Prop pitch .VS. outdrive gearing questions
 

OK - I've done some basic searching here and haven't seen any real discussion about this - what's the relationship / benefits/drawbacks between prop gearing and performance - not the lower (higher numerically) outdrive gearing slows prop rotation thus lowers speed so increase pitch to compensate, change prop pitch for best performance at a target RPM, etc.

What I'm wondering is what's better - to spin a prop faster at a lower pitch or to have a higher pitched prop spinning slower and why ? I'm thinking that there's got to be some magical balance in there between slip/pitch/gearing, etc....

Anyone care to take a stab at it??

often wondered the same thing myself, somebody's gotta know

good question! im curious to see what people say!

I'm no expert on this but have been told it's better to turn a larger prop slower than a smaller prop faster, the larger prop spinning slower is more effecient giving you lower slip#'s.

A 1.5 drive will mutiply the torque to the prop more than say a 1.36 drive. So go with a slower prop needing higher pitch. Once you get to huge HP then you need a lower ratio, you will run out of pitch.

There is a discussion similar to this on the Scream and fly forum. I would think bigger ptch in an I/O application where you dont want to rev the motor too high, but they are saying the oppsite on an outboard, since you dont want to load the 2 stroke down. seems they like RPM over lugging the motor, I agree for the outboard, but in an I/O, I would run as much pitch as ya can and still reach your lower end of the max RPM Band.

The effect on the motor is the same... not an issue of lugging vs. spinning the motor. Your prop would have to be changed to accomodate the gear pitch.

What it's going to come down to is prop hydrodynamics.

I don't really know what would be better, but I do know that the discussions regarding diesels on high performance boats have led me to believe that a lower pitched prop spinning at a higher RPM, comparable to a gas boat, is preferable. There is going to be a sweet spot, though, and it's going to depend on the resistance of the water to the prop blades turning vs. the efficiency of the prop blade at a specific angle. The intricacies of prop design will play a big part as well.

I think what they were referring to is that on a 4 stroke you dont wanna spin it up too high due to the valvetrain, whereas on a 2 stroke they like to rev, and dont like to be loaded down with the big pitch props, at least not so much to where they wont come up to the reccomended RPM at WOT, for instance, it is OK to run it at 4000 RPM(at part throttle), but not to where it is loaded down by the big prop, so that it only reaches 4000ish at WOT.

Think the Question being asked is why change gear ratios. We change ratios depending on length of run and speed wanted. More midrange lower ratio.Tall ratios speeds are normaly higher but takes forever to get there. You can actually spin a prop to fast in which prop slip will rise and handling will go away. The idea is to find a compromise at your motors torque and hp. You want the motor to be able to use all of its potetial.

Also big props load the drives alot so most try not to run over 32 pitch on a bravo style drive. Brake alot of drive this way.

I believe the 'sweet spot' or ideal combination varies by application. Hull design, prop type (3, 4 or 5 blade), max engine rpm where peak horsepower occurs, etc, and the combination of all these play a part.

I have read a couple threads related to this discussion and the consensous is for peak performance you are better off turning a taller prop before lowering your gear ratio (as long as the prop pitch is available). I think of it kind of like drag racing. My car ran a lot faster with a 5.13:1 gear and a 32" tall tire compared to 4:11's and a shorter tire. Probably doesn't relate to boats though!:p