Morning all,
The last episode of this chronicle concluded with a picture of the broken rudder case and the statement that it “Might be a little while before the next report…”. My apologies to those of you who were buoyed by that last statement! But you know where the delete key is. Actually, I see that this note has got kind of long and technical…you are forewarned, so check where the delete key is now if you need to. (Though you may want to go to the pictures first for some amusement.)
You may also recall from the last episode that Alec was the purveyor of destructive kite loads. He is redeemed. Thanks very much to him and Marcel for helping with SS experiments on the weekend!
Nacra/speeds/loads
Design of a much stronger rudder case for the Nacra is under way. That needs a little tweaking before construction starts. So, meantime, we’ve experimented with handling SS (single skin) kites.
Although SS kites have been used on the Nacra, their intended use is on a bigger vessel. Launching any kite from the Nacra, given the small platform, will be difficult other than by drift launching – and drift launching an SS is not going to happen! Having said that, if the Glidefree lifting hydrofoils work well, for the speeds I’ll be looking for the kite size required is not so great that it can’t be launched normally to a harness then transferred to the boat.
Perhaps “…the speeds I’ll be looking for…“ requires clarification. AW and I did 18 knots on the Nacra in displacement mode with a 14m kite – then jumped off and went for a swim around the Manly leads…kiting on the boat was more fun. That kite, on 50m lines, was on the edge of being dangerous to handle on a harness in the 20knots or so we had that day (OK, it was arguably on the wrong side of “..on the edge of being dangerous…” as I did have to trigger the primary QR once as I headed for a breakwall…a 14m kite arcing through π*50m ~155m in 20 knots is not to be messed with!). We’ve done 15 knots in about 15knots of breeze with a 12m SS with 3 crew on board (25m lines). The point is that the extra drag in going from 15 knots to 18 knots, and hence the extra grunt required, is significant – if you don’t have lifting foils!
Without much science/modelling, I’m anticipating that 25knots when foiling (and before breaking bits!), will be possible with a kite size that can be safely handled on a harness. Higher foiling speeds will require bigger/more powerful kites. Although that will be quite possible (eg drift launching), those speeds will also require much stronger boards…which will be quite a bit more problematic. So, “…the speeds I’m looking for…”?: – essentially 25knots in the medium term with a kite that you could use safely if you kited with a board.
Bigger boat
BUT…a bigger boat with winches to handle line loads….that’s a different story. More on that bigger boat in a later missive.
Mast launch of SS
A bigger boat would provide an adequate platform to use a light stayed mast for launching – which would be stowed when not in use. This video shows 2 windsurfer masts, one inserted in the other. Total height is about 8m. Although the kite is controlled manually, a production version would have the 2 lines (yes, only 2 lines) going to winches through blocks either side of the mast. The video shows launching both with a sock and without. The best system seems to be to have a wing tip on the halyard – it’s not unlike launching a spinnaker. That is, pull up the head and when the breeze fills the sail the other 2 lines, brace and sheet, take the load. In the kite case “the other 2 lines” are the kite lines and when they become tight, the halyard is dropped to run free.
A production version of this will have the halyard block at the top of the mast on a quick release shackle. So when the kite starts flying the block will be released from the mast and the wing tip line will be secured, under no load, to the flying line on that side of the kite. (This would only be 8m or so long, depending on the size of the kite.) To recover the kite, it would be winched in (did I mention that after launching, both lines would be winched out to optimum flying length? Perhaps 100m for light downwind sailing and more like 15m for upwind sailing.) to the point where the wing tip line can be accessed. This would then be taken off the flying line and lead through a chute into which the kite (with the myriad bridle lines!) is doused – again, very similar to the way in which a dinghy spinnaker is retrieved.
Stacked SSs
I’ve stacked LEIs and Arcs (not much). The LEIs worked surprisingly well and arcs are legendary. So I have to try SSs. But the reasoning is different. SS kites are, unlike LEIs for example, scalable. So, as long as you can handle it, you can make it more or less any size – certainly as big as I could possibly want. But one speculates that perhaps it is possible to stack smaller kites to get the same power as a single large one but with easier handling. Very probably a recipe for disaster in the making …but …
The video (same one as referenced above) shows a 4m SS (4 line) under a 2.5m SS (2 line). The thinking here is that a small one can be launched, from a short mast for example, quite easily and safely. The mast height required is determined by the bridle length which is, broadly, determined by the square root of the kite size. A larger kite, perhaps much larger, can be attached to the flying lines of the small one. When the smaller kite’s flying lines have been let out far enough to take the lower kite past it’s bridle length, the lower kite’s flying lines can be tensioned so that it flies. Similarly, the lower one can be collapsed by the flying lines from the upper one when it’s time to recover it. Launching and collapsing the lower kite worked very well – quite surprising for a first test of another whacky idea. On the other hand, the upper kite was relatively unstable when the lower kite was flying. More experiments, and even some thoughts and design, required here – Michel/Gavin? I think the separation between the upper kite’s flying lines on the LE of the lower kite needs to be less…and the kite separation could be greater.
I have to be an actuary for a while and any of you who have read this far need to get a life.
Regards,
Trevor