Props for 34 convertable

[PaulR]

Currently we are running 21-24's with a slight cup (4 blades). We are getting to many rpm's out of the engins. Boat is a 34 convertable (2000) with 315 yanmars w/ zf transmissions. Props are spinning up to 4000 rpm's where the engins are rated for 3800 rpm's. I herd that 3900 rpms would be acceptable when on the pins. The engins were rebuilt a few years back as part of a suit against yanmar, and I guess the running gear changed at that time.

We started with 21-23's and moved up to the 21-24's with a slight cup. I get the feeling that going up to a 21-25 would not do very much. Can we run a 22" prop and still push 3800rpms?

It seems like everyone has their own opinion, including a few different yanmar tech's we have talked to. Any info would be helpful. We want to bring the engins and running gear down to the magic number 3800, and hopefully maxamize speed and efficency.

Paul

[Rusty]

I recommend you get in touch with a prop shop that works with the Rolls Royce Engineering department concerning props. I recently went in this direction with my 32 ft. Open and had very good results.

I supplied the boat specifications and current running conditions. The numbers came back from the engineering department and the props were altered as recommended bythe prop shop. The boat then perfromed very well. Note, the props were not computer balanced, but worked from hand. I am not against computer work but would only do so at the last resort due to costs.

We have very good prop companies in our area with decades of experience.
Good Luck
Rusty
Lafayette, La.

[PaulR]

Thanks for the info Rusty. That is something to check out.

Does anyone here run a 34 convertable with 315 yanmars? I know we dont have the only one built...

[Dave K S]

I repowered my 1989 342 Tournment in 2000 with 315 HP Yanmars. These are 6LYA-UTE's rated at 3300 RPM's. The 2000 6LPA-STE Yanmar is rated at 300 HP at 3800 RPM's. The new 6LPA-STE 's are now rate at 315 HP. This is the same engine they just changed the HP rating. I would not let my engines run less than 3400 RPM's at WOT with a full load of fuel, gear and people on the warmest days of the summer. In your case I would not let your engines run below 3900 RPM's with a full load of fuel, gear and people. The risk of overloading these engines could mean premature engine failure. You will not really save fuel. Diesel engines burn fuel based on engine loading. If you increase pitch to lower your RPM's and run at the same RPM you will burn more fuel and go a little faster. If you lower your RPM's to 3900 you will probably gain less than one knot in speed at cruise. It will not hurt to run these engines with a WOT of 4000 RPM's. The engines have governers that will not allow you to over rev them.

[PaulR]

Interesting, thanks for the info