What pitch should i buy?

[rhino_160]

Ordering a new prop and need some advice on the pitch: 22' Pontoon (not tritoon) 2011 70 HP 4 stroke Suzuki DF70 outboard. With 4 adults and half tank of gas it runs 4400 RPM wide open. The motor is rated 5000-6000 RPM Currently have a 13" with 17 pitch propeller. My question is should I got with the 11 or 13 pitch to get my RPMs up??? What's your advice? Thanks

 

[Sea Rider]

Definitely, but which one ? Some states 150-200 revs gained per 1 pitch, others states more. Woul be ideal to test both and stay with the one that gives at least middle wot numbers with currently loaded pontoon. You need a prop maximization to better the water performance out of that combo.

Happy Boating

 

[steelespike]

Seeing as a pontoon doesn't plane;I think rpm may be hard to come by. I'm leaning toward an 11".
What is the present speed? You might do a test run with a light load and get us the rpm and gps speed, remind us of the 17" prop.
If the 11 over revs you can throttle back a little.If the 13 lugs your stuck and shouldn't run it excessively until you get a prop for better rpm.

 

[jestor68]

A 13 pitch would be my recommendation.

 

[rhino_160]

with the current 17 pitch on a 22' Pontoon i am getting 20 mph with 2 people at 4400 RPM wide open throttle. with 4 adults i get 18mph with the same setup. I am torn between running an 11 or 13 pitch with the cupped blades. I am trying to achieve a higher top end speed being i am only turning 4400 rpm right now. I do get the occasional cavitation with the current 13"x17 pitch Solas aluminum prop. Any advice is welcomed. THanks

 

[hwsiii]

Rhino, at 20 MPH you have a prop slip of 37% and that is extremely high and at 18 MPH it is worse, even for a pontoon. Which exact model of Solas prop are you running and we need some pictures of your motor height relative to the bottom of the pontoons using a piece of wood between the pontoons to make sure the motor is not too deep in the water as well and creating excess drag from the motor. Is your occasional cavitation in turns only or when you are running straight lines as well.
Usually cavitation means the motor is not too low, but your prop slip is so high we need to check this.

I show your motor to have a top RPM of 6,000 and a gear ratio of 2.59, please verify this information.

Here is a drawing showing how to measure motor depth.
http://forums.iboats.com/forum/gene...r-performance-and-ss-versus-aluminum?t=369057

H

 

[steelespike]

Hi H good to see you.
Wouldn't a prop that was too much pitch tend to have higher than normal slip numbers?
Especially a 17 being up to 6" too much.

 

[jestor68]

Your primary concern should be getting your rpm up to between 5500 and 6000 rpm in order to develop full HP and not be lugging that motor.

A 4 blade prop should reduce the tendency to ventilate(better grip).

 

[walleyehed]

You will also find you can "Over-shoot" the pitch curve when the torque comes in on a 4-stroke...especially a zuki... I'd lean hard to a 13P with near zero rake, or at least very little rake. I've had a rise of 800rpm with just a 2" change in pitch on a similar set-up.