Paul and Paul,


Funny enough I'm actually working on this aspect right now. But I'm trying to go for the most simple solution. I think such a thing is possible as we F16's really don't need to adjust our mainsail settings much for the downwind legs. Certainly not to the extend the A's have to do this. I think we F16 sailors can get away with optimizing our settings for upwind and just use what we got with under spi. Any prebend fine-tuning here is not going to make much difference at all.

However I need more time to develop the model and test it against real life measurements.

But I think I can give you guys a quick fix for in the mean time.

Assuming your sails behave almost the same as the A-cat sails, then I believe that the A-cat sail tops stay more flat when they derotate their mast in very light winds. Our masts have more difference in stiffness between fore-aft and side-to-side then the carbon A-cat masts. This means that when we derotate our masts our tops fill up, meaning the draft increases considerably and our leech stands up. Both these things are bad in very light winds. Here you want to have a rather shallow draft over you sail with a significant amount of twist (open leech).

In order to correct this we alu F16 mast sailors need to pull on heaps of downhaul to bend the top in its stiffer plane. The bend flattens our sails and also opens up our leech. However, if you are still sailing with the prebend settings for more mast rotation and stronger winds then the middle section of your mast will now bend away to far or too little. I haven't determined which one it is. However it is pretty certain that the prebend is not where it must be for the right draft in the middle of the sail. It is here either too deep or too shallow. My guess is that it is too deep. At least that is what I experience on my own boat in very light winds and I have to sheet out more to correct somewhat for this. Drawback is that my top leech opens up too much as a result, the top section of my sail is now generating less drive then it is able to. This works relatively well on the water, especially when suddenly the wind drops away and you have to do something with the controls that you've got, but still there is something to be gained here. As said earlier your rig may just a well have too little draft. So check for yourself which it is in your case.

The only quick solution for me is to INCREASE pre-bend before I go out in light winds. A more engineered solution would be to stabilize the bottom section of my mast more by changing the spreader setup but that is too much work for this time. So I just turn-up my diamond wire turnbuckles (just as landenberger wrote in his tuning guide). I still haven't done this often enough to give you guys a good estimate on how much is needed.

By increasing prebend when expecting to sail in lights winds the draft in the middle section is reduced. Additionally there can be a sufficiently large added flexing of the mast top to open up the leech even more without affecting the draft there. But this latter reaction is still being researched, it can't quantize it yet.

Anyway, the combined effect is that you can reduce mast rotation a little to reduce profile and then apply downhaul so to reduce draft in the top and open up the leech, both of which were negatively impacted by the derotation. The negative effects on the middle section are then corrected by proportional adjustment of the mast prebend you made on the beach.

You should now have a pretty small profile combined with a shallow draft and the leech has significant twist to compensate for windshear ; this is fast in really light winds . NOTE : we are talking about really light winds here, not just simple light winds. Think 0-5 knots.

Of course the mainsheet is sheeted on very lightly.

After the light winds have passed you just release some diamond wire tension (the amount you added) and you have regained your medium and heavy settings again. Simply counting the numbers of nutt or turnbuckle rotations will be sufficient.

Of course it can be that your sails are to shallow in light winds and then you guys must do the opposite of what I described here, you'll be reducing diamond wire tension for light winds then.


Some A-cat sailors are doing these adjustments while sailing (most when turning to sail another course) using the system shown in the picture. I suspect most of this setting is tuned when going from upwind to downwind, because here their sheet tension changes alot. From on hard to rather loose.



Personally I think we F16 sailors can mostly do this tuning on the beach and then forget about it for the rest of the day. This because we only need to adjust this setting for the winds and not when switching from upwind to downwind, we have the spi there remember ? That thing takes care of everything we may lack in mainsail trim on downwind legs !

That spinnaker has now become an integral part of the F16 designs. It allows us to cut corners (complex tuning systems) and thus produce cheaper and simpler boats without sacrifizing performance. The extra sail area we got up there will limit the hit we get when we decide to sail without the spi, but that only goes so far. In the really light stuff sail area is not that important, proper sail trim is, hence the experience that we can't really stay with the A's there. Of course the fact that the spi won't fill in these conditions is contributing strongly to this as well, we just lost 5% performance there. But by fiddling a little with your mast settings you should be able to win some back.


Good luck,

Wouter