Diesel Doctor

[Lefty]

Hi;
I'm looking to find a replacement for my 4-108 - it's time has come & gone.
I'm considering a Beta-38. Other than getting corresponding comparible speed & torque values, how do you calculate the required torque for a sailboat?
Length - 40'
Waterline - 27'
Beam - 11'
Displacement - 20,000# (US)
Prop - Luke 15" 3 blade folding

I'd like to get the lightest weight engine that "will do the job".

Thanks,