GRF's rig school

some here might die of shock!

Windsurfing sails and masts perform in different arenas, some for fun
leisure application, some for competition, some for One Design.

But the basic essential ingredient is always matching or more accurately
mis-matching by design, the luff curve of the sail to the bend curve of
the mast.

Windsurf masts are pre bent to assist in the stiffening process, load them
one way and they're less likely to distort the other way.

The subject matter that raised this discussion was the mast of the DI
which bends both ways.

It's not pre bent, we're supposed to believe on the one hand it bends off
sideways upwind to cope with spilling gusts from a big fat headed sail,
whilst then not bending forward because a bloody great kite has been
hoisted up it?

Or when bending forward it some how miraculously holds the exact
shape to continue to provide the stable leading edge that big fat headed
sail would need.

Well my simple point was that it doesn't it's not efficient, whatever is
gained by hoisting the spinnaker is lost by fall off from the main, maybe
not in equal amount, but significant.

So I now have to spend time explaining to y'all the fact that I just happen
to have thirty plus years studying these rigs in order to what?

Justify an opinion?

Ok, I've avoided rushing into this reply, partly because I needed some sleep last night, and partly because I quite enjoyed going back through my old notes and books.  I will try and keep this as coherent as possible.  Now I appreciate that much of what you say is somewhat tongue in cheek, and perhaps a little facetious, however it worries me that people will take it to be true when your scientific basis is wrong.  I will freely admit that my knowledge of fluid mechanics is not complete, but I like to think I have a reasonable enough grasp of the basics to correct some of your mistakes.  There seem to be a large number of things you are moaning about without actually putting together a serious argument for your reasoning behind any of them, you flit to the next problem you perceive before you have finished exploring the previous one.  That aside I will try and deal with each of your points in turn. 

For your first comments on the D-One rig we first need to consider two things, firstly the shot was taken, in what looks like a decent breeze, some of your friends are out in the background so it must be north of 20 knots, i.e.: the top end of the boats operating range, and secondly, it is a photo - a single moment in time, and is with limited context and thus only viable for the most basic of analysis. 

You question the need for the upper roach on the mainsail leech when it is twisted off and the mast is bending forwards of its upwind position and to leeward.  There are a number of reasons for this.

Firstly being at the top end of the wind range it is designed for it will be bending significantly, now that is not to say it cannot be engineered to deal with higher winds, but if that is not the design brief then it cannot be said to be a failure if it does not do something it is not required to do.
Secondly the kite will have a major impact on the onset flow around the mainsail.  If nothing else given that the kite is fractional (not mast head) the apparent wind angle at the head will be significantly different than lower down the main where there is a more significant interaction with the flow from the kite. 
Thirdly, the extra sail area in the upper roach is used upwind far more than downwind, the kite more than doubles the total sail area, and the total amount bladed off will be less than a square meter by the looks of it, this will simply sit in line with the flow around it, not creating much if any driving force, but also having a minimal drag effect. 
Fourthly, the main, is as much about balancing the helm once the kite is up as it is about creating driving force, the driving force from the kite will be significantly higher than the main, the point of having the extra sail (the kite), is because its the one doing all the work.  However, going back to my earlier point about interaction between sails, the two sails together are more than the sum of their parts.  They create a single composite lifting body. 

Next you comment that the mast is "bent out of shape".  Out of which shape? It’s upwind shape, its static shape, its unloaded shape? Of course it will be, it has a kite flying off it and it has less mainsheet induced leech tension as it has been eased for a start.  (I am trying to work through your points in order, however I will jump forward to another of your arguments relating to flexible masts and progressive bend).  Looking at the two photos of downwind sailing the point of maximum bend is around or just above the second batten up, and after the third batten up the mast is virtually straight again, this is the signature of the modern "progressively" bending mast, where the aim is for it to be more like a bow rather than having a floppy tip - which I agree with you is not 'fast'. 
I am not convinced by your argument that windsurf rigs are significantly more stable, certainly notwhen taken in proportion to their relative size - Besides I doubt even you would be using a sail as big as the D-One main in 25 knots, (the upper leeches on the sails here look pretty mobile to me here: http://www.youtube.com/watch?v=OcyifOVKP8w ), this may not be the best example I grant you-it was the first thing that came up, but I don't think I've seen any dinghy rig set up even half well bounce around that much.  True a lot of it seems to be coming from impact loading from waves rather than wind load, but it is definitely happening, and as you have said yourself the mast bending and the leech going loose is not efficient, so the point remains. 

You say that you can hold up 12meter rigs by hand, I don't deny that, but I somewhat doubt you'd be holding up something that size as the breeze increased.  There would be no point, the biggest drag factor on a windsurfer is the aero drag on the person sailing it, you simply need enough sail to get you consistently fully planing and then extra sail area is going to be more hindrance than help, but you knew that already, or people would only have a 12 meter sail, and not bother with the rest of them. 

You say that sails are, or should be, cut to deal with variations in wind speed, if this is the case, why have a mast that will bend at all? Curved yes, but you could make pre-bent windsurf mast that was completely rigid and would not 'flap about' and yet this is not the case, I suggest that having some flex in the mast, to allow the rig to react to gusts and lulls is vital to make the whole thing handle able. 

