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Thanks for initiating the discussion on hull
speed and, conveniently, using my launch as the
example! Before building the boat I performed much the
same calculation and arrived at much the same result
as to theoretical maximum hull speed. The designer
specified the speed as "up to 6 knots". The hull speed is
actually just about 6 kts.<br><br>I have always
understood, however, that "hull speed" was not quite as
inflexible as you describe. My various displacement boats
over the years have always been able to exceed hull
speedeven the ones I rowed!<br><br>I think of it more like
riding a bike. While youre on the flats it takes more
power to go faster but the application of that power
generally results in a fairly rewarding addition of
speed.<br><br>When a boat hits its hull speed its like encountering
a hill on the bike. (Its actually very much like
this since the displacement hull is in effect trying
to climb the hill of its own bow wave as it tries to
increase speed, right?) So as you try to peddle the bike
faster AND uphill it takes quite a bit more effort for
even a little increase in speed. And for our
displacement hull its as if the hill gets steeper with each
little increment of speed you try to add. At some point
you are working VERY, VERY hard to go even just a
little bit faster. Just aint worth it.<br><br>This
particular thread started because of my stating that I had
installed a prop that was less than ideal (I think) and was
wondering what effect a change of prop or shaft rpm would
have on performance. But, like most of us electric
boaters, the goal is not so much faster as FARTHER. If a
change of prop/shaft speed could convert my most
efficient cruising speed from 4 kts. to 4.4 kts with the
same amp draw, then Ive added significant miles to my
range.<br><br>There are some photos of the boat at:
<a href= target=new></a> which were taken at the launch in Aug. I have
some later photos with the boat underway, under power,
but, unfortunately, I need to find a friend who can
scan them for me. <br><br>Larry

Hello Larry, I looked at the pictures "great
job"...<br>One direction you may take to improve the efficiency
(cruise speed and range) of you launch is to look into a
more efficient propeller once you determine the right
size and pitch relative to shaft speed. Most of the
large Prop/Wheel manufactures (Mich. and Nat.) design
props to be spun faster then electric installations
"normally" provide. 1400 rpm for a 14~16 prop is just
breaking into the 60% eff range, below this <1000 rpm
eff. can drop to 40%. (this means you only get 40% of
your power to the prop). The standard recommendation
in the e-boat circles (and diesels) is to turn a
larger prop slower to gain efficiency. For this to work
out though you need a prop designed to be turned
slower to start out with. Sailboat props, steam powered
props, small diesel work boat props are (some not all)
designed to be turned at lower rpm's, much closer to the
proportions needed for electric power. But, the first thing
you need to know is: Is the motor
over-loaded,un-loaded or right on for amp draw and rpm (motor
amps).<br>Holiday Greetings and Regards,<br>Joe