nebbian said..

FreeRadical said..

However, this is all pretty much contrary to many of those boat tests, that show the rotating propeller creates less drag.

I don't think so. Your diagram and mine both show a crossover point, where the most efficient way to go is to change from freewheeling to fixed, or vice versa. They both show that a high speed prop with a high pitch will be better if left to freewheel. They also show that a low speed prop with a low pitch is better left locked. Note that the boat tests above used a high pitch prop. I bet that if they used a low pitch prop then they would have a different story to tell.

The crossover point on mine is 2/3 Pitch/Diameter. The crossover point on yours is at about 20 degrees. I wouldn't be surprised if they mean the same thing when measuring near the tip of the prop.

Yeah, I think I'm arguing with myself??

You need to manufacture a disc brake to fit onto you coupling lassarare. I spent hours thinking up such a device on my last yacht with hydrolic gearbox , went from a timber wedge to the spanner onto the coupling! Peace for a long passage but not so nice if you need to start the donk in a emergency!

Would the old cable pull brake from a push bike give enough holding grip?
Easy to mount and run the cable to have quick release in an emergancy, or in a real quick emergancy I'm thinking just start the motor and the rubber pads would just fail.
A disc would add spinning weight and harder to set up as would need a solid mount for caliper and if slightly out of aline would be a pain,

Maybe something like how prams lock their rear wheel?
It's a pin that pushes into a hole in a disc.

The advantage of this system is that you can locate the holes so that the prop would always be locked into the right orientation.


I'd use a split collar around the prop shaft, each half of the collar having a hole for the pin. Then have a stationary solenoid to actuate the pin, hooked up to the engine ignition (or separate switch if you wanted to get fancy and not destroy the pin as you turned off the engine I guess).


Doesn't seem that hard to arrange to my uneducated eye

What ever method of locking the shaft is used, from a mechanical point of view locking it has to be a good idea. A free wheeling prop is going to make extra wear on the gearbox bearings and seals.

What about a pawl and saw tooth sprocket? When the engine is driving the pawl on the correctly oriented sprocket which is fitted to the shaft will ride up and over, meaning you can start it in a emergency and flick the pawl out of the way at your leisure. When the engine is not running the prop will try and spin the shaft in the other direction and the pawl will hold it in place. Just mark the uppendicular position on the shaft, rotate by hand (engine off) and engage pawl.

nebbian said..


Are you sure that the prop spins the other way when freewheeling?

No, of course I'm not sure, I just come up with crazy ideas, other people can sort out my thinking.

Dammit, seemed like a good idea at the time ...

LooseChange said..
What about a pawl and saw tooth sprocket? When the engine is driving the pawl on the correctly oriented sprocket which is fitted to the shaft will ride up and over, meaning you can start it in a emergency and flick the pawl out of the way at your leisure. When the engine is not running the prop will try and spin the shaft in the other direction and the pawl will hold it in place. Just mark the uppendicular position on the shaft, rotate by hand (engine off) and engage pawl.

Or just one groove in the flange and a pawl that drops in like automatic gearboxes have when selecting "Park" [ automatic gearboxes have more than one groove or slot of course]. The pawl could be spring loaded and operated remotely by cable or cord.

nebbian said..
Maybe something like how prams lock their rear wheel?
It's a pin that pushes into a hole in a disc.

The advantage of this system is that you can locate the holes so that the prop would always be locked into the right orientation.


I'd use a split collar around the prop shaft, each half of the collar having a hole for the pin. Then have a stationary solenoid to actuate the pin, hooked up to the engine ignition (or separate switch if you wanted to get fancy and not destroy the pin as you turned off the engine I guess).


Doesn't seem that hard to arrange to my uneducated eye

I think I am going to go with this idea.

I will insert an aluminium disc, with two opposing holes on the outer edge, between the coupling with the solenoid mounted to the gearbox or brace.
I must have the ignition switched on for the engine stop solenoid to work so there is no chance the pin will be sheared off.

When the ignition is on & engine running the solenoid pin is retracted & when I'm finished with the engine & ignition off the pin is relaxed & up against the disc waiting til the shaft rotates & the holes line up with the pin. Shaft is locked with the prop lined up with the skeg & when I hit the ignition to start the engine the pin is retracted.

Foolproof .....................................I hope!!