"bowgus" wrote...
Seems to me I once calculated 32 lbs is about 1/4 hp ... if I find the
equations, I'll post ... but don't hold your breath :-)

I recall once I tried figuring it on my MinnKota 42 once, and got about 2/3 HP.
I, also, don't have the calculations handy...

John R Weiss wrote:

"Jim Woodward" ...
Since one HP is 550 ft-lbs per second, that suggests that one hp is
5.5 pounds thrust at 100 feet per second or 6.25 pounds at 60mph
(statute miles, here).

That feel OK, but my college days are in the distant past -- any
comments?

Brian Whatcott wrote...

Yes, ignoring scaling constants:

Horse power = thrust X speed

It appears there are a few means of conversion...

My MinnKota EX42 is advertised to put out 42 lb thrust at 12V and 36 amps. What
is that in electrical power consumption, converted to HP?

It also pushes my 21' boat at about 2.5 - 3 knots at full thrust on flat water.
What is that in power output or work done?

I don't have my calculator and conversion constants handy, but at first glance
the electrical power draw appears considerably higher than mechanical power
output or work. OTOH, maybe it isn't pulling all 36 amps when pushing the boat
at max speed (I assume the prop RPM is electronically governed).

that works out (at 100% conversion efficiency) to .57 hp. figure the motor at about
85% efficiency (at best). and you are at just under 1/2 hp.


--
Beer, it's not just for breakfast anymore.......

What no-one has considered here is that powering calcs are usually done from
the other point of view:

My boat has X lbs of drag at 5 knots, how many horsepower do I need?

Factor in windage (the lee shore situation), foul hulls, appendage drag, prop
efficiency (an oxymoron if ever I saw one) and other such truck like Taylor
Wake Fraction, and you have a real problem on your hands.

Doing it in reverse? Priceless....

Steve

Stephen:

In this biz, that's as simple as it gets...

We have worked for many of the small trolling motor and propulsion equipment
companies, so here's some more insight into trolling motors.

1. Trolling motor drives are typically less than 1 hp. One of the most
powerful is Minn Kota's e-Drive at 2 hp (see www.minnkotamotors.com and look
for the motor spec chart).

2. Thrust figures are indeed for maximum thrust at low speed (as one would
expect from their principal market - recreational fishing). These products
(including the propellers) are not typically designed to push boats at
higher speeds.

(As an aside, we designed a custom prop for one of the mid-90's AC
syndicates to use in side-by-side testing. They wanted to match drag figures
as closely as possible, so they were looking to add some 30-35 lbs thrust to
one hull. The existing trolling motor props lost all of their thrust as boat
speed increased, so we did a prop with much higher pitch to get a better
speed-rpm-pitch match where they needed to be. Who knows, there might be a
nice market for replacement props with higher pitch for small boats.)

3. Unlike gas or diesel engines, these electric motors have their highest
torque (and power) output at lowest RPM. This further complicates the
ability to run at higher speeds, since an increase in revs (associated with
getting to higher speed) doesn't buy you more available power.

4. If you need to get over 2 hp into the 5-10 hp range - and you've got some
deep pockets - you can consider the Briggs & Stratton gas outboard or a new
elec outboard from eCycle (www.ecyclemarine.com). There are other elec
outboards on the market, as well.

Regards,

Don

"Stephen Baker" wrote in message
...
Don McPh says:

snip technicalities

Don, The OP asked for a "simple formula".......

;-)

Don McPh says:

Stephen:

In this biz, that's as simple as it gets...

snippage...

I know, Don, I know... ;-)

Steve
Stephen C. Baker - Yacht Designer
http://members.aol.com/SailDesign/pr...cbweb/home.htm

On Tue, 16 Sep 2003 17:56:16 -0700, "John R Weiss"
wrote:

"Jim Woodward" ...
Since one HP is 550 ft-lbs per second, that suggests that one hp is
5.5 pounds thrust at 100 feet per second or 6.25 pounds at 60mph
(statute miles, here).
//
Brian Whatcott wrote...

Yes, ignoring scaling constants:

Horse power = thrust X speed

It appears there are a few means of conversion...

My MinnKota EX42 is advertised to put out 42 lb thrust at 12V and 36 amps. What
is that in electrical power consumption, converted to HP?

It also pushes my 21' boat at about 2.5 - 3 knots at full thrust on flat water.
What is that in power output or work done?

I don't have my calculator and conversion constants handy, but at first glance
the electrical power draw appears considerably higher than mechanical power
output or work. OTOH, maybe it isn't pulling all 36 amps when pushing the boat
at max speed (I assume the prop RPM is electronically governed).

