Variable Pitch Propellers

Dora~Jean · Posted: Sat Oct 20, 2007 7:46 pm Post subject: Variable Pitch Propellers

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!

rogerbum · Posted: Sat Oct 20, 2007 10:44 pm Post subject:

Variable pitch props are available and used for larger boats. See for example this wikipedia article on the topic. The main issues are expense and maintenance of a more complicated system.

starcrafttom · Posted: Sat Oct 20, 2007 10:50 pm Post subject:

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.

starcrafttom · Posted: Sat Oct 20, 2007 11:20 pm Post subject:

I also found this link but it seems that this product is for go fast boats. you have to be able to travel at 40mph plus in order to get the full effecct

http://www.land-and-sea.com/marine/torque-shift/torque-shift.htm

and heres another one but again for bass boats and races boats or i/o

http://www.aerostarmarine.com/outboard.html

and this may be the way to go for a 25ft c-dory in the 200hp range.

http://www.swordmarine.com/Products/default.htm

Dora~Jean · Posted: Sun Oct 21, 2007 1:43 am Post subject:

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:

starcrafttom · Posted: Sun Oct 21, 2007 3:17 am Post subject:

the props for the cruisers where all for i/o and inboards with v-8s the smalles pitch they had was 19".

Jimbo · Posted: Sun Oct 21, 2007 7:23 pm Post subject:

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

Sea Wolf · Posted: Sun Oct 21, 2007 7:51 pm Post subject:

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.

thataway · Posted: Sun Oct 21, 2007 11:00 pm Post subject:

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.

Dora~Jean · Posted: Mon Oct 22, 2007 12:26 am Post subject:

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.