"Roger Long" wrote in
:

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.



But the point is, as I load the shaft alternator, and therefore the
freewheeling prop, the boat slows more and more as the load increases,
terminating in worst case drag as the prop stalls and stops.

As the load decreases, and the prop turns faster, speed increases. If you
take this beyond the natural drag of the transmission and cutlass bearing
limiting prop speed, by adding slight power to it, there comes a point at
which the prop crosses zero drag trying to pull the shaft out and starts
pushing harder and harder on the shaft as speed increases, where drag goes
through zero and turns into THRUST.

I've never seen it plotted. It might not be a linear function but appears
not to have any weird peaks or valleys in the curve....
|\
D | \
R | \ It's not linear, I'm sure.
A | \ Lots more physics going on.
G | \ Somewhere it stalls.
| \
|------\--------------"freewheeling drag"
0 PROP SPEED - (prop turns = boat speed = no drag here)
| \
T | \
H | \
R | \
U | \
S | \ Down here, somewhere it cavitates.
T | \

God I hate text graphing....(c;