Thought experiment

[Rob_Webb]

How about this?

Like you I've also always been confused by this. Also agree with others that over-riding consideration is to reduce noise and wear.

But reading this made me think about about the issue in another way. Try this.....

Imagine boat sailing along slowly (say 3-4 kts) with engine off. If there was a pressure guage inserted into the thrust bearing of the prop shaft it would show negative pressure i.e. the movement of water would be trying to gently pull the prop and it's shaft back out through the hull. This represents high drag on the boat.

Now start engine and engage forward drive. If you increased to full power the pressure guage in the prop shaft would show positive thrust and the boat would accelerate.

But as you gradually increased power from neutral to full power the thrust in the prop shaft guage would pass through a neutral stage as it transitioned from negative thrust (when prop was trying to drag itself back out thru hull) to positve thrust (accelerating boat).

My thinking therefore tells me that regardless of why the prop is turning (engine or water flow) the drag is gradually reducing from a maximum (when the prop is stationary) to a minimum (when it's pushing the boat along). And so when under sail alone, if the prop is freewheeling (even slowly) due to water flow, then it has got less drag than when stationary.

So locking the shaft will slightly increase drag again.

Any sense?

Of course the simplest answer is just to get a folding/feathering prop. Then the whole problem goes away!

 

[Lakesailor]

Re: How about this?

If you go back a couple of posts I think my solution gives a practical method that takes in account all other influences. It would measure the net effect of locking the prop.

 

[ChrisE]

Just experiment

Also being simplistic, I wonder why we repeatedly ask this question. If it bothers you why not try out sailing with revolving and not then get an answer?

I know for a fact that my boatie goes faster with locked prop but then I have a folding prop.

 

[Vara]

Re: Just experiment

Hear hear,bollocks to the theory,go empirical,suck it and see, not difficult to do.Calm day get boat up to reasonable speed turn off propulsion and note time taken to get down to a lower speed, propellor locked and then propellor freewheeling,repeat several times.If any significant difference go with that.If no difference then lock prop to stop wear on eardrums and cutless bearing!
FWIW I think that prop locked is better for boat speed and wear and tear.
If you go down the Rogers route of locking off prop with mole wrench, tie ignition keys to handle of same to prevent starting without removal.
When I was young and keen about these things when boat was out of the water I lined up the prop with the dead wood and marked the shaft so that it lined up underway.