12 ? 15 pitch is gives more rpm than 11 ? x 17 pitch

[NascarNHIS]

My question is what prop size and pitch should I use for 5500 rpm WOT.

I had an old OMC prop p/n 386786 -12 ? 15 pitch that gave me 6000 plus rpm WOT. I thought that was a bit high for rpm on this motor. A friend gave new Michigan Wheel MATCH-p/n011028 11 ? x 17 pitch to try.That only gives me 4500 4600 max WOT,

The diameter got smaller and the pitch increased the rpm droped, so I am lost in how to calculate for my 5500 WOT goal.

SPECS

1989 EVINRUDE 48SPL 48 hp
Mfg. 4500-5500 RPM WOT range per the manual
2.42:1 gear ratio

Boat = 1989 Sea Nymph SS175 FISH & SKI Aluminum (same as Starcraft SS)
Base weight 800lbs dry, beam 6?10? length 17ft.
Normal load = 800 lbs. (2 fat guys, lg cooler of beer and food 3 batteries, 2 tackle boxes, 2 small anchors, trolling motor, 15 gallons of gas safety gear etc.)
Deep Vee, bow rider, do not know degrees of deadrise

I want to stay with aluminum, please DO NOT recommend a SS prop.


thanks in advance.

 

[steelespike]

Re: 12 ? 15 pitch is gives more rpm than 11 ? x 17 pitch

It would help to know your gps speed and rpm with both props.Don't forget your motor is part of your load,about 200 lbs.
Rule of thumb a 1" pitch change results in "about" 200 rpm change. Decreasing pitch, lower number ,increases rpm.
Decreasing pitch increases power speed result will depend on previous results.Your 17" extreme results are different because your going from old to new
The new prop h very likely has better grip(lower slip) But is also way to much prop so your results a skewed.
Generally as pitch increases diameter decreases,Diameter is a secondary consideration if the prop is intended for your motor
the diameter will be right.
I think a 15" is very close to what you need and a new15" prop may produce slightly less rpm because it may have lower slip numbers.
The Solas Amita or Rubex would be a good choice. Rubex has a removable hub.

 

[eavega]

Re: 12 ? 15 pitch is gives more rpm than 11 ? x 17 pitch

You might also consider going to 15" P Stainless Steel. My experience with aluminum vs Stanless Steel with the same pitch is that it will decrease your RPMs ~300 depending on the actual props. Stainless steel flexes less so it produces less slip. Also weighs more so it requires more of the motor's power to keep it turning. I had a 70 HP Outboard that I originally ran with the original 17" P aluminum (standard prop from factory) which ran at 6100 RPM WOT. I switched to a 17" P Stainless Steel Michigan Wheel prop and dropped my RPMs to 5800 WOT. It will depend on the physical configuration of the prop itself (the SST prop had more pronounced cupping, and weighed about 5 Lbs more than the aluminum one), but it might drop you right into the "sweet spot" for that motor more so than a higher-pitched prop. Agree with Steelspike that a 17" P is probably too much prop for your setup. I had a similar weight 15' fiberglass tri-hull with an Evinrude 50 HP, and had to drop from a 17 P aluminum to a 15" P aluminum to get even close to 5500 RPM.

Rgds

 

[steelespike]

Re: 12 ? 15 pitch is gives more rpm than 11 ? x 17 pitch

Prop weight doesn't affect rpm,motor power. It may over time add the slightest wear to the drive components.Driven correctly
even that will be minimized.