If you put the battery in the back of the boat use #8 stranded wire. This will cover you should you ever decide to upgrade your TM to a 55lb motor. If you are positive you will never upgrade, #10 will work. Don't forget to put a 40 amp fuse or circuit breaker ay the pos side of the battery. While marine grade wire is by far the best the stuff they sell at Lowe's or Home Depot will work just fine.