Seems like there must be something wrong here.
<br><br>First, how does the calculation relate to hull
type/configuration? Theoretical hull speed on a 23' displacement boat
is going to be a lot below 8 kts. In my case,
5.8kts. I have always understood (and experienced with my
other displacement hulls) that to get 38% above hull
speed is going to take incredible amounts of additional
HP, if it's possible at all in the real world. I
don't really think that 8kts is a realistic
target.<br><br>More to the point: <br><That means that a<br> 1.4:1
Ratio would be ideal. anything less means that you are
wasting<br> power..... If your motor is not geared at 1.4:1
then you should either<br> change the ratio or select
a smaller prop.<br><br>I'm turning through a 2.4:1
pulley reduction. I had been concerned that this ratio
was too low, rather than too high! One of the major
manufacturers of electric boats (who will remain nameless but
the field is limited, right?) wrote me a letter at
the beginning of my project saying that "the
efficiency of a slow-turning propeller is what is required
to improve the efficiency of the electrical system
to a point where it is competitive in performance".
They use a 20 X 30 prop (!) and, if I recall
correctly, turn it with 10:1 reduction.<br><br>Elco
materials (NOT, I might add the mfr. referred to above,
which limits the list of possibilities
further!)illustrate a 6kts speed target, with 2:1 reduction and an
800 rpm target yields a prop with 12.1" ideal pitch.
They say "if propeller speed is more than 1000rpm
consider a gear or belt reduction to increase propeller
efficiency."<br><br>So my thinking had actually been the opposite: I
might improve performance by increasing prop size and
decreasing shaft speed--not the opposite.<br><br>I will say
that the speeds and draws I seem to be experiencing
out on the water are very similar to the calculations
presented for the Elco. But I am not at all turning maximum
rpm's on the motor and, at least with this particular
prop, find that the increased rpm's generate greatly
increased amp draw without any proportionate (although
there is SOME) increase in speed.<br><br>So...do I
optimize by slower (rpm) and bigger (prop) or the
opposite? Put that way, it sounds pretty
fundamental.<br><br><br>Larry
