Let's try the whole thing again. In a completelyideal world.
Taking first the prop itself, an obvious conceptual model is a screw. In
an ideal case, one rotation of the screw advances the whole thing some
distance in the axial direction, into the wood (water). That distance is
the pitch. At a constant rpm, the higher the pitch, the greater your
velocity. At a constant velocity, the higher the pitch, the higher the
rpm. [Hmmm, you're right. Sounds a lot like a gear.]
As for the engines, they invariably have a power output which, as rpm
increases, starts low, grows to the maximum, and then falls off slightly
until max rpm is reached. Note, however, that this nice curve represents
_maximum_ power available at a given rpm; the _actual_ horsepower needed
to maintain that rpm is a function of the load. More load (e.g., a
headwind) requires more horespower.
Staying in the ideal world, it'd be nice if our engine could always
operate at or near the rpm at which its max horsepower is available,
regardless of speed. And that's what a "transmission" tries to do.
Since we don't have transmissions, you get to pick the boatspeed at which
you'd like to have maximum power available and then determine the prop
pitch which would make that happen. Picking that boatspeed is not as
simple as it appears at first blush, even in the ideal world. Do you
really want to be at max speed in conditions which require max power? Is
less than max power enough to attain your max speed,whatever it is? [nice
article BTW, I'm gonna read it again] And on & on. Not to mention
departure from all these idealizing assumptions!
One way to get around the dilemma is to use a bigger engine so that extra
power is available at any rpm. Hence the use of 8 or 9.9 hp engines on a
P19. Another way is to figure that you'll only motor in & out of slips
and in calms; even a headwind is wind & you'll just _sail_ that dadgum
sailboat. Thatapproach would call for 2 hp or less (electric?) on a P19.
Or anywhere in between.
Not that it matters to us. Usually the very small outboards appropriate
for Pottering have only one or two prop choices ... and they're mostly
plastic, so that you can't even give many $ to your local prop basher to
reshape things.
And all that produces the rule-of-thumb: get the lowest pitch prop
available for your engine. Nobody wants to go slower than us!
Regards,
Bill Combs
WWP 19 #439 (August 1987)
"Ursa Minor"
Fort Walton Beach FL
ttursine@gnt.net