I'm in the process of buying a used boat. I'm looking at a 18-19 foot glass boat. I'm still looking, but I want a boat with probably a 115 hp or up depending on what I find out there. That being said, there is a small lake near my son's house that has a 40hp limit and we plan on fishing that lake regularly. My plan is to use the trolling motor when we fish that body of water. Im wondering if switching to a   36v motor is the way to go or just stay with a 24. I don't necessarily need the extra thrust, but will I get enough more battery life to make the switch beneficial?

I ran a 24 volt for 14 years.  With good batteries it would pull a 20’ boat all day long. 

The way to go is to not skimp on the batteries. 2 good quality group 31's and you should be good to go.

Sorry, what is a good group 31?

This https://www.batteriesplus.com/productdetails/battery/marine-and-boat/deep-cycle/bci-group-31m/sli31mdc

 

The above is a wet cell made by Deka and a d**n good battery at a d**n good price. You can also get a AGM or gel cell. I can't recommend any of those because I have only used wet cells.

12 hours ago, JeffD said:

Sorry, what is a good group 31?

Marine batteries are grouped by size some of the most common are group 24, 27, 29, and 31. The bigger the number the bigger the battery and the more power it can hold. 

 

As far as 24 or 36 it depends on the boat itself. Some might not have enough room for 3 batteries, others it's not ideal but you could make it work, and some you can throw them in no problem. Another factor would be how long your average trip would be. I've got a 17 foot Ranger with an 24v 82 lb thrust trolling motor running on two group 29 batteries, and I can run that 10-12 hours fishing without any issues. Your biggest draw though is going to be when you're running it on high moving from place to place. 

My size 27 12v weighs 59lbs and wonder what a 31 weighs.

16 minutes ago, OnthePotomac said:

My size 27 12v weighs 59lbs and wonder what a 31 weighs.

I did a little poking around online - group 31s seem to be 75-85lbs for the most part. Optimas come in 'light' at around 60 for a blue top.

Wow, a lot of extra weight.