Prop Diameter vs Pitch Relationship

[Cheechako]

(planing hull) My little powerboat has a 15 x 19 prop on it. It doesnt get to max rpm (4500-4800)... far from it. The Previous Owner said it should have the other prop that came with the boat on it. That prop is 16 x 17. Here's the question: is the 16 x 17 a prop that will give me more RPM? It's less pitch by 2", but it is an inch bigger diameter...? is there a rule of thumb for dia vs pitch comparison (ie: if i have a 15 x 15 prop and want to use a 16 x ? prop, would a 16 x 14 be equal performance to the 15 x 15? or something like that?)

[CharlieJ]

The generally accepted rule of thumb is changing the pitch 2" will achieve a 300 to 400 rpm change. Increasing the pitch lowers the shaft rpm and decreasing the pitch raises the shaft rpm. Unfortunately this becomes a cut and try exercise. An experienced prop shop, can greatly reduce the number of cut and try cycles.

[Exile]

What about the influence of prop diameter on engine rpm?

[atoll]

Originally Posted by Exile

What about the influence of prop diameter on engine rpm?

i seem to remember 1"diam change = 2" of pitch change

[CharlieJ]

The rule of thumb is 1" of diameter (up or down) is equivalent to 2" to 3" change in pitch.

[deckofficer]

atoll,

This is hard because 99.99% of the time you are always right on with advise. Pitch is how you reach a desired rpm, diameter up to a point, larger is better for less slip. On most recreational boats pitch is measure in inches, so an 19" prop with zero slip (that doesn't happen) will move the boat 19" for each revolution. If that is too much of a load on the engine, then the engine will not reach its top operating rpm. Trawlers and other efficient displacement boats will swing as large a diameter prop that will fit, as slow as gear reduction will allow, that is why you see steep pitched props on them going through a 4 to 1 gear box reduction. On the container ships our max engine rpm is 118 and we are direct drive so the screw turns 118 rpm. We have the lowest slip numbers by turning a large diameter prop slowly.

If Cheechako can fit a 16" diameter prop, he should go with that and reduce the pitch until his engine can reach its top operating rpm without going over it. Cupping a prop can give an incremental pitch increase, but he needs to decrease pitch.

[deckofficer]

OK, now I see CharlieJ who works in the industry agree with atoll, so maybe I need some schooling.

[CharlieJ]

Bob #6: +1
Recommend Dave Gerr's excellent "Propeller Handbook" as THE authority on propeller design.

[deckofficer]

Originally Posted by CharlieJ

Bob #6: +1
Recommend Dave Gerr's excellent "Propeller Handbook" as THE authority on propeller design.

I'll have to pick that handbook up, but in the meantime could you help me with the concept of increased diameter equating to a 2X increase in the results of pitch change because I am lost on this one.

[CharlieJ]

No, Bob...based on the info provided, you are spot on. In general, for a normal (not surface piercing, super cavitating, or other exotics) propeller, I think that the generally accepted design practice is to fit as large a diameter prop in the available volume, ensuring proper tip clearance, etc., and then adjusting pitch to achieve engine WOT.

[deckofficer]

Originally Posted by CharlieJ

No, Bob...based on the info provided, you are spot on. In general, for a normal (not surface piercing, super cavitating, or other exotics) propeller, I think that the generally accepted design practice is to fit as large a diameter prop in the available volume, ensuring proper tip clearance, etc., and then adjusting pitch to achieve engine WOT.

Whew! I thought the grade I received in NA at CMA was a gift I didn't deserve, Thanks. So back to the OP, he should go with the 16 x 17 for increased engine rpm.

[Cheechako]

Originally Posted by atoll

i seem to remember 1"diam change = 2" of pitch change

Wow that much huh? That figures.... The boat had a 15 x 19 on it. I have a 16 x 17 also. It appears that would be "no change"!

[deckofficer]

Originally Posted by Cheechako

Wow that much huh? That figures.... The boat had a 15 x 19 on it. I have a 16 x 17 also. It appears that would be "no change"!

Put the 16 X 17 on, you will see higher rpms and a much improved hole shot.

[Cheechako]

Yeah, I might as well give it a shot and see what happens....

[River Cruiser]

Having the correct prop also depends on a well tuned engine & clean hull. Also over time the load on the boat can increase as we bring things aboard & leave them & that heavier load can decrease wot rpm. This would be more noticeable on a smaller boat.