Diameter Vs Rpm for the new guy

Greetings.
I have a question regarding the effect of diameter on rpm.
The setup;
I have an "adventure" mini speed boat. Like the addictor 254, but a foot longer @ 9 feet. Im running a 1975 Johnson 75 stinger, short shaft, small gearcase, 1.87 gearing, with a foil for planing help, no tabs, AV plate 1" above hull bottom.
New tach, GPS for speed. desired RPM is 5800.

When I got the motor, it had a oem 12.5 X 13 cupped on it. It would run 6100+ at 39mph WOT.
I bought a hustler 12.25 X 15 cupped for it, and now get 5200~5300 at 42~43mph.
I am thinking it's too much prop, (I couldn't find a 14p anywhere), and since my boat is so small & light, maybe 600lbs before I put my skinny butt in it, that going to a smaller diameter would unload the motor a little with less area & drag, and allow me to run more pitch. Much like airplanes. A big flat prop for the stearman, and a small, high pitch prop for the glass homebuilt. I think I want to trade diameter for pitch, for my application.

Given the above rpm info, what would I expect going to a 11.75 X 15, or 17,
or maybe I can find a 11.5D somewhere.
An inch of diameter is equal to how much pitch, for a given rpm.

Thanks in advance for taking the time to respond, I know there's thousands of folks out there asking the same prop questions over & over.

-Dan

 

I asked this same question to "Michigan Wheel" Prop manf.

Was told 1" change in Dia. is roughly equal to 1" pitch change. (about 400 rpm)

Prop Dia is kind of a function of Load. i.e. As the total load or weight increases the prop Dia should increase with a goal of optimixing efficiency or minimizing Slip. Too much Dia reduces slip at the expence of Thrust. Too little Dia will increase Slip (decrease efficiency)

After Min Dia is determined, Pitch is adjusted to provide Max. Thrust and optimim RPM.

I would guess 11 1/4 -11 1/2 Dia.is probably the smallest you should try.

 

I think I may be in the to much diameter relm.
So of the available sizes I've seen, 11 3/4 is the smallest, without going to 19p.
If you are correct, and 1" diameter will affect rpm by 400, then 1/2" should put me up 200. Guess I better leave the pitch at 15.
If Im getting 42mph at 5300rpm with 1.87 gearing, what is the %slip ? apparently, a certain amount is desireable.

-Dan

 

Dan,

Based on the no's provided, your slip is about 2.2%, which is exceptional and probably indicates that your prop may be too large. Most fast, lite hulls are in the 4 - 6% range. By decreasing Prop dia. and increasing slip, Speed will be DECREASED,if ALL else remains the same. Prop dia reduction will however increase rpm, increasing speed more than what was lost due to slip.

Hope this is clear.

Keep in mind, A Full Cup acts the same as adding about an inch to the pitch. Therefore a full cup 15P acts the same as a 16P (at full speed) and should be calculated that way.

Take a look at:http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

good luck, John

 

Thanks, John, this is what I thought all along.
I just ordered a 11 3/4 X 15 SS prop, on closeout for $75.
We'll see what the resulting rpm is.

-Dan

 

Zero slip? More prop theory

Is it possible Im getting near zero slip?
I used prop calc with my numbers, & that's what I get.

I put on a rapture SS 13 3/4 X 15, and here are the results.

Im getting 43-44 mph @ 5700 rpm. Measured several times, over distance. The most I saw was 44.5, but at a standstill, the tide was running .8mph, so I'll give up a knot for that.
This works out around 0% to 1% slip. Am I missing something? Is this possible, with a big motor on a small boat? Is this desireable?
I need to run 1 trim tab on the left because the tourque at speed rolls the boat to the left, even with me sitting on the right.

I feel I should be running an even smaller diameter prop, with more pitch, to get some slip. I think it would help reduce the tourque effect on the hull as well. However, Im unable to find anything smaller in diameter, without going up to 19"p. I think that will bring the rpm down too much.
I can't buy a prop that will slip more than 2% for my combination, it would seem.
Your opinions welcome.

-Dan

 

Dan,
Are you using a 13 3/4 dia or 11 3/4 dia?

With the No's. given, you are actually getting about 3.7% slip @ 44.5 mph or 4.7% @ 44mph


5700 RPM
15 P. (CUP) - acts like a 16P.
1.87 gear

Don't forget a Full Cup 15P. should be figured as a 16P.

A smaller prop with more pitch could help your Torque effect but would probably require a gear change to maintain desired RPM.

John

 

John,
I miss-stated the diameter in the last post. It's 11 3/4 X 15 SS
I also forgot to factor it at 16" for the cupped style, so that makes more sense.
I find it suprising that some think diameter has little effect.
(rant not directed specifically at you)
The area & therefore the loading of the blades is directly related to the diameter. By using a smaller diameter prop, the load on the engine is reduced, allowing more power available to go to pitch, at the expense of slip, or efficiancy.
The engine can turn a smaller diameter prop faster, so you can absorb that with more pitch. Going from 12 1/4 to 11 3/4 brought my rpm up by about 300. Just what I wanted.
I believe I could get better results for my application however, if I could find a 11 1/4 X 17. I believe I could turn the 2" pitch increase to similar rpm, by reducing the diameter. So I would not need a gear change.
My opinions are based on my aerodynamic wisdom, and so while I may be wrong to apply it to hydrodynamics, my instinct tells me Im not far off.

Thanks for the help, folks.
-Dan

 

Dan,

I believe you're on the right track. If you could turn a 17P. (cupped) @ 5700 motor rpm, speed will be 50 mph with 4% slip. Even with 8% slip you're at 48 mph. Also, you may need to reduce weight somehow to make the smaller prop work (reduce slip).

Good Luck, John