The advantage of the cam-based system with the widest implications beyond the Moth fleet is the ability to integrate variable gearing into the foil linkage, rather than having to adjust the control ratio manually with a conventional linear arrangement. Compared with the control precision of the cam, standard flap control can be relatively vague with adjustments often required as conditions change; as a result Smith’s own boat (left) is set up more simply than the norm (top left). The shockcord cam return (top right) is external but everything else is tucked away inside the sprit and bow mount now being manufactured by John Ilett’s Fastacraft (above). Forestay tackle is for rake control. A lot of trial and error (opposite) went into the new system… involving a fair bit of swimming for the writer
drag and maintains a more constant lift) combined with as much lift as you can handle dialled in to the foil itself. 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 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-style system and design in better control timing and movement. What we need is for the flap to remain on full for longer for good initial lift, then to switch off to the correct amount of lift and control movement 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. Here the cam system comes into play.
Let’s first look at the simple make-up of the cam system (Fig 2, overleaf): 1. The cam is 3mm alloy plate machined to a designated control profile and incorpo- rates a ‘May stick’ on shock cord 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 very rigid 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 no longer needs to be adjustable while sailing. 4. The bell crank at the centreboard head should be approximately 1:1 ratio – and it’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 there to guarantee the full switch off when flap pressure is minimal! 5. Then it’s pushrod down to the flap.
Cam vs lever The overall movement of the flap at its control point (where the pushrod attaches to the flap) can be up to 14mm maximum 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, 10°. With the Ride Height in Fig 3 (overleaf)
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 (low) speed of 1mm per 10° 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 (more if you feel you need it) over the last 12°. 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 which immediately jumps to a speed of 3.3 over the first 10° 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 in Fig 1
which roughly shows the more linear action of the lever system dialled back to the same optimum ride height speed of 1; however, this 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
SEAHORSE 55
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98
