Having built many mono hull boats thought it time to try a catamaran as they say 2 is better than one.
After searching many sites I could not find any plans so I turned to mr Google and after looking at hundreds of images set to making a scale model then drawing up a mid section profile full size.
With the help of a neighbor a past Olympic champion sailing a Tornado I finally got what looked like a good profile and an economical one using 6 mm 2400 x 1200 gaboon marine ply.
With the stern and one forward bulkhead I only needed one mid section form made from MDF.
The bows I keep short to give me max water line length. I did have some trouble bending the ply on the lower section of the bow and found what we call bendy ply 5 mm and great for the job. I used cable ties in the stitch and tape method.
Glassed the outside of the hull and inside the two bow sections. I set the outboard in as my man cave shed is only 5 m long so managed to get the cat to 4 m.
I had read a lot about the problems with a centre outboard on cats with cavitation etc with aerated water through the tunnel. With no surprise this was true and it took four or five adjustments to solve the problem.
I lowered the outboard, gave it a little more trim to keep the bow up and put a water deflector just in front of the outboard only 150 mm long and 100 mm wide shaped like a sharp bow. Some water splash deflectors to keep the spray down and success. This solved the problem of water hitting the front of the outboard that is usually out of the water on a mono hull.
It proved very successful and I only get cavitation when heading straight down on a following sea in waves over 200 mm . I have found I only need to run off about 30 degree and cavitation stops. You can see from the photos I have fitted fold up wheels on the back (sea legs) and a removable set for the bow. If anyone is interested I am happy to share my design. The one thing I would change is have a little more sharpness on the bows. With my new 30 hp mercury light outboard BANANA SPLIT cruses at 20 knots with top speed over 25 knots.