Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.

Roger Long


I will second Roger on this. The dinghy experiment would be valid on a
very naiive level of thinking and the experiment with the fishing lure
is invalid because the boat propellors is NOT freewheeling. There is
quite a bit of friction involved. In some cases the gearbox get too hot
to hold a hand on it because the transmission fluid is not pumped around
and while lubricating it is also transmitting the heat away.

- Lauri Tarkkonen


Actually, the dingy experiment is on the right track. It seems to me
that the energy dissipated in the prop/shaft/transmission/oil train is
irrelevant. That is energy that *would* had been dissipated in
redirecting water flow. Look at it simply; you have *one* energy source,
and that's the water 'stream' past the prop (not quite accurate of
course, since the overall drag will affect the 'stream' velocity, but
it's an easier way to envision it). The amount of energy extracted from
that source will be the result of 1) frictional losses, 2) inertial
forces, i.e. energy required to redirect the mass of water striking the
prop surfaces (not straightforward to calculate), and 3) impact forces
(momemtum, if we're thinking in terms of a 'stream'). Now, the only
issue is whether the drag at the prop is more or less when fixed. So
*if* the freewheeling prop has less drag than the fixed prop, then so
will the turning prop attached to the prop/shaft/transmission/oil train.
To the extent that energy is dissipated by the drive train, the prop
will be rotating slower, and the system will behave somewhere in between
fixed and freewheeling.

Now, whether the fixed or turning has less drag, under any particular
set of parameters, I don't know. I suspect it's a wash. Clearly a
totally freewheeling prop would have less, as the impact forces are
reduced, as is the angular deflection of the water stream, leaving
mostly frictional losses. I suspect though, as appears the case on my
Catalina 30, that the prop when turning significantly slower than it's
pitch rate (i.e. dragging the shaft and trans with it) creates nearly
the same drag as the fixed prop.

Keith Hughes