help me choose the right propeller

[500dollar744ti]

i have a 1990 cobia san marino 225ecs, it's got a 270hp, 350 magnum alpha one. currently it has a 14 x 19p stainless prop and turns 4350rpm WOT. that equals 48.6mph with trimming, 3 people and half tank of fuel. supposedly the dry weight is 3800lbs, does that sound right?

i really think i need a different prop because bottom line, the hole shot sucks. once it's on a plane it's nice but getting on a plane is like wait for it, wait for it, then it flops down.

engine is rated for 4600rpm max WOT, based on my searching this site and other sites i think a 14 1/4 x 17p is a good place to start?

agree/disagree? any recommendations based on your experience would be greatly appreciated. i would like a better hole shot without much sacrifice in top end. economy is nice but not terribly important to me.

 

[500dollar744ti]

Re: help me choose the right propeller

thank you mods for putting this in the proper location, i didn't see the propeller sub-forum when i first posted.

 

[500dollar744ti]

Re: help me choose the right propeller

no recommendations? i thought i might also be able to stay with a 19p and pick something that might give a better hole shot? i can tell you everything you need to know about picking the right turbo for your race car but i don't know the first thing about picking a propeller.

 

[wellcraft-classic210]

Re: help me choose the right propeller

Without have the same boat as you its hard to say with exact certainty.

I tried a variety of props on my boat when I purchased it and read what I could. It seems each one had strengths & weaknesses.

If you go with a 17p it will most likely help with you hole shot & make your max rpms raise 3-400 rpm . If the props are made by different manufacturers the blade design will add a variable as well. As you may know it is important to find a prop that allows you to stay within the recomended max ranges for motor longevity.

I am frugal & found I could buy them off craigs list & resell them if I did not like them for a similar price. I ended up using a Vortex 4 blade which adds low end power. It helps me stay on plane at lower rpms & save gas but I lost 4 mph on the top end. I also run aluminum as it protects the drive should I find an unexpected rock. I am not concerned with the top end as I prefer not to run my boat too hard.

All that said; its trial and error to find whats best four you-- good luck -- hope that helps some.

 

[jestor68]

Re: help me choose the right propeller

Boats over 3000 lbs capable of under 50 mph should use the larger diameter(15"+) props for better load carry performance.

A 15-1/2 X 18" will run almost the same WOT rpm as a 14-1/2 X 19", but produces better static thrust than the smaller diameter prop.

A 15-1/2 X 17" will produce much better hole shot and raise your WOT rpm by at least 200 rpm. You should not lose much(if any) top speed.

 

[500dollar744ti]

Re: help me choose the right propeller

i decided to go up 1" in pitch with a slightly larger diameter, got a Solas 14.75 x 20. i'm going to check my base timing, do a full tune up and i just rebuilt the carb. i can always keep it as a spare and go from there. the gist i get from searching is that prop selection is mostly educated trial and error.

i will report back with my findings.

 

[jestor68]

Re: help me choose the right propeller

To improve hole shot, you went UP in diameter and pitch from a prop with(in your words) poor hole shot that is operating very near the minimum rpm.

That is an interesting solution. Good luck.

 

[500dollar744ti]

Re: help me choose the right propeller

reading reviews and comparisons led me to this prop. i'm at the experimenting stage, prepared to buy one or two more to find out what i like best.

 

[wellcraft-classic210]

Re: help me choose the right propeller

You may want / need to lower the pitch ( not raise it) to get the perfoarmance you are looking for / epecially if you increase the Diameter.

 

[jestor68]

Re: help me choose the right propeller

The rules are simple. Reducing diameter/pitch raises rpm. Increasing diameter/pitch lowers rpm.

 

[500dollar744ti]

Re: help me choose the right propeller

i'll still be within my WOT RPM range (theoretically) with this new prop. i found my base timing to be off, i'm hoping for a few RPM out of that and new set of correctly gapped plugs. i found that my current prop is out of shape (just got the boat) and it's really noticeable at higher RPMs, it's not even qualified to keep as a spare. because i'll need a prop and i want to have a spare prop, if this new one doesn't work as i'm hoping, it will become my spare and i'll get something else.

 

[Scott Danforth]

Re: help me choose the right propeller

take your old prop to a prop shop and have rebuilt. many marinas have test props

 

[500dollar744ti]

Re: help me choose the right propeller

^^^ that's good info, thanks. what do they do to fix it? is there like some press they put it in to re-shape it or what?

 

[Scott Danforth]

Re: help me choose the right propeller

nothing as sophisticated as a press. think more black-smithing and metal bashing. they have blade pitch blocks and a hammer. they also have a welder for the missing chunks.

here is a good video of the process for prop repair http://www.youtube.com/watch?v=eq9PwAgDroQ

 

[500dollar744ti]

Re: help me choose the right propeller

Boats over 3000 lbs capable of under 50 mph should use the larger diameter(15"+) props for better load carry performance.

A 15-1/2 X 17" will produce much better hole shot and raise your WOT rpm by at least 200 rpm. You should not lose much(if any) top speed.

the 20p i got provides the same (poor) holeshot as expected but top end increased 4-5mph, mind you that's heavily loaded. it will be a good prop for just me on the boat trying to get somewhere fast.

next i'm going to do the 15 x 17" (couldn't find a 15 1/2 x 17" to try) as suggested by you, i will expect a much better hole shot and slightly decreased top end so i'll report back.

the type of person i am, i'll have a shelf full of props and i'll make the excuse that i need all of them because they each serve me a different purpose.

 

[wellcraft-classic210]

Re: help me choose the right propeller

Some info you might find helpful;

I just tried a 15inch 17p & like it on a 3100 pound 5.0 litre and found it;

--Planes out at a few hundred rpm less than the 14.5 17p / a few more hundred rpm than a 14.5 18p 4 blade
--Gives good low end but not quite as good as a 14.5 18p 4 blade
--It does have better performance on the top end over my 14.5 18p 4 blade
--It dropped my max rpm to about 4400 from 4800 with a 14.5 17p.

Jestor 68 was right on in his post above / This ones a keeper for me ( along with all the others !!!)