Controlable Pitch Prop

Maybe the answer is a folding prop where variable folding also gives effective variable pitch.

 

Sorry for starting the thread then running away but Easter family responsibilities called. Mr. Efficiency, the greatest feature of being able to manually change the pitch on the blades while underway is to properly load a diesel engine. Diesel engines do not like light loads at low rpm. (motorsailing)It causes carbon build up within the engine and can greatly reduce the it's life. With a variable pitch prop you can vary the pitch to better match the engine loading and rpm., both as a tool for motorsailing and fine tuning the drive efficiency at any load or speed. This can not be accomplished with a feathering or fixed prop set up. One byproduct of CPP/VPP is the elimination of the marine gear(transmission). However having one does possibly allow faster forward to reverse shifting and in some cases would replace the need for a reduction gear. There are forum members that could better explain and have posted the operational set up in more detail, but this is a general drift of what it is all about.

 

Reversible also includes feathering, though ? Did you mean folding, in that context ? The idea has several aims then, including fine tune the prop so the engine runs more happily, so it is always revving busily and loaded, and have less resistance than a fixed prop under sail only, all of which makes you wonder why the market has little or nothing to offer.

 

I'm sure the issue is the cost of a variable pitch prop.
There is a company that sells one for ski boats, it costs $2,000
which is about 10x what a fixed prop costs.
It is only a 'two speed' prop also, increases pitch at a given rpm,
so a ski boat can pull skiers up with more torque, but achieve a higher top speed when it adjusts. Great product, if it where only maybe 2x the cost, might be more common.

 

No not folding just feathering. Basically the same idea as used in aircraft that are able to change pitch and feather if the engine shuts down to prevent drag. One would think it would be a commonly available system as it was at one time for smaller craft in Europe but it seems even there world markets have reduced the demand. Possibly and i'm hoping so, high fuel costs demanding more efficient smaller craft drive systems as well as battery driven craft will promote a comeback. It would be great to locate a reasonably priced unit for my build. It would be the ultimate motorsailer propulsion set up.

 

There was a sterndrive being touted with a two-speed gear option I have seen somewhere, but yes, the cost is always a deterrent.

 

Do you lose a lot efficiency-wise by dropping the gearbox with a reduction gear ? That kind of cancels the extra cost of the variable prop.

 

And there my good man you have hit the nail on the head. No efficiency is lost but more likely gained with less gears turning in an oil bath especially where no reduction gear is required. I'm getting a bit out of my knowledge zone now but if I recall in many situations the reduction gear is not necessary resulting in another positive of weight saving.
Well gentlemen at almost 2 am here I bid you good night --reminds me of my HAM radio days ---Geo.

 

Goodnight, I am still a little sceptical that losing the reduction would be OK in all cases.

 

Quite right too. The reduction gear is a torque multiplier. This enables you to swing a bigger prop at a slower speed. IMO this is still necessary for boats moving at displacement speeds. Ski boats etc - no idea and no interest either.

PDW

 

I'm all for them, they are great.
Very effective on a motor sailor

 

I think I would stick with the reduction box for most slow boats, but for slow-revving engines it might not be that important.

 

>This of course obtaining max. drive efficiency and motorsailing without the worry of carbon.<

The perils of underloading are well known . More frequent oil changes are required .

A >cruising prop< large diameter and bigger pitch can help bring the load to the engine at lower RPM.

After all most motor sailors never operate at max rpm like a ski boat .The tach will need to be marked for the new max RPM.

>Use a two blade prop and the old vertical behind the keel trick to reduce drag. However there is still a lot of drag plus the fact loss of drive torque when compared to a three <

A large diameter 2 blade prop is more efficient than a 3 or 4 blade as it has less induced drag.

If it has blade enough area for the HP being used thats it , extra area is only more drag.

These CAN be set up almost no drag with a prop brake behind the deadwood.

It is dangerous set up with two wrenches to sail at 5 or 6K and twist the prop back and forth to find the position where it doesnt want to turn , and all you are paying for is prop surface area drag. Mark the shaft , set the prop brake for that angle of the shaft.

The cruising prop with higher pitch helps here too as the high pitch is easier to drag thru the water.

The usual hassle with sail folding or feathering props is they are designed for minimum drag under sail, for the racers.

That they sort of function under power is about it, although advertising BS will claim each is Gods Gift to propulsion.

Perhaps the HYDE ( LUKE co) prop , designed for propulsion under power that also feathers would be a better deal? M/S were more common in the 1920 era .

http://www.peluke.com/marine-hardware/boat-props/

These are expensive , but very robust and CAN be factory rebuilt.

On a budget I would opt for a large diameter 2 blade cruising prop, with wide enough blades to accept the max HP the engine can produce.Check Skeenes.

You may need a bigger than 2-1 reduction to spin an 18-20 inch wheel.(My guess)

I would install a prop brake , and look for a used Hyde .

The Hyde will be older so may require a fatter shaft than is common today.

Happy hunting.

 

For controlling engine load, I suspect that a multi-speed gear box is more practical. Might even be be more efficient than a CPP (which also has losses).

 

You mean an automotive box ? It still leaves the prop dragging when on sail alone, though.