In "Flemming Torp" fletopkanelbolle2rp.danmark writes:


"Dave" skrev i en meddelelse
...
On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long"
said:

When
the prop is freewheeling, it is producing energy

A fairly basic principle: energy can be neither created
nor destroyed.

Isn't it right, that when the boat is sailing (just for sail
that is) you need some energy to keep the propeller from
turning - i.e. to keep it fixed - and that gives a certain
drag on the boat ....

My intuition tells me that when you ' loosen the grip' on
the propeller shaft and let the propeller turn freely, it
reduces the drag on the boat. Right or wrong?

You do not create energy by letting the propellor run. In one answer
earlier it was pointed out that a stalled propellor is very different
than a rotating propellor. If there were no friction in your propeller
shaft when it is running freely the energy needed to spin the propeller
could be less than the energy needed for the water to around the
propeller blades. If you have a two blade propeller and it is in
vertical position behind your keel, the water flow behind the keel is
already somehow turbulent and the drag is fairly small. Then in most
cases it will cause less drag if it is not rotating.

There are so many facotrs involved; the sahpe of the blades, the pitcc,
the aspect ratio the shape of the rear of your boat etc. so it is
impossible for someone even very knowledgeable person to give you an
answer just by reading your description of the situation.

There are (fairly reliable) measurements of several sailors sailing and
then stopping and letting the propellor to rotate about the changes of
the speed. The problem is that in some cases it is faster to let spin
and other cases (boats) it is faster to not let to spin. In a case of a
three bladed propellor in a 40 feet Colin Archer the difference has bee
almost one knot.

So I suggest you keep on recording speed differences, there must be some
situation where you wave and weather conditions keep for more or less
constant for some 20 minutes, so you can stop and let your propeller to
roatate by putting the gear on and putting it on neutral.

You keep saying that you do not know physics. There are plenty of good
(and interesting and well written books) it would be easer to choose
from the answers of this group the relevant ones if you were not
ignorant. I can promise you that the speed of your boat is not affected
by the "votes" given in this discussion.

- Lauri Tarkkonen