Lbs of thrust vs. horse power Lbs of thrust vs. horse power
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Boats and Motors

    Lbs of thrust vs. horse power
from Tboss  
2/27/2001 10:56:00 AM

Rated:

 Electric motors are usually rated by lbs of thrust

gas motors are by HP

Question: a Minkota elec motor with 90lbs of thrust is roughly equivalent to what size gas outboard?

(have a 20ft cuddy and am wondering if one of those large electric trolling motors that mount to the outdrive will actually push my boat - know that a 9.9 hp Merc will do it)


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   No direct conversion... from Jeff Southern (206.67.247.211)  2/27/2001 11:27:00 AM
 There is no direct conversion between thrust and hp. However, most 50lb motor are about 1/2hp. The 67-80 are about 3/4hp and the 100lb might be close to 1 hp.

However, you cannot directly compair the hp of an electric to a gas engine. An electric motor produces it's full hp across most of it's rpm range. A Gas engine produces it's peek hp in a very narrow rpm range and falls off sharply outside of that range. As a result, an electric can spin a higher pitch prop in the lower rpm ranges that a gas engine would have trouble spinning.

After all of my research It seems to break down this way.

a 1hp electric ( properly geared ) will push a boat at NON-PLANING speeds about like a 5hp gas engine.

a 5hp electric will act like a 10-15hp gas engine.

All electric boats are non-planing. There is smply not enough energy in a battery to plane a boat for any distance.

I built an electric outboard for an electric fishing boat that I am working on. I put a 1hp 24v electric motor on an old evinrude 8.5hp that had a blown power head. This combination will produce about 200lb of thrust. Because I am using a constant current controller, I can also run this motor at 36v and it will produce about 2hp without over heating. It produces about 225lb of thrust at this voltage level. So you can see that I do not get much thrust increase even by doubling the hp.

The big difference is that the trollling motor does not have any gear reduction. This means that you have to spin a low pitch prop very fast to produce much thrust. That is why a 100lb trolling motor doe not seem all the much stronger than a 67lb trolling motor on a heavy boat.

I suspect that my converted 1hp electric out board running on 24v would push your cuddy about 5-6mph for about six to eight hours with eight group 27 12v batteries ( 50lb each ) for about 6-8hr. My goal is to try and get 10mph or so with a 40-50mile range out of a small efficient displacement hull fishing boat.

If you use trolling motors, I would think a twin 50lb setup would work better than a single 100lb unit. Also two 50lb units could actually be cheaper because you don't need variable speed because you will probably run them full speed most of the time.

Fosters ( a board sponser ) might can hook you up with two rebuilt units at a good price.

If you want more details, send me an email at: sranger@bassfishingnetwork.com


   Jeff, this maybe totally irrelevant but.......... from Spotted Bass  2/27/2001 11:36:00 AM
 didnt the first MinnKota All Terrain troll motors that were 65 lbs thrust have 2.5hp stamped on them near the directional arrow on top on the motor? Seems like the guy I used to fish Anglers Choice with had one of these on his BassCat Sabre..........just wondering.

SB


   I remember... from Pat  2/27/2001 11:40:00 AM
 The Beast by motor guide was compared to a 5hp gas engine...


   Horsepower from PropR (152.163.205.37)  2/27/2001 11:54:00 AM
 I've seen an electric advertised at 3hp before, but I don't remember which one although I believe it was a Motor Guide.


   A good site to visit... from Jeff Southern (206.67.247.227)  2/27/2001 12:42:00 PM
  This site has a much better explination of comparing an electric to a gas engine.

http://www.rayeo.com/index2.htm

As as actual hp is concerned, no trolling motor really needsto produce much over 1hp. The reason is simple. A 1hp motor can push most boats ( from 12' - 25' ) at their hull speed. Hull speed is a function of how fast a boat can travel at non-planing speeds before any increase in hp is simply wasted in producing a bigger wave.

You will find that most bass boats hit a wall at around 4mph because of the eddi current that forms around the transom. If a 50lb motor will go 4mph, a 100lb motor on the same boat might go 4.5mph.

