Square head vs pin head rigs
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Square head vs pin head rigs

Square head vs pin head rigs

Square heads have been around for thousands of years, Vikings and the sails they had on their long boats, and I’m sure the Polynesians and their Proa’s preceded that also.

The Dhow Rigs are equally as old, “leg of Mutton Rigs”, formed into the original “Bermuda Rigs” in the 17th and 18th centuries.    Then came “Gaff Rigs”.

What I refer to as a Pin Head rig is really a Marconi Rig, developed mostly in Germany at the beginning of the 20th century.  Most notably “triangular sails”.  Manfred Curry in his 1946 book shows lots of photos dating back to 1935 of his father’s boats, and those boats had developed significant “roach” (curvature in the leach of the sail supported by the battens) and a level of mast bend.

I will now skip 50 years of development and I acknowledge that this is a great dis-service to many developers including my father.

In 1981, I tried and sailed a 2-handed 18 foot skiff (Prime).  Over the next 3 years that developed and what became apparent was with only 2 people, you simply did not have enough hands to do everything, so we needed to make the rigs automatic.

2-handed Prime 18ft skiff

The development happened at an extraordinary rate, 10 years later a GRP 18teen with a few carbon straps (to hold the mast), went around the std NE course in Sydney Harbour in under 50mins.   The “straight line” distance is 15.5nm, it was a heat of the World Championship, 3 windward legs, 2 downwind legs around the island, 1 straight to the bottom, not too many gybes, but plenty of tacks.

Sure, it was a highly refined boat, many aspects of it were honed but the biggest contributing factor was a highly developed “pin head” rig!    That record time still stands today and not even foiling Moths or MC32s can come close to it.

I doubt a square headed boat will ever be able to achieve that sort of time for a few years to come.

So, the very refined “pin head” rig was a bit of mathematical poetry.

First issue was the way the mast bent, biggest contributing factor was the height of the hounds, and altering this 20-40mm (in a 10-11m mast) made considerable difference to the point where there were 2-3 keyways (attachment points) for the forestay that could be selected depending on conditions.    We always tried to get an “exponential” bend, so the lower sections (alloy tube) bend far less than the upper mast (GRP wound tube).   We developed a whole system of % and depth to define this bend, and we would alter it by 0.5% occasionally.

Once we knew the luff curve of the mainsail, and the rate at which it would bend under different loads we then designed the roach of the sail.      They had to match, and there was again a very definite multiple, so if say there was 222mm of luff curve, we would have 222 x (say) 3 = 666mm of roach at that height in the sail.


© Sport the library/Jeff Crow
Sailing-18ft, Hayman Island 1993
AAMI/Mast Mount

So why am I going into such detail?

Because of the analogy of a gust, and what do we want to happen in that gust.

Your standard gust lasts 9-12 seconds, it increases the wind speed on average by 25-30%.   So, if the mean wind is 10kts, then the gust is likely to be 13kts, and in 99.99% of cases, it will be skewed 3-5° off the mean wind direction.   The reason for that is it comes down from the layer or 2-3 layers above, and almost by definition those upper layers are screwed by Coriolis, so they are at different angles to the lower layer of wind.

But the most import thing about a gust is 80-85% of the energy is in the first 0.5-1sec.

You’re sailing in 10 kts, fully powered up, gust hits at 13 kts. Force goes up by X² so it goes from a nominal level of 100 to something approaching 169, and it happens in under 1 sec.    Do nothing you capsize, simple.    If you’re good and you can see it, you luff a little, and your sheet hand prepares to ease the sheet.   Get it wrong, you capsize.

With an automatic pin head rig, because the load increases, the mast bends more, and it bends more at the top than at the bottom.  If you have the roach right, then it progressively flattens at the head more than it does at the foot, so the CoE automatically drops, increasing your Righting Moment, and that in turn helps to “resist” the increase in force and again this all happens in under 1 sec.

The other side of the gust, the automatic rig also powers straight back up, long before even the fastest human being can tension the mainsheet.

By the time we got to 1993 and the AAMI 18teen foot skiffs, we had it, so you were actually pulling the mainsheet on in the gust, which meant the rig automatically flattened even more in the gust, which led to much higher pointing angles, and acceleration.

The “pin head” rig theoretically closely approximates an ellipse, therefore as you move towards the tip, the span-wise “pressure” loading drops and you minimise tip losses, vortices and the like.   If we were as good as the birds, we could probably effectively end plate with all the aspect ratio benefits that brings.

But we are nowhere and will never be as good as nature and the birds!

The first time I saw a square head rig was in the late 80’s.   Chris Cairns’ loft was upstairs from our factory and he was using the square head to try and emulate what we had with the pin head rig but on a Tornado which at the time had a tree trunk of an alloy rotating section mast.

It would not bend, so he used the bias of the sail cloth and the square head to emulate the head de-powering benefits of the pin head.  It was very clever, and it required some quite extraordinary detail in rocking the head panels to get the desired result.   We still do this today in the 49er mainsail.

With the advent of carbon masts, the ground rules changed.   The UTS (Ultimate Tensile Strength) of carbon and glass are in fact quite similar.  What is different is elongation to break.    Glass, if laminated, with epoxy or vinyl-ester is 5-7%, good Carbon is 1%.    It’s just a whole lot stiffer.   If you are looking for a mast to bend consistently at 4-5% and you’re using carbon as your base material, you either need to thin down the wall thickness or reduce the diameter.    But the overwhelming reason for using carbon is weight.   The base tube weight is approximately ½ that of its alloy counterpart and ¾ of the weight of the GRP topmasts.   And now, with advances in building techniques the cost is coming right down.  In 5 years’ time, any relevant class will have a carbon mast.

Thin wall tubes are fragile, so in 90% of the cases in the marine mast business diameter has dropped 15-30%, wall thickness has gone up from around 1.7mm to 2.2-2.5mm and this new breed of carbon mast is now extraordinarily tough and light with huge longevity.

But, the topmast simply does not like to bend anything like the old GRP topmast.

At the same time material science has evolved so that the Mylars used in sails have similarly advanced, and the extent to which you can dial in bias, warp and weft is simple and common place, especially if you are buying 1000’s of meters for classes like the 49er.

What we now do is design the bias stretch, we rock the head panels so we get the warp/weft aligned exactly and then complement that with the bend of the carbon mast to achieve something approximating the sort of automatic gust response of the pin head rig.

There is a lot of science to this.   The biggest contributing factor is the length of the square head.   Most sailmakers like excessive squarehead length.   Our experience of the 49er and FX sail plans, is that if you go too big, you lose “range” and the ability to alter the shape beyond a rudimentary level.    In the 18teens which are even more extreme than the FX, this has become very niche, they are fast in one setting and they sail the boats, so they are in that setting as often as possible.

This is not necessarily a bad thing, it’s evolution, and that process has many years to run.

If you get it right, then you may also endplate the top of the sail effectively, and that leads to Aspect Ratio benefits and considerable changes to CoD and CoE considerations.

Going back to the 18teens, they are pursuing this now extensively with mast head battens that are not too dissimilar to a 787-wing tip which when all is said and done, are copies of birds, think cranes at altitude in gliding flight.

We still have a long way to go, it will get refined further over the next 5-10 years.

It’s been 25 years since we put a cuff on the bottom of a 49er main, that is still surprising us as to its effects, square-heads will be a longer road yet again.

Julian Bethwaite
November 2017

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