Need some help with prop / V8 OMC gear ratio

[Spajn]

Hi,

We have a 1970´s North American Offshore S-22, a classic daycruiser with V-bottom that looks kind of like a Bertram and is quite heavy (but not by far as heavy as a Bertram).

The engine (Johnson 300 V8) is mounted at standard height with the anti cavitation plate slightly (1/4 inch) above the boat bottom.

On the motor I have a 14 1/2" 20" Raker prop that followed the motor when I bought it.

I'm getting roughly 35-37 knots at just under 6000rpm, if I'm alone or with 3 passengers and both fuel tanks full doesn't affect the top end speed. If I trim the engine out slightly the engine revs more but the speed doesn't increase. Overall this prop is very sensitive to cavitation and I have to be really careful with the trim.

I think I've read somewhere that the gear ratio of the Johnson/Evinrude 300 is 1,78 , is this correct? If so I'm getting 30% + slip with this prop. (!!!!)

I've understood that I'm going to have to use this prop as a spare and get another one.

The question is which pich should I begin with when trying out new props? 22"? 24"?

What kind of top speed is reasonable to expect with this type of boat and motor?

Any specific prop type you guys could recommend? Perhaps the raker isn't the best type for this kind of boat?

I know almost nothing about props so all advice is very apreciated. What I'm primarily looking for is good economy cruising.

With best regards/
Alex in Sweden

[Spajn]

http://www.boatlife.se/forum/uploads/post-989-1176122616.jpg


This is the boat.

[Spajn]

http://www.boatlife.se/forum/uploads/post-989-1178463250.jpg

[Spajn]

Forgot to mention that the engine is mounted immediatly on the transom. I have no setback and no jackplate.

[kimswang]

Just try different props and see wich one brings you to Skagen quickest for a cold one on the pier. 22" would be a good start, and you might want to lower the engine an inch just to try it out. Best of luck there...

Normannen i Las Vegas....

[STV_Keith]

Make a call to a good prop shop guy. I use Kevin at R & R Prop Shop (http://www.randrpropshop.com). They will likely have done something like this before and can recommend a prop and/or engine height change that may help you out. +1.702.564.7502

[Spajn]

A prop guy that I spoke to is sending me some different 24 and 26 inch props for me to try out during the weekend, we'll see how it works out.

Does anyone know the lower unit gear ratio? It would be of some value to me so that I can calculate slip and theoretical speed with different props.

[kimswang]

Gear Ratio = 17:30 (.567)

[Spajn]

That would also imply 1,7647058 Gear ratio, Sh-t, then I really am getting 30+% slip with the propeller I'm using right now.

Thanks a million for the info Kimswang. It's going to be really interesting to see the results with the different props. I just installed a brand new Tach in the boat and am bringing the GPS along to see what i kind of numbers I get.

What kind of speed to you guys think I'll get with this boat/motor?

[kimswang]

When I had two of them on the back I got around 50MPH with Raker 20". I now have three of them on the back and is testing some different props, plus a few extra parts.. Should be 70 withing a short time, and 80 next year.

[Spajn]

When I had two of them on the back I got around 50MPH with Raker 20". I now have three of them on the back and is testing some different props, plus a few extra parts.. Should be 70 withing a short time, and 80 next year.


That is one hot setup you got there! What kind of boat is that?

[kimswang]

It's a 34 Scarab SS. Fishing boat with a center console..