Clever stuff going on
Working closely with foiler Moth pioneer John Ilett Australian sail designer Phil Smith has been investing a lot of time and energy into a better – and safer – flight control system that offers longterm advantages for every type of sailboat that is moving away from dependence upon Archimedean principles…
The Bugs Cam Up until now the near-universal 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 about
54 SEAHORSE
40-45° 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, reducing lift and bring- ing it back to the chosen ride height. Over the years a lot of work has been put into working out how to deliver a smooth action that gives the best control and fast trim. Depending on where the forward
control lever is angled in relation to the wand determines which part of the wand’s arc you use (there is approximately 80° of useful wand movement) and this can to some degree vary the timing of movement from the wand to the flap (see Fig 1, overleaf). As the wand drops (with the boat
rising), using the rear quarter (A) of the arc 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 wand action speeds up. So with this action lift is held 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° 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 is adjustable to change the ratio between them and so vary the amount of movement delivered to the flap from the wand – what we refer to as the gearing speed. The control rod between the wand lever
and bell crank is also adjustable, which 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 affect the speed of control delivery. If you back off rod length and so reduce 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 slower. Increasing lift with the control rod while
shortening the wand to maintain correct ride height brings the wand further for- ward 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 and too 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 plain confused in moments of extreme manoeuvring. Fast trim at ride height is the slowest
control speed possible to maintain height control (minimal flap movement is less
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
