...Or anyone else who can explain it:

Here is a diagram that was posted in the landyacht forum a little while ago:


This shows that a pinhead sail (high aspect) will have a higher lift to drag ratio than a fat head (low aspect) sail.

So why are we using fat head sails with lots of twist?

Possibly something to do with the mast not being vertical but racked back.

I did read an article in a mag once which explained where the twist of sail came from.

Going from memory, there was some sail makers doing sail testing on the top of their 4WD roof.
They had sails rigged up on the roof and drove along with strain gauges connected to the sails, on one test the bloke up on the roof had to get to something underneath the top corner of the sail so he lifted it slightly allowing the sail to vent or exhaust at the top of the sail.
When this happened the bloke monitoring the gauges noticed the change in forces on the sail.

I'll go for a wade through the mags in my shed to see if I can dig up the article in question and post it.

Slowie no doubt would have more technical details in his never ending search for more Power

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nebbian said...

...<<<<

So why are we using fat head sails with lots of twist?

Think it's got more to do with control, and tip vortex.
If the head of the sail powers up too much it has more tendency to catapult the rider.

A twisty head helps stop the flow over the top of the sail from the high pressure side to low. reducing tip vortex.

Also a high aspect ratio sail is "twitchy", think I've got an old 4.0m you can try if you don't believe me.
If I read your diagram correctly, think that also bears this out, look how narrow the power ban is for the high aspect ratio sail. The angle of incidence is much more critical, making it much harder to sail in shifty, gusty conditions.

A fully stayed sail can cope better than we can with power in the head, so what works for yachts doesn't necessarily work so well for us.

I think that it is self evident. Assume that the graph accurately represents a polar diagram of forces acting on a sail. Then assume that AR=1 best represents the planform of a modern sail. Then assume that the angle of attack (alpha) is 38 deg on a speed run. QED The modern windsurfer sail has the highest lift to drag ratio at approx 1.5.

or possibly empirical tests have led to the current planform.

While your waiting for Slowboat to give you the answer, here is something I saved from "Matthew" (a sail maker) on the Boards forum about aspect ratio you might find interesting:

http://www.boards.co.uk/forum/forum_posts.asp?TID=19796&KW=Hi+screamer+yea&TPN=3

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Hi screamer yea thats been a long held phallacy about the low aspect ratio sail for longboards, and indeed there are times that low aspect ratio is beneficial but we have found, quite succesfully(ok very), that the optimum allround performance aspect ratio is a lot taller.
In the old 7.5m racing days we had 3 versions, the allround 0-30 knot sail, which was/is reasonably tall ( 5.15 mast 2m boom), a midwind which was optimised for about 11-15 knots which was shorter on a 490 mast and a 220 boom, with a lot of width at mid span, this was very rapid in those comfortably powered up middling conditions, but lost out really badly if the wind dropped lower than 8 knots, the third sail was the dedicated low aspect strongwind sail, again with a 490 mast but a 235 boom ( pointy looking thing) this was quick if the wind was over 20 knots, but dire if under 10 knots.
The most succesful sail by far was/is the tall one as it was/is better in dirty air and especially for the uk across a range of conditions, this was proven over many years in hard fought racing.

Decrepit's explanation "If the head of the sail powers up too much it has more tendency to catapult the rider." Is backed up by a physics 101 'equilibrium of moments' look. ie the righting moment of the fully hiked out rider equals the toppling moment of the rig. The moment equals the force times the distance of action. So if the sail force is centred lower in the rig then a greater force can be balanced by a rider of fixed weight and dimension.



A tall sail might be more efficient, but you can generate greater force in a shorter sail.

If you're sailing in light conditions where you're not fully hiked out a taller sail of the same area would be better, but a larger less efficient sail with its force centred lower - better again.

Probably impossible to engineer, but rather than sheet in and out to maintain equilibrium from a fully hiked position, we'd be better off with a dynamically telescoping foil that we could hold sheeted at the constant optimum angle.

well I got up a bit late. Ian hit the nail on the head. Thanks maaate.

Its all about maximising the lift from the sail with the available righting moment from our body geometry and position.

