Left: the Full Lift Zone shown corresponds to Fig 2 opposite. The boat has risen on full lift to the end of the Full Lift Zone 1 marked on the cam (opposite), the wand paddle is now approximately 200-300mm from the bottom of the boat at this stage (zone 1 is the only segment of the cam with a continuous radius with no gearing change, so the cam has held the follower still and the flap stays on full). The cam follower is now about to roll through into Zone 2 on the cam where the lift will steadily dial down as optimal ride height is approached


Left: the Ride-Height Zone corresponds to Fig 3 opposite. The system has settled into Ride Height Zone 3 on the cam and it’s here that all the planets must line up. The goal is now to achieve the slowest control rate possible that enables the ride height to be maintained. This is the time when the foil that you have chosen of the right size/type for your weight/skill should be in its most efficient performance window. This Ride Height Zone should be as wide as practicable


Left: the Full Off Zone that is shown here corresponds to Fig 4 opposite. The boat has strayed up too high and the wand has flicked right forward to full-off at the end of cam Zone 4. The boat will now be dropped back to ride height ideally as smoothly as possible. You can tailor each of the curves machined into the cam so the boat reacts as you want across all of the flying range


from taking the slow gearing right forward and having no emergency off ‘notch’ right at the end – having too much switch-off can drop the boat too much… and the rebound can be a killer. So a slow ride height gearing is very


Tuning the cam Switching to a cam system means taking onboard the fact that the cam can deliver the best manual control movement possible over the full range using a wand as a sensor. It’s all about developing a tuned boat before the race starts and then getting down to heads-up racing rather that having your head in the boat pulling string. If you feel you need to introduce adjustable gearing to survive, or go faster, then the cam profile is not correct, so make it correct. You can develop a cam curve that involves other controls while sailing and still get better results than with a lever, but that’s not the point of going to a cam, it’s about accurate simplicity. The only time you should adjust bell


crank ratio is to very finely match the overall cam movement to the overall flap movement. If the cam has 14mm in its design and you only need 12mm the control rod arm connection point can be moved up to gear down its overall move- ment to 12mm (or re-profile the cam to suit). This is easily checked ashore. On the water you check that the foil is


giving the correct lift at ride height. If the wand is too far back and engaging too much with the zone 2 lift bump you need to increase control rod length to move the wand further forward (approximately 40° back from vertical) and the cam follower will move closer to the centre of the ride


height zone. Then it’s just wand length to suit height and you’re ready to go (it’s just like going out and setting the jib cars). So far all the foils I have used with the


cam have responded in the same way to the same cam. Mind you, they all had flap area-to-main foil ratios that were similar. Foils that are pushing the boundaries of


minimal flap area along with fused tips are going to be harder to get control from regardless of the control system choice. Foils of this nature are fast because they actually water down the effects of poorer control inputs and would be less affected by lift leakage purely because the flap area is smaller and so less of the overall foil area or amount of lift is disturbed. My take on this is that if the control


movement is minimal enough at ride height and the foil flap is stiff enough, then the flap is almost ‘invisible’ in the ride height zone 3 and so can be kept at a more effective ratio of area to actually be of assistance when needed for control. A larger flap needs to move less to have an effect on control than a smaller one, so if correct attention to detail is observed in construction of the foils and cam speed I fail to see a problem in having a larger flap. Then you have speed and control – this will become more obvious with time. To date I have decreased the amount of


overall flap I require from 13mm down to 10mm. Most of this reduction has come


stable but it can’t cope with sudden drops in height; with the cam system the lift bump behind ride height quickly turns on lots of lift to arrest the drop then quickly turns back off to kill the lift at ride height. This minimises boat drop and rebound, the latter the reason why I’ve now been able to eliminate the late extra flap switch-off on my boat, preferring to rely on the right amount of rudder trim to come down more slowly to ride height and not slam down and need full lift to stop nosediving or a big rebound later. (A trip to the moon). In the future it may be good to work on


a hydraulic line to the flap from the cam follower. It gets rid of the bell crank and any control rod flex and friction issues and would allow centreboards to be raised while still connected to the cam system. That’s one of the beauties of keeping the system at a 1:1 ratio. Every cat class that wants to foil should


really be using twin T-foils that are angled out about 8-10° (around the angle a Moth naturally cants to windward) and a cam control system similar to that described. Going a step further, on crewed boats if they employ a hydraulic line they could also raise the windward board, or both foils for launching and landing. They would foil earlier… and more safely. But I guess they’ll work that out in time too. Safer foiling, everyone.


Phil ‘Bugs’ Smith Avalon Sails, western Australia


q SEAHORSE 57


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