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...
Upwind performance is a bit disappointing, most especially with
the short rig. I've had lots of correspondence on this, and no two
people seem to agree. Probably because there just isn't enough data!
I read about your difficulties with the reefed rig, and have done
some simple calculations that may shed some light on the problem. As I
see it, induced drag should be lower (as a percentage of total force),
not higher, in the reefed condition. Induced drag is proportional to
lift coefficient squared, and if airspeed is doubled lift coefficient
must drop by a factor of 4 to maintain the same side force. This should
reduce induced drag by a factor of 16. The lower aspect ratio will
increase drag, and parasitic drag of rigging and such will rise, but not
by nearly as much. Side force is of course limited by righting moment
and will be nearly constant in upwind conditions regardless of wind
speed.
So what do I think it is? I think the dominant effect is an increase in
side force on the foils due to the reduction in effective rig height by
reefing and/or allowing the upper leech to open. Since righting moment
is constant, any reduction in rig height will proportionally increase
the side force that can be resisted by the fixed moment. In upwind
conditions, the critical boatspeed is roughly constant (just past
planing speed), and every rig height reduction increases the force
required from the foil, increasing its induced drag and putting it that
much closer to stall. Things to try might include consciously footing a
bit to increase speed when reefed or using a larger daggerboard. I
suspect the foils also because of your previously described tacking
difficulties, again due to asking the foils for more force than they can
provide at low boatspeed.
Read about your rig/reefing problems in AYRS Digest. A quick
& dirty calculation:
Aspect ratio is defined as span (= luff) squared divided by sail area.
For a practically rectangular sail as yours, this is the same as
luff/foot [(luff x luff)/(luff x foot)].
Induced drag is directly proportional to aspect ratio, so when you reef
down to AR= 2.26, induced drag is increased by 2.54/2.26%= 12% -
assuming that lift remains the same. Induced drag is proportional to the
square of lift.
But the lift cannot remain the same:
If you are fully powered in 8 kn. of wind with the full main, lift must
decrease a lot to allow you to cope with 16 kn., even with the reefed
sail. So even with the decreased AR induced drag will be much less:
This something else will need to be a) flattening the sail as
much as possible b) sailing/sheeting at a narrower angle to the wind
(you need to pinch to keep the boat upright, or sheet even further
outboard).
Roughly, the kinetic pressure on your reefed sail will be about 3 times
bigger in 16 kn. than in 8 kn. This means lift coefficient must be 1/3
only, for the same heeling moment. Since induced drag is proportional to
the square of the lift, induced drag (and associated tip vortex) will be
only 1/9th of what it was in 8 knots, even if AR is a little smaller. In
light air, induced drag is a big issue, in heavy air it is not...
Huh? Complicated, but I don't think investigating into the tip vortex
will help solve your problem. The reason must lie elsewhere. My gut
feeling is that your case illustrates well the problem of the cat rig:
If you make big enough a sail for the light air, you cannot cope in the
breeze, because adjusting power is so much more limited than with the
sloop rig (jib). To be able to handle a cat boat efficiently you need a
Finn-style free standing rig with heaps of bend, and a more triangular
sail.
Your choice of a rectangular sail makes it worse. First of all, when you
reef, aspect ratio goes down. With a triangular sail it remains
unchanged. Even more important, when you try to feather (twist off) a
rectangular sail, you lose all power. With a triangular sail, you dump
the top part, but the foot still remains powerful. In other words, the
triangle is much more forgiving & gives a wider groove, while your
fathead sail keeps you right on the edge of not going anywhere, or
tipping over. The fathead sail only makes sense if you are limited in
mast height and sail area at the same time - else it's better to make a
longer mast and a sail with less roach to get the same area...
There is a loss around the boom... but I don't think the height makes a
big difference. A phenomenon called side edge suction makes up for some
of the losses. One of the most surprising results we had in the wind
tunnel tests we did recently was that lifting the jib up away from the
deck had hardly any influence on performance...
I still think (intuitively more than with reason) that your problem is
in the difficulty to depower the rectangular sail - it's either on or
off, more sensitive to the angle of attack than a triangle... a very
interesting topic for a study, if ever I find the time.
