... yes, there are many variables ... weight of the dinghy, how does she sit and go through the water, sea state, upwind, downwind, wind speed & direction, how far back she trails. On longer trips, we'll put the dinghy (Caribe L9 Inflatable that weighs 100 lbs) on the foredeck. We never tow the dinghy with the outboard on, that always goes to the rail, and generally we'll put the gas tank & anchor bucket on deck. If it's rough, it goes on deck, particularly when downwind.

I tend to think it is less than 1/2 knot.

Cheers, John