Bethwaite360 | WHY DOES SAIL AREA HAVE TO INCREASE IF YOU CHANGE TO A SQUARE HEAD?
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WHY DOES SAIL AREA HAVE TO INCREASE IF YOU CHANGE TO A SQUARE HEAD?

WHY DOES SAIL AREA HAVE TO INCREASE IF YOU CHANGE TO A SQUARE HEAD?

The 29er Carbon Rig – Status Update on the page below is getting increasing traffic, started at about 2/day, it’s now well over 16/day and there is a lot of interest.

If I read the questions correctly, there is quite a bit of interest in the process by which we select the size of the rig, particularly the height and the workings around a square head.

What we know is that if you simply reduce the weight of the rig, by whatever means, (in this case by switching out Alloy for Carbon) and do nothing else, there is an “empirical fact” that you need less crew weight to drive the same rig.    That weight reduction has been of the order of 5-6% in the case of a stayed rig (like a 29er).   (On a rig like a laser, it’s a lot more, because the reduction in over-all weight of the mast is so much bigger!)

If the desire is to maintain the existing weight bracket, then one way to do that is to increase sail-area, especially up high, and probably the most effective way of doing that is a square head.

Square heads have other advantages also.

Recently we have seen the advent of a rash of new designs in aircraft, the 787 and the A350 being right out there.   My pilot friends tell me the fuel burn of these machines is dramatically less, they are that much less draggy, and a lot of that has to do with how they “wash-out” the wing tips.

We could do that also, but it would be expensive and we are not flying at 40,000ft so it is not the most effective way of resolving the problem given a skiff.

A well designed square head has a lot going for it and “washing out”, reducing “induced drag” is a very fortunate side effect.

Little known fact but bearing-away with a square-head 49er is a lot easier in 20+ kts than the old pin-head main, and I can attest to that personally.

Take an existing 49er.    The mainsails are being retired because the shape in the lower ½ becomes undesirable, mostly because it is flat off the luff, and hooked in the leach.

We never get to the point where the upper ½ loses shape or becomes undesirable before we lose the lower ½!

Even the changes that are due to come into effect straight after the Olympics, in the 49er, in which we will dramatically alter the manner in which we transfer the downhaul loads up the rig will not alter this phenomenon.

The lower ½ of the mainsail ages out probably twice as fast as the upper ½.   But a well-designed upper ½ (square head) prolongs/doubles the age-out factor of the lower ½.

So why?    When a gust hits a conventional (pin-head) rig, most of the effect is to increase leach tension, and as that increases it’s like tying a rope between the head and the clew and then tightening it.

The whole mast bends!    As it bends it pulls luff curve out of the main, as the main goes flat, loads go up, the greatest unsupported area in the main is the lower ½, so that region is subjected to maximum load and gets tired soonest!

2016 Nacra 17, 49er and 49erFX World Championships in Clearwater, Florida - Racing Day 5

With a square head rig, same gust hits the rig, the first thing that happens, if it’s designed well is the load or the leverage on the back end of the square head increases more than anywhere else, as a result it “feathers”, it moves to leeward until its aligns itself with the wind.  As it does that it pulls the mast head aft, as it pulls the mast head aft, bending the mast in the top ¼, it flattens the upper main, sure the loads go up, but the cords are so much shorter the cloth can handle that extra load much more easily than the lower sail and also it’s exactly where you want the sail to flatten off, up high, where it has the greatest effect.

When Chris Cairns first did this in the Tornado class it was very, very clever!

Square top Tornado

The added bonus, is a Carbon Square-head mast will bend around 2%, the existing pinhead rig bends double that, and the mean diameter of the Alloy rig is around 66mm, mean diameter of the Carbon rig is 48mm and it’s a X² law.      And that is the biggest reason why a carbon mast will last theoretically indefinitely whereas alloy has a very defined life span.   Just go ask the aircraft industry.

That’s why we should be going to a square head.    It is also easier to set up, bear aways at the top mark are easier, and it will last that much longer.  It’s just a no brainer!

Re rig height.    The B14 went Carbon, but then also increased sail-area without increasing mast height.   And maintained crew weight (nominally).

If we can limit the increase in mast height to around 50mm, then we don’t have to alter fore-stay lengths, spinnaker hoists and you could pull a new main up the old mast, not sure you can pull an old main up a new mast, it will sail, but you won’t be fast.

I am presently engaged in some other rig design issues not too dissimilar and it now appears this is, in fact, possible.    A lot of this comes from the FX class.

So don’t ever say the 49er has no bearing on the 29er!

Julian Bethwaite

 

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