I'm thinking about getting a new trolling motor. A 55# will work for my boat, but I'm thinking about a 70#. It's a small open bow ski boat.

The only reason for a 70# would be to troll for trout with a down rigger. A 55# will pull my boat about 2.4 mph at full thrust. I need to go 1.8 mph to troll, but the 55# runs out pretty fast at full thrust.

I don't want the 70# because of the extra batteries. Currently, I can have a backup battery, but with a 24v system I wouldn't have room for two more backup batteries. And I'm afraid that two batteries on a 24v system won't last as long as two batteries on a 12v system... not sure...

What is your best guess:

1. How long can a MK 70# pull my boat at 1.8mph?

2. Do you think I could fish a 12hr day with a 24volt system?

the 70# 24volt hands down and yes it will last way longer than 2 batteries on 12 volt. It has nothing to do with speed(which the 70 will be way faster than the 55) it has to do with range. and the 24 volt will run way longer even at hight thrust than the 12 volt will anyday, even with smaller batteries on the 24volt. If you already carry and extra battery than why not go with the 24volt and you not need to carry back up batteries.

24V motors use less amperage per pound of thrust than 12V motors.  The same goes for 36V over 24V.

the 70# 24volt hands down and yes it will last way longer than 2 batteries on 12 volt. It has nothing to do with speed(which the 70 will be way faster than the 55) it has to do with range. and the 24 volt will run way longer even at hight thrust than the 12 volt will anyday, even with smaller batteries on the 24volt. If you already carry and extra battery than why not go with the 24volt and you not need to carry back up batteries.

X2

Looks like I will spend the extra money and buy the 70#. Thanks for the help.

I disagree on the 12v not lasting as long.If you invest in a digital trolling motor it will draw less amps than a non digital,also if you throw those acid batterys in the garbage and buy a agm battery that alone will extend your usage time,agms take less time to recharge also and cold weather doesn't affect them like regular batteries.I have left mine in the boat all winter,hooked the charger to them and they were fully charged in less than a hour.

If that's a typical open bow ski boat, those sit higher out of the water than a typical bass boat.  That means the boat becomes a sail in the wind.  I tried a 55lb 12v on my 18 1/2 Champion for a couple years.  Then I switched out to a 65lb 24 volt.  There's just no comparison.  With just the 10 lbs extra the difference is night and day, especially in the wind.  I've noticed no extra battery consumption between the 2.  I'd definitely go with the 70lb, 24v.  

Since you already carry 2 batteries then you don't have an issue.  Get the 24V and be a happier camper.

The important thing about batteries is reserve amps.  Be sure to use deepcycle batteries with lots of reserve amps (180-200).  

I am pulling a 21' boat with an 82# thrust 24V trolling motor.  I can run into the wind all day long and not notice a drop in power at the end of the day.

I'm thinking about getting a new trolling motor. A 55# will work for my boat, but I'm thinking about a 70#. It's a small open bow ski boat.

The only reason for a 70# would be to troll for trout with a down rigger. A 55# will pull my boat about 2.4 mph at full thrust. I need to go 1.8 mph to troll, but the 55# runs out pretty fast at full thrust.

I don't want the 70# because of the extra batteries. Currently, I can have a backup battery, but with a 24v system I wouldn't have room for two more backup batteries. And I'm afraid that two batteries on a 24v system won't last as long as two batteries on a 12v system... not sure...

What is your best guess:

1. How long can a MK 70# pull my boat at 1.8mph?

2. Do you think I could fish a 12hr day with a 24volt system?

Save yourself a bunch of money by buying the 12v and just getting a pair of small drift anchors for when you want to troll.  I use my 60hp for walleye by using two bags, one on each side amidships in front of the console.  W/O the bags I can't get down below 2.4mph, with them, I get right down to 0.8mph.  Its a lot easier using the wheel to steer while trolling, especially in a chop.

Some good info here guys. I am contemplating the same choice but my issue is my boat stays on the water all summer and I'm 200ft from the nearest electric outlet so going to a 24v system would mean hauling 2 batteries to the garage to charge them but I dont know if going from the 40lb thrust to a 55 is going to be enough of a difference on my 17 1/2ft bass boat??

Dont know if they make them anymore but mine is 12/24v system