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MSPbass

trolling motor basics?

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I use a Fish Cat Cougar float boat, but recently inherited a 12-14' jon boat. Even though the Cougar has a motor mount, I hadn't ever thought of using one. But now I'm thinking about doing some of those bass boat mods to the jon and a motor would be nice. I know nothing about trolling motors. Could I use one motor for both boats? I fish small bodies of water in a metro area. Often gas engines are prohibited. I don't think I need an outboard, but I'm not sure how much power a trolling motor puts out. I know it would move the float boat fine, but do folks use just a trolling motor to putt around in bigger boats? What if there were two 200lb passengers? And then, it looks like you have to buy a motor, battery and then a battery charger? How much thrust would be needed for a jon? 12 or 24 volts? I have no idea how these new fangled motors work!

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If both boats have a transom, then a single motor can be used on either boat. The motor is secured with a couple of clamps and installs easily.

Motor power is expressed as the amount of thrust (in lbs) the motor will provide when the boat is not moving. It provides some indication of how fast the motor will accelerate your boat from a stop. Most 12 volt motors will draw about 1 amp of battery current for each pound of thrust. A 24 volt motor will draw half as much, but you need 2 batterys. A small boat will not get much of an advantage going to 24 volts.

The run time you get on a battery charge is determined by the Ampere Hour (AH) rating of the battery, and how much current the motor draws from it. A 75 AH battery will deliver 75 amps for 1 hour, 37 amps for 2 hrs and so on. I use a grp 24 battery that is rated at 75 AH. It will push my boat around a lake most of the day. Your battery will last much longer if you avoid discharging it more than 50% and you recharge it as soon as you get home. Use a marine deep discharge battery. Wallmart sells a grp 24 battery for about 50 bucks.

Not all motors are going to provide the same effeciency. One 30 lb motor might run all day, and another only 3 hours. The difference is in the way they throttle the motor. The most effecient throttle is the continuously variable (CV) throttle. It converts the 12 volts to a lower voltage to operate the motor at less than full trottle and is 90% effecient. That is the motor gets 90% of the power at any selected speed and the controller uses 10% of the power making the voltage conversion.

The OTHER method is the step throttle, usually 5 speeds forward and 3 in reverse. This design switches resistors into the power lead thus lowering the voltage available to the motor. At half speed 6 volts is applied to the motor and 6 volts is lost on the selected resisitor. Here the effeciency is 50%. At 1/4 speed 3 volts is applied to the motor and 9 volts lost across the selected resistor. Here the effeciency is 25%.

So in the above examples, when operating at 25% power, the CV motor will run 3 1/2 times longer than the 5 speed motor. At full power they are about the same. Most of my time is spent as less than half throttle, so I use a CV motor. It more than doubles my run time on a charge.

Either a 30 or 40 lb motor will drive your boat. They are inexpensive and offered by MinnKotta and Motor Guide. The bad news is MK offers only one 12 volt motor with a CV throttle, the 55 lb Vector or Vantage. The smaller motors are all 5/3 speed motors. I am fortunate enough to be able to design my own CV throttle for the MK40. You probably do not have that option. So for the equivalent performance a 30 or 40 lb 5/3 speed motor with an extra battery will run your boat all day.

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Wow, dude. That was awesome. Thank you.

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Holy cow! The MK Vantage at $1200 is bad news.

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Ok, first off, thanks again for that detailed explanation Sneaker. It helped, greatly.

Second, the first place I looked was Cabelas. I now see that Bass Pro has a few Minn Kota's with their "Digital Maximizer" feature; which "controls the draw of power at infinitely variable speeds." They have a couple in the $4-600 range. But they also have the "Maxxum" transom mount motor.

