Winch power and torque

The word ‘power’ is, in my opinion, misused in Chris Beeson’s otherwise excellent article on modern easy-care winches (YM, August) and the literature of some winch manufacturers.

A statement in the article reads:

“The winch number refers to its power ratio…”

This implies that a winch with the number 4 would give 4 times as much power at its output than that input by a crew member, a truly magical device which would earn its inventor a lot of money! Power is the rate at which work is performed. In the case of a crew member input to a winch this is (torque × rotational speed), torque being the tangential twisting force on the winch handle times the length of the handle. The term mechanical advantage would be a correct term to use. So if a winch number is 4, with a given length of winch handle and drum size, then a force of 10 lbf on the handle would give a sheet pulling force of 40 lbf, ignoring any small mechanical losses within the winch.

Another statement reads if you replace a 10″ handle with a 12″ one, then you will get 20 percent more power. Not so, you will get more torque and hence more sheet pulling force. But the sheet will be pulled in more slowly for a given amount of input power.

Although my comment is perhaps a nit-pick, I would like to think that the correct terminology would be used in such a prestigious yachting magazine as YM!

Roger Castle-Smith, Chartered Engineer