Horsepower/thrust

[Rodburner]

I'm looking for a kicker motor for my boat. I don't want one that is too big but sure want one that is big enough. My 67# thrust trolling motor pulls me fast enough. My question is what horsepower would be equal to the 67# thrust? Is there anything to go by to determine what would be equal or do you just guess? Thanks~~

 

[tashasdaddy]

Re: Horsepower/thrust

a 4hp would do good, at idle, but going against the wind and current you could speed it up. now if you are accustom to a bow mount trolling motor, a transom mount you may not like, due to control problems. you may do better adding batteries as back up.

 

[Rodburner]

Re: Horsepower/thrust

I've been using the bow mounted trolling motor to troll with but it has lost a lot of power. I bought new batteries and it didn't help much. They hold charge longer but it doesn't push the boat like it did before. I used it last summer and it started losing power at the end of the season. I thought the batteries were just getting weak but I guess the motor is getting worn out. I don't think they are inteded to be used for trolling, are they? My thinking is, if I'm going to spend $500-$1000 why not buy a gas motor. Another thing, the rheostat, or speed control, on the bow mount doesn't seem to be working on all #'s. It goes from 1-5 and from 3-5 it doesn't change the speed. Where could I find a rheostat? Thanks tahsasdaddy, for the info, that's kind of what I was thinking. Do you know a good place to get one? Besides e-bay~~~

 

[JB]

Re: Horsepower/thrust

Not really clear what your question is, Rod.

Each 1HP of gas engine theoretically makes about 75# thrust at full speed.

Sound like the controller on your mystery troller is not making full power.

 

[ondarvr]

Re: Horsepower/thrust

More info on the boat would help, also what type of fishing you want to do and what bodies of water will you be on?

 

[reelfishin]

Re: Horsepower/thrust

Need the brand, year and model of trolling motor too.

I know Motor Guide used separate windings for various ranges on some motors and the lost of one or more speeds can be either a bad winding in the motor or a bad control switch.
I've also seen a few Minn Kota motors lose their fixed magnets within the motor itself, this causes the magnet to drag the armature and if kept in use, it will overheat and burn out the control and the armature. Not to mention drain the batteries fast.

 

[Rodburner]

Re: Horsepower/thrust

Mine is a Motorguide. I would say it's at least an '89 model. It's a 67# thrust and use to work great. I did use it for trolling and figured it would cash in some day. I thought the rheostat might be bad but I see what you're saying reelfishin. I think I've lost more than one speed.
The boat is an '89 Procraft 18' and the only fishing we do is in smaller lakes. We troll for walleye or hybrids and fish for crappie some. I want the kicker motor for trolling only. I was thinking 4 hp would be plenty but wanted some more advice. Thanks for all the info it's greatly appreciated!

 

[Silvertip]

Re: Horsepower/thrust

The most obvious thing to do is remove the prop and check for fish line wrapped tightly around the shaft, behind the seal, and hence creating a water leak into the motor case. Happens all the time. Even if the line doesn't get into the motor, it creates a tremendous amout of drag. That drag causes current draw to spike and hence drains the batteries quicker. Trolling Motors are to be used exactly as you have been using them. Why else would they call them trolling motors? Because of the drag I mentioned, there is a likelyhood that one or more of the speed coils (not separate windings) burned out. Those speed coils are separate wire wound resistors that when used in combination provide various motor speeds. Not very efficient in these days of pulsed motors like the Minnkota PD. The speed coils are out of the circuit when running on the fastest speed so they don't affect that function.

 

[reelfishin]

Re: Horsepower/thrust

..........Those speed coils are separate wire wound resistors that when used in combination provide various motor speeds. Not very efficient in these days of pulsed motors like the Minnkota PD. The speed coils are out of the circuit when running on the fastest speed so they don't affect that function.

I have a few older but very clean Motor Guide 34lb thrust motors which have lost one or more speeds, one of them lost high speed. I've replaced the burnt switches, but other than simply removing a melted switch, the motor still lacks the same speeds. Where are the resistor windings and are they repairable without replacing the whole motor itself? I only found a complete motor listed from Motor Guide.

I do prefer the pulse or "Maximizer" motors from Minnkota since they run so much longer on a charge, but the older motors are fine on small ponds here. I have had a dozen or more older Minnkota motors with fixed magnet separation in the motor. If you catch them in time, they can be succesfully epoxied back in place and most likely they will never fail again. The original glue looks more like rubber cement, it's soft and peels away pretty easily.
I lost a 3 hp motor like that, it was only a year or so old at the time, I was a good distance from shore and way upstream, I kept giving it more power thinking that the battery was dying or that the current was fighting me, but what had happened was that the magnets were dragging the armature, by the time I got to shore, the motor was toast, the control head had melted wires and the circuit board had burned up. I never heard a sound since he water hid all the grinding. Being that it was on a small pontoon boat, the normal noise and vibration from the tunnel also didn't alarm me to what was happening.
When that happened, I checked my other motors and found every one of them had loose magnets which were close to dropping off. A call to Minnkota got me no where either. This was back in the early 90's.

 

[Rodburner]

Re: Horsepower/thrust

Great info! I take the prop off after every fishing trip and haven't seen any line. I'm going to tear into the motor this weekend and see what I find out. If I find anything I'll let you guys know. Thanks again for all the great info!

