Logo
FULL SITE MENU

Propellers pitch

Pitch is the angle of blades.

In purest theory a 17 pitch (inch) prop will push the boat forward 17 inches for each full rotation of the prop - so the 21 pitch will move it 21 inches forward for that same rotation.

Again, in purest form if your motor was turning say 5000 rpms and you had 2:1 gears (so making 2500 rpms at the prop), the 17 pitch would give you about 40 mph and the 21 pitch almost 50 mph.

But, that's just "theory" and just because your motor can "turn" a 17 pitch without bogging doesn't mean it can turn the 21.

Your motor has a Wide Open Throttle (WOT) range. On many motors it's in the 5000-5500 rpm range. The "more prop" (more pitch OR more diameter) that it tries to turn, the harder it has to work. There comes a point where the motor just can't turn the prop because it's "pushing too hard" and the rpms fall out of the range - when that happens the motor bogs, carbons up, and drastically shortens it's life.

The opposite is true. If the prop is TOO EASY to turn (not enough pitch OR diameter) and it can "exceed" it's max rpms and literally blow itself apart.

So you want to find the pitch and diameter that leaves the motor "maxed out" WITHIN it's WOT range.

Changing the pitch or diameter by 1 (one) adds or subtracts about 200 rpms (so if you are making 5200 rpms with a 17P, you will make about 5000 with an 18P - move up the pitch, rpms go down and vice versa).

Also a Stainless prop flexes less than aluminum or composite, so all else being equal you "drop" about 50 rpms if you switch from aluminum to stainless. Other things also effect the rpms - cupped blades shave 50 rpm and going from a 3 blade to a 4 blade will shave another 50-100 rpms from your max.

If your rig gets into it's WOT range with a 17 pitch prop with the boat operated the way YOU normally run it, then there is no way that it will be able to turn a 21 pitch without bogging out (that would drop the rpms by 800 and likely well below your minimum WOT range). Same with the other way around - if you are in the WOT range with a 21 pitch, you would be over-revving with a 17.

You need to know a couple of things when you change a prop:

What is your max rpms the way YOU typically run your boat (typical gear/passenger load).

Is that rpm withing the WOT range?

Do you have a little room to play around and still stay in the range?

What are your trying to achieve in changing the prop (a 4 blade gives you a better hole shot - so good for water skis or pulling tube - 3 blade gives you better top end speed etc)

You can't look on any chart or ask someone and get an answer that applies to YOU - you need to run your boat, get the rpm answers and go from there.

Even if you and I have identical boats/motors we might use significantly different pitched props - you might run by yourself "most of the time" - I might load up 4 x 200 pound "buddies", fill the livewells etc and be pushing 1200 more pounds than you - you would need "more pitch" and I would need "less".
 
Back
Top