We have a 10.2' and I belive it should work for a 12' no problem.
It will fit, but it will definitely hang out over the back by 3 or 4 feet. I suppose that is no big deal, but you should be aware of that. I used mine for a 16 ft canoe, but it hung off the back by about 6 ft, even after moving the front stop bar as far forward as possible. So I bought a 10 ft piece of steel tube and used it instead of the 5 or 6 ft tongue that comes with it. That allowed me to adjust it so my 16 ft canoe did not hang 6 foot over the back. Also, notice the two carpet covered boards? They are flat and straight, so they don't fit the curvature of the hull very well . . . just contacting at arc point of the hull . . . again, no big deal, but when it sits in the hot sun all day, the weight of the canoe pushes the floor of the hull up to conform to the flat boards. I may make some modifications for that. Also note there is no front roller for the bow to rest on. I had to buy a roller and make a bracket to support the front of the canoe. I also put a rubber V stopper on the front vertical stop bar. Finally, I added two brackets to the back to hold a strap . . . painted the whole thing black with silver wheels and put two BMW logo's on the wheel bearings just for fun. Otherwise, very happy with the trailer. Good Luck!
I have converted the trailer to carry my 17 foot canoe by replacing the tongue with a longer one. The canoe hangs over the rear 3 feet which works with a canoe, but perhaps not with a boat. Also, the canoe only weighs 75 pounds.
Should be no problem. It’ll hang out to the sides a bit and extend over the end of the trailer as well. I have a 10’ by 5’ beam sailboat on mine.