Electric Propulsion
 

Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?

Electric Propulsion
 

Paul Squire says:

Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?

Yes, but it involves speed as a variable.
Be very careful with manufacturer's figures for thrust, as it is usually taken
at very low (if any) speed, where the thrust is highest.

Steve

Electric Propulsion
 

On Tue, 16 Sep 2003 22:51:57 +1200, "Paul Squire"
wrote:

Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?


Yes, ignoring scaling constants:

Horse power = thrust X speed

Brian Whatcott Altus OK

Electric Propulsion
 

I have no experience with Electric Propulsion, but have seen some
interesting technical info in the Vetus catalog. They also describe
their system at:

http://www.vetus.com/frame-cat.htm

then select the Electric Propulsion page.

MW

Electric Propulsion
 

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?

Jim Woodward
www.mvfintry.com
Brian Whatcott wrote in message . ..
On Tue, 16 Sep 2003 22:51:57 +1200, "Paul Squire"
wrote:

Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?


Yes, ignoring scaling constants:

Horse power = thrust X speed

Brian Whatcott Altus OK

Electric Propulsion
 

Jim:

You've described the conversion of force into power units - and it is all
OK. You'll note from your equation that thrust-per-hp is inversely
proportional to speed. You can generate more thrust at low speed. But if you
approach zero speed, you get infinite thrust - which, of course, means we're
missing something.

When considering the relationship between a propulsor's developed thrust and
its absorbed power (from an engine or motor), you have to introduce a number
of different efficiencies. In somewhat simple terms, from engine/motor power
to applied thrust, you pass through mechanical energy losses (mostly
friction and heat in shafting and transmission), propulsor energy losses
(such as friction and non-useful rotation of some of the water), and thrust
application losses (where the propulsor's suction side actually creates a
detrimental "suction" on the hull).

So, your thrust needs to be reduced to account for these losses - even for
small electric trolling motors and propellers. You'll need to include the
efficiency, and these overall system efficiencies also change with speed. At
running speeds, a good typical system efficiency is 65%. As speed approaches
zero, however, efficiencies also approach zero to keep the original
relationship from getting out of hand.

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
email: dm~AT~hydrocompinc~DOT~com



"Jim Woodward" wrote in message
m...
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?

Jim Woodward
www.mvfintry.com

Electric Propulsion
 

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 :-)

"Paul Squire" wrote in message
...
Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?

Electric Propulsion
 

SI may be a nuisance to some folks - but it shines here, because there
is NO scaling constant in this relation:
power (watts) = force (newtons) X speed (meter/sec)

Now let's put in all the scaling constants to US customary units
(Can't say British, or Imperial, any more...)

1 watt X 1HP/746watts =
1 newton X 1kg/9.81newtons X 2.2 lb/kg X
1 meter/sec X 39.37 inch/meter X 1ft/12 inches X
1 mile/5280ft / (1min/60 sec X 1hr/60 min)

And this boils down to
0.00134 HP = .2243 lb X 2.237 mph
or even more succinctly:
1HP = 374 lb X mph
(Your solution gives 6.25 X 60 = 375 lb X mph - close enough)

As James Watt defined the HP as a horse capable of exerting a force of
550 pounds at a speed of one foot per second (which is about 50% more
than a good horse can do for a shift, without then falling down dead)
your figure of 5.5 lbs at 100 ft/sec is *exact*

Brian Whatcott Altus OK

On 16 Sep 2003 11:22:30 -0700, (Jim Woodward)
wrote:

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?

Jim Woodward
www.mvfintry.com
Brian Whatcott wrote in message . ..
On Tue, 16 Sep 2003 22:51:57 +1200, "Paul Squire"
wrote:

Is there a simple equation between lbs thrust (as specified in electric
outboard motors) and Horsepower?


Yes, ignoring scaling constants:

Horse power = thrust X speed

Brian Whatcott Altus OK

Electric Propulsion
 

"Stephen Baker" wrote :
Don McPh says:

snip technicalities

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

;-)

Actually, I asked if a simple formula existed. I had a sneeking suspicion
that the answer would be no. I often recall the words of a colleague I was
working on a bank's data model with almost 20 years ago: "For every
complex, intricate & detailed problem there is a solution that is simple,
concise, to the point, and wrong."

I am pleased with the technicalities in the responses to my question as I am
always keen to learn. And I have now learned that thrust can only be
compared to HP if distance & time are considered.

I haven't quite reached full understanding of the responses and will think
on them more. In the meantime, I'd be grateful if someone who did have a
good understanding would be kind enough to do the calculations for the
particular case I have in mind.

The boat is a Monarch 17 foot trailer sailer. In cruising trim it displaces
2500lb on a waterline of 15'6"(L) x 5.4"(B) x 1'0"(D). On flat water 2HP is
adequate, 5HP would be normal, but if I ever expected to find myself trying
to motor off a lee shore into 30 knots I think I'd want 8hp. Lets use 5hp
for this exercise which equates to 3250 watts. Unfortunately makers of
electric trolling motors rarely specify wattage, preferring to specify lbs
of thrust. Trolling speed is 3-5 knots which is also the speed that the
Monarch would motor at so we're lucky there.

My gut feel is that we'd be looking at an electric trolling motor designed
for a 25-30' inboard launch. This is larger than the Monarch but the
trolling motor is not expected to push the vessel off a lee shore - the main
motor would be called in to play for that. I have no science to back up
this guess and do not know what thrust the manufacturers would recommend for
this sized launch but it feels about right.

So, If I've understood the technicalities, the variables necessary to
calculate the thrust required a

5 Hp at 4 knots. = ? lbs thrust.

Paul Squire.

Electric Propulsion
 

"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).

Electric Propulsion
 

"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...

Electric Propulsion
 

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.......

Electric Propulsion
 

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

Electric Propulsion
 

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".......

;-)

Electric Propulsion
 

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

Electric Propulsion
 

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

Electric Propulsion
 

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

Electric Propulsion
 

"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?

Electric Propulsion
 

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?

Electric Propulsion
 

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?