 

[Steboe]

Re: Horsepower/thrust

Briggs & Stratton claim 150lbs thrust for their 3hp electric

Ken

 

[tashasdaddy]

Re: Horsepower/thrust

and are extremely noisy. if want to hear a lawn mower, i will cut the grass.

parts here: http://www.mikesreelrepair.com/index.php?cPath=45_34

 

[Silvertip]

Re: Horsepower/thrust

Fish line wrapped between the prop and motor housing will also slow them down -- and cut the seal so water can get into the motor. Pull the prop and check.

 

[steelespike]

Re: Horsepower/thrust

and are extremely noisy. if want to hear a lawn mower, i will cut the grass.

parts here: http://www.mikesreelrepair.com/index.php?cPath=45_34

Thats a 3hp electric.Didn't know thay had one.
I thought it was discussed that motor problems caused battery drain.
He said batteries hold up but speed in down.sounds like a controller issue to me.

 

[marquette]

Re: Horsepower/thrust

i'm not sure what the actual conversion of HP>#thrust is but i tried my 2hp johnson as a trolling motor on my Crestliner Commander. it was 16ft 9in and much like the Lund Rebel only a little bigger. boat and motor weighed about 900 lbs with out gear. i was fishing in canada where i couldn't recharge my trolling motor battery. it was a pretty dismal failure. first off after an electric motor the noise drove me nuts. again i was going from a bow mount to a transom mount and steering was more difficult and i had gotten used to being able to control speed much more subtly. i would say it took about half throttle to troll at 1-1 1/2 mph. so either the HP>thrust conversion is low or the prop was not as efficient or bow thrust is more efficient than transom thrust. if i knew i was always going to be in a place where recharging batteries was not going to be a problem i would stick to electric.

 

[bundick]

Re: Horsepower/thrust

I've had this thought for a long time.
I wonder why somebody hasnt come up with a Weed Eater motor attatched to a small car's 12V Alternator?

That would give some Trolling motor voyager a heckuva long range with a few gallons of gas.
You'd get off on the beach for lunch, and let the battery charge while you were having a bowl of soup!

 

[pvanv]

Re: Horsepower/thrust

Horsepower cannot be converted to thrust. HP is a measure of work (force, distance, and time). Thrust is force only.

 

[OptsyEagle]

Re: Horsepower/thrust

"Horsepower cannot be converted to thrust. HP is a measure of work (force, distance, and time). Thrust is force only."

That is true, but how I calculated my 29# trolling motor was to measure the current draw at maximum speed. It was about 30 Amps. Since it was powered by a 12V battery, I knew it would provide 360 watts of work. Since 1 Hp equals 746 watts, I simply calculated that my 29# trolling motor would put out the same power as a 1/2 Hp engine.

Maybe crude, but that is how I calculated it.

 

[reelfishin]

Re: Horsepower/thrust

The problem is that so many companies rate their motors more for sales and advertising purposes rather than true scientific ratings.
A MinnKota 3HP is in the 50lbs of thrust range give or take. The output can also be reduced by a lack of amps feeding the motor.
Other factors come into play when comparing small motors, whether electric or gas. The size of the prop, torque produced, motor weight, and total hp will all need to be factored in. Then you have to consider prop pitch and cup as well.
Most trolling motors are rated as a unit, prop pitch is fixed and not adjustable as on most gas outboards. Electric motors also have a more constant output than does a gas engine as well as a more steady power curve.

I have a 3 hp MinnKota trolling motor and two 3hp gas motors, one an air cooled 2 stroke Aqua Bug, and the other an Evinrude. The Aqua Bug and the 3HP trolling motor are pretty close in speed, the trolling motor gets up to speed faster but the added weight of the batteries is far greater than the 2 quarts or so of gas it holds. The Evinrude is by far the fastest and most powerful, it runs nearly twice the speed of the other two, is in the middle as far as weight, but uses a larger diameter prop than the Aqua bug. The Evinrude 3hp will actually start to plane out a 12' flat bottom jon boat with two guys in it, the other two just keep it moving at a steady pace. The Aqua bug is the lightest, and I often take it along as a back up in case the battery dies or I need a bit more power.

 

[pvanv]

Re: Horsepower/thrust

I agree with optsyeagle and reelfishin. Skipping the theory of series-wound versus shunt-wound DC motors (in theory, series-wound units can produce unlimited power -- limited only by the power source), the manufacturers blur the engineering truth by rating electric trolling motors in a different manner (thrust) than gas outboards (HP).

Seems like a deliberate dodge, intended to dazzle and confuse the customer. After all, when buying a land-use electric motor, it is rated in HP, not thrust. Aside from just a few trolling motors (the minn-kota 3 hp and the torqeedo electric ob for example), we are left with an apples-to-oranges comparison when shopping for small motors. The only way to get a solid HP number from a thrust-rated electric is to do as optsy did, and actually do an on-the-water test, followed by some math. Of course even this testing method ignores the differences in power/rpm curves, as well as prop differences.

Reminds me of the (gross hp) versus (brake hp) rating differences in ob motors. At one time, the Jap outboards were pounding the heck out of the US units -- only because their rating system gave HP at the prop, where the US guys were rating at the crank. The difference can easily be 10-15%, due to losses in the transmission of power to the prop. Of course, today, all ob's are supposed to be rated at the prop.

Then there are the differences in noise (gas motors are usually noisier) and weight (the batteries for the electric usually add up to more weight)...

Returning to the original question then, probably any small ob (commonly available at 2.5 hp and up) would achieve or exceed the forward performance of a 67# electric. Depending on propeller geometry, these motors will probably not be too efficient in reverse, but then many of these motors don't have a reverse gear -- you spin the entire unit around backwards.