Designing a fast open deck catamaran

Beside struggling upwind , shallow water board handling is important. Did you start your build already ? Board and boardbox materials ?

 

Shallow water capability definitely is important. My design currently have draft of 0.55m at 8.8 ton displacement. I suppose with boards partially pivoted up I could still somewhat sail upwind in as little as 1m water although using engine probably makes more sense in such conditions because with rudders also partially pivoted up there would be huge weather helm force. I haven't started building anything yet, it is brainstorming phase currently. Hangar where I will build also needs to be cleared from junk. Boards would be wood core covered with fiberglass like most homebuilt boats have. Board boxes would be plywood covered with fiberglass.

 

Why would you look to Mumby as an example? A guy with no credentials and no website selling knockoff Shuttleworth catamarans made in a Philippine jungle? Sure, they might be fine boats with a long track record, but maybe not the best optimized.

Someone in the cruising forum said their Mumby 48 does 5.37 knots VMG in 15 knots true. Compare that to the Shuttleworth 35 cruising cat which does 7.3 knots VMG in 15 knots true. Mumby claims the Mumby 48 has seen a top speed of 22 knots, while the Shuttleworth 35 is claimed to have seen a top speed of 24 knots with a crew of 9 people onboard. One would think the much larger Mumby would fare much better against the smaller Shuttleworth design.

Here are the polars for the Shuttleworth:



Shuttleworth, by the way has this to say about daggerboards:

"There has been a tendency in multihulls to reduce the size of the daggerboards on the basis that they sail faster and, hence, need less area to provide the same lift. I have never agreed with this philosophy and in my designs I am making the daggerboards as deep as the structure will allow. I would consider 11 foot draft as normal for a 60-footer, with the board down. Bruce Banks, sailmaker for Livery Dole 111 said "The only boat that could come near to this for pointing ability is a 12 Meter." That is high praise from one of the sailmakers to the Victory syndicate. Therefore the boards on this catamaran are larger than usual, with the intention that both should be used in light airs, retracting the downwind board as the wind increases."

Now lets compare the daggerboard of the Shuttleworth to your design: