the problem with this one is that it is short. maybe too short.

The bed on this is roughly 4 feet long. If you compare the pictures of the two trailers you'll see that the 8 foot trailer's bed is made of two sections, and this trailer is made of just one. Almost everything else is identical.

With a 16 foot kayak placed on an 8 foot trailer you get an overhang of 4 feet front and rear. The trailer's tongue is about 5 feet long, so you could actually support the leading end of the kayak also, by simply bolting a rest or a prop to the tongue. If you slide the boat forward a bit, your overhang at the rear of the trailer is just 3 feet.

with a 4 foot trailer bed your overhang is 6 feet on each end. Your support in the middle is about what you would get with car-top carriers, so it is sufficient, but that huge overhang at the tail-end can be a bad thing. Unless the boat is mounted higher, you risk scraping the boat on the ground when navigating launch ramps and driveways. Boats longer than 16 feet would be the most at risk.

With either trailer you can do a little modification to the tongue and splice in some additional material to etxend it. Taking it from 5 feet to 8 feet would be fine for the 4x8 trailer. Then you would have 8 feet of trailer bed following behind an 8 foot tongue. That would be fine for an 18 foot or even a 20 foot boat. To get the same, or similar advantages witha trrailer tha is 4 feet shorter you'ld probably want to extend the tongue to 12 feet. that is quite a stretch, and it really changes the balance of the rig by placing the wheels quite a bit further back. Since the boats are quite light, there would probably not be a major weight/balance problem, and some boat trailers do have such long tongues, so it might work for you.

The big advantage you would lose with the shorter trailer is the ability to bring 4x8 panels of plywood, and 8 to 10 foot 2x4's home from the store. And getting the supplies is important when you consider the next boat you'll be building. Can't stop at one, of course.

PGJ