fede

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

fede

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?

fund razor

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.

bowtie

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.

Griff

I have seen trim tabs set a 1/2 inch differently, but drives are always dead center.

fede

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.

monstaaa

run a left handed prop

jpclear

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

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.

jpclear

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