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Old 09-01-2005, 12:08 PM   #1
jimh
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Default 101 lb thrust equals how many HP?

So I was thinking of adding a trolling motor or kicker to my driftboat for the times that I go through very slow water. I might even use it to run up river a bit. But, I have a question, 101 lb thrust equals how many HP? If even a 101 is relatively weak, it might be better to buy a small kicker and not deal with the weight of the batteries. Afterall, you can buy 2.5 to 3.5 2-strokes for about the same amount as just the trolling motor and they only weigh about 30 lbs.

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Old 09-01-2005, 12:33 PM   #2
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Default Re: 101 lb thrust equals how many HP?

about 3-4 hp...I had one and it worked ok if it was not windy at all..might as well get the gas powered motor with the integrated fuel tank. The electric needs a battery and you'll be about the same weight either way. The only caveat is if you fish places that don't allow motors, like the Nestucca.
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Old 09-01-2005, 12:50 PM   #3
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Default Re: 101 lb thrust equals how many HP?

We used to push our koffler wide bottom around with a yahama 4 hp. 2 cycle with the built in tank, and it worked great for what we were using it for. We took an extra gallon of gas, and that was all we needed for a full day of trolling on a high lake. We also put an older minn kota electric on for non-motor lakes, and that worked too. Probably just depend on the water you will be fishing. We saw a guy on the mcenzie(sp?) a few years ago with a 100 lb thrust 36 volt trolling motor, and he could push his 16' drifter up a small rapid, or all the way back to the landing. :shocked: He also carried 6 batteries, so might not be ideal.
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Old 09-05-2005, 09:04 PM   #4
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Default Re: 101 lb thrust equals how many HP?

Approx 70 lbs-thrust = 1HP
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Old 09-06-2005, 11:11 AM   #5
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Default Re: 101 lb thrust equals how many HP?

This thread has shown up before. I've seen 17 lbs to 1 Hp on this board as well but in reality, horsepower is a unit of power and the pound is a unit of force. That said, the more commonly stated # I have heard is 36 lbs of force yields about 1 Hp in most marine motors.

With enough scientists and engineers on here, and I know you are there, this could really turn into a large debate. The real "conversion" is enormously complex.


That said, I have a 50-lb Minny on my 15-foot DB and you can nearly ski behind it.
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Old 09-06-2005, 11:41 AM   #6
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Default Re: 101 lb thrust equals how many HP?

Moman is right, you cant really compare them. But in a driftboat application, my 101 36v pushes my 17' DB about like a 8hp gas at half throttle, maybe a little better. But that doesnt mean its 4 hp
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Old 09-07-2005, 06:21 AM   #7
Kevin Turner
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Default Re: 101 lb thrust equals how many HP?

I installed a Minnkota E-Drive / 48V on my pontoon. MK gives it a 2hp rating, but no thrust rating. KT
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Old 09-12-2005, 03:57 PM   #8
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Default Re: 101 lb thrust equals how many HP?

On the back of a db, we have used a 40, 55 and 74 minkotta. Going down stream or for side drifing they all worked fine.

Going up stream in a fast current the 74 worked ok.

With a 101 you need 3 batteries. So this is a lot of wt in the boat.

Do you need a 101 or do you already have it.

As far as your question, I was told by Ollie damon that 15 to 16 # = 1 hp with a fully charged battery.

Giz...
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Old 09-12-2005, 04:57 PM   #9
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Default Re: 101 lb thrust equals how many HP?

I've now got a Riptide 101 on the way. Yep, 3 batteries is pretty heavy, I hope it works out ok. I should know in a couple of weeks.
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Old 09-12-2005, 05:20 PM   #10
SeanD
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Default Re: 101 lb thrust equals how many HP?

I had my doubts too when I ordered it. You will love it!!
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Old 09-12-2005, 09:53 PM   #11
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Default Re: 101 lb thrust equals how many HP?

Straight from the Minn Kota website:

Quote:
How do I convert thrust into horsepower?

Many people think that the more pounds of thrust they are getting, the faster the boat will go. This is not exactly true. With trolling motors and outboard motors, boat speed (and acceleration) depends on horsepower and prop pitch. By changing prop pitch, you can trade acceleration for top speed. Our trolling motors have props designed for maximum acceleration. When you hit the power, the boat responds immediately. This also allows you to hold the boat in high wind conditions. Top speed with our motors and props is about 5 mph. Small boats will reach this speed with most of our motors. Larger boats require our largest motors to approach this speed. Comparing our motors to gasoline outboards is difficult because most outboard props are designed for much higher speed.
and

Quote:
How does thrust compare to horsepower?

Horsepower is a measurement of “work” being performed.
One horsepower is a unit of measurement equal to 550 foot pounds of “work” per second.

There is no direct correlation of thrust to horsepower. Contrary to what you may have been told, fifteen pounds of thrust DOES NOT equal one horsepower.

As noted in the previous definition, thrust is simply a static measurement of force.
and finally (and possibly most importantly)

Quote:
How does thrust compare to speed?

“My buddy has a 36 lb thrust motor, we have the same size boat yet his motor moves his boat just as fast as my 42 lb thrust unit. Why is this?”

Again, thrust is simply a static measurement of pushing or pulling power, and higher thrust does not necessarily mean greater speed.

