What prop pitch to run what rpm

[acuraguy81]

I would like to know what every one is running there 4.3 motors at and what pitch props?

I have a 2050 sl chapparal cuddy 4.3 omc cobra with 21 pitch, max rpm 4000-4100. How can I get more rpm for top end?

[HT32BSX115]

Well, another prop question for the prop forum!


You have to know your drive ratio use a prop slip calculator and you must verify your tach and speedometer before you do anything with the prop.
http://www.go-fast.com/Prop_Slip_Calculator.htm

If your tach is correct, and the engine is otherwise running correctly (and you're not waterlogged or dragging an anchor.....) you are WAY over propped. (pitch is too HIGH)

Put a 19 or 17p prop on it and see how it does..... I would do a 17 first.....

I used a 20p (19 re-pitched and cupped to 20) stainless steel prop on my previous King Cobra and ran 60 mph at 5000 RPM.

I still have it and a nearly new 17p stainless. (although it didn't run as fast with it.)

If you lived up here in the NW you could try them.


Did the engine EVER turn any faster with your current prop?...... if it did, you have something else wrong with your engine or boat.....

[acuraguy81]

same prop i need to try some thing different i will see if i can find a 19. the motor is new runs great. ps its a .30 over 4.3 im running it at 12 degrees adv on the timming seems to run best there is that ok

[HT32BSX115]

I can't speak to your timing.

If you get it too far advanced though, you can have a detonation event and quickly destroy the engine.... (running at 4100 WOT would do it pretty quickly)

[acuraguy81]

im not that far at all

[acuraguy81]

bump

[acuraguy81]

got a 19 pitch gonna try it

[acuraguy81]

it has a 19pitch on it already duh i looked again to double check. What should i do try a 17pitch?

[acuraguy81]

bump i got booted to this forum off the main now no one can see it

[steelespike]

bump i got booted to this forum off the main now no one can see it

The prop forum is pretty active so the slow posts do slide down pretty fast.
By the seat of my pants I think you are allready down a step in pitch. 21 being normally the fastest pitch but not good for much else.19 a little better for loads and light water sports.The 17 imo would be for serious water sports with a load,with a caution to watch the revs with a light load.Even if I'm off by a size the 17 would still not be considered
a pitch for normal operation unless you run with a pretty good load most of the tiime.
I would suspect you have something holding down your performance.
Is the bottom clean and smooth?Motor in excellent tune.Timing set and advacing as it should? Have you done a compression check?

[sethjon]

If you go much more than 21 the boat will take forever to plane.

[acuraguy81]

it is a fresh great running 4.3 .30 over that is tuned to specs and has a strong bottom and runs hard out of the hole even with 6 people and 3 coolers with the 19 pitch but still only got 4100rpm. cant figure this out???

[steelespike]

A strong runner that doesn't reach rpm I would suspect the tach unless it has been varified?

[acuraguy81]

im thinking that too. How can I check my tach. PS my dash is a serious pain in the *** to get apart takes hours and tears things up when taking it apart. That is why Im not to jump on the tach. but it could be