12 V X 36 A = 432 W
432 W X 1HP/746W = 0.58 HP

IF notice IF 0.58 HP is the electrical rate for 2.5 kt ( = 2.5kt X
1NM/hr / 6080ft/hr X 1hr/60min) = 253.3 ft/min
\so the required HP at 100% efficiency is 253.3/550 HP = 0.46HP

So the efficiency might be 0.46/0.58 (power out/power in) = 79%
which is rather high.....

Alternatively,

42 lb thrust from 0.58 HP would give at most 7.6 ft/min
which is 0.075 kt. Hence, unsurprizingly, the maker is quoting
quasi-stationary testing.

Brian Whatcott Altus OK

On 17 Sep 2003 02:52:45 GMT, ospam (Stephen Baker)
wrote:

What no-one has considered here is that powering calcs are usually done from
the other point of view:

My boat has X lbs of drag at 5 knots, how many horsepower do I need?

Factor in windage (the lee shore situation), foul hulls, appendage drag, prop
efficiency (an oxymoron if ever I saw one) and other such truck like Taylor
Wake Fraction, and you have a real problem on your hands.

Doing it in reverse? Priceless....

Steve


Once you seize the idea that if a vessel (of any sort) is not
accelerating wrt the water mass, then the total drag = total thrust.

Brian Whatcott Altus OK

"D MacPherson" wrote...

(As an aside, we designed a custom prop for one of the mid-90's AC
syndicates to use in side-by-side testing. They wanted to match drag figures
as closely as possible, so they were looking to add some 30-35 lbs thrust to
one hull. The existing trolling motor props lost all of their thrust as boat
speed increased, so we did a prop with much higher pitch to get a better
speed-rpm-pitch match where they needed to be. Who knows, there might be a
nice market for replacement props with higher pitch for small boats.)

YES! There is definitely a small but viable market! There are a BUNCH of us
with 21' SeaPearl sailboats who use various electric motors as auxiliaries. I
am almost positive I could get significantly more speed with a higher-pitched
prop, but have found nobody who makes one. Any chance of getting one made for a
MinnKota EX42 for a trial?

John:

Let me think about it for a bit. Can you define "BUNCH" for me? How to
approach this will depend to a certain extent on the potential numbers
involved (i.e., one-off vs production).

Regards,

Don

"John R Weiss" wrote in message
...
"D MacPherson" wrote...

(As an aside, we designed a custom prop for one of the mid-90's AC
syndicates to use in side-by-side testing. They wanted to match drag
figures
as closely as possible, so they were looking to add some 30-35 lbs
thrust to
one hull. The existing trolling motor props lost all of their thrust as
boat
speed increased, so we did a prop with much higher pitch to get a better
speed-rpm-pitch match where they needed to be. Who knows, there might be
a
nice market for replacement props with higher pitch for small boats.)

YES! There is definitely a small but viable market! There are a BUNCH of
us
with 21' SeaPearl sailboats who use various electric motors as
auxiliaries. I
am almost positive I could get significantly more speed with a
higher-pitched
prop, but have found nobody who makes one. Any chance of getting one made
for a
MinnKota EX42 for a trial?

I would guess initially a dozen who might take an exploratory plunge, with a few
dozen possibilities after that.

I've copied this to our e-mail group, so I might get a better idea of immediate
interest.

Pearlers: How many might be interested in a source for a high-pitch prop for
the electric motors on their SeaPearls?
------------------
John Weiss
Seattle, WA
remove NOSPAM from reply address

"D MacPherson" wrote in message
...
John:

Let me think about it for a bit. Can you define "BUNCH" for me? How to
approach this will depend to a certain extent on the potential numbers
involved (i.e., one-off vs production).

Regards,

Don

"John R Weiss" wrote...
"D MacPherson" wrote...

(As an aside, we designed a custom prop for one of the mid-90's AC
syndicates to use in side-by-side testing. They wanted to match drag
figures
as closely as possible, so they were looking to add some 30-35 lbs
thrust to
one hull. The existing trolling motor props lost all of their thrust as
boat
speed increased, so we did a prop with much higher pitch to get a better
speed-rpm-pitch match where they needed to be. Who knows, there might be
a
nice market for replacement props with higher pitch for small boats.)

YES! There is definitely a small but viable market! There are a BUNCH of
us
with 21' SeaPearl sailboats who use various electric motors as
auxiliaries. I
am almost positive I could get significantly more speed with a
higher-pitched
prop, but have found nobody who makes one. Any chance of getting one made
for a
MinnKota EX42 for a trial?