Prop size

[BAZILDON]

Hi to All. I have a steel cruiser/ trawler 34ftx11 with a four foot draft. I have just fitted a new engine 2.8 turbo of 120hp A Japanese Diahatsu DL52 out of a fourtrack 4x4.previously had a 6354 perkins which never ran well!
My question is: the prop is 3 blade 24x16 do you think I should change it?

 

[Gdavid]

What ratio transmission?

 

[Comodave]

Welcome aboard. You need to give us more info. What horsepower is the engine? Gear ratio? What RPMs at WOT are you getting now with the current prop?

 

[DavidM]

Welcome aboard. You need to give us more info. What horsepower is the engine? Gear ratio? What RPMs at WOT are you getting now with the current prop?

Plus what is the rated rpm’s for your new 120 hp engine.

In general you should prop to hit about 100 more than rated rpm’s at wot.

A good prop shop can adjust the pitch of your existing prop (within limits) to achieve that.

David

 

[Brooksie]

I think you will need to:
I imagine your 6.354 was 130 hp in it's best days and your new engine is 120 so, on the surface, it would seem to work. However your new engine most likely develops its hp at 25-35% higher RPM.
You could change your transmission to turn the propeller at the same old speed but allowing the engine to come up to its' proper rpm. But that is a rather backwards approach. Armed with the proper information, your prop shop may be able to re-pitch your existing prop to function w/ your new engine.
At any rate, as mentioned above, DON'T overload your engine; if it doesn't come up to rated rpm at full throttle STOP and have your prop re-pitched or replaced.

 

[Xlantic]

boatdiesel.com has a prop calculator but you will need to take a $50 annual subscription.

 

[Hippocampus]

Defer to Dave Gerr for all things prop related. Above posts offer excellent advice but before messing with it would get an outside opinion from a knowledgeable pro who isn’t gaining business from his answer.

 

[Peter B]

Welcome to TF, and I have now moved your thread out of Welcome Mat to an area more relevant to the specific nature of your issue so you'll get more responses.

 

[PierreR]

Off the top of my head the prop seem undersized for the 6354 engine. I would not have guessed it to hit full power before the governor kicked in. I could be wrong and I lent my copy of Gerr's book to my nephew a couple of months ago so I can't easily calculate it.

 

[Art]

Welcome!

You had it on test runs yet? That's something I'd do to get a base line of [during a slack-tide] GPS SOG compared to rpm's turned; as well as rpm and SOG at WOT.