There has been a good discussion going on on seabreeze around the impact of channels on a twin tips performance.
The concensus seems to be that channels are about increase the grip of the board and is especially noticable when your fins a small or riding finless. It seems to me that they act like very long thin fins so that their action is felt along the full lenght of the board rather than just at the tips.
From the board analysis work I've been doing they wouls not seem to contribute greatly to the stiffness of the board although if the channels are built up from stiff material is may help. However, this would seem to be secondary to the flow control it provides.
It seems to make sense that the smaller radius channels ( smaller than the radius of the single concave) would mean that the angle that the bottom layer enters the water when edging will be greater and hence the increased grip. However, from my limited experience if you don't have sufficient rocker in the board,this stepper angle of attack will force the board up and out of the water and you'll loose your edge.
I have also read about the lossy effect of cross flow. This is where the water flows across the board instead of down the length of the board so that less lift is generated by the flow. I'm guessing that the channles might also redirect the flow a bit and help improve lift.
So how to add channels to the bottom of a DIY board?
I haven't got my head around this one yet. Single concaves are fairly easy with the aid of a solid rocker table. However, the channels add a new level of challenge. In production boards I believe they use precise aluminium molds that have surface contours in place and then they load them up with up to 90 tonnes pressure for short periods ( 20 mins) so everything gets squeezed together nicely. The DIY rocker table is more limited to say the least.
Thinking out loud, some of the approached might be:
i) lay the baord up with minus the last bottom layer of glass so that the rocker and concave are largerly held in the board. Cast of carve the channel profiles in a seperate piece of material ( foam, balsa or just build up from scap glass and resin) then laminate the seperate channels under the last bottom layer
ii) Double bagging. Carve or build the channel profiles into the core prior to adding any laminate. Add laminate to the bottom of the board with the board on a flat surface.Put the flat board in a vac bag and suck out the air. Then place the core facedown on the rocker table and cover with a second vac bag and vac. clamp the baord down to the rocker table to introduce the rocker and the concave.
iii) Create the core by laminating very thing 2-3 mm layers but do so on the rocker table so that the adhesive between the layers puts the rocker and concave into the board prior to putting the laminate on. Then you could put the laminate on without the need for clamping it to the rocker table. The issue with this one would be the the spring back of the core in the absence of the exterior laminate.
iv) Carve the entire board out of a single piece of wood using a CNC machine so that there is no need for a rocker table. CNC machine would be need for this one.