Speed is a factor of prop pitch and motor R.P.M.
Given motor R.P.M. under load, and Minn Kota’s 4" prop pitch, the approximate speed that a motor will push/pull a small boat can be calculated.

The formula for this is:
.85 (4x) x 60 = Miles Per Hour
12 5280 (Motor RPM = X)
Hopefully this helps a bit. If the motors are only rated for a top speed of 5 MPH, even the biggest electric motor may not push you upstream very quickly as a lot of sections on a stream are moving downstream at velocity's near or greater than 5 MPH. If you are putting it on a driftboat, I would buy a big electric motor as drifters are not terribly hydrodynamic when being pushed from behind. Sort of like trying to push a wedge through the water.

On the other hand, my 1973 Chrysler 9.9 2 stroke almost gets my 16' Koffler up on a plane...well at least as much of a plane as you can get from a big wedge of aluminum pushing it's way through the water. I've got a 5 gallon tank with a hose long enough to keep it up front for balance. A single tank will last me for days. I have to think that an electric motor with 3 batteries weighs more than a gas outboard.

Good luck on whatever you decide to do!
TF
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Old 09-14-2005, 06:20 PM   #12
Got One!
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Default Re: 101 lb thrust equals how many HP?

I'll include some links to elec. motor info, since most boaters know
less about them than about the familiar low-hp gas engines.

Let me say right up-front that if, as you say, you wish to go
up-stream against a reasonable current, in a loaded boat, the elecs.
are at a distinct power disadvantage.

To make this clearer, you can roughly equate 750 watts with 1
hp.(this ignores efficiency losses in the unit). This would equate to
about a 62.5 amp draw with a 12v. system(1 battery) / a 31.25 amp
draw in a 24v. system(2 batteries) / a 21.8 amp draw in a 36v. system
(3 batteries.).

The "lbs-thrust" rating of the trolling motors is a relatively
less-accurate method of comparing to "horsepower' in the gas-motor
sense, as so much depends on the prop, boat speed, etc., when any
attempt is made to measure thrust.
So, obtaining the full-throttle amp-draw spec. for a given motor will
give you the best approximation of the actual power of the motor, in
hp terms. Effectively applying that power to the water is a whole
'nother thing, too. (Where the prop-meets-the-water, ...discussed
below).

You can see from this that when you even think of comparing elecs
with even small gassers, you are gonna be talking some SERIOUS
amperage (biggest motor many batteries/mucho weight), if you want
to approach the power of the gas engines.

A 1 hp elec is a very powerful one, by today's standards. Finding a
prop to match your boat/speed, etc., from the available (limited)
choices designed for these elec motors, would be sheer good fortune.
These motors/props are designed to move heavy(bass) boats,
slowly,...something in the neighborhood of 3-5 mph. A (relatively)
lighter driftboat attempting to operate at a speed great enough to
make decent progress against any real current, is a different beast
altogether. Since prop-design is very "condition
specific"(speed/load/rpm), it is not an easy match to make.

The small(4 - 9.9 hp) gassers, originally mostly used on john-boats,
etc., have much more power, and are usually propped to efficiently
apply that power at greater speeds. (And chances of finding several
props to "try" on your rig, at your speeds, with your loading, are
greater).


When you compare the weight/bulk/cost of the number of batteries
needed to power the larger elecs., add the weight of the motor
itself, and then compare the total to the weight/bulk/cost of a small
gasser + fuel Tank, ...then see how much useful power-per-lb. you are
ending up with, you may feel the gas engine is the obvious choice.

Of course the relative silence of the elec can be a plus when you are
being swept seaward or over the falls, ...people will be better able
to hear your screams! :-)

here just in case you're curious about what's going on out there in
the "electric boat" world:

http://pages.intrstar.net/~brb/submot.html
http://www.mindspring.com/~jimkerr1/sebc&t.htm
http://www.mindspring.com/~jimkerr1/sebref.htm
http://groups.yahoo.com/group/electricboats/

Good luck,
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Old 09-14-2005, 08:21 PM   #13
jimh
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Default Re: 101 lb thrust equals how many HP?

Thanks for all of the feedback so far. There seems to be several of us interested in this topic and probably more lurkers!

There are a couple of advanatages to the electric that can help it overcome the disadvantages of less force. The force required to move up river is substantially different going bow first compared to stern first of course the driftboat is a somewhat funny shaped boat in that regard. Rotating the head of an electric lets you take advantage of this. As we probably all know reverse on an outboard isn't very efficient. Also, the balance can affect the required force to move the boat up river. Any one who's rowed very much has experienced this effect. The electric also has an advantage due to balance.

What swayed me to buy the electric was the ability to control the boat floating downstream and the relative lack of maintenance. Yes, the gas motor can be used for short bursts, but for a momentary adjustment it is somewhat lacking since you have to start the gas motor, and you only have to twist the throttle of the electric to get the minor correction. Gas motors don't require a lot of maintenance but the electric only requires charging.

In any case, I've got another week to wait to get my motor/batteries/charger, and I get to try it out! Yippee!
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Old 09-17-2005, 10:14 AM   #14
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Default Re: 101 lb thrust equals how many HP?

Quote:
Approx 70 lbs-thrust = 1HP
That is more like the correct number.
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