Next you make a point about windsurfers having been working on mast bend and its relation to luff curve for thirty years, is that all??  Yachts have been experimenting with bend characteristics, and matching luff curves for over 100 years, but you knew that already, didn't you?!
Even if we only go back thirty years, I am sure you will remember the 12meter British AC challenge yacht 'Lionheart' which had a significant amount of mast bend designed in, although I think the then IYRU banned it (a political problem not a scientific or engineering one), someone can correct me on the exact details of that, development being prevented by politics is not a reason to ignore its existence. 

Progressive twist is nothing new either, it has been the goal for decades, all the different ideas have been working towards this in some way.  The pace of development has been different between the two disciplines, I grant you.  Boats have moved slower, in part because of their size and relative expense to work through a number of ideas.  But more so (and this is especially true for dinghies for which the size-cost issue is less) because of class rules.  Windsurfers for the most part have developed free of prescriptive class rules, and because they are smaller and cheaper it has been far easier to work through a vast array of ideas at full scale, and as such they have, to some extent, stolen a march on boats in terms of rig development, especially for high speed. 
If you are right that there is not enough communication between mast and sail makers than that is a project management problem, again, not an engineering one, but I doubt somehow that with the investment most new boats get that this is the case. 

I will treat your comments ("Have forgotten more about sail design than clearly the simpleton that built that rig will ever know. " and "As an efficient device for dealing with a range of conditions, given the level of sophistication the other control features of the hull potentially bring to bear, it is quite frankly a joke.") with the contempt they deserve.  So far I have not read a single thing which you have written that implies you have any real understanding of the mechanics of sail design, please do prove me wrong though.  The very fact that there is not too much going on at deck level to control the rig implies that the rig its self is doing a good job of looking after its self. 

The very fact that you mention laminar flow around foils in water means you really do not understand the structure of water, let alone fluid mechanics that well at all.  I will let you (look it up) and while you are at it I suggest you have a look at Reynolds number, and its calculation too. 

As for the leech being in tension, I would agree that in those shots there is not enough, but as Tim and I have already discussed, learning the correct amount of mainsheet which is required to achieve that is a process which is ongoing, and is matter of operation, not design, because unlike a windsurf rig, this can be adjusted (main and kicker) far more easilly as you go along. 
To me it does not look to bad, the only section of leech which is 'loose' is across the top of the square head of the sail.  A function of square head mains is the top corner, held out by the top batten, is the first to blade off, and the load transfer of leech tension from the head of the main bypasses this roach and straight-lines to the end of the second batten down.  But as I have mentioned before, the loss here is minimal in terms of area, especially compared to the total area downwind.  Indeed it's interaction with the tip vortex may indeed be of some benefit in terms of driving force, the same as with the square head windsurf sails as they bounce around in the tip vortex. 

This, by the way is my next research project, trying to understand what exactly happens to the tip vortex when you have a square head main which is moving around.  Much as I would love to do this with RANS code I don't think I'm going to be able to access a computer big enough to do that so it’s going to be a bit more simplified for now.  I'll let you know how I get on. 

Happy to accept all polite critisism and correction. 

Phil

Sorry Phil gave up reaing halfway through your post, too long.....

Originally posted by Chris 249 As a windsurfer and boat sailor, I have to agree with the dinghy sailors.Dinghy sails are different because they have to be different, so that they can cover a wider wind range and achieve the lift needed to drive a larger hull in winds that are often lighter. Sure, a Moth foiler sail looks like a windsurfer sail - but that is because the lift/drag characteristics of a foiler are closer to those of a windsurfer, not because foiler sails are 'better'. There are quite a lot of things that dinghy sails do better than board sails, and vice versa. You can't compare them directly, any more than you can compare the wing of an airliner to that of a fighter.Grumpf, you are COMPLETELY wrong about dinghy sailors not taking care to match masts and sails. They have been very well aware of varying mast bend since at least the 1950s; so aware that they could see the bend in their wooden Finn masts change according to humidity. This is all documented in black and white, and has been for decades, since Von Hutschler's breakthrough with bendy rigs in Stars in the '30s.

Get into the context of the discussion, I'll humour you since you know a
little of what I'm trying to get across,
the context of this discussion is "Flying a Kite from an unstayed rig which
has been designed to be bendy to
depower a fat head sail."

Now my other experiences have been that the masts have exactly not
been considered when the sails has
been designed, now they are my personal experiences, I dare say
FinnSailors shaved wood off the masts to
greater and lessor degree.

But we are living in changing times, as dinghy world (read the beginning
of the post) begins to come to terms
with the greater speeds with increasigly forward apparent wind vectors,
and some are not dealing with it.

I maintain the rig of the D1 has not dealt with the issue.. Hence this
discussion has come about.

So how would I have dealt with the issue? If it had to be an unstayed rig?

1st I'd have had a mast designed to be unidirectional in it's bend, and
2nd I'd have controlled the for-aft
bend to twist off the sail up wind using luff curve mis match a la
sailboard rig.rather than permittng sideways
bend under load. That for aft loaded pre bend would have helped support
the kite.

But then I'm not pretending to be a boat designer - am I?

And this, isn't 20 knots..



I'd say it's barely fifteen..

Phil, there are a lot of points you make, a lot I'm not about to dispute, it
will however take a bit of time to go through them one at a time, will you
be able to stay awake long enough tonight?

Edited by G.R.F.