Prop size difference

Jacko9370 · 25 August 2016, 18:12

What difference would the following props make to my boat please;

12.75 x 17 4 blade or
13.5 x 15 3 blade

Thanks

jyasaki · 26 August 2016, 15:43

A) What are you running now, and what kind of performance are you getting? WOT RPM?

B) What do want to accomplish?

jky

Jacko9370 · 27 August 2016, 10:21

Currently have the 3 blade prop on, about 5-5500 rpm is about 28-30 knots.

Wondered what the difference would be with the other one.

Thanks

paintman · 27 August 2016, 10:24

What engine is it, what's the manufacturer's recommended WOT range for the engine & is the current prop within that range?

Nos4r2 · 27 August 2016, 10:32

Looking at the numbers,you'll be slower with the 4 blade.

Diameter nearly equal, more blades and more pitch, so you'll lose revs.

Jacko9370 · 27 August 2016, 21:01

Thanks, that's the simple answer I was after

jyasaki · 29 August 2016, 03:49

Pitch: 15 to 17, you'll lose 400-600 rpm or so.

Diameter less by 3/4" should gain you about 300.

3 blade to 4 should drop you by another 250 or so.

Net: Should turn about 300 to 400 rpm slower at WOT.

That all assumes you are well within the power curve, and that all "standard" effects actually pan out. They rarely do.

jky