Prop vs. prop?

[smithrock]

Would the results be the same on a SS 13dia. with a 20 pitch versus a SS 14.5 with a 19 pitch? Witch would give higher RPM's and top speed?

 

[Silvertip]

Re: Prop vs. prop?

No. How could we possibly know since we have no idea what prop you have now, what boat you have, what engine it is powered with, what your current top speed is and what engine rpm is at wide open throttle. Read this and you will realize why the question you asked can't be answered with any certainty.

http://forums.iboats.com/showthread.php?t=158083

 

[smithrock]

Re: Prop vs. prop?

No. How could we possibly know since we have no idea what prop you have now, what boat you have, what engine it is powered with, what your current top speed is and what engine rpm is at wide open throttle. Read this and you will realize why the question you asked can't be answered with any certainty.

http://forums.iboats.com/showthread.php?t=158083

I have a 14.5 x 21 Alum on there now. Tried SS of same size with the same results. Boat is an '85 Chaparral V6 4.3L Alpha 1
WOT is 4000 at 40mph (gps) I'd like to get it up to 4400 but not lose any top speed (would like to gain top speed if at all possible)

 

[andy1canada]

Re: Prop vs. prop?

Howdy,

I've got a new 4.3/MPI and max rpm is just under 5000. Sounds to me like you are over-propped. Check your engine's max recommended rpm and shoot for close to that.

From what I've learned, SS (of the same dia/pitch) will loose you a hundred or two rpm. Don't ask... I don't understand it either. You'd think a prop with less friction would cut the water better.

If you want better hole-shot (yankin' skier's & tubers) try a 4 or 5 blade. They can also lift more of your boat out of the water and hold better in turns etc.

My boat weighs about 4000/lbs loaded and I'm swinging an alum/merc 18-pitch 4/blade at about 4900/rpm @ 45mph. And that's with a fairly rough boat bottom. When I finally spring for a new SS and clean the bottom off and paint it, I'm confident I'll hit 50.

Hope this helps,
Terry

 

[Silvertip]

Re: Prop vs. prop?

That engine should spin to 4800 with no sweat and a 21P prop should not be dragging rpm down that far unless it is really beat up or it's been marked incorrectly. You might also want to verify that the tach is correct. A stainless prop of the same pitch as an aluminum of similar design will sometimes lose a little rpm because the blades on an aluminum prop flex more than stainless, hence they will pick up some RPM over the same pitch in SS.

Make sure the throttle is opening fully and the engine is in a proper state of tune.

 

[Tail_Gunner]

Re: Prop vs. prop?

That engine should spin to 4800 with no sweat and a 21P prop should not be dragging rpm down that far unless it is really beat up or it's been marked incorrectly. You might also want to verify that the tach is correct. A stainless prop of the same pitch as an aluminum of similar design will sometimes lose a little rpm because the blades on an aluminum prop flex more than stainless, hence they will pick up some RPM over the same pitch in SS.

Make sure the throttle is opening fully and the engine is in a proper state of tune.

Made simple as he state's, becasue of how flimsy alum is a 21 pitch will deform to a 20 pitch, SS will not deform in the same manner. There ya go, now add some cuppiing which drain's your motor's output and you get even more of a drag.
It take's a real expert to what's best for your boat. And why it's so hit and miss,