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Hull Shape Considerations

By Julian Bethwaite

A very interesting conversation developed on Sailing Anarchy week or so back and it has had me think about what is planing upwind, why can some boats do it and others can’t, even though they have similar righting moment (RM), and to a lesser extent, as we have just gone through a re-tooling process, what are acceptable variations to a one design concept.

It’s a biggish topic, but all inter-tangled because I’m not quite sure people get it.

I come back to the fundamental issue which is – what is the biggest difference between air and water?    Both are fluids and most naval architects would have you believe they’re similar.  Why?  Because most research has been done on wings and fuselages of aircraft.  A few % points difference in Coefficient of Lift or Coefficient of Drag makes a huge difference to fuel burn, so it’s worth investing $b in research because the savings are worth $b’s.   Whereas, the sum with respect to a sea-freighter, it’s still important, but a few % difference gets negated by how much cargo can be carried.

Air is compressible whereas water is not.

This even affects the car industry where water in the cylinders can bend crankshafts.

So when it comes to boats that are going to plane upwind, as compared to a boat that does not, mm’s count.

Going back to Sailing Anarchy, the argument centred around what was planing upwind?  Crack any boat off say 30° and spring the sheets, and within reason, most boats will plane.  Technically they are going upwind so they are planing upwind.

But the more meaningful definition to me and I believe to the assembled multitude on Sailing Anarchy was a boat that can plane upwind achieves a meaningful higher VMG than they would have had they remained displacement sailing and pointing.

So a couple of definitions:

RM = Righting Moment.  How far you move the crew weight and how much weight you move sideways from the CoB (Centre of Buoyancy)

hull-shape-diag2Righting Moment and Sail Carrying Power – click on diagram to enlarge

VMG = Velocity Made Good.   Very few sail driven boats can sail straight into the wind (a few with blades can, but most would consider those boats “out there”, and they are pretty slow) so you need to tack.  If you assume 45° tacking angle, that means for every 100m sailing through the water you are actually making ≈ 70m to windward (sine 45°). If you did that in 30 secs, you are doing 2.3 m/s VMG.  1knt ≈ ½ m/s (or 1.7ft/s) so your VMG is approx. 4.6knts.  Speed through the water is 100/30/0.5=6.666 knots.

This would be a good example of a Flying Dutchman sailing in displacement mode upwind.

So the next definition is displacement, often referred to as Hull Speed (HS).

Start from the simple formula, HS = (√ water line length) x constant.

Because my father was a pilot and speaks in imperial (feet and knots) I will stay there but there will be metric equivalents.   So length in this case is in feet, the constant for salt water is ≈1.4 and the result is in Knots (fresh water is very slightly different).

So take a 29er LWL = 14ft, so √14 = 3.741 x 1.4 ≈ 5.238knts.

So what does that mean?

Because of the nature, the viscosity, density and environment of the water that a sailing boat operates in, as it moves across the water it pushes the water down and out (from the surface of the hull at the bow, the bow wave) and what flows out, must flow back! It does and the time it takes for it to go out and come back to ostensibly the same point, in the case of a 14ft LWL boat, travelling at ≈ 5.2 Knts, that water flowing back (the stern wave) happens at the transom.     So if you travel slower than that, the stern wave will form in-front of the transom, if you travel faster than that it will form behind the transom.

If the stern wave happens behind the transom, i.e you are moving faster then HS, the boat will go nose up, as the stern falls down into the trough in front of the stern wave and you end up sailing “up the bow wave”!

Just like pushing anything uphill, it’s hard work and the heavier it is, the harder it is to push.

Same goes with a boat.   The result is you get a pretty significant spike in the drag curve.

So the attached graph is based on hard empirical data, peer reviewed, yada, yada, yada.

It is the 470, done May 2016 and it is the 49er, pre re-tooling 2005.

hull-shape-diag1Drag curves for 470, 49er and 29er hulls – click on diagram to enlarge

Picked these 2 boats because they are very similar in terms of weight, particularly hull/rig/foils weight (near identical) and they are also pretty similar in waterline length (LWL) but mostly because we have the data to hand.

The 29er curve is my best calculated guess.

With the 470 you can clearly see the hull speed (HS) bump, so for a 470 to exceed HS it needs a considerable increase in grunt.

The 49er has a much flatter drag curve at this point, what my father often referred to as a dynamically humpless hull!   49er has almost double the “grunt” (more correct word is ‘righting moment’ (RM), less correct is ‘power’) than a 470, but if you now look at the 29er curve, because it has similar RM to a 470 and would have a similar if not slightly lower drag curve than a 49er (because it’s lighter) then the reason why a 29er can’t help but plane upwind, most of the time, whereas a 470 has to hold off until circumstances are favourable, becomes obvious.

Switching direction a little, what is an acceptable tolerance when we retool?

We start with what is realistic.  Moulds move, does not matter how well they are built, +/-1mm in the length of a 49er/29er has to be accepted.

So when we re-tool, I insist on an absolute minimum tolerance of +/-1mm under the chine, it matters there, because water is uncompressible and that 1mm moves the water alongside it, and so on, and so on!

How far, the best example I have was Helen, who worked for me for 15 years, besides being an accomplished glider pilot and really amazing with FRP she also represented Australia at the 1976 Montreal Olympics in the one man (woman) scull, think we now call them K1’s.  In the St Lawrence seaway, at 10m deep, they achieve much faster times than in the rowing channel at 4m deep.    So if a K1, at its weight, affects water 4m down, a 29er has to be moving water more than that.    Add to that, Helen still paddles but these days up and down the Danube, often in a 2 woman K2.   Though it’s not meant to be competitive, can’t change her so they exploit the pressure wave, so when the water is restricted from moving down and out, it heaps up beside the boat, forming a pressure wave, and apparently the “grunt” required to paddle up this wave, by a following K2, is near insurmountable.

Coming up from the chine with a 9er, because they are sailed flat, it’s less critical, so +/-1mm at the chine, ¼ of the way up 2mm, ½ way up 3mm, ¾ of the way up 4mm and at the gunwale +/-5mm.

With respect to the deck, provided you don’t increase the beam, industrial tolerance here is +/-10mm and it’s now more about ergonomics than hydrodynamics so that’s just fine.

Those are my parameters, not ISAF’s or ISO’s.    I have imposed them every time we re-tool.

Both the 29er and 49er are re now represented accurately electronically and we can photo scan anytime relatively cheaply to verify a hull if we need to.

But it’s also horses  for courses.  Some people believe it’s a cube law, because it’s 3 dimensional, but I will hold with convention and accept it’s a square law, even still a 29er that regularly exceeds 20knts (occasionally 30) is bending double or triple the amount of water that the 470 does, so it’s only fair that tolerance is +/-1mm (where it counts) whereas the 470 is +/- 17mm over its entire surface.

I won’t get into “humpless hulls” now because it will consume another 3-4 pages, maybe next week’s edition.



Sails – Single Source vs Open

By Julian Bethwaite
Porto17 2015

Favourite topic of the month is sailmaker selection.

For as long as I can remember I have been pursued re open sail making versus single source sail making.

I have been wined and dined, but also accused, slandered and occasionally vilified.

As a result, I have developed a system that allows me to accurately judge the benefit of being a single source sailmaker class as opposed to sail supply governed by rules, and then a system that simply identifies who is the best sailmaker to award a contract to if we choose a single source supply.

The first thing we do is ask the sailors.

In the 49er class this is pretty easy as it’s a Fb driven association.

We tend to do this every 8 years seriously because the sailors like stability and it is approximