Variable Pitch Props

I have a newly constructed 16' runabout for which I have purchased a 90 hp 4-stroke Yamaha. Looking at similar boats at shows - as well as using on-line propeller selection guides - it seems a 13" dia x 19" pitch is what I should start out with. There are available, however, adjustable pitch props, such as the composite "ProPulse" prop. The manufacturer claims efficiency is comparable to standard props, plus some other good stuff.

An adjustable pitch prop seems like a good place to start since I could quickly determine the optimum pitch required for this particular boat/engine combination. I would like to know if anyone has any experience with these adjustable pitch props - in particular the ProPulse. I would like to hear about any good or bad experiences.

 

Sorry I can't. I have never design CP prop but FP prop.

 

Generally, if you've bought the outboard new, the dealer will have a selection of props that you can try before you decide on the final one.
'adjustable pitch' props, such as those you mention can never be as efficient as a fixed prop throughout their pitch range, and are generally used on boats that are marginally underpowered. The lower pitch setting allows the boat to get up onto the plane, then when certain revs are reached it 'changes gear' and allows the boat to get up to higher cruising speeds.
If your boat / motor package is properly matched, you shouldn't need one

 

The type of variable prop that shifts automatically is, as Will said, generally to help an underpowered boat climb to plane. Another variety, that is somewhat less common, has the pitch set manually by a hand-adjusted gear ring on the back of the hub. This type is usually for smaller boats (under say 50hp) that operate with both very light and very heavy loads, so you can increase the pitch running solo and decrease it when loaded down. They, too, are not nearly as efficient as a fixed prop, but can be convenient if you would otherwise be changing props all the time. Usually a good prop dealer will let you try a few until you find one that works well. (They try to keep good customer relations, that way, you come back to them when you hit the rocks with it!)

 

witch with a complex vario prop nobody likes is offcourse more expensive
varioprops do however give good overall performance working like say an automatic gearbox

 

Seems like you could do the same thing by putting a locking torque converter in the drivetrain somewhere.

 

The torque converter's an interesting idea, if a tad inefficient. I think you'd run into cavitation problems with it though. A torque converter (essentially a combination of a hydraulic pump and turbine) can greatly increase the output torque when the engine is spinning faster than the output shaft. Allowing the engine to rev high, and then using the converter to apply a much higher than usual torque on the prop, might tend to just spin the prop and cause cavitation. It's a similar effect to when you gun the engines from a standing start with a 2-speed transmission in low gear; if you apply too much torque too early, the prop loses its grip on the water.
The torque converter's main purpose is to provide a 'slippable' fluid drive so an automatic transmission can shift without jamming. Since the water around the prop already provides a fluid linkage, I think you'd be better off with just a two-speed box rather than a torque converter.

 

I don't know about inefficient - the converter on my car is 96% efficient even when it's not locked up.

As far as cavitation goes, I don't see that as insurmountable. Torque converters come in different stalls. The stock converter on my car stalled at 1300 rpm - in other words, it only allowed the engine to rev 1300 rpm higher than the output shaft on the torque converter. The converter I have in the car now stalls at 3500 rpm. I can't make full use of it without putting slicks on the car, though. Anyway, the point is you could match the converter to the prop and motor so that you'ld be able to rev the motor to get more torque on launch, but not so much that you cavitate the prop.

 

Additionally, to add to the above post, seems like a correctly designed prop can handle a lot of torque without slipping too much - how much torque do the top fuel drag boats put to those tiny little props they run?

 

My thinking is that the composite variable pitch props could be a fast solution in case of prop break while navigating. You can substitute it easily. It´s a cheap solution to come back home. You won´t get the same performance like with traditionals, but i´m thinking that life should be longer (no corrosion)

 

Check this site : www.air-composite.com/

They have the Contur Propeller that is somehow pitch controlable.

 

I haven´t understand the concept 100 %. Are the contur prop variable pitch? What do they mean with "adjusts its pitch automatically without external setting mechanisms". Is the pitch adjusted only one time or is it possible to adjust it when needed?. As it seems to me, you will only have a pitch position after adjusting (most convenient as they say), but you won´t have the possibility to change it after that. The challenge would be to find the propeller which you could automatically change the pitch without doing any other operation more than pressing a button onboard, and not lifting the drive unit to fix the blades again, like you have to do with existing variable pitch props. The challenge would also be to find the most simple, quick and cheaper adjustable pitch system. Composite could be again the solution. Do air-composite offer this? Anybody knows more about these props?

 

I found this quote on the contur website:

What it sounds like is they designed the propeller blades to flex depending on the load - lots of torque, low speed, the blades flatten out and reduce the pitch. Once you're up to speed, the blades unflatten and increase the pitch for more speed at less rpm.

This brings up an interesting question. What do the blades do at wide open throttle?

 

I've got a variable

I've got a composite variable pitch prop that's manually adjustable from about 16 to about 21 pitch, plus the blades are individually replacable. I't not as efficient at 19 as the stock stainless cupped prop on my old 135 small case Evinrude, but it's a good backup prop, and a good test prop to determine proper "real" prop pitch.