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deermaster

some more noob trolling motor questions.

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have a couple more questions and would appreciate any input, and appreciate the help on the other thread.  so here are my questions.

1) if i get a 36v trolling motor, is my only option to wire 3 12v batterys together?  

2) how much does a 12v battery weigh, and what is a rough price for a decent one?

3) on a 15' v hull aluminum boat, with 2 people, how long can i expect to run, wide open, not stop, on a 109lb 36v minn kota with one (or 3 12v) battery fully charged, no head or tail wind?

4) and finally, for a boat using a trolling motor as its primary propulsion, would a bow mount or transom mount give me the most control in wind?

thanks for bearing with me, i am new to all this and want to be sure i know what its capable of before spending the money.

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Never run a trolling at max, unless emergency and return 75%. Nothing kills a batterys  quicker.

36 volt requires 3 batterys .

Running at 30 - 40 % with a 109 will be 2 days maybe more.

Running your batterys to zero will hurt your batterys . You will get about 1 year. Charge after every outing with good charger 15/battery 3-4 years.

You need 6 guauge wire to get the most out of a 109. Kit are available at BPS.

Garnet

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so going by what you said, would running a 109 at 50% get me 4 miles total? with some battery left? thanks.

also, is 50% power in a 109 enough to push my 15' aluminum v hull and 2 people, in good weather?  would push my boat and 2 people into a 20 mph wind at a higher setting if necessary?

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I have used a MK40 12 volt motor for primary power several years and here are a few things i have learned in the process. MK favors a lower pitch prop of large diameter to give lots of static thrust and good 'coupling' to the water. Most of their props have a 4 inch pitch and will push or pull a boat at about 4-6 mph. A 4 inch pitch prop will move ahead 4 inches for every revolution of the prop if the boat has no drag. The MK40 motor is rated for 40 amps max current.

Your motor will draw maximum current from the battery when the boat is tied to the dock and the throttle at max. As soon as the boat begins to move the current will drop. This is because the motors are constant rpm/volt motors. With 12 volts applied, mine turns 1800 with the prop in the water or out of the water. The motor at full throttle is doing less work when the boat is moving than when it is not. Mine typically drops from 35 amps at full load (boat stopped/max throttle) to 28 amps when at 4.3 mph. So it is operating at about 70% of its rated power at full throttle and 4.3 mph (28/40).

MK seems to limit the motor current to about 40 amps for all 31/2 inch to 4 inch diameter motors. This is probably the best they can do with the size of the brushes that will fit in the available space. So to get more power, they have to go to a higher voltage. The maxim 101 motor is rated at 37 AMPS at 36 volts. If it behaves like a 12 volt motor, it will probably draw about 30 amps on a moving boat at full throttle.

A group 31 battery (12 volt) weighs 70-75 lbs and is usually rated at 100 amp hours. You will need 3 of them. The battery is good for a limited number of discharge/charge cycles and typically are specified to be good for 200-300 cycles when discharged to a 20% charge condition. However you can double that number of cycles by limiting the discharge level to 50% according to Trojan Battery. So lets assume you discharge to 35%, you get an easy300 cycles and can use 65 amp hours each trip.

This battery set will run your motor at full throttle for about 65ah/30A = 2 hours at about 4-5 mph for a distance of 9 miles. At half throttle you will use 15-20 amps and get 3.5 mph for about 3.3 hours for a distance of 13 miles. So slowing down a bit gets you further and is easier on the battery.

The above estimates are based on what i have measured on my boat. Scaling up the 12v to 36 volts and my 12 footer to your 15 is not a precise process, but I think the numbers are realistic, and help full.

As to horsepower- a 36v motor at 40 amps uses 1440 w of power. It takes 746 w of power to equal 1 hp, so the motor uses 36 x 40/746=1.44 hp of electricity. The motor is about 85% efficient, so you get 1.23 horses at the prop. My info shows all MK motors above 55 lb thrust have continuous variable throttles and I am assuming you are not using a 5/3 position throttle motor.

I prefer a stern mount motor as it allows a better balance of the boat. Have fun and enjoy the fresh air and quiet operation of the electric power.

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sneaker, most bass fishermen prefer a bow mount electric motor for more precise boat control and better "balance" with the battery/s positioned at the transom.

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I agree that a bow mounted motor provides better boat control. I have fish a bass boat for years and wouldn't do it any other way. However when the electric is your primary power I think the stern mount will be a safer way to go if the water gets rough, and it is more useful at the launch/recovery site.

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With a 109 you have the beast.

Not sure if the 109 have a ramp up to your power setting if 50% hits in one shot away from your balance you will be standing on air.

It's a beast and you will be spoiled.

Garnet

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Nice write up sneaker.

Answers to questions.

1.      yes 3 batteries is your only option

2.      you could save some weight by using smaller deep cycle batteries instead of the larger ones, but this would reduce you run time / range.

3.      refer to sneakers write up.

4.      depends on weather you are fishing or traveling.  For the slow moving along a weed edge or other structure / cover you are picking apart bow mount with a stern rudder is best.  For traveling rear for the reasons Sneaker mentioned.

Don't forget you will need about $250 worth of battery charger to handle the 3 batteries.

Opinion

Holy cow that's a lot of motor for a 15 ft aluminum hull.  Have you considered the 2.5hp and a 45 ish trolling motor? The 2.5hp will increase your range and probably weigh less than the 2 extra batteries needed for the 109.  Scratch that.  The 2.5 won't get you on plane so it won't be significantly faster. It would increase your range and lighten the load, but the overall cost would be more and it would complicate the system thereby adding to maintenance.  Get the trolling motor you want then save up for a gas motor that will get you on plane.  

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