In the link below I talk about issues with the sail upwind. Its been a nice summer so far, reasonable breeze, but not too much. Except last night. 12 knots gusting nearly 30 and taking about 20 seconds to do it... In fact the gusts were so vicious I nearly stayed ashore, but it looked as if it was calming, as it does in the evenings in the UK, and so I rigged the small sail and set off. Late for the start for various reasons I quickly reached down 50 yards to the start and set off up the beat. Hey, I thought, this feels OK. Lots of wind - probably 20knots, but controllable and the boat seems to be planing nicely. Further analysis, it is planing well, I've got good height and I've just overtakjen the RS400 that started (also late) just ahead of me and several other things. Then I got round the windward mark, then it got very windy, then I gybed, then it got very very windy, then I got blown flat twice in 3 minutes on a beam reach and had to get the safety boat to give me a tug as the lee shore of the reservoir was getting awfully close... Game Over.
But the upwind performance was very encouraging. So what's changed? Basically the daggerboard. The original foil was a high aspect ratio high speed section and stalled out like crazy, making tacks difficult. So Andy made me a new board with a more forgiving section shape and a considerably longer chord. This, with experiece, seemed too large, so I've shortened it so that, compared with the original, the current foil is slightly larger in area, but a bit shorter and a lot greater chord. Plus its a more moderate section.
One swallow doesn't make a Summer, and I'm not sure I've sailed in that much air before, but it is most encouraging...
Rig Issues I posted some thoughts on the rig problems in various forums. Here are some of the suggestions.
I've now sailed the boat a lot in different conditions. Depowering the rig is fine, it all works nicely as predicted. The usual thing with a large roach mylar sail - yank on the cuningham, the top mast bends, top of the sail goes flat and the leech opens. The leech is pretty easy to control with the kicking strap, and certainly with full sail everything seems satisfactory.
Once the wind has got up enough to justify reduced sail with the stump things change. Offwind is fine, no problems at all, but it almost seems as if a switch is turned off when you point the boat up high, and it seems to be more difficult to get the boat in the groove and planing upwind steadily in 15 knots than in 7! My second opinion agrees with this, and we've talked it through a bit. We think something very dramatic must be happening to the whole rig in this regime - it seems as if suddenly there is a lot of drag from somewhere. As its obvious that it can't be the hull it must be the rig or foils - maybe both. The obvious conclusion is that the short rig is too stumpy, and has got so low and is so over-elliptical that the induced drag has gone through the ceiling. Another possible contributing factor might be that the centreboard - a NACA 63 series foil, is operating outside its ideal range as a result of the rig lift/drag curve and is overloaded.
With the short rig the ratio between foot length and luff length (which I believe isn't strictly aspect ratio but is obviously associated with it) changes from 2.54:1 to 2.26:1, and this is a fairly squat sail. By contrast the same ratio on my smaller roached (so not directly comparable) Cherub mainsail is 2.92.
I suppose the first thing to do is to gather some data. If the problem is induced drag then I believe I'm right in saying that this will be associated with a very large tip vortex, which is not the easiest thing to test on a full size rig, unless one just happens to have access to a 7m wind tunnel. I don't! I wonder if its possible to learn anything about the behaviour of the tip vortex with long ribbons on the leach, or even small pieces of ribbon on yarn? I don't really see myself managing to create a smoke generator that will enable me to have smoke indicators of flow at the top of the mast! Has anyone done anything with this?
This isn't going to be too easy to test out or rectify, short of an entire new mast and sail, taller and thinner... The lesson here is that there is more to designing a reefing rig than just drawing an ideal full sail rig and then providing a two foot slab off the bottom. Instead you've got to look at the rig in detail in both positions and - as ever - work out appropriate compromises. I suppose a starting point will be to try some different sails and see how things work out. I also wonder if one reason why the RS600 sailors seem to have more or less abandoned using the small sail in any weather conditions is related to a similar effect? F the small rig is relatively inefficient upwind it would make hanging on to the big rig a better idea because the extra loss in the gusts when dumping even more power would tend to be counteracted by the gain in efficiency in the lulls, plus of course the extra power on the reaches and runs.
Having sailed the boat six times in conditions from about 4 to gusting 18 knots
The rig has come out pretty well on the whole.
The mast may be a little stiff at the top, and perhaps the leech doesn't open to depower the rig quite as freely as I would like, but on the whole, so far, its good. It may well be, too that more flexibility at the top would have compromised other things as we don't have the topmast bend control that the cap shrouds give the Bethwaite flip-top rigs.
The sail is outstanding. Its quite the best sail I've ever owned. When you consider how little information Caws had about the boat to work from I think that is no mean achievement, and a credit to his understanding of this sort of rig. The sail is powerful and astonishing close winded and sets very evenly and easily. I suspect that the boat actually planes on a beat in less wind than it does on a dead run, which I guess has to indicate very low drag.
The bare mast tube weighs 5 kg! After a lot of umming and aahing I've decided on 12 m2, stepping down to 10.25 m2 for the step. This might be a slightly larger jump than would be optimum once I've learned to sail the boat properly, but I rather fear that's going to be quite a long interval... Given enough practice and so on it might be possible to go a little bigger all round, bearing in mind the RS600s now hang on to 12.3 m2 in 25 knots but I'm not at all sure. I suspect that larger area might be a little big, and the smaller one a little small. Who knows?
Anyway the dimensions that Caws will peg out for the first sight will be roughly:-
luff 6.1m leech 6.15m foot 2.4m foot round 100mm leech offsets 1050mm 825mm 500mm reef height 670mm
The profile is pretty much like a current generation Moth sail with strip around the leech of ~ 450mm, perhaps a bit more at the head, with the head at 90° to luff to end of first batten for a square topped sail. These dimensions are very similar to an RS600 sail - Bloodaxe measured one, but the RS sail has more luff round and a smaller roach.The details also need thinking about at this stage.
The most problematic detail is what happens to the luff in the area that will reef. The 600 has no luff rope but heavily reinforced cloth/tabling that rolls up neatly, but means that there's a gap between mast and sail which is highly inefficient. I was toying with the idea of combining this with a zip-on cover, a bit like the bottom of a 49er sail, but it would have to have two lots of zips, and it gets rather complicated. What I've decided is that we'll leave the luff groove right down to the bottom of the mast, so that it sockets into the stump (thus handily stopping the mast rotating in the stump) However we will cut a couple of lengths out of the luff groove for 3 or 4 inches above the gooseneck in both mast positions, and when the sail rolls up the luff rope will run out from that. The luff rope for a carbon mast is a bit thinner than for an alloy mast, so it shouldn't be too much of a lump when rolled up. The first reef will tidy up with a zip in the same way as the RS600, but I don't think I'll bother with that for the second one, which shouldn't get used too much. With the solid lump of the first reef there the second one should, I think, stay rolled round that well enough with a lashing or webbing buckle front and back. I'm going to aim to have the boom a little over 800mm off the deck, which seems fairly normal. We'll aim to have the aft end of the boom a little higher than the luff end, for more headroom aft...
I'm now even less decided about sail size than before. A friend now has an RS300, and pitchpoled it comprehensively on its first outing. It seems that the large rig version of the 300 is by no means underpowered... On the other hand the 300 has an unstayed rig, and I'm not sure how much that affects the dynamic of how the rig can shed power. The fact that my local club has just had two weeks running of 15 to 20 knots frequently gusting 30 also tends to make me want to think small. No doubt the fact that its been 5 knots all winter up to this is why up to now I've been tending to the larger sizes. Sailing upwind in such conditions with a conventional single hander rig definitely isn't my strong point - I seem either to spend so much time feathering up into wind that there's little forward progress made, or else, if I don't feather up, just getting blown over. Some of this could well be due to the lousy rig on the current boat, [Note - 3/6/98. I spent a day going over the rig on the current boat, and by means of some very odd batten tension settings have managed to persuade the 20 year old sail into a more reasonable shape, which has made about a 20% improvement in speed upwind in a decent breeze]but I think the major handling challenge for me on the new boat will be upwind. Upwind in heavy airs is definitely the conditions where a self tacking jib would make things a lot easier, and that's always a possible future development... Mind you having said that downwind in heavy airs on this boat might just be a challenge too, because I should not be at all surprised if there turns out to be a definite tendency to go mining bearing away from the windward mark.
I've been talking to a few people about rig sizes and so on.
The trouble is that there isn't a great deal of experience in the UK on the subtleties of varying rig size. It seems that the RS600, which is the closest relevant boat, doesn't use the step all that much, and the talented sailors can hold on to the full size rig upwind in anything up to the top of Force 5 (20knots ) plus. I'm told that in the 18 footers the change down from the biggest rig tends to be around 15 knots, whilst in Australia the twelve footers, which may have 4 sizes of rig plus a slab in the smallest mainsail, tend to be on their second rig in the 10 to 20 knot range.
I'm also trying to get hold of some data about what speed ranges actually get encountered in the UK, but this isn't easy. Data available seems to be as arithmetical means, whereas what I'm interested in is the distribution of different wind speeds. Also, as Bethwaite notes in High Performance Sailing, what interests the meteorologist about wind speed isn't what interests the dinghy sailor.
My interpretation of Bethwaite is a typical sea breeze comes in at around 6-8 knots and builds to maybe 15. Obviously you don't want to come in halfway through a race because the sea breeze has kicked in, so I'm thinking the big rig will need to do for up to 15 knots, and be able to manage a bit more with really OTT depowering measures. The step down, on the other hand, probably wants to be usable from about 12 -13 knots, but optimised for 15 to 25, which is as much as many clubs sail in these days. I'll also have a second slab in the rig, which won't have a matching "telescope" position for the mast, which will be very much a "survival conditions/sail home when its howling" sized sail.
I think the boat has about the same righting moment as the RS600, which means that the sails probably want to be a little bigger - maybe 10% - to bring the changeover point down to the top of a sea breeze.
The Moths and the Int. Canoes shared various events this year, and Andy Paterson has commented that although the Moths were only about 65% of the speed of the Canoes upwind, the Moths were definitely making big gains downwind because they were able to go much deeper downwind than the Canoes (both Classes tack downwind). This rather confirms my view that, if we can get a una rig that has good depowering ability and gust response upwind, perhaps by using some of the twelve foot skiff ideas on panel shaping and luff round in the sail, then the advantages of the una sail downwind will more than outweigh those of the two sail wind upwind and on very close reaches.
On the whole I've decided not to have too radical a rig, with so many other new factors in this boat. The conclusion I've come to is that the first rig for this boat - there may well be others in the future - will be as follows.
Mainsail only.
Sail Area a little over 130 sq. ft. (12sq.m.)
Mainsail Luff around 21 ft (6.4m)
Mainsail foot around 8 ft 6" (2.6m)
This should give a reasonably large roach sail in line with current Moth and Cherub sails in the UK., but not as extreme as the latest 12 foot skiff sails in New Zealand, which I suspect must be combined with a pre-bent topmast.The mast will be a non-rotating carbon mast, basically round in section but with an external carbon tube as sail track faired in to make effectively a pear shape.
The mast will be very heavily tapered above the hounds, and reasonably stiff below, and will be supported by spreaders, lower shrouds and a prodder. The Lowers and prodder will be attached to a lower section running to a few inches above gooseneck height, and this section will fit over the main mast section, and the main mast will "telescope" down into it to reduce the luff length and sail area for stronger winds. The idea of this, rather than the sleeve used by some other adjustable masts, is that it will be possible to remove the shrouds and change the mast height quickly and easily without having to remove the mast from the boat.
The rig is still a long way from being settled.
The basic themes are that the boat will need
· Very generous sail area for light airs
· Excellent gust response for what will undoubtedly be
a somewhat twitchy boat.
· Some means of efficiently reducing sail area for strong
winds.
· A rig that depowers smoothly and cleanly.
Of course how to achieve this is another matter.
The following developments are of interest in various classes,
and all seem to me, in their various ways, to provide useful possibilities
to explore. Unfortunately there is no way of combining all of
them.
· Frank Bethwaite's Over-rotating "flat back"
section masts of the early to mid 1970s, familiar to some from
the Tasar. This rig type is, as far as I'm aware, only really
being developed in the Australian NS14 class, and there's very
little information available in the UK.
· Current International Moth rigs, using a normally rotating
pear section mast or a round section mast with a sleeve luff
camber induced sail and a flexible top mast for gust response.
· Julian Bethwaite's "Flip top" skiff rigs,
featuring a top mast with a considerable amount of pre-load and
thus bend. This gives a "dual rate" spring effect and
a rig that depowers readily when the pre-load is overcome, but
is stiff up to that point.
· The very latest Twelve Foot Skiff rigs from New Zealand,
which have an unusually flat mainsail combining extreme roach
with a prebent top mast, together with careful use of panel shaping
and luff round shaping in the sail. This gives them a rig that
depowers readily and cleanly at the top, but yet minimises twist
in the lower two thirds of the sail, maintaining pointing ability.
All of these rig types have their disadvantages for this type of boat. The principle disadvantage of the skiff rigs is that they are optimised for boats that always sail with the apparent wind forward of the beam, which is unlikely to be the case in a non-spinnaker boat, especially in lighter conditions.
Other significant decisions are whether to have a single sail, or to have a self tacking jib as well, and also in the matter of twist. In the UK Cherubs currently sail with very low twist rigs that are basically either on or off, and small changes in wind direction can make the boat very tricky to handle. Moths, on the other hand, have rigs that, although superficially similar, are set up with a great deal of twist, and an on/off rig like the Cherub rigs would probably be exceedingly difficult to manage on the twitchier Moth hulls.Building the Hull
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· Page Last Updated 19 May 1999