Be wary of people trying to sell a boat, they will exxagerate the speed, and "undere estimate" the weight.
If you look at crossing times in the ARC
, not many boats average much more than 6 knots nearly regardless of size. It depends on the crew, how hard they push and of course sea state and wind
I never plan on more than 66% of hull speed
, and carry stores for 40% hull speed
. For a 50 footer, 66% of hull
speed is around 6.3 knots.
Most of the charter
style boats will not do more than around 50% of windspeed, so you can also look at average wind
speeds, and not count on doing more than around 10 knots boatspeed for longer periods. With more than around 25 knots of wind on the ocean you will probably be looking at reefing and going more for comfort than speed.
Have a look at the data on the Multihull
Dynamics site, buy the figures for the boats you are interested in. They might not be exact but will give you a good picture of how they will perform compared to each other.