UK Moths when we started mothing used to reef in 20 kts +, with a
slab off the bottom, or use a low roach full luff sail. (Caws had the
storm sail, I used a slab) performance..... with both was similar.
Gutless downwind, and much slower than full rig, but less pitchpoling,
especially with the storm sail. upwind performance similar, and slightly
faster than full rig, generally a bit easier to cope with the big gusts
without flapping ( which is very very slow and can lead to blowing
over.... all drag and no lift) But now in the age of the T-foil ( and
also we are more experienced ) the races get cancelled before we need
the small sails. So my experience of the reefed rig was that it was
easier & faster upwind, and slower downwind. ( but the smaller rig
was only needed to stop the pitchpoles) I suppose your reef is bigger in
proportion. The stop/go thing beating may be too much kicker. Moths use
fairly slack kicker with lots of twist in big winds, sheeting fairly
close to centreline. The sail is backed at the top, but pretty flat. (
the kicker is probably at the same setting as medium winds, but it seems
slack because of the twist from the wind loads). The sail twists off and
looks funny, but there's still plenty of power
Sailing a 14 or 49er is different.... they have jibs!.... the main
doesn't do much, the jib makes the boat go... you can pinch, flap the
main and it'll still go. I think perhaps you're pinching too much. When
we come back from the nationals, we are sailing upwind much freer...
because the others do it, and its faster. Gradually, at home, we sail
higher again , till next year, and we remember again. I haven't made my
big endplate yet... perhaps you should try it. the 49er sleeve probably
extends the luff effect so aspect ratio increases and end plate effect
is from deck area etc. generally more efficient. Lots of older designs
have supersquat rigs, with pin head sails, so their real aspect ratio
must be very small, and tip losses huge. ( and they are slow too!)
After much work-wasting discussion with a friend who works on
this sort of thing (albeit on a very much smaller scale) we've come up
with some (useful?!) suggestions: As a wind/smoke tunnel doesn't seem
feasible, some sort of larger particle tracking velocimetry might be the
way forward - if you can get some synchronised digital cameras, then
sailing in the dark in a heavy snowstorm under your own illumination
should do the trick. We couldn't think of a way of measuring the
pressure distributions over the sail (which would give a good indication
of what was going on). Alternatively, a slightly more radical option
would be to fit inclined winglets (a la Airbus) to limit the losses.
Would certainly be a talking point at opens.
(These are extracts from a phone conversation).
Aspect Ratio - seems unlikely - not too many gains to be made over about
3, and have got to consider effect of hull/water
Twist/aspect ratio effects can be experimented lots by playing with
twist...
Leeway/board effects might be significant try board up with big rig try
checking out leeway
I am not an aeronautical engineer nor an international calibre
sailboat designer , but I'll give you my two cents on induced drag.
the airplane people use a formula that goes like this : Cdi = CL2 / pi *
ar * e CL2 is coefficient of lift squared pi is 3.14 out as many decimal
places as you can tolerate ar is aspect ratio e is efficiency factor of
the planform in use. .8 for rectangle 1.0 for elliptical run the numbers
if you can calculate cl and then multiply by your sail area and you get
induced drag in pounds (if I remember correctly.) The bottom line is
that induced drag is inversely proportional to aspect ratio so your 2.26
rig has 12% more induced drag than the 2.54 rig. If that number is tiny
than 12% more is still tiny. I wonder if the fact that the rig is not as
tall and "sees" less wind due to wind gradient is as much a
factor? Both of them together may then become noticeable upwind.
I was going to suggest that you try the sail in a less dense
medium in heavy weather & ! I sailed the 300 yesterday with it's
reef in for the first time and it was a lot more manageable even though
it a very small reef (probably around .70m2 It certainly made the gybes
survivable due to the lower c of e. My guess is that your problem is the
wopping roach that you've got, ok if its opening and closing with the
gusts but when its windy I suspect its fully open most of the time which
has got to help induce the air flow upwards into a vortex, way beyond
the directional differences from the boundary layer.......must admit
I've never been convinced about that anyway and have always tried to
keep the leach under control - almost parallel and I'm convinced that's
good for upwind speed due to minimal drag. Even more so I've you're
going to power upwind by dumping main rather than feathering. That's the
best thing about the 300 the kicker is really powerful.
