Re: Adding a 3rd toon
you could also look at a shorter center tube that won't interfere with your exsiting transom, you get the additonal flotation without having the expense of having to buy and actual center tube.
A true center tube has a transom in the back which can add alot to the price, if you just go with the tube you could find a 16' toon and just bolt it up.
Aslo unless you get a center tube that was designed for your specific boat you will have a real hard time retro fitting one, either the brackets won't line up or the transom will be far forward/back, there is alot to consider.
We had a really old school one at work, 13 years old, I made it 1 1/2'' to short and it wouldn't work so you need to be really carefull with what you get.
