Hello: We have a 27 Catalina, fin keel.
This past spring, we towed her from Montana to Florida (and back). 6,300 miles in total. We had a double axle trailer with 3500 lb axles. So . . . yes, we were a bit overloaded. Electric brakes on one axle. Our tow vehicle was a Dodge 3500 (one ton) diesel.
The trip went very very well. I did have to switch a couple of the tires around because they were wearing a little unevenly. I have new axles on there with the grease zerk on the spindle ends; these are awesome because they inject new grease directly into the inner bearings and flushes the old grease through the outers. This is a very good system - way better than "bearing buddies".
I stopped at a highway scale at a truck stop and found that the weight on the trailer wheels was 8,100 lbs. That is with a pretty heavily provisioned boat. No hassle at all with oversize. Despite going through 21 states, not one time were we stopped and checked/measured/questioned. No hassle there at all.
I do know that we were overloaded by a thousand pounds or so. it worked, but I felt we were "on the edge". I would strongly recommend either 3 - 3,500 axles, or 2-5,000 axles.
The dodge (cummins diesel) handled the towing job admirably. No problems there at all. We cruised along at 65 mph without any problems at all - even in the mountains. We averaged 11.9 mpg over the whole 6,300 miles.
I see that a lot of the discussion involved bunks and launching. For us at both launch and load, we used a travel lift/crane.
I would be happy to send pictures or answer any questions.
Regards, and good luck!
~markb