prop size for sailboat outboard

Hi , We have a 24 ft. sailboat , 6000lb disp , with a 2005 yamaha 9.9 4 stroke long shaft, behind the rudder in open area, what pitch/size prop should we have on there ? Better 3 or 4 blades ? liz

 

I think there are two bottoms for that Yamaha motor, in the long shaft version. The low speed lower unit is geared down, suitable for displacement boats like yours. Note that the output speed is lower----- and so the prop is bigger than the standard version.
Which model do you have? The output RPM is different depending on which one. If in doubt, call a dealer and give him the model number, or visit the website--- the lower part looks very different; bigger and beefier..
The prop for the lower speed "thruster" version is the one you want with that model.
If you have the higher speed version, it won't be efficient with any prop, though there may be some choices better than others.

Alan

 

Thanks , its the big thruster lower unit ,thankfully. apparantly , the dealer I bought it from didn't KNOW that there even were two lower units. So with that in mind , can you suggest a prop size for the 24 ft , 6000lb Bristol I mentioned earlier ? Liz

 

The standard prop the motor comes with is probably very close to what you need. I gather you have damaged the original prop? !0 hp is about right for your boat, to achieve near hull speed. Therefore, the prop original to your motor is probably about right. If your boat (which sounds like a Bristol or a Pearson or a Cape Dory) has a website (they all seem to), I would ask there for the particular pitch, size, and blade configuration. Every hull is different, and it looks like you don't want to guess.

Alan

 

Oops--- didn't see it was a Bristol. Okay--- check out any website for Bristol owners.

 

Be happy to hear your geuss. Havn't had much luck on websights, so lets hear your thoughts...thanks liz

 

I have a program for the calculation. I need the engine RPM, transmission ratio, waterline length (guessing about 18-19 ft) and whether you would sacrifice some top end speed for a four-blade high thrust prop (for example, do you sail in river currents against tides? Solas has them in aluminum for the high thrust Yamaha, 7 or 9 pitch, but I can't find your high thrust Yamaha prop shaft RPM anywhere.
Frankly, a call or an email to Solas or other prop manufacturers would answer your question quickly.
Again, why do you need a prop?

Alan

 

The specs. list gear ratio # that I don't get, i am used to the #s with larger inboard diesels. 13:2.08 is what yamaha sight said. Used to seeing things like 3:1 and such. We are in Wilmington where theres lots of river and current to play in. The 4 blade is what we are thinking about for the exact reason you mentioned. The motor was sold set up for little well boats the gill netters use around here as a kicker.hence fast prop. The dealler knew more about Mars than boats.Rpm list top range as 4500-5500 ,I believe we have about 20 on the waterline. We are not getting to hull speed as is. Our bottom is clean.{ha ha}Whats the wtr temp up there at this time of year? Always wanted to get up there - hear its mighty pretty......

 

Your ratio is 2.92:1
The engine RPM max is about 5500.
Your motor appears to take a max diameter of 10" or so. The most efficient prop would be too large to fit as a three blade, so a four blade is recommended. The recommended pitch is 8.5.

10" x 8.5 four blade.

 

Thank you Alan , thats right on with what we thought. We sure do appreiciate your time. Be safe. Liz