You can not calculate HP from thrust alone. Horsepower is proportional to
force (thrust) times speed. One HP = 550 ft-lb/sec. For example, if an
electric motor can push a boat with a thrust of 35 lb. at a speed of 5 mph
(7.4 fps) the horse power delivered to the boat is:

35 x 7.4/550 = 0.47 HP

Actual motor power would be higher since there are losses from friction,
prop slip, etc.

Unfortunately, I suspect the manufacturers rate the thrust at zero speed
(highest thrust value) so you can't calculate motor HP.

"bowgus" wrote in message
le.rogers.com...
Question ... when I did my 36 lb Min Kota calculation afew years back,
the
answer I got was about 1/4 hp. Would you mind sharing your calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs Minn
Kota Endura motor, about the same thrust as a 2.0 or 2.5 hp gas
engine... OK, it doesn't plane. I'm not so sure it would plane with the
maximum 3.5 hp motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle, you
don't have to mix oil in the gas can, and their exhausts smell better.
I
use a no-cycle outboard on my inflatable: an electric trolling motor.

I like Mr. Ohms method (used by Loyd) ... convert watts to hp, e.g. ... the
35 lbs thrust Minn Kota is rated 34 Amps max so assuming 12V across the
motor = 408 watts (P=IxE). And converting that to hp by x 0.00134102209 :-)
= 0.55 hp. The calculation for my 55 lb Minn Kota is: 45 Amps x 12V = 540 =
0.72 hp. So good old ohms law calculations say 0.72 hp is the absolute max
(as Loyd pointed out, there's gotta be some mechanical loss in there
somewhere) ... let's call that good old "K".

So hp = f (((I*E)*1.34E-3) - K) :-)

Thanks Loyd, that works for me.


"Bill Sc" wrote in message
om...
You can not calculate HP from thrust alone. Horsepower is proportional to
force (thrust) times speed. One HP = 550 ft-lb/sec. For example, if an
electric motor can push a boat with a thrust of 35 lb. at a speed of 5 mph
(7.4 fps) the horse power delivered to the boat is:

35 x 7.4/550 = 0.47 HP

Actual motor power would be higher since there are losses from friction,
prop slip, etc.

Unfortunately, I suspect the manufacturers rate the thrust at zero speed
(highest thrust value) so you can't calculate motor HP.

"bowgus" wrote in message
le.rogers.com...
Question ... when I did my 36 lb Min Kota calculation afew years back,
the
answer I got was about 1/4 hp. Would you mind sharing your calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs Minn
Kota Endura motor, about the same thrust as a 2.0 or 2.5 hp gas
engine... OK, it doesn't plane. I'm not so sure it would plane with
the
maximum 3.5 hp motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle,
you
don't have to mix oil in the gas can, and their exhausts smell
better.
I
use a no-cycle outboard on my inflatable: an electric trolling
motor.