A MAX40T is a 40lb thrust, 12V with 36" shaft and goes for $290. That's more my speed, if you know what I mean. Actually, I was hoping I could get everything for under $300. The MAX55T, with a 45" shaft, jumps up to $375. You sell the variable speed motor very well. (I liken it to recording MP3s at a variable bit-rate; which is much more efficient kids.) I think that the extra money now will save me money in the long run.

http://www.basspro.com/webapp/wcs/stores/servlet/Product_10151_-1_10001_21688_200002001_200000000_200002000_200-2-1

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I disagree a little bit. I have a cheap $100 endura 36# thrust. According to the above explanation it should run the same amount of time on speed 1 as on speed 5. I don't find that to be the case on the water. But sneaker has amp meters in line and has done more research than me.

Now I could be wrong as I have never taken apart a motor head, but when investigating the wires coming out of the switches on speed 5 the most number of wires were activated. While on speed 1 the least. Now using different size magnets and in the motor and turning them on and off in different groups according to speed selection one could achieve a fair efficiency at all speeds. I believe there were 3 switched wires going to the motor head. 3 magnets of different sizes could achieve 5 speeds.

Differences aside and in effort to answer the original question. Unless you want to get fancy a jon boat is light and even with 500lbs of person and gear you should get about 2 to 3 MPH with a small 12V trolling motor. That will get you a mile in about 20 minutes. I tried a 55 endura and I didn't see that much top end difference over the 36.

Modified for clarification. 3.5 mph vs 4 mph GPS in a canoe.

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Hmmm. Thanks for the input, Surfer. More to think about.

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Not an engineer, but I've owned a bunch of different MK motors. Call it CV, Pulse Modulation or Maximizer, what ever you like, but it will let you run longer at most speeds for a given battery size. The slower you tend to run, the greater the savings.

On the speed deal, not much difference on the first 4 levels of forward regardless of which unit (over all thrust), but there is definitely a difference at speed 5. This is because on units like the Endura, the first 4 speeds are roughly equally split over just 50% of the rated thrust. For example, if you have a 40# thrust unit, speeds 1-4 will be about 5#, 10#, 15# and 20# thrust respectively. When you switch to speed 5, you'll get all 40#. May not seem like a big difference just riding along, but put a GPS to it to get actual speed difference and it will be noticeable.

Last thought, just my experience, but you will almost never go wrong or regret getting a larger TM.  ;D Sooner or later they'll be an occasion where you'll wish you had larger than whatever you get.

-T9

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Yeah, I've been reading up on things and I think variable speed is the way to go. For me at least. I can also see that the prevailing sentiment for trolling motors is definitely: the bigger the better.

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Well surfer has made some good points that are worth a little more effort on my part.

You are right in that the motor will run longer at speed 2 than at speed 5 with either a CV or 5/3 speed control. Lets assume that speed 2 places 3 volts on the motor and 9 volts on the internal resistor. The motor voltage has dropped by x4 (compared to speed 5 it is 12/3) and so will the current drawn from the battery. So the motor will run 4 times longer on speed 2. But the resistor that was switched into the power lead will consume 3 times the power that the motor does since it drops 9 v and the motor only 3v. So the power from the battery is 12v x 3v/motor resistance (12x(V x I).

On the CV motor, the controller converts the 12 v to 3 v with no power loss (rough estimate) and the battery power is now 3v x 3v/motor resistance . Thus in this example the CV motor can run 12v/3v = 4 times as long as the 5/3 motor when at speed 2. To be a bit more accurate we can assume a conversion loss of 10% in the CV converter so the improvement is only 3.7 times as long as the 5/3 at speed 2. MK makes the claim that the improvement is up to 5 times longer at some speeds, and I think they are right. The slowest speed will give the greatest improvement.

You can tell the 5/3 T/Ms are permanent magnet (PM) motors with the following experiment. Disconnect the battery connections and set the speed to 5. Spin the prop with your hands and note that it spins freely. Now connect the 2 motor leads together, and spin the prop again. Takes a lot more effort?? That is because the motor is now operating as a generator and you have shorted the output, so it is working hard and you can feel it in the prop. CV motors have some electronics in the power leads and may not act the same since the drive circuit may not be active.

