BUGS CAM (a cam style control system for foiling boats)
Up until now the main method of flap control for the main foil on Moths (and other T-foil boats) has been via a form of lever or other attachment to the wand that scribes an arc to deliver an initial control movement through the control rod to the flap.
As the boat rises the wand drops and swings forward taking the control rod with it, in turn reducing the flap angle on the main foil so eventually lift is reduced and the boat stops rising when lift pressure equalises with the weight it has to carry (we call that ride height). The wand is usually at an angle of approx. 40-45degs back from vertical trailing on the water’s surface at this point so if the boat tries to rise further the wand will continue to move forward further reducing lift and bringing the boat back to a safe level. If the boat drops the wand is forced back and more lift comes on to keep the boat up. Over the years a lot of work has been put into working out how to deliver an action or timing of movement that gives the best control and fast trim.
Depending on where the forward control lever is angled in relation to the wand determines what part of the arc you use (there is approx. 80degs of wand movement available to usefully utilise) and this can to some degree vary the timing of movement from the wand to the flap. (See Fig.1) As the wand drops (with the boat rising), using the rear quarter of the arc (A) flap movement is slow to begin with and speeds up towards ride height. This effect is further enhanced by the fact that the wand is acting in the same way in relation to the water level, as the boat rises the wands action speeds up. So with this action lift is held on longer and control action is faster at ride height. Using the lower quarter of the arc (B) gives the most linear (even) movement through the range, and using the forward quarter (C) flap movement comes off fast then slows toward ride height.
Behind this is the bell crank, a two armed 90 degree lever that links the control rod with the final pushrod that goes down the centreboard to the flap. The length of either the wand lever or the control rod arm of the bell crank can be adjustable to change the ratio between them and so also vary the amount of movement delivered to the flap from the wand, this is what we refer to as the gearing speed. The control rod between the wand lever and bell crank is also adjustable, this independently tunes the amount of flap that is forced on and so is a lift control, along with a wand that is adjustable in length and a lift/ride height control as well. These last two cross over in their jobs, and adjusting them can also effect the speed of control delivery. If you back off rod length and so reducing lift, but maintain lift with wand length the wand will be longer and angled back more under the boat and the action of the wand is then slower. Increasing lift with the control rod while shortening the wand to maintain correct ride height brings the wand further forward and the action is faster. Lots of stuff to fiddle with.
The main problem using the arc system is that overall the control movements are too linear or smooth to give us the control we always imagine we need, that’s why we include dials and gearing systems to try to fill in the gaps if possible, the problem being that you can get very lost in the numbers and just plain confused in moments of extreme manoeuvring and you have an intense time helming your way out of trouble. Fast trim at ride height is the slowest control speed possible to maintain height control (minimal flap movement is less drag and maintains a more constant lift) combined with as much lift as you can handle dialled in to the foil. This is fine if the conditions are glamour but there’s not much of a safety net if things turn nasty or you have some full on manoeuvring to do. So you pull some strings and rearrange your settings to increase the gearing (speed) to gain some more control, but then you upset your nice smooth fast trim with a slower bumpier one. You rob Peter to pay Paul.
So, enter the cam. In simple terms it lets you escape the restrictive control patterns of the arc stylesystem and design in better control timing and movement. What we need is for the flap to remainon full for longer for good initial lift then to switch off to the correct amount of lift and controlmovement at ride height to maintain good trim and speed, this along with control built in to best deal with the more hairy moments we experience.
Let’s first look at the simple makeup of the system with Fig.2
1. The cam is 3mm alloy plate cut to its control profile and incorporates a May stick that shock
cord is attached to pull the wand onto the water’s surface at the right pressure. (Tension 1.)
The cam is locked onto the shaft that the wand pivots on and so turns with the wand.
2. The cam follower is a ball bearing pulley wheel set in the end of a 20mm plastic rod section
that in turn runs in two round Teflon plain bearings now within a bowsprit tube.
3. The control rod is a stiff carbon tube (windsurfer batten tube is good) that runs in a direct
line from the cam follower through the bowsprit tube and “service” tube into the boat and
to the bell crank. The rear section of the control rod should be adjustable for fine tune of lift,
but doesn’t need to be adjustable while sailing.
