On Fri, 28 Jul 2006 02:36:00 GMT Brian Whatcott
) wrote:

Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Thrust depends on the propeller dimension. Bollard pull (Google gives
plenty hits) is commonly used with tugs, but conditions have to meet
minimal requirements to measure.

My previous boat was (minimal) faster with its 11 hp then a identical
boat with 16 hp. Bollard pull with my boat give a 150 kg, the boat with
16 hp 100 kg.

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

Thrust and boat speed don't behave proportional, why else bollard pull
for tugs.

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.

Possible indeed. I think efficiency is best tested by measuring the
supplied power at different speeds, but requires an ammeter.

--
Richard
e-mail: vervang/replace invalid door/with NL.net
http://web.inter.nl.net/users/schnecke/