Sail in Forward, Reverse, or Neutral?

[elcap373]

I have a Hurth ZF 10M transmission on the 24 hp Universal diesel in my Newport 33. The ZF 10M manual indicates the transmission should be in forward or reverse when sailing – not in neutral. I would think it would be advantageous to sail with the prop in neutral to let it free-wheel and reduce drag. Is it truly necessary to operate under sail in with the transmission in forward or reverse?

[jrd22]

The prop shaft free wheeling turns the output shaft of the transmission but in most transmissions that does not provide lubrication or cooling which can do a lot of damage.
We put ours in reverse to prevent it from spinning (Volvo).

[rocinante33]

In neutral, the trans may not lubricate as it should. Since the input shaft is not turning (only the output shaft) the oil in the trans may not circulate to the bearings.

Also, the freewheeling prop is not really less drag than a prop which is stopped. I know it is counterintuitive, but the spinning, freewheeling prop is more drag.

[tager]

That is counter to my intuition, can you explain why the spinning prop presents more drag?

[Zanshin]

tager - coming from flying this was also counterintuitive for me. This topic is a popular one and there have been a number of studies done that come to different conclusions. The one that I found to be rigourous in method was in a study done at the University of Strathclyde and can be found in pdf form at http://strathprints.strath.ac.uk/567...ints005670.pdf

[pvanv1]

The Hurth will not be damaged by sailing in Neutral. One of the few transmissions where that is the case.

[newpbs]

I'm not sure which Hurth I have but the manual specifically states that the transmission should be in reverse when sailing.

Paul

[nickmerc]

It is correct that a spinning propeller creates more drag than a stationary one. I too fly airplanes and it is very well demonstrated when your engine quites. You will glide significantly further in a fixed pitch propeller airplane if the prop is stopped.

Also, weather the bearings receive lubrication or not, there is no need to turn the shaft when not producing power. It will still wear the bearings needlessly.
________
How To Roll Blunts

[k1vsk]

I'm not sure boat props follow the same theory as those on airplanes. Being a compressible fluid, the analogy to that of airplane props (in air) falls apart and if you were to substitute the dynamic properties of water vs. air in the equations of the Styrathclyde example, the coefficient of drag changes inversely.

Regardless, this issue has been debated and argued for decades and so-called experts constantly disprove each other. Seems to me you should either get a folding/feathering prop if drag is of great concern or risk the transmission by not following the mfg recommendations. Your choice.

[MSN2Travelers]

I crew on a Pearson Flyer and the manual for the boat indicates the transmission should be in reverse too.