Variable Pitch Propellers

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Dora~Jean

Active member
I've got a question that's been nagging for some time now, maybe its already been explored. If the drag decreases after a boat is on a true plane, then thereafter continues to decrease as the speed goes up (except for wind drag), why don't propulsion systems offer transmissions or variable pitch props? Maybe I'm all wet and the 'total drag' goes up with speed nearly in proportion to the fixed propeller's propulsion.

With today's prices and tomorrow's predictions on fuel prices, you would think this would be a very attractive feature that a boat or engine mfr could capitalize on. Seems C-Dory's would be the perfect candidate for a prototype!
 
I found this on a web site but have never seen constant pitch props for a small boats but it would be a intersting idea.

There are also controllable pitch propellers (CPPs), where the blades can be rotated normal to the drive shaft by additional machinery at the hub and control linkages running down the shaft. This allows the drive machinery to operate at a constant speed while the propeller loading is changed to match operating conditions. It also eliminates the need for a reversing gear and allows for more rapid change to thrust, as the revolutions are constant. This type of propeller is most common on ships such as tugs where there can be enormous differences in propeller loading when towing compared to running free, a change which could cause conventional propellers to lock up as insufficient torque is generated. The downside of a CPP is the large hub which increases the chance of cavitation and the mechanical complexity which limits transmission power.

For the smaller motors there are self-pitching propellers. The blades freely move through an entire circle on an axis at right angles to the shaft. This allows hydrodynamic and centrifugal forces to 'set' the angle the blades reach and so the pitch of the propeller
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Thomas,

The Aerostart Power Pitch Pro all SS, two speed, auto-shifting prop is what I was thinking about. Like you said, their testimonials and advertisement definitely cater to the go-fast, bass boats and similar. But if you click on the "Cruisers" link, that provides info for our kind of boats, like the following:
The Power Pitch® Pro cruiser version of our auto-shift transmission propeller is specifically designed for wide beam, full cabin cruisers and other heavier applications. Starting off in low pitch dramatically reduces the time to get on plane and in high ptich the engine rev's are lower at cruise. Fuel economy typically improves from 10-20%.
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But whoa, at $1995 per prop, I'd have to save a lot more gas than 10-20% improvement!!! Still, something worth considering. Thanks for the research.
 
My brother has a 18 foot Hourston with a 350 I/O in it, turning a 21" 3 blade stainless prop, on one of the Okanagan Valley lakes. He knew his boat mechanic really well and talked the guy into letting him try a variable pitch prop that he had brought in, for a couple of weeks. It took him 2 days of playing with all the adjustments to tweak it to his boat, then the damn thing jammed a couple of times. He has trim tabs, leg trim adjustment and a vacuum gauge to check economy. He used a hand held GPS for before and after results. Gave it back to the mechanic after a week. Results were lousy.
Jimbo
 
An underway variable pitch prop would have got me excited (bad choice of words!) several years ago, but the KISS Principle looms larger with time and experience. Just too darned complicated for a boat of this size and cost.

Joe.
 
Actually in most boats the total drag goes up as the speed increases. Wind resistance is a considerable factor in the C Dory lines. The lower profile racing boats, drag boats and bass boats, might be a better candidate. There is also increasing hydrodynanic drag also--plus it depends on the sea (wave) conditions, wind direction etc.

I have owned several controlable pitch props--there are also several "auto props"--which will change pitch with RPM and drag. The ones I have owned have been on motor sailors, where the sails add a certain amount of the needed power, and then a small amount of engine power will push the boat up in speed, with a higher pitch prop. In these displacement boats they work well.

David Gerr addresses this in his books--and comes to the same conclusions about the increased drag with speed.
 
GREAT, I'm no longer nagged by this question, now I can sleep better! I had this design dream of using my hydraulically controlled throwout bearing from my Jeep to vary the prop pitch from the dash (manually at first until I could automate with a computer-controlled feedback system).

I agree with Joe, non-KISS, plus ROI is way too low...! Thanks all for the information.
 
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