not reaching minimum WOT with new Prop

[aviationMx]

Boat: 2001 ssi 216, 5.0 l gm efi rated at 240 HP.
Props: :Replaced a 3 bladed 14.25 inch 21 P prop with a alpha 4 14 inch 20 P.

At full throttle with one person and 30 G of fuel, I can barely reach 4200 rpm- I think the range is 4200 to 4800 for WOT.

At first I wasn't too worried because I needed a tune up but I have now completed installing all new ignition parts and new TBI injector seals and checked the butterfly position is fully open at full throttle. Motor starts easily and seems to be running to spec (although perhaps small for a 22ft open bow). ..The 3 bladed prop put the RPM around 4700 and both props or not SS.

the Alpha 4 planes out fast and pulls tubes easily at lower speeds, not to mention better handling at low speeds but I'm concerned that I'm putting stress on the 305 motor w/ 375 hours. Do I need to reprop or is this acceptable ?

Been reading this forum for years, first time posting,


Chris
SML VA

 

[Maclin]

Welcome Aboard! :welcome:

Conventional Prop-tuning wisdom certainly points to a lower pitch. Since you are worried about it then I think you do need to try a lower pitch 4 blade in the same series from that brand.

I did a very basic prop calc, using 10% slip, 1.6 ratio, and 4200 rpm just to get a baseline. Then ramped up the RPM to 4600 and it showed an 18 pitch would get the revs up to where you need them. Should get about the same top end as you have now with the 20p. Should get even better holeshot, and be able stay on plane at slightly lower speed.

Good luck!

 

[jimmbo]

Before you start buying props, find out what your engines WOT rpm range is. Being a fuelie, it might be 4600 - 5000

 

[aviationMx]

Before you start buying props, find out what your engines WOT rpm range is. Being a fuelie, it might be 4600 - 5000

Jim- Found my ops manual and it has 4400-4800 RPM for the 5.0 (carb/EFI) WOT

Maclin- Thanks for the calculation, I'm assuming that the 18 P for a 4 blade is still 14 inch diameter?

 

[Maclin]

Should be the same as the 20 pitch if from the same manufacturer. Diameter is not much of a factor, the designers have that all figured out.

 

[aviationMx]

roger that, thanks again...i just took the boat out on smooth water ( not the norm her at smith mountain ) and all planed out I still only got 4200 rpm...

 

[Sea Rider]

Are you boating at high elevation lakes, rivers, whichever ? if so, at which level ?

Happy Boating

 

[aviationMx]

Are you boating at high elevation lakes, rivers, whichever ? if so, at which level ?

Happy Boating

800 ft here at smith mountain