A theoretical question...

[XcoM274]

If I want to generate exactly 50 lbs of thrust on a trolling motor, which motor will take the least battery juice? A 50 lb, 75 lb or 100 lb? Not that it affects anything I'm buying, just a challenge question. Does running a motor at full, 3/4, and 1/2 power make that much or any difference on the battery?

[Jig Man]

If I want to generate exactly 50 lbs of thrust on a trolling motor, which motor will take the least battery juice? A 50 lb, 75 lb or 100 lb? Not that it affects anything I'm buying, just a challenge question. Does running a motor at full, 3/4, and 1/2 power make that much or any difference on the battery?

I'm not sure that you can get answers without motor specifications.  If they all had the same amp draw then the 100 lb would only be running at 1/2 power and would draw a lot less amps than the 50 at full power.  The 75 would fit somewhere in between.  It also depends on whether the units are the same volt systems as 12v draws the most amps.

You have to know the motor's amp draw and the battery's reserve amp capacity before you can estimate the length of running time.

For example, if you had a tmotor that draws say 30 amps at full speed and a battery with 150 reserve amps it should run close to 5 hours on high.  As you use slower speeds you use less amps.  That tmotor may only use 10 amps on low and you battery might last 15 hours.

I think I am confused now. ;D

[Guest avid]

Minn kota states that it draws approx. i amp for each pound thrust at full throttle.

the maximizer reduces that by alot.

[bassboy1]

A 70# has the best efficiency of any 12v or 24v.  It can run 3 times as long as the 50, but using only twice the batteries.  As far as endurance, the MK maximizer can help a lot.  You see, conventional motors use the same amount of power whether set to fast or slow.  If on low, the extra power is changed into heat, which can also fry the motor sooner.  The maximizer only uses the power needed for the speed setting and therefore batteries will last longer.

[NBR]

It seems to me that except for efficiencies in the motor and prop tp converttorqoe to thrust they would be the same.

[Siebert Outdoors]

Thats easy one. 100. Your drawing from 3 bats at 50%. The 75 would be 2 bats at 66% and the 50lb would be 100% on 1 bat.

so would it make sense that say your motor ran at 50amps all three

100lb   would be 16.66amp per bat

75       would be 25 amp per bat

50       would be 50 amps

[XcoM274]

According to bassboy, NBR is right (assuming you don't have the maximizer) and all motors draw the same amps. Now again this question was theoretical, but 100 and 75 lb motors usually only run on 24 and 36 volt anyways. If you ran them all on 36 volt, would it be different?

[Siebert Outdoors]

I'm still sticking with the 100lber.  It would be running at the least percentage to power percentage.  In turn producing the most effeciency.

Its the law of deminishing returns.  The closer to 100% you get the more power it takes.  Just my opinion though.

[Pa Angler]

Here's another comparison, It's cheaper to run a 10,000 BTU A/C on 220V then to run one on 110V a 12V motor will not last as long as a 24V motor which will not last as long as a 36V motor I have a 74lb 24V motor which lasts longer than my friends 45lb 12V motor using the same size batteries my boat is 17'-4" long and 94" wide where his is 16' long and some 70 inches wide maybe this may shed some light.

[RecMar8541]

as you increase voltage, amp draw drops.  In the case of the 120v vs the 220 v ac.  the 220 is cheaper to run, because it will draw 1/2 the amps of the 120v system.  

In the case of the trolling motor.  As with most dc motors there are 2 types of power control modules, manual and electronic.  With a manual it draws the exact same weather you are at top speed or low speed (these are outdated for the moost part)  and there are electronic ones that are like a car, the more speed you use the more power you use, and less the less.

[Ann-Marie]

as you increase voltage, amp draw drops. In the case of the 120v vs the 220 v ac. the 220 is cheaper to run, because it will draw 1/2 the amps of the 120v system.

In the case of the trolling motor. As with most dc motors there are 2 types of power control modules, manual and electronic. With a manual it draws the exact same weather you are at top speed or low speed (these are outdated for the moost part) and there are electronic ones that are like a car, the more speed you use the more power you use, and less the less.

NO.  In your example on the 120/220V AC, they cost nearly the same because you are paying the power company for WATT-HOURS, not AMP-HOURS.  The current will be 1/2 but the voltage is twice and when you multiply to get watts it is the same amount.

I think that the original question was about EFFICIENCY not current.  For the same power a 24 volt is twice the voltage but half the current so again the amount of power used is theoretically the same.

So why put two batteries in series to run a 24 volt motor instead of the same two batteries in parallel running a 12 volt motor.  Theoretically it is the same amount of power WATTS or amp-hours battery capacity (1/2 current x twice the voltage).

The reason is wire gauge and thermal losses.  If you are losing 2 volts in all the wiring, including the wiring INSIDE the coils in the motor, then on a 12 volt battery you are getting only 10 volts doing work for an efficiency of 10/12 or 83%.  If you lose 2 volts on a 24 volt battery your efficiency goes up to 22/24 = 92% efficiency.

The magnetic force in the motor windings is measured in amp-turns.  That is the current flowing through multiplied by the number of loops of wire in the coil.  But the space in the motor cavities to hold the wire is more or less constant.  Since you can use smaller wire on a 24 volt motor you can get more turns of wire on the coils and increase the magnetic pull more than the reduction of current for 24 volts.