The trailer will not keep her down. I've seen boats on trailer being tipped over in less wind.

WHen push comes to shoove the best way is to get 4 or 6 pieces of timber (perferably some girder thickness and strength) of say 3 to 4 ft length and bury them horizontally some 3 ft down and attach some high strength line to them. Strap your boat to these lines or strap your boat to the trailer really well and use these 4-6 lines to tie down the combo. It will be a bloody hefty hurrican if this boat decides to go flying.

Also remove your trampoline that helps alot.

And make sure that there is as little slack in the lines as possible. The less movement the combo can make the more resistant it is to being pulled away.

Flying debri and falling trees is a different problem though.

Wouter