Nice post by Patrik Diethelm (Patrik Boards, former designer at F2) on some of the considerations in designing a board for windfoil
Source: https://www.seabreeze.com.au/forums/Windsurfing/Foiling/Which-2021-2022-Slalom-Foil-Board--?page=3#4

Hi everyone,

just got an email from a customer asking me to reply on this thread...

There is a lot behind designs... - have fun reading !

Cutouts:
1. Slalom/Speed foilers rake their foil mast up to 4? to prevent crashing when touch down at high speeds. This means that the front wing points down when just floating and makes it very difficult to just increase board speed and take off. The much bigger cutouts sink the tail so the front wing is back on a normal angle of attack - not more angle. For Freeride foiling the masts are raked around 0?-2? and therefore need smaller cutouts. With big cutouts the tail would sink to much and give the front wing to much angle of attack which again makes it harder to take off - either the board shoots up to much or/and the front wing stalls.

2. The big cutouts have much less drag when touch down but important are also the angles coming off the rail and it need to be enough so the water can not follow the surface otherwise it is "sucking" and creates lot of drag.

3. An other important detail for early take off, acceleration, top speed and less resistance when touch down is to have sharp edges where the water flows off. Tuckeds have lots of advantages but definitely not in the last 200-300mm off the tail - even 1mm round has only negative influence except the purpose is to reduce planing, acceleration and speed. For entry level Prone-Foilers it helps to be connected with the wave/white-water until the rider is standing on the board and then pump and take-off. If the prone-foil board is fast with good acceleration the rider need to get up quicker otherwise the board takes off while the riders is still on his belly.

4. Similar to point 3) rockered tail shapes also change how the board floats/planes and helps changing the angle of attack of the front wing when pushing on the tail. But rocker is definitely the enemy for planing/acceleration and top speed and even 3mm more rocker on a small wave board tail feels like a lot and reduces planing, acceleration and speed significantly. Most wing boards have lots of rocker even 50mm and the planning, take off and touch down suffers a lot. Not to mix up with a "tail-kick" (starts behind the fin box and is never more then 2mm) which helps to reduce the wetted surface in the front of the board and increases a lot of speed but looses stability/control in stronger wind and chop.

5. Again similar to point 3 & 4) tapered outline curves help reducing tail width and volume to sink the tail but in flight mode the board is angled down to the upwind rail and makes the curved rail like an extrem rocker and sucks up the water when touch down and creates much more drag. At high speed touch downs it sometimes feels you get pulled to the front out of the straps.

6. Straight outlines feel like a catamaran hull when touching down and re-bounce easier but if the rail is to long it will be harder to "re-bounce" back up and the back of the board/rail will push/hold the board nose down.

There is more but lets say those are the main points for cutouts

Aerodynamics:
1. "With foiling the board is not so important as it flies" - I hear that a lot and makes my job as a shaper useless -right? Well, how many sailors can not get a stable run and blame the foil setup or the sail? How many test their setup all over the place - straps and mast-track back and forward, longer and shorter fuse, more or less tail wing lift, more or less mast rake, etc., etc. Also with the sail, t much power in the head pushes the board nose down? To much backhand pressure = to much back foot pressure lifts the board nose? Ever though about that Fin-Windsurfing likes the board slightly onto the leeward rail and get wind under the board to get the "fly over the chop effect" but foiling has the upwind rail lower and the wind is pushing onto the deck surface? Take your bicycle, ride 10km/h and hold a 1m2 big 3mm ply-wood into the air and feel what happens

2. To reduce as much possible wind-attack surface the foil-board need to be much shorter then a fin-board. But not to short as the nose will dig at high speed touch downs and to just lift the scoop will negatively effect the planing and the aerodynamics of the "frontal wind". The floating stability of Foil-boards is much better then Fin-boards as the foil feels like a stable anchor. So more about 200-300mm shorter then fin-boards seems to work well.

3. Board width depends on the leverage over the foil. With 1000 front wing a 1m wide board feels great but with 350 front wing the rider has to much leverage and "over-rolls" the board to the upwind side and need to compensate with sailing technique. At the moment PWA riders use tiny front wings but more forward with the same fuse size to still get enough lift for planning.

4. Deck concave's give stiffness and lower the mast-track position for more control. But concave's on the deck feel like a spoon catching the wind and push the board down. A lower mast-track feels more controllable and does not catch so muh wind but the water needs to flow out and the edges are more fragile while stepping on them which makes depth and size critical as well. A convex deck also gives stiffness and let the wind flow over it better. The deck shape is also important in the tail area and the flatter the easier the jibes. Concave / convex are mostly annoying when pumping, jibing even for worldcup stars.

5. Thin rails feel like a sheet in the wind and sink / suck water when touch down - thicker rails feel more like a ball in the wind and float / re-bounce better when touch down. Of course with convex deck and big rails the volume distribution and board weight need to be considered.

6. Angled deck/rail stands give better connection under the foot i.e. more control but it is harder to move the foot in and out of the strap when jibing. In general Freeride boards have flatter decks as jibing is more important then speed.

Construction / Prices / Warranty:
Which factory does not really matter as every brand has the choice of materials and constructions in any factory. If something is not well built the only reason is to cut costs to be able and have more money for marketing... ??
Prices vary if costs are cut and if a brand sales direct or trough a distribution network and with this comes as well the service and warranty handling.

Bottom line is too either care about price, believe in marketing and results or know what you are buying...

Cheers,

Patrik