Weed Fins

The Urethane Fin Base system is easier to use than HPR5 & likely better, specially for production purposes.

HPR5 epoxy is softer than the Urethane system, however the mechanical properties of the HPR5 fin base can be modified, the quanitity of graphite/ calcite added to HPR5 tends to govern the finished fin base hardness.


HPR5 is hard to work with, requires heat to make it pourable & to allow the air bubbles to escape. Our fin base molds are heavy steel which are pre heated allowing all air to be removed without using a vacum process.

Also I would advise not to use chopstrand or fibers in the fin base, thus only traps air.

light wind fins use this. they bend to windward under load. Formula fins for light wind use 45 deg bias carbon to minimise the twist but allow the fin to bend. They do this when theres not enough apparent wind strength to use vertical aerodynamic lift sources- eg gliding through lulls.



Rolling the board to leeward increases the aerodynamic angle of attack of the board, which increases the nose lift causing it to pitch upwards. Overall this is quite unstable especially in chop, and it creates an easy opportunity for ventilating the low pressure side of the fin when going through chop. The feeling of rolling the board to leeward is more about reducing the vertical force applied to the board and taking the weight with the rig. The board itself is not actually railed over much. No I'm not talking about 30m long old skool raceboards that are not planing.



The sail is running much higher L/D than the fin due to the higher lift coefficient (from higher AOA). Compare that to ~1 degree for a symmetric fin. Using the fin to generate hydrodynamic lift is better than using the board, but the aerodynamic lift from the board and the sail are even more efficient. Remember the board is running in ground effect with the surface sitting several degrees higher than the trim angle (due to the rocker). We need this aerodynamic lift to counterbalance the pitching moment from the rig. Ever tried sailing a board thats snapped in front of the mast track?

Also, hydrodynamic drag is working directly against the direction of travel. Perfectly coupled to have maximum impact on our efficiency. Aerodynamic drag is works in the direction of the apparent wind, and if we are sailing forwards the total aerodynamic forces (WRT the axis of travel) can only be resolved as sideways and forwards. Aerodynamic drag therefore serves to increase side load whilst eroding thrust, which wastes our limited ability to generate moment.

The optimum setup has minimum vertical hydrodynamic lift (and therefore drag), maximum forward force =>apparent wind more from the side=>going downwind, maximising use of our available leverage: high sail AOA for max L/D and highest coupling of sail moment to forward force, and also minimal side load which creates fin load which (unless in the drag bucket) increases drag which difrectly slows us down.

It may seem strange but all of the actual drag working against the direction of travel is produced by the parts in the water. The importance of aerodynamic efficiency comes in minimising the lift requirements on the hydrodynamic lifting surfaces since lift=> drag.

Sorry for going OT.

Fred, I've tried several urethane systems and they all shrink to a degree. I overpour the mould by the shrinkage amount to compensate, and then trim off the sides with a knife. I also use vacuum degassing. Some of the urethanes are very flexible so I use chopped glass mat on them. The main issue I have with urethane is the curing schedule. It varies a lot between formulations, so you have to be careful to get it right. Rubberised epoxy is probably the way to go but I havent used it yet.

Please keep it going guys. I'm so far out of my depth here it doesn't really matter but it is really interesting. Thank god for clever people or we would all be sailing downwind.