21 pitch vs. 17 pitch

hi there
i just got a 1984 searay seville 120 hp, i know not great but you gotta start somewhere.
any ways the prop that came with the boat was a 17 pitch but baddly damaged so i replaced it with a 21 pitch.
after a couple times on the water i've noted a top speed of between 55 and 60 km/h but rpm only hitting just over 3000.
i did not run the boat with the 17 pitch due to damage howerver the boats redline is 4000 rpm.
so i guess what i'm asking is if i get a new 17 pitch prop can i expect the same top speed in the proper rpm range?
and can someone explain how changing pitch can effect rpm's?

thanks

Re: 21 pitch vs. 17 pitch

Prop selection is like picking one gear in a car to do everything.
Lower number pitch is like a lower gear higher number like higher gear.
Very basically higher pitch may be faster but with less power and too high will cause the motor to lug.(run below suggested rpm).
Lower pitch will have more power but less top speed.And too low will over rev.(allow to the motor to operate above rated rpm range).
The idea is to select a prop that lets the motor rev freely within it suggested rating.
Rule of thumb 1" pitch change will result in about 200 rpm change. So your 21" lowered rpm about 800 this further lowered the speed increasing slip and more boat in the water creating more drag. And the motor is operating below its rpm range further
complicating the performance.
A 17" prop should help get the motor back to its rpm and the boat should respond better as well.
Have you adjusted trim (usually up) to improve speed?
It may be cost effective to have the 17 repaired at a prop shop.

Re: 21 pitch vs. 17 pitch

To turn the 21 pitch prop fast enough, you'd need about 200+ HP.

A clean, fresh 17P prop should get you squared away.

Re: 21 pitch vs. 17 pitch

The 17 pitch prop turning 3700 rpm will move at the same speed as the 21 pitch prop at 3000 rpm.
If the 17 turns at 3800 rpm it will be even faster.

The engine will generally produce more power at 3800 rpm than at 3000 rpm.
More power means more speed.
Less could actually be more in this case.

Re: 21 pitch vs. 17 pitch

thanks for all the advice, let the search begin for the 17 pitch prop

Re: 21 pitch vs. 17 pitch

UncleWillie said:

The 17 pitch prop turning 3700 rpm will move at the same speed as the 21 pitch prop at 3000 rpm.
If the 17 turns at 3800 rpm it will be even faster.

The engine will generally produce more power at 3800 rpm than at 3000 rpm.
More power means more speed.
Less could actually be more in this case.

Click to expand...

This is the unknown about propulsion. Sooooo many factors playing, some in your favor some against.

So as Uncle Willie said....you are running too much prop (pitch too high); it's lugging your engine....rpms are low.....HP = rpms x torque and a constant of proportionality. Okaye, lugging the engine you don't have the available hp you are supposed to have due to low rpm's so you suffer in performance.

Cut back on the pitch, unload the engine, rpm's go up, available hp goes up, result is more speed and performance because more speed pushes more of the hull out of the water, reducing hull drag, allowing what hp is available to do a better job as it is pushing less load translates to more speed, less boat in the water , less drag..etc. etc. ....it just goes on and on.

Mark