Now before someone gets the wrong idea, I am not saying that a 50lb will compair to a 100lb in normal use. A 100lb can run against a current and the wind much better than a 50. However, this thread started as a discussion of using an electric to push a boat at speed. As far a top speed goes, there is suprizingly little difference between the two.

For example:

A 14' John boat has a hull speed of around 4-5mph. A 1/2 hp engine can easly push this boat at that speed. However, if you add a 5hp engine, top speed might go up to 7-8mph. In other words you added 900% more hp power and only gained about 1% more speed. The rest of the hp went to making a large wake behind the boat.

Now if you add a 10hp motor, you can plane the boat. The speed might go up to 15-25mph. This is because the boat is no longer using all of the hp to make a big wave....

As stated above, some motors may be rated at 1/2 - 3hp, but the usable power in these motors is more like 1/2 to 1hp.

Without gear reduction, most electric tolling motors really do not produce much additional thrust above about 1hp. Gear reduction allows you to spin a higher pitch prop slower. This increasses efficiency at NON-Planning speeds. However, energy is lost due to friction in the gears. This means that there is a point at which further gear reduction is a self defeating use of energe.

By calculation and my own tests with my little electric outboard, 2:1 seems to be about right.


   Wow - lot of information from Tboss  2/27/2001 1:47:00 PM
 lot of good info, thnx.

My main interest is whether or not a 100lb thrust electric motor can push my 5500lb Trophy W/A at least 1 1/2 to 2 mph.

Most of my fishing up here is trolling for salmon - have to deal with tide, current, waves and wind quite a bit that Trophy has a pretty tall profile, wind really pushes it around - have seen a couple of people using electric, thought I might try it.

Cabellas has a trolling unit that mounts directly to my Alpha outdrive, wondering if that would work -


   15# Thrust = 1 HP. from Scott (205.188.192.156)  2/27/2001 5:48:00 PM
 That's the formula I've had in my head for many, many years.


   Speed from PropR (64.12.104.52)  2/27/2001 7:15:00 PM
 I believe there is a hp to pounds thrust conversion. I think some people are trying to complicate things. Not to go to war with you again Jeff, but I've had two 12 foot jon boats one with a 5hp and the other with a 4 1/2hp before rating at the propshaft started. The 5hp would plane the boat if I shifted my weight forward a little and would go about 15mph. The 4 1/2 would try to plane at about 12 mph but couldn't. Granted you said 14ft jon boat, but with a 14 footer, I probably wouldn't have had to shift my weight forward. I've rented a 14 foot aluminum boat many years ago with a 5 1/2 hp Johnson which is the quivalent of a 5hp today, and it seemed to go closer to 20mph although I realize speed on the water is deceiving. I don't believe that the gear reduction thing holds too much water. The hp to pounds thrust may not be an exact science but something used for comparisons.


   Well PopR... from Jeff Southern (206.67.247.126)  2/27/2001 8:09:00 PM
 I am simply relating the information that I have read and seen first hand while designing an electric boat. I was trying to put in the context of the application of a heavy boat and very little hp to push it.

Any yes, spinning a large diameter/pitch prop slower is more efficient at low speeds that turning a smaller pitch prop faster. As a matter of fact, most early trolling motors did have some gear reduction in them before more efficient and more powerful motors were produced. By slow speeds, I mean 4-10mph. I though I was fairly clear about the non-planing speed thing, I guess not. I have no doubt that a 5hp electric could easly plane a 14' John boat. The problem is that it would consume about 250-300amps at 36-48v to do it. That would mean that a typical bank of group 29 batteries would last about 10-15 minutes max at this much power draw. That is why I refered to the "USABLE" horse power that can be generated from a trolling motor. It may very well be capable of generating more hp for short periods of time, but 1/2 - 1hp is about all the batteries can deliver over a reasonable period of time.

