New propellers: need advices

[Philippe]

Hello, I am new in the forum. I had restaured a Bertram 30 that was damaged in the Haiti tsunami. She is now in Panama, and I am now eliminating the GM 7.4l 350hp 454 (they had installed truck engines....) to move to YANMAR 315hp 6LTPA STP2. Current propellers are 18 x 16. Reviewing some past posts it seems that I should change them for different prop... 18 x 19 was referenced in one of them.
Sorry to ask my question in (perhaps) a silly way, but is this size correct? Some advice? Thanks a lot!

[Tony Meola]

philippe

Welcome

Hopefully one of the members is familiar with the 30 Bertram Hull. As you are aware the V is not as deep on the 30 as it is on the 31. Usually you will find the props being run are pretty close to square.

For instance with my 31 with 270 Cummins I am running 20 x 23. Some are running 21 inch props.

Hopefully one of the guys with Yanmars will jump in. You should also take a look at Boat Diesel.com. If you become a member, they have a prop calculator you could use.

How much room do you have know between the prop tip and the Hull?

[Tommy]

You don't mention gears, but on my old 31 with Yanmar 315 HP coupled to 2:1 gears I ran 20X23 4-blade props and got great performance.

[ed c.]

I have the same layout as Tommy, great performance after 2200 hours.I

[Philippe]

Thanks. I will make the measures and follow your advices! Great community here!

[Philippe]

Just had new engines arrived. Gear is at 2.43.... my shaft is 1 3/8... engine at 315hp... is it compatible with 4 blade 20 x 23?

[mike ohlstein]

The 2.43 gear is going to cost you some speed compared to a 2.0. But what I would really worry about is all that mechanical advantage snapping a shaft. Do you know what alloy the shafts are?

[TailhookTom]

I agree with Mike -- you are going to need something like Aquamet 22 for shaft alloy or go up in shaft size to 1 1/2"

Tom