposite pairs of sprayrails.

You may be interested in this short thread:
http://rib.net/forum/showthread.php?t=9339
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Old 05 May 2009, 18:51   #15
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Quote:
Originally Posted by 1978SeaRider View Post
Also... is there an off set calculation or is it rule of thumb ?
Before you go down this route there was a valid comment a few posts back about the height of your engine specifically the anti vent plate (the plate over the prop) above the bottom of the transom. If it is below the bottom of the transom or level with it you may want to try lifting it until it is two inches above.

As for moving the engine over I would suggest you would need a couple of inches offset. If you want the job done right then call Osprey and ask ROy to put you in touch with Mike at Wotton Glass Fibre who builds all the Osprey hulls. He would have probably built the hull in the first place and would do a top job of filling the old holes. He would also be able to advise on offset.

http://www.ospreyribs.co.uk/contact_us.html
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Old 05 May 2009, 19:27   #16
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Excellent advice and info, thank you very much.

I will start by measuring the offset, engine height & prop size. I'll also check the steering shaft bushes and/or the tilt tube this weekend and go from there.

And as for the benfit of an over sized helm... I'm working on that !
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Old 05 May 2009, 20:10   #17
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Originally Posted by Mollers View Post
If it runs true in flat water it's not a trim issue. If it's bunging you to the left when the hull leaves the water it's a torque reaction off of the prop. I've experienced this big time with Phantom powerboats, but not with either of my ribs.
My Humber does that, I figured it was probably torque reaction but it scared the sh*& out of me the first time it happened, only happens when you jump a wave at over about 20 knots and get the hull more or less right out of the water - the rest of the time it is fine and a quick flick on the wheel keeps it flying straight but it does get a bit tedious. I figured that probably everything did it, not having much else to compare it to round here!
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Old 06 May 2009, 02:57   #18
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I'm fairly sure that offset and cav plate height won't be the issue as the boat runs true in flat water. It very much sounds like prop torque, as for a cure?

We didn't manage to fix the problem on the Phantom, but we didn't really try. I'd start with the prop.
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Old 08 May 2009, 08:47   #19
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Another factor is the amount of negative trim - the more you trim in, the more it will land on the left.
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Old 08 May 2009, 16:52   #20
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Its a lot of power for that length of hull .. throw some ballast in the bow, like 3 x 25 litre drums of water (or spare fuel ) .. or the odd teenager if you can find em .. that should cure most of it .. not ideal but it works then trim like the bejesus, and youll land good
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