I am aware that a boat is not like a caravan BUT.
The recommendation from the Caravan Club to it's member's is that the
Caravan maximum loaded weight should be no more that 85% of the unladen weight of the car towing it.
This is why more and more people are moving to 4X4 which are not recommended by some of the Caravan manufactures.
Sorry if this upset the people with small cars intending to tow long distance.
BUT IT IS ONLY A RECOMMENDATION