4. The bell crank at the centreboard head should be approx. 1:1 ratio and its’s good to have the
control rod arm extend up a bit to fix a second shock cord control (Tension 2) to make sure
the cam follower chases the cam all the way to off (flap right up). If your control rod is nice
and free running then this is almost not needed, there is plenty of flap pressure all the way
to the end of the ride height zone to keep the cam follower in contact with the cam at all
times, It’s mainly to guarantee the full switch to off! (there is very minimal flap pressure
5. Then it’s pushrod down to the flap.
Fig. 2 corresponds to Full Lift Zone in Fig. 5
The boat has risen on full lift to the end of the full lift zone 1 on the cam, the wand paddle is approx.
200-300mm from the bottom of the boat at this stage (this zone is a continuous radius, no action, so the cam has held the follower still and the flap stays on full), the cam follower is about to roll through zone 2, the dial down of lift to arrest the boat at ride height.
Fig. 3 corresponds to Ride Height Zone in Fig.5
The boat has settled onto ride height zone 3 and it’s hear that all the planets must line up. The goal
hear is to develop the slowest control speed possible that enables ride height to be maintained. This is the time that a foil you have chosen of the right size/type for your weight is set to be in its most efficient section for performance. This ride height zone has to be as large as possible.
Fig. 4 Corresponds to Full Off Zone in Fig. 5
The boat has strayed up too high and the wand has flicked right forward to full off on the cam at the end of zone 4. The boat will be dropped back to ride height. You can design a curve at the end of this movement to have the boat react as you want. It’s best if this is like a “notch” at the end so the cam follower stays well away from it during normal ride height.
Comparing the Cam to the Lever System.
The overall movement of the flap at its control point (where the pushrod attaches to the flap) can be up to 14mm max at the flap. With the Cam you can design to this or any amount of overall movement required for a particular foil. The Cam is like a curved graph so we can now see what that movement and its speed is.
Let’s start a new language by calling it a speed over a duration of rotation, say 10degrees.
In Fig. 6 Control Speed you can see a cam profile and the cam follower in ride height zone.
3. Over the duration of this zone the cam follower moves the flap at a speed of 1mm per 10degs of rotation, so we can call that speed 1 and it’s linear over the whole zone, this appears to be a good gearing at ride height. If the boat rises and the cam goes all the way to the end of zone 4 the flap drops off 2mm (or even more if you feel you need it) over the last 12degs. So that’s an average speed of just under 2, but it isn’t linear as its switch off speed increases towards full off (With the last flick forward of the wand the flap goes off like a switch). If the boat drops from ride height the cam rotates back bringing the follower In contact with zone 2 that immediately jumps to a speed of 3.3 over the first 10degs back but slows to a speed of 2.1 that tapers to 0 (no movement) as it moves onto zone 1 of
full lift. This helps keep the boat from dropping too far and pushes it back to ride height as quickly as possible.
Compare that action to curve A which roughly shows the more linear action of the lever system dialled back to the same optimum ride height speed of 1. You can see that it lacks the handy lift switch off (a lesser issue) and the reserve of lift close at hand if the boat dives (a big issue). Usually to combat this when conditions get rough gearing is increased and the result is curve B. As you can see there is now full flap movement to off and more lift available sooner but ride height speed has now increased to 2, too much for a smooth ride height trim but still doesn’t match the response speed of the cam which can be 3.3 when needed.
Sailing With Bugscam.
On the water once the boat has been set up right all you need to do is adjust wand length to whatever ride height is desired and do a bit of rudder trim. On occasions I may tweak control rod length between races (just some fine tune, usually after changing between foils and I’m just resetting ride height trim to suit) but I don’t touch it during racing, my boat is free of excess control lines. Downwind it’s really interesting to play with the combination of wand length and rudder trim. If you keep the wand long and you are high you tend to use a bit more rudder to keep the boat planted. This pushes the bow down and the wand further back meaning the cam follower starts to engage with the faster zone 2 with plenty of control action and feels OK. But if you shorten the wand to drop the boat a bit more
then back off the rudder lift, the bow comes up letting the wand move forward so the cam follower stays on the geared down and very slippery zone 3, it feels like the wand is out of the water but it’s still in and active. This is the trim to target up and downwind, the control zones are there ready to assist when needed, remember it’s the wands position relative to the cam zones that you should focus on.
