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Prop size and pitch advise

Posted: Sat May 02, 2020 3:30 pm
by deavoll
Hi i am looking for some advice for a new prop for my boat.

The boat is a chris craft 19'' racing runabout replica that i build a year ago.
the specs are:
19"
320hp marine power 350
1:1 velvet drive
LH 1" shaft
current prop: OJ legend 12x15

I have a OJ legend 12x15 at the moment. This is the only prop i have tried so far.
The prop works pretty well and the boat gets up to 78kmh but it gets a little bit unstable at high rpms.
the boat pulls nicely up until 4500rpm and once it goes past that it slightly slows down and feels like it pulls to one side slightly and continues to rev up to 4800rpm but the speed doesn't increase and it feels a bit unstable.
I am looking for some advice on what prop to try next.

OJ recommended trying a 12x16. this seems like it could be a good option.
A local prop shop said they could add some more cup to my current prop. also a good option?
ACME recommended a 13x12.6. this seems like a big jump from where i am.

I would like to try a ACME prop to see the difference and thought that their 12.50" x 15.25" Wake/Ski 3-Blade Propeller could be a good option.
It is a slightly bigger diameter, slightly more pitch and cup than my current prop so might be good?

Any advice would be really appreciated.

Thanks, Troy


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Re: Prop size and pitch advise

Posted: Sat May 02, 2020 3:47 pm
by Bill Edmundson
All I can tell you is that any 12" dia. prop. may start tip cavitation at about 4000 rpm. An inch of pitch may cut you 200 rpm. Those are just rules of thumb, for 12" prop., that I've seen on this forum. I'm sure others will disagree.

Bill

Re: Prop size and pitch advise

Posted: Sat May 02, 2020 3:51 pm
by Roberta
Great Looking boat!! I'm running a 13X13 OJ Legend Force four blade on my Torpedo and it seems to be a good match. I am using a Crusader 5.7 V8 at 330 hp and a 1:1 Velvet drive Liberty A and 1" diameter shaft. I top out about 5000 rpm and about 50mph.

You may be cavitating your prop at higher RPMs. My prop is a full cup. I originally ran the same size prop in a three blade version. It cavitated and ran too many RPMs. The extra blade gave me better hole shots and kept the RPMs slightly below max, where you want to be. The four blade is also very smooth.

Roberta

Re: Prop size and pitch advise

Posted: Sat May 02, 2020 5:02 pm
by kens
I have not bought an ACME, but their support seems to be top notch.
They told me if I buy one of their props, they will trade, swap, repitch, re-cupp or whatever, until I am satisified.
The people I know that have bought ACME did receive good support.
I do not believe that 4000rpm is a majic number for onset of cavitation, because there is just too many successful boats running above 4000rpm.
that 4000rpm number is an old wives tale.

Re: Prop size and pitch advise

Posted: Tue May 05, 2020 10:48 pm
by deavoll
Thanks for the replies,

It makes more tempted to move up to a 13'' prop.
I live in New Zealand so i can't really try and swap props which makes it a bit harder. It would be really nice to try a few options.
The 13''x12.6" ACME suggested might be about right.

Re: Prop size and pitch advise

Posted: Wed May 06, 2020 3:42 am
by neel thompson
Troy... On my Gentry which is about the same size and weight as your boat, I had a 1:1 trans, 310 HP small block, one inch shaft, and finally got the WOT RPM to 5200 running a three blade prop 13/15. I originally had a three blade 13/13 on it and the WOT RPM was too high. One thing to keep in mind is that with a 13/15 prop you will be making some speed at idle and when docking you will have to go in and out of neutral to keep the speed down a little. Top speed was over 50 mph (never got there). You have a beautiful boat there... Neel

Re: Prop size and pitch advise

Posted: Wed May 06, 2020 4:59 am
by Roberta
Like Neel, I was also concerned about higher speed at idle and didn't want to go too high with pitch. The 13x13 three blade OJ was not limiting my RPMs enough, but I was having trouble finding the right prop. I have right hand drive and that limited my choices a bit. The 13x13 four blade OJ worked out well. The Acme has larger blades on their three blade design, making the bite nearly the same as the OJ four blade. I think the Acme 13x12.6 three blade or the OJ 13x13 four blade will work well for you. You'll get good speed and good hole shots and should limit your RPMs properly.

Roberta

Re: Prop size and pitch advise

Posted: Wed May 06, 2020 11:41 am
by Rich Coey
The Acme 13x12.6 three blade is what I have. 19ft Monaco, 300hp, 1:1 velvet drive, wot 5000 rpm, top speed about 55mph.

I have tried several different props and this one worked best for me for overall use.
If we are mostly going to pull tubes or skiers I use an Acme 13x11

Rich

Re: Prop size and pitch advise

Posted: Fri May 08, 2020 10:30 pm
by deavoll
Thank you very much for the replies, it's really helpful seeing the different setups.

cheers, Troy