The solution might not be on the shelf for purchase, but I'll throw out what I would try.
A typical PM motor can be operated in both counter clockwise, and clockwise, directions with ease if the brush holder is in an almost neutral position. Most holders are not designed to be moved (but that can be changed) so best efficiency in either direction is a matter of advancing or retarding brush contact with the commutator pads.
Using clutch bearings and a dual pulley or gear setup, two separate drive ratios can be used by a simple reversing switch. As others have said "electric motors will only draw amperage equal to load resistance".
If a single prop can be found that offers a good compromise in performance at the operating speeds mentioned, then this setup is really easy to do with "off the shelf" parts from some motion control dealer.
I have put a lot of time into 8D batteries and high wattage inverters, 120v to 240v stepup transformers powering simple and cheap treadmill motors, also expensive well made units 4.4+hp. A somewhat low cost system is within reach, but requires knowledge of how everything works and what level of performance and duration is needed.
Efficiency is everything, but it is not a reason to run from some good selections that are much less in cost and need some attention more often. Just my thoughts.
Best wishes
Ron L
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> John,
>
> Good info. However, Neil want's to be able to run at 8.5kts with the same motor, batteries and prop as well as long distance. Like you, I can give advice on how to achieve either one of those goals, but not both with the same system.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, John Paramore <watertoyz@> wrote:
> >
> > I think you might want something like the Utility Marathon boats from
> > the early '90s. Rules limited competitors to 125 pounds of lead/acid
> > batteries...2 batteries aboard, one ashore for pit-stop change-outs.
> > The initial marathon ran 12 hours with the winner turning about 72
> > miles. The next was reduced to 6 hours, but competitors turned up the
> > amperage wick to run the same distance.
> >
> > The most successful boats were long slick hulls running 1 hp Bosch
> > premanent-magnet motors and big slow turning props. The Cloud/Mischke
> > winner ran a U-jointed shaft to make clearance for a 14 inch peddle-
> > boat propeller...think airplane prop...that turned at about 300 to
> > 500 rpm. The NOPEC runner-up initially ran a huge marine three-blade
> > wheel, maybe 10 inch diameter with lots of pitch, and used chain-
> > drive gear reduction. Later they switched to a custom made tractor
> > outboard with a three-blade aero-prop. I'll send some pictures off-list
> >
> > John
> >
> > On May 19, 2011, at 7:35 AM, F Neil Simms wrote:
> >
> > >
> > > .... What I'm looking for is a motor configuration that will drive a
> > > light, long boat (24' x 2' at WL, under 500lbs all up) at high
> > > efficiency (90%+ for the motor), with prop speeds in the 300-700
> > > rpm range. I would like this boat to be capable of both high speed
> > > (relatively, say 8 to 10 mph) at around 1 to 1.5 kilowatts, and
> > > long distance cruising at very low power levels - 50 to 100 watts....
> > > Any suggestions?
> > >
> > > Neil S.
> > >
>
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