Most well designed FD trawlers require about 3hp per ton.
Hummmmmm Willy- Assuming your statement is backed by knowledgeable marine architect, said as you have provided actuate data on many occasions, I have to ask from a point of "What"??????
My boat comes in at a weight of approx. 14,000 # (7 S, Ton). and is designed by Monk, a noted marine design/architect, who purposely designed our boat for that period of time of fuel efficiency was the driving force for economical, cost savings, hulls.
However, I don't believe the designed intent was to drive these hulls with 21 HP engines.
The original power for our boat was a 58 HP Perkins with a 3:1 gear turning 2150 RPM to achieve a hull speed of 6.9 knt. Your stated calculation may be a theoretical point as it is inconceivable that to achieve hull speed of our boat with 21 HP would require some formula of gears/wheels that escapes my desire to compute.
Could you extend your remarks to encompass the 21 HP potential?? Is the 21 HP a theoretical result, or is it a reality?
Thanks,