The world speed record for an electric boat is currently 71mph. It was set by a modified 12' hydroplane running a 30hp electric motor at 144v. The batteries would last for about 3 miles at this speed. This is just enough to make two 1 mile passes betweeen the traps. However, I thought running 70mph with a 30hp motor and about 1000lb of batteries is still fairly impressive.

20mph from a 5hp motor? That must be a very efficient hull on those tin barges.... I don't think an Allison can match that hp * weight / speed ratio.

The reason that there is no direct conversion between the two is simple. Hp is a measure of work over time. ( ft-lb per sec ) Thrust is simply in force in pounds. The amount of time that the motor can generate the thrust must also enter into the equasion to make any reasonable comparison possible.


   20 MPH with a 5 1/2 !!!!!!!!!!!! from tx_basser  2/27/2001 8:48:00 PM
 Shoot my 15ft with a 35HP, would barely do 21 MPH with 2 people


   Lets Make this easy.... from Jeff Southern (206.67.247.129)  2/27/2001 9:38:00 PM
 Here is an accurate conversion for an electric Trolling motor to Hp.

Electrical Power = Volts * Amps = Watts

746 Watts = 1hp

The MinnKota 107lb Trolling motor is rated to draw a max of 45 amps at 36v.

45amps * 36volts = 1620 watts / 746 watts/hp = 2.2hp

The above formula assums no losses due to heat and friction. The true hp would be about 2hp. This is as accurate as it gets.....

For comparison:

A 12v 50lb Minkota 50T pull a max of 40 amps. This = about 0.6 hp

A 27v 74lb Motor pulls a amx of 46 amps. This = about 1.2 hp

TBoss,

Sorry about the log winded answer and the side thread with PopR.

To answer you original question, a 100lb motor with three good group 29 ( 80lb each ) batteries and a good 10 amp three bank multi stage charger to keep the batteries in good shape should easly push your boat about 3-4mph for about 1.5 hours.


   This is just in case... from Jeff Southern (206.67.247.129)  2/27/2001 9:51:00 PM
  Just incase PopR still thinks I am wrong....

I took this directly from the Minkota Web Sight at :

What" target=new1>http://www.jwa.com/motors/minnkota/faq.html

"...

Whatthrust?

Thrust is a static measurement in pounds of our motors pushing or pulling power. Minn Kota's published thrust ratings are laboratory measurements taken by using a load cell and with voltage equivalent to a fully charged battery (or batteries) applied directly at the motor brushes.

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. 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 pound 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)

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 5mph. 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.

..."


   My bad.... from Jeff Southern (206.67.247.129)  2/27/2001 9:57:00 PM
 I looked in the wrong column on the Trolling motor spec sheet.

The Minnkota 101lb trolling motor is only rated to pull 37 amps at 36v.

Therefore, the actual hp calculates out to approx. 1.7 hp


   Quite a thread above from PropR (152.163.213.211)  2/27/2001 11:08:00 PM
 Wow, thanks for the explanation. Excuse some of us for having a hard time not getting through all of that though. You've obviously done some research. I guess 150 hp and 175 hp could be equal due to each could put out the same thrust depending on the pitch of the propellor. I think I've got it now. Thanks again.


   A little less scientific from MajBach (206.172.59.188)  2/28/2001 11:50:00 AM
 Boss, ideally it would be great if you could borrow a trolling motor to try it before you go out and buy one. I once mounted my 31lb trolling motor to my 25 foot Sea Ray (5400 lbs). It moved it, but slowly, as you might have expected. Maneuvering was better than you may have suspected, it was fairly good. But in the presence of medium winds or waves, it would have been useless, especially to fight a current. From this experience, I would expect that even a very strong motor would have been inadequate in wind or high current conditions. But if you're looking for something to maneuver in calm conditions, a 75 lb or better motor may suffice.


   Source for steeper-pitched trolling motor props. from Kyle Stringer (63.76.98.12)  4/26/2001 4:50:00 PM
 I have not been successful in locating a source for props. that would be optimized for speed and not take-off for trolling motors - does anyone know where I could get a steeper-pitched prop. for a 54# thrust trolling motor (the boat only weighs 350#'s fully loaded!)


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