Sailing with low gearing is really very stable, with less reaction to waves it’s like the waves disappear and so have less impact on the boat, the reserves of lift control need to be close enough to either side of the what we could call the ride height “platform” and there ready to put the boat back on the platform only if it strays off it while gybing or dealing with troughs or backs of larger waves, the extra lift coming in strongly is also a benefit while foil tacking (badly in my case) I fitted the cam system to a bowsprit on my boat and did it in a way that utilised the old control rod when the wand was mounted on the side of the bow (this is how the boat was in Garda) but in testing after we came back I had come to the realisation that I was getting some deflection in the new longer control rod and with that a loss of lift at critical times, this leads to miss information between the cam and flap. Upwind just when you need to be pushing off good solid lift to stay in trim it fades and temps you to dial in more lift than you need, then on occasions you have too much when the pressure backs off and the rod straightens, downwind the boat can press down too much and then rebound back too much causing you to look else ware for solutions. I have recently fitted out the boat in the way I had always intended it to be done if you were building a boat from scratch (it was really easy in the end and the way it should be done on any existing boat) and the results were incredible, so much more stability with the boat doing what it’s being told, a good lesson in making sure that no matter what control system or foils you use, any loss of lift due to flex in control rods, wands, flaps and foils cannot
be tolerated. The change was removing the old control rod that came in on a slight angle from the bow side and fitted a “service” tube from the bow to the bulkhead lining up with the bell crank. The bowsprit tube now slides into this at the bow and is a very strong attachment method. A new control rod of stiff carbon tube now goes from the end of the cam follower straight to just before the mast stump (the adjustable wand control lines also go down this tube and into the cockpit, much neater), then it becomes a short section of 5mm S/S rod and passes through a 5.5mm hole in the mast stump, this acts as a midway low friction guide. The end of this S/S rod is threaded into another section of carbon rod that continues to the bell crank connection fitting that is threaded in the opposite direction
and this becomes the adjustable part of the control rod. Getting that sorted eliminated any loss of lift due to rod flex and keeps rod movement friction to a minimum. After this change I sailed with the same cam and foil and couldn’t believe the improvement. As the boat was more stable without the lift changing under load I was confident to re-profile the cam with a larger ride height zone 3, this keeps the faster control gearing zones either side more distant creates a larger “sweet spot” of fast trim to aim for.
Tuning to the cam
Switching to a cam system means taking on board 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 some heads up racing rather that your head in the boat pulling strings when you can. If you feel you need to introduce adjustable gearing in to survive or go faster, then the cam profile is not correct, so make it correct. You can develop a cam curve that uses other controls while sailing (other than the wand and rudder trim), and get better results than with a lever system, 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 fine tune match the overall cam movement to
the flaps 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 movement to 12mm, or re-profile the cam to suit. This is checked before you go out. On the water you check that the foil is making the correct lift at ride height. If the wand it 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 (approx. 40degs 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 start (it’s just like going out and setting the jib cars)
So far all 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 where similar. Any foils that are pushing the boundaries of minimal flap area along with fused tips are going to be hard to get control from regardless of the control system choice. Foils of this nature are fast because they actually water down the effects of bad control systems input and would be less effected buy 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 has come from taking the slow gearing right forward and having no emergency off “notch” right at the end, having too much turn off can drop the boat too much and then the boat has to recover from that drop, the rebound can be a killer.
So a slow ride height gearing is very 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 minimizes 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 the boat float out to off and 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 nose diving 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 possible 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.
Cat classes that want to foil should really be using twin t-foils that are angled out about 8-10degs (around the angle Moths naturally cant to windward) and have control systems like this. On multi- crewed boats if they use a hydraulic line they could raise the windward board if they wanted. They would foil earlier and safer but I guess they’ll work that out in time too.
Safer foiling everyone,
Phil “Bugs” Smith
April 2018 (this article published in Seahorse Magazine April 2018)