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. Depending on what part of the arc you choose to use, can vary the style or timing of movement from the wand to the flap. As the wand drops, using the rear quarter of the arc, flap movement is slow to begin with and speeds up towards ride height. Using the lower quarter gives the fastest and most linear movement through the range and using the forward quarter, flap movement comes off fast then slows toward ride height.
Now, what we are after is for the boat to have strong lift to ride height and good control when at ride height, simple. Looking at the choices of arc positions the rear arch at first seems the right way to go, it holds lift on well and shuts off lift reasonably too, and it is ok. The lower arc choice turns off lift too fast and the movement is too linear (explanation of that coming). Using the forward arc works well if you jam on so much lift (with the dial increasing control rod length) that the wand can’t go back any more than the paddle being 200-300mm below the bottom of the boat. This means that you have max lift to break free of the water and then lift comes off quickly to ride height, lift is also available quickly on the drop of the boat. Downside, the wand jamming back on the system can cause failures and there is some drag when not foiling (not that that matters much).
The main problem using the arc system is that the control movements are too linear or smooth to give us the control we imagine we need, that’s why we include dials and gearing systems to help, 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 stress (you know what I mean!) To have strong lift to ride height then have the boat settle to ride height with good control (that’s where gearing comes into it) is not a linear action overall.
So, enter the cam. In simple terms it lets you escape the restrictive control patterns of the arc style system and design better control timing and movement. What I was after was for the flap to remain on full for longer then switch off to the correct position and timing to maintain trim, in affect creating a better, stronger bubble or cushion of lift to sail on.
To start with I measured the amount of movement in the flap at the point that the control rod connects, my foil was using around 12-13mm from full on to full off. This then was to be the range of movement that the cam needed. The bell crank then needed to be made a 1 : 1 ratio so that what was happening at the cam was what was happening at the flap (you could eliminate the bell crank altogether with a cable type connection to bring the direct drive through). Next was the design of the cam, with 13mm or so of movement to build in, the choice of radius was important. If you go too small the “ramp” from on to off can be too steep which causes some basic mechanical issues and control could get harsh and twitchy particularly in the direction of lift coming back on, in reality the larger the cam the easier it is to “tune” your designed control curves and the action is smoother and engineering wise more robust.
So I built the cam system with all these factors in mind and cobbled it all onto the bow of a Prowler Zero. This boat has been a bit of a test hack so was sporting a good foil to work with. In my mind things should go like this, put foil in boat and connect control rod, make sure you have full flap with wand back for take-off (control rod is still adjustable but no dial), throw boat in the water and go sailing only needing to adjust rudder trim and wand length to suit, no dial or gearing adjustment necessary. Now on the first test sail the first thing that the cam made stand out was that I had way too much AOA on the foil, the boat was sluggish getting up and pressured I could drive it off the foil, it was stalling, also with the much shortened bell crank my control rod was bending under the extra leverage the flap had and so I was getting mixed messages back from the boat. So AOA was then backed off to a new sane amount and I modified the control rod set up to suit. Next test sail the boat just jumped up to ride height now I had found a more suitable AOA for the foil, switch off at ride height wasn’t aggressive enough so a re-profiling of the cam was needed (the non-linear nature of the right numbers was even greater than I anticipated), I did however rotate the cam back a bit to address this on the day and got things working quite well, I was a happy camper.
I made a few more changes to the system to improve reliability and guarantee smooth operation, re-profiled the cam now I was getting a better idea of the numbers that give the intended control, and hit the water again. Well…. It was as good as I’d imagined. The boat was strong in lift with great control and ability to maintain trim, it is much faster now too as the foil is in a much better section at ride height, and tacking and jibing is far better too, going down the Swan River in 25kts+ wind doing 30 odd knots is amazing. I keep getting nervous thinking somethings going to give but it doesn’t, the brain snaps before the boat and going into a jibe is now a time for quiet contemplation before you unleash it on the next leg!
But racing is where it counts, and I’m happy to say that the old Prowler is totally on the pace and some. As I’m getting more familiar with the way the boat trims and its new capabilities’, and of cause getting my own act together again after winter, I’m getting harder to beat, and the Kiddies don’t like it, being pasted by a 56 year old grandad is not high on their list.
What I like is that the boat has become so easy to sail, I’m happy to say all I need is rudder trim and wand length control. There is no need for an adjustable rod (dial) or gearing while sailing. Set the rod length for max flap and you’re done, check you have the best AOA for your foil and your weight at lift off (now made easier by the extended controlled lift zone, no flap comes off in this), you need to find the most minimal AOA with satisfactory lift which then not only lifts better but the section will be better at ride height too, and less flap up is needed to keep you down so more control also. Dial in the cam to switch off as late as possible and then set up the ramp off curve to give the desired gearing for control. I cannot see at this stage why you would need to change the gearing once you’re set. The other day we were out testing and we had marginal conditions to start with and ended with 28knts, all was fine through the range but time will tell. You can however tune the gearing “feel” by rotating the cam on the wand shaft, rotating the cam forward delays switch-off and increases lift so your wand will be more forward, shorter and so faster acting. Backing off the rotation means switch-off is earlier with the cam producing less lift so the wand will be more angled back and longer to maintain good ride height so the action will be a bit slower or softer.
Anyway the next step (and some are doing this now) is to fit this system into a bowsprit, not so much as I think that the bowsprit is that important, but it’s a neat way fit the system to the boats, and any extra control gained by the bowsprit registering greater pitch movement can’t hurt. This will be completed by end of January so stay tuned.