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Outdrive offset

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Old 05-22-2006, 03:16 PM
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Default Outdrive offset

Hi guys, I try to post here the very same question I posted on boatdesign.net since I got no answers there.

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Hello, here it comes,the good old 1.000.000 $ question,hope someone is able to give an answer based on direct experience.
We are in the process of installing a single 450 hp sterndrive on a 25' Racing Rib.
The prop is a 28 pitch and the engine delivers 450 hp (+ or - ) @ 5800 rpm,reduction 1.5
Rib dimensions are:
25 lenght
6.3 beam (inflatables excludede)
8.4 beam including inflatables.
I have to mount the sterndrive with an offset to contrast the turning effect
created by the prop thrust and high power of the engine.
The prop rotates clockwise so i would offset it's location to port...and now the 1.000 k $ question: how much???
Once the hole is done is done and it's not possible to apply trial and error here....
Thanks to everyone who will give it a try as usual!

If you are curious you can see the RIB here:
http://www.naumacosdesign.it/25%20rib.htm
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Old 05-23-2006, 12:41 PM
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Default Re: Outdrive offset

Ok...let's try it this way...could you guys that have a single engine high perf boat tell me if the leg is dead centre or slightly offsetted to one side?
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Old 05-23-2006, 12:53 PM
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Default Re: Outdrive offset

I had one. It was dead center.
I only noticed the difference at extremely low speeds.
(like 1mph)

I also noticed that it turned differently in reverse as well.

never noticed any pull one way or another at speed.

Good luck.
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Old 05-23-2006, 10:08 PM
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Default Re: Outdrive offset

Always centered,thats how you get your x dimension. I can see how prop torque could effect that thing though. Maybe a Bravo 3 or Volvo Duo Prop.
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Old 05-24-2006, 01:18 AM
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Default Re: Outdrive offset

I have seen trim tabs set a 1/2 inch differently, but drives are always dead center.
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Old 05-24-2006, 03:18 AM
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Default Re: Outdrive offset

Thank you all guys,yes I'm thinking about a different flaps set up port and strbrd,usually it's very easy to notice the prop thrust effect with heavy sea when the boat goes in and out of the water.
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Old 05-25-2006, 08:50 PM
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Default Re: Outdrive offset

run a left handed prop
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Old 05-25-2006, 09:10 PM
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Default Re: Outdrive offset

Now I'm going way back to the early '70's. But when we would set-up a shallow "V" , Hi-perf Switzer, Bob and Dave suggested that we clamp the outboard on so that the the prop shaft C/L was about 1/2 inch starboard of keel line with a RH rotation prop. They would actually scribe the transom off center to help me with the set-up if they knew what I was going to run on it. Hope this is a bit more help than confusion. --- Jer
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Old 05-28-2006, 11:49 AM
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Default Re: Outdrive offset

thnks again for this second round of suggestions, after a lot of searching and asking I decided to put the leg dead centre,quiet conservative as far as the X dimension is concerned (16 1/2 inches) the leg is a bravo xr.
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Old 05-28-2006, 11:11 PM
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Default Re: Outdrive offset

Quote:
Originally Posted by fede
thnks again for this second round of suggestions, after a lot of searching and asking I decided to put the leg dead centre,quiet conservative as far as the X dimension is concerned (16 1/2 inches) the leg is a bravo xr.
Yep, that's just what I would too. It sure was a lot easier to re-clamp an outboard if we didn't like it, than would be to completely rebuild the a$$end of a hi-perf stern-drive. --- Jer
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