Hi Darryn,

the upper limit would be the load able to lift the bows out of the water times a safty margin.
Let say your 100kg boat is 5m long and it centre of gravity is in the 2.5 from the stern and the sailor weights 80kg and sits 1.5m from the stern. Than the maximum load would be 100*2.5/5+80*1.5/5=74kg. Apply a safty factor of 5 for dynamic loading and you have 370kg.

Cheers,

Klaus