4 blade aluminum to 3 blade stainless and slight decrease in pitch

[praeses]

Hi, and for starters:

1) Performance issue you are trying to correct: Top Speed
2) Current prop manufacturer, model, aluminum or stainless as a minimum. Evinrude (stock) Aluminum
3) Current prop diameter and pitch (required). D 10-1/2" P 14"
4) Wide open throttle RPM and speed with an average load (very helpful) 5200RPM
5) Engine/drive make, model, year, and HP Evinrude 35HP Oil injected 3-cylinder
6) Boat make model, year, length and weight Custom aluminum RIB 12' ~ 800lbs

I have a very light RIB (aluminum hull) that planes extremely quick with virtually no bow lift. I am mostly aiming to gain a slightly higher top speed (1-2mph) by going from a 4 blade to a 3 blade (stainless, if it will be more efficient) and if possible reduce side crawl as over long distances through narrow canals it noticeably tracks sideways. That may not be reduced with changing to a 3 blade prop, as I understand. When talking with my dealer and what I have read so far, I am currently operating below my peak RPM of 5500RPM which would be desired with my regular load. One dealer says I should go 2 pitch numbers up (4 blade to 3 blade) but no changes for stainless which would put me at a 16p. The other dealer says I should go for 1 pitch up for going to 3 blade, and 1 pitch down for going with stainless due to less flex which would put me at 14p. Neither dealer is willing to let me test any props as supposedly props for this engine are rare. I am leaning towards the 14p stainless. So far Solas props are the only ones I have been able to source both locally and online.

Part # 2231-100-14

Any thoughts would be appreciated.

Note: I guess I was a bit tired when I posted, I didn't mean slight decrease in pitch, I meant slight increase in RPM

 

[jestor68]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

If you desire to GAIN rpm, go down in pitch. If we were dealing with the SAME brand and material, I would say when switching from 4 to 3 blade go up one inch to maintain rpm. To increase rpm, you would stay the same.

The Solas 3 blade will run about the same rpm, but with more speed.

 

[steelespike]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

The wot rpm and gps speed would help evaluate your your setup.
With no bow rise out of the hole I wonder if the boat may run too flat at speed.

 

[praeses]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

The wot rpm and gps speed would help evaluate your your setup.
With no bow rise out of the hole I wonder if the boat may run too flat at speed.

WOT RPM is 5200, I am aiming for 5500 with my typical load. the maximum I have recorded is 46km/hr ~ 29mph or 25knots.

As for bow rise, I have 4 more adjustment holes to tilt the engine up, but currently it will dolphin after ~ 35km/hr if I go up a hole.

 

[praeses]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

If you desire to GAIN rpm, go down in pitch. If we were dealing with the SAME brand and material, I would say when switching from 4 to 3 blade go up one inch to maintain rpm. To increase rpm, you would stay the same.

The Solas 3 blade will run about the same rpm, but with more speed.

Would a 14pitch stainless and a 15pitch aluminum 3 blade net about the same speed or would the stainless be appreciably more efficient (providing the blades are thinner)? The cost of the prop doesn't concern me so much, but the softness of the aluminum seems like it would be handy.

 

[steelespike]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

I think usually going from al to ss "they" suggest the ss be the same pitch.
Going to a 4 blade usually suggested to drop one inch in pitch. It would seem that you would drop an inch for the ss 4 blade.
Changing material,brand and number of blades really makes things un predictable.
Before changing props have you tried raising the motor. The anti vent plate just above the prop should be barely visible
at speed.
Also you can buy or make wedges to use to gain or lose half a notch of trim.

 

[praeses]

Re: 4 blade aluminum to 3 blade stainless and slight decrease in pitch

So I tested before and after within about 30mins and these are the results:
Before: (stock: 4 blade aluminum 10.5x14")
Top RPM: 5400
Top Speed: 48km/hr
After: (Solas New Saturn: 3 blade SS 10x15", supposedly cupped)
Top RPM: 6100 (too high)
Top Speed: 54km/hr

This is the highest pitch I can find for the motor. I suspect going down slightly in diameter made the rpms go up higher than I anticipated. Hole-shot is good, considering the light weight I don't think hookup was ever a significant issue now it can just spin up a bit quicker. I was expecting my prop slip to change going from a 4 blade aluminum to - 3 blade aluminum but it appears as if that's not the case for me with these two props.

So far vibrations are the same at the same rpm, however it is louder at 6100 vs 5400rpm. I am going to see if the local shops here can re-pitch it to 16 ideally or 17 as I don't want it revving that high.