Strapping surf board under TT lashed to the axles

The axles probably would handle it, but I would be concerned about warranty issues since your TT is new. Hauling a load strapped to the axles might void any warranty claim you had if something went wrong. I think a better idea would be to construct some type of lightweight metal frame or "box" underneath the trailer to place the boat into. By the way I really like those porta-botes. Never owned one but they seem like a great way to take advantage of lakes, rivers, ect without towing a full sized boat.