i just picked up a 18' jon boat with a 40 hp motor that i want to get rid of...the boats floor is covered with 5/8" treated wood so it pretty heavy.---i want to put a mud motor on it but dont know how big of one to get, can anyone help???

Go with the biggest possible. If you don't due it now, you will immediately want to uprade. The type if terrain will determine the make mudmotor you should go with. I noticed this was your first post. Welcome to the disfunctional family
Uncle-J

The biggest one you can buy with what ever funds you have.

x3 on the biggest possible.

don't be scared to go smaller than you like based on your budget, just be prepared to go slow if you have to go with a smaller one for now.

If you will read thru some threads, you will see some speeds for boats of your size and then you just have to estimate your weight differences, hull size, shape, and style to guess what you might do with similiar motor.

If you will post up some exact info on the boat (width, crimped bottom or not, side height, transom height, what's in it,etc...maybe even some pics) and a potential motor you are looking at, I am sure there are some boys on here that can give you an idea of what speeds to expect, and if you can get on plane, etc...

Don't be afraid to rip that floor out and go without or replace with aluminum. It would depend on the boat as to how much I would spend fixing the boat up though.

And welcome... It is addicting. Get ready to spend some money!

Good luck

thanks for the advice fellas. will try to post some pics in the next few days