As for twist, well its good for a few things- the main one in my opinion is to lower the COE (centre of effort). With a sail there are a lot of constraints. One of them is getting adequate material tension in the areas creating the most lift (to keep the shape mechanically stable). This is hard to do without a relatively tall mast and some luff curve. The twist keeps the COE lower. Thats the main reason for "more downhaul=> more control". Its because "more twist => lower COE". Not always though- it depends on how the sail/mast is designed. Theres also the dynamic component to consider- ie how the COE moves with sail loading. Getting this right makes a comfortable sail. Ian- we use dynamic twist as the simple mechanism to achieve a degree of your telescoping idea.

Thanks Slowboat and Ian, that explains a lot.

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frant said...

QED The modern windsurfer sail has the highest lift to drag ratio at approx 1.5.

No, the modern windsurfer sail has the highest lift coefficient, not the highest lift to drag ratio. The highest L/D ratio is the point on the curve that's steepest when looking from the origin, in other words, the point on the AR=6 curve that's at an alpha of 10 degrees.

Nobody's quote from the UK sailmaker is valid for the longboard conditions the quote refers to. But have you seen the conditions those sails are designed for? Gliding around upwind/downwind courses on glassed out lakes in <6knots!

The gaff rig is the closest of the examples to the planform of a windsurfer sail used in planning conditions. It has the most lift, it works at high angles of attack (35* apparant , about what you have on a speed course), it has a smooth shallow lift curve so is not twitchy and all the other reasons Decrepit and Slowy gave.

Stable platforms like C class catamarans and heavy keeled yachts can have very tall rigs and use the high aspect efficiency for going close to the wind but that is not what a speedsailor needs.

Onya Slowy, you've got to get up early to beat us on the east coast. (did i say coast, I'm dreaming). But you seem to be hinting, that, but for need of a long bendy mast to nicely distribute tension in the lower part of the sail, we might be better off with even lower aspect rigs in powered up conditions?

Has any manufacturer spent a bit of R & D trying to get a lower aspect rig to work? Or is the Naish Boxer as far as has been tried?

You'd still want twist, just all scaled down in the vertical direction.

But even with tip flex we are still generating some power up there, in the wrong spot as far as leverage goes and at a less than optimum angle of attack. Because if there was no force generated up there the tip wouldn't flex. That is unless the twist up top is generated by extra tension down below as the gust hits?

That would be nice if the top feathered completely to zero angle of attack in a gust due to tension increasing down below. Does it??

Cut away sails got a short run many years ago. Cut out a bit of lift at the top but still have the long mast to hold shape in the bottom. Did they experiment in taking the idea even further , using kite strings to maintain tension from batten #1 or #2 to the mast tip? If it did work they'd surely figure out how to hold bottom tension on a short mast.

But maybe Decrepit's tip vortex point is part of the reason aspect ratios have settled as they currently are.

It is interesting to note that Yellow Pages Endeavor had a quite high aspect sail (wing). It was obviously very efficient for the amount of righting moment available in that configuration.
It is also interesting to note that one of the major differences in Macquarie Innovation is that the designer has gone with a lower aspect wing and a wider beam for more righting moment. They calculated that more power at the expense of slightly lower efficiency was what they needed to go even faster.

My brain hurts

I have used naish boxers .Still have one and a half of them.Anyway the 5.8 i have rigs on a 400 mast.Although i didn't have a gps at the time i found the boxer topped out in speed compared to say a severn S1.Which has a 430 mast.The board was the hypersonic.I could tell the s1 was much faster as the holes in the boom started to whistle like a flute.
So all the techo stuff that you guys are talking makes sense.The wider the sail is in the head ...the more drag you have when it comes to your apparent wind.Like the guys where saying in the landyacht thread.

Oh the servern was a 5.7 and i was swapping rigs on the same day to see which felt faster
My 2 cents although i'm not educated in the way of aerodynmical engineering and techi thingamy stuff

I was on a low aspect trip a few years ago. Ben Severne and Jesper were kind enough to make a couple of protos for me. The sails worked quite well. But there were some areas that suffered- especially in the light wind operation. There was no issue in the top-end department though. I raced on them until they were practically shredded. We learnt that there's more to it than just lowering the aspect ratio.

The larger head on the KA is not heavily loaded. It can however be lightly loaded and the leverage generated helps to increase the twist and keep that less useful moment under control. Much of the twist is induced from loading in the middle of the sail though. They are very fast and efficient over a large range of conditions despite the larger than normal heads.