[Jeb]

As already stated...it depends a lot on your use case.
If you are using it mostly as a rudder and pushing with a gas kicker in the back, you can get away with less. But if you are using spot lock, trolling in windy conditions, etc...then you will be disappointed.

When pushing the motor to the higher thrust setting under the latter conditions, you have to remember that your current draw (amperage) goes up and the efficiency goes down. This is the laws of physics (specifically ohms law). As the current goes up, you have higher loss that is just given up by heat. That is why if you look at the wiring & breaker charts for the motors, the wire size & breaker requirements are less when you make the jump from 12v to 24v (specifically from 55# to the 70#).

 

[Jeb]

Officially, Minn Kota does NOT provide this data, only Max power draw.

https://minnkota-help.johnsonoutdoors.com/hc/en-us/articles/5975498407319-Trolling-Motor-Amp-Draw-

Some folks have tried to chart this...but honestly, its not accurate - its just a guestimate.

Calculating Motor Run Time

A common question people have when trying to select a motor is “How long will this motor run out on the water?” To answer this question, it's necessary to know two things: a battery's amperage hour rating and a motor's amperage draw. Keep in mind that trolling motor run-time is not an exact...

www.trollingmotors.net

Edited...added chart from Garmin:

Force Trolling Motor Owner's Manual - Motor Thrust and Current-Draw Information

www8.garmin.com

Even the Garmin chart, lists a caveat...this measurement is only in calm water...once you change the environment, your power draw goes up. Therefore, these numbers are not really 'real world' accurate...they are just a guestimate too.

Why is it not accurate? Because there are a lot of variables, such as current, wind, temperature, etc.etc. It all factors in the actual power draw. Oh...and as your lead acid battery discharges, the voltage drops, and then guess what, the amperage goes up. (Watts law goes into affect here...power stays constant...voltage goes down....amperage goes up to maintain the same power).

Additionally, the motor controller uses Pulse Width Modulation (PWM) to control motor speed (i.e. it is not a steady voltage/current applied its variable).

I've said this before in the tech forum and I'll say it again. Unless you use a Shunt inline with your battery, you really have no idea what your power draw is, nor your battery depth of discharge.
Google "Victron smartshunt" as an example.

Without a shunt, you are just guessing. Its like having a fish finder without a transducer...you may think there are fish there, or its so many feet deep...but that is a guess without a transducer

 

[Jeb]

I have a Minn Kota Riptide Terrova 80lb Thrust 70" shaft powered by two 12V Costco AGM batteries on my Hewescraft Sportsman 180. I have used them for a year (approx 10 trips). So far they are holding up. Most days I can get approx 6-8hrs of power. But on windy days or when I fish in rapid current, then I get approx 4-5hrs. Been thinking about upgrading to either one 24V 100Ah LiFePO4 or two 12V 100Ah LifePO4 batteries. I installed a NOCO Genius Genpro10x 4-bank charge that can charger either lead, AGM or lithium batteries so I am set on the charger. However, deciding which brand of LiFePO4 to use gives me a headache

Hi there. May I suggest if you want to talk LFP batteries, perhaps we can work on the long thread going on in the Tech forum.
Trying to keep a single thread that has battery reviews, charger reviews, etc. Otherwise we have like 20 different threads all over the place

Lithium batteries suggestions

I am in need of advice / experiences from those who have purchased lithium batteries for their boats. My fiend has Battle Born brand and is happy with their performance - but admits this brand is pricey. So I researched lithium batteries on web and in reading the articles from various boating...

www.ifish.net

Your LiFePo4 battery options (for marine use) are getting better each week. Seems like everyone is jumping on the LFP bandwagon.
Battery 'type' & chargers are a whole separate discussion...


For the OP, the moral of the story is, go with a larger than expected bow mount. If you undersize, you'll regret it. Most of the tournament fisherman that run bow mounts all day long have found that the sweet spot is about 30% power. That seems to be where the motor runs the most efficient. If you go with a smaller motor, then its is running at higher speed and you'll loose efficiency and your runtime goes down the toilet.

There are just too many variables to try to even recommend stepping down. Stepping up to the next size is going to be a better experience especially if you plan on fishing in various conditions (i.e. not on just the perfect flat lake days with no wind, no current, etc).

-J