centre board for nacelle

It is my first post so here goes.
I wjsh to do away with my full length mini keels and replace them with a centreboard or dagger board in the centre of the cat
My nacelle is touching the water and run to the middle beam and is solid grp 12mm thick .
What would be the cheapest way to get better<any>windward preformance without taking up yo much cckpit space
How deep would the foil have to go
Could i try and buy a second hand case of a f28 or similar production boatTThe clearance is 800mm water to deck
The cat is a crother 10 sports beach marine and does not sail to windward

 

From what I understand the Len Surtess catamarans use a center board in a central accommodation pod. It might be worth a look at, for ideas. There is a thread for the Len Surtess Catamaran, as well Len has his own web site. the thread at present can be found on page 2 mid way down.