2 prop numbers - same pitch?

[Grant Brown]

1993 Mercruiser 4.3 Alpha 1 gen 2. Boat is a 20 foot deep V aluminum, about 1500 pounds wet.

Prop that is on it is a Quicksilver 1918P21 - is this a 21 pitch?

Spare (free) prop looks like it's an aftermarket. One side in large stamped numbers has 1567. The other side in smaller stamped numbers has 4878 120 A40 19P - is this a 19 pitch?

How does the difference in pitch affect performance? Ideally I would like to increase economy when cruising. With the Quicksilver prop on it now, I generally cruise at about 3200 - 3500 rpm and 27 - 30 mph. I do not ski or tube much.

Thanks,
Grant

 

[steelespike]

Re: 2 prop numbers - same pitch?

Both appear to be Mercruiser; One is 19 pitch other is 21.
Assuming similar designs the 21 will lower rpm ABOUT 400.from the 19.
Generally the higher pitch will have slower hole shot higher top speed
assuming the motor can turn it up.Lower pitch will have better hole shot
lower top speed.These are not absolute.Generally a prop that provides the best top speed wiil get the best mileage.Because the boat moves faster at the various rpm than with a prop that provides less speed.
Sometimes a 4 blade prop will help lower end performance and my improve cruising speed mileage.Because the boat may stay on plane with less power.
The best cruising speed can be anywhere from just on plane to signifigantly higher.Trial and test is the only way to tell the best without a flow meter.

 

[Grant Brown]

Re: 2 prop numbers - same pitch?

So essentially the higher pitch number is like a taller (lower number) final gear ratio on a car.
I'll probably try the 19P just for kicks to see what it does.
Thanks!

Grant