For a fast displacement design there are currently two options. The 16-1 rule
is for Catamarans and is mainly used on larger commercial ferries. There are a number of 40ft-50 ft power catamarans (Schionning)using this system.
The problem is the narrow hulls are not really suitable for accomodation in the hulls and they "hobbyhorse" a bit in shorter lengths.
I have developed a triangular hull design which has a very fine angle of entry which continues back all the way to the stern.
The very fine bow angle of entry negates any bow wave being formed allowing for much higher than displacement speeds.
The wide stern stops hobbyhorsing, this allows for fast displacement to be used in lengths under 40 ft.
The design is not suitable as a monohull
because the narrow beam in this case 7ft 6inches for a hull length of 39 ft is relatively unstable. So I added outriggers.
My original version weighed in a 5 1/2 ton and the 2X 30 hp Honda
outboards were too small. Top speed of 8 knots.
I have recently fitted 2 x 90 hp tohatsu outboards but i am still in the "running in" period plus I need finer pitch
I did achieve 12 knots with both motors running at 3,000 rpm
and a very interesting 8 knots will one motor running at 3,000rpm. Max revs for these motors is 5,500rpm
An interesting thing happens as the boat reaches the 10 knot
mark, the whole boat lifts up and the stern wave is significantly reduced.
It could be that the bow wave "is formed" but much further back lifting the whole boat (this was a comment by another CF member)
The boat is NOT planeing because the bow is further in the water
than the stern (the boat has a flat bottom) No hydrodynamic lifting is involved as with a planeing hull. Plus I am using a lot less power. My calulations so far show a 25 hp /ton whereas a planeing boat needs 60hp / ton just to get up on the plane.