So how about the 'bigger is better' approach? Like Surfer I have put a 26, 30, 40 and 50 lb motor on my boat. The 26 gave me a max speed of 3.8 mph, and all of the rest gave me 4.4 to 4.5 mph. In addition the battery draw was the same for the 30 to 50 lb motors. How come??

Most of you have noticed that if you pull a running T/M out of the water it does not 'rev up' like a gas motor would. That is because PM motors are constant rpm/volt motors. If you double the voltage on the motor it will double its rpm. It makes no difference if the motor is in the water or not. All of the above motors were measured and they turn 150 rpm per volt applied. So if you apply 10 volts the motor turns 1500 rpm. What has this got to do with max speed?

M/K and perhaps MG have chosen to use props with 4 inches of pitch. Each revolution of the prop moves it 4 inches thru the water. With 12 V applied the motor turns 1800 rpm. So a perfect boat will move 1800 x 4 inches = 7200 inches or 600 ft per min or 6.8 mph. Most boats under 17 ft have a hull speed of 6.5 mph or less, and the 4 inch prop is a good match for them. So when I went from a 30 to 40 to 50 lb motor, all were spinning their prop at 1800 rpm, and had a pitch of 4 inches so all achieved the same max speed. And since all were doing the same ammount of work, all drew the same battery current. Except for the 26 lb motor. This motor could not draw 30 amps as it was designed to draw only 26amps at 12 v, so it was a bit slower.

So it appears that the only way to get more speed out of a T/M is to find a prop with a higher pitch. Of course if you want to go faster the motor will draw more current, so the motor must be rated for the additional current. These 12v motors usually draw 1 amp per rated pound of thrust, so The 40 and 50 lb motors will safely turn a 7.5 inch prop and draw 40 amps (on my boat), when the 30 can not. Finding the right combination of pitch/ Diameter/ 2 or 3blades for your boat is a bunch of work and most fishermen are not that eager to get an additional 1 mph of max speed. But I have the time and so worked at for a while.

So if your motor at max speed draws about 1 amp per rated lb of thrust, you have a good match for your boat. How can you tell? Borrow a digital VOM (volt-ohm-meter) or buy one at radio shack for 25 bucks. Put a 12 inch 10gau stranded piece of wire in the ground lead of your motor at the battery. Measure the voltage across the 12 inch wire(clip on to the wire, not the terminals). Your meter will indicate 1 millivolt for each amp of battery current. Now you are all set to play. Good luck. :o

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Maybe I should stick with oars. Ha! Just kidding. This is great info.

However, I am a bit confused now. If a 30, 40 and 50lb motor use up the same amount of electricity and delivers the same speed with the same prop turning at the same rate, what does a 50lb motor do that a 30lb motor can't?

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Here is the part number and applicable model numbers for a Minnkota prop:

MKP-6

PROP NO.:

MOTOR DIA.:

PROP NUT KIT:

#2061125

3-1/4"

A

35A, 35TA, 40TA, 65TA, 35T, 50T, 50MXT, 65T, 555T, 555TA, 565T, 565TA, 30AT, 33AT, 36AT, 36AT/H, 365T, 365TA, 365TMB, 765T, 765MXT, 765MXTA, 865T,

865MXTA, 665T, 665MXT, 30EX, 36EX, RT27/S, RT36/S, RT40/S, RT40/SC, 40PD V2, 36PD/C, 36PD/AP/C, 40PD, 40AP, EDGE 40, EDGE 40/H,

ENDURA 30, ENDURA 34, ENDURA 36, ENDURA 38, ENDURA 40, ENDURA PRO 32, ENDURA PRO 38, 40AT, 40AT/H, MAX 30T, MAX 36T, MAX 40T,

POWERMAX 30, POWERMAX 36, VECTOR 40

According to sneaker's "theory" the thrust rating of the motors is a lie and your boat won't go any faster with a 12V motor if you get the highest thrust rated motor instead of the lowest thrust rated motor.

I wonder why my jon boat is faster with a 55 than it is with a 36? I can tell the difference by the wake it makes and the rate I pass stationary objects. I haven't done a GPS speed comparison, but it doesn't appear to be any reason to do so.

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