When you're powering the 300 upwind the apparent moves forward and you
keep having to dump.......to the point when the main is almost where the
shrouds would be ! then *suddenly* the sail completely collapses and it
takes an enormous amount of sheeting to stop yourself from going in to
windward! I don't have the upper body strength to sail like that for
long buts its a lot faster than the feathering technique.
Long woollys from the leach & on the sail in the head reason are
going to be your best bet to see what's going on.
There seems to be a large difference between your small and big
rigs. It'll probably take a while to sort out when you should change
down. I think the aspect ratio isn't to low as the light weight crews in
the 12 footers get down to round 2.3 for their number 4 rigs and they
don't seem to have problems with tip losses, but this in 20+ knots.
One technique I've found for tacking a large roached main in a blow is
to let the vang off a bit just before a tack, holding the leach tension
with the main sheet, then going through the tack as normal and once
you're on the new tack and making progress pull the vang back on.
Letting the vang off means that the sail can twist off easily and hence
reduce the drag. The only problem with this method is that the boat has
to be setup with a really good vang system.
Sailing yesterday with the big rig in moderately overpowered
conditions... Still difficult to get the boat really in the groove,
Occasions when I was seemed to involve pointing very low, too low to be
competitive. Its noticeable that once planing fast its possible to bring
the boat up closer, but its very critical.
Had a little time to do some development sailing after the race, not
enough for full parameters. Sailing with a very tight leech, much less
twist than I normally have boat seemed to plane a little more readily
higher, but was - naturally - intolerant of gusts shifts etc. and needed
lots more work on the mainsheet. Sailing with the board up a bit was
odd, it didn't seem any more doggy than with the board right down, but
it did seem extremely difficult to point. Trying to work out whether the
boat is making leeway is difficult from out on the wing because you're
so much further out from references and shrouds. The rudder flow does
appear to be making a bit more of an angle of attack when the boat isn't
"in the groove" , but its so difficult to tell, and the rudder
is loaded - i.e. does take sideforce.
Someone is working out some numbers on centreboard loads for me, and it
does seem possible that the board is overloaded. This boat has a good
amount of righting moment, and load on the foil should be proportional
to righting moment...
I'm personally beginning to incline towards foil overload as a possible
issue. Pity its so difficult to test! Fortunately (well not fortunately,
but you know what I mean - the boat was engineered and constructed so
that ripping out the daggerboard case and putting in a larger one is not
a major structural exercise. I shall think and test some more though,
because its still time off the water and money - my Cherub daggerboard
is, I think, way too big.
I was probably the instigator of the induced drag theory, which
has been discussed quite intensively. One should be cautious about
applying "classic" aircraft theory here. The classic
expressions are valid for wings operating in uniform flow (which sails
don't) and "aspect ratio" is an artificial figure which can
lead one to a flawed understanding of induced drag.
I'm fairly confident that the lack of pace is not due to hull drag. Even
a relatively broad reach in lightish conditions, with little drive
available, popped the boat onto an easy plane, so I don't think that
there is any question of an excessive drag hump.
The square top main adds a great deal of area where there is most wind
(and remember the dynamic pressure, which supplies the aerodynamic
force, is proportional to the square of the wind speed). To my mind,
this can have three possibly important consequences, all related to very
careful control of twist.
If the added area is being worked very hard (ie. too little twist, and
remember the effective angle of attack is different, due to the wind
gradient) it loads up the tip tremendously. This can give one a very
distorted load distribution, which could push up the induced drag
significantly.
It might be possible to stall the upper parts of the sail much earlier
than the rest, due to too little twist. Some tufts could clarify this
quite easily.
The extra sail area at the top may simply be adding too much viscous
drag, due to the higher wind speed at that height, if there is too much
twist to take advantage of the available drive.
Personally, I don't think too much of theory number 3, but experimenting
with a lot of twist and tufts could illuminate the question.
The centreboard remains a bit of a question. But if the boat is going
slowly due to a rig problem, it might overload the centreboard, which
would make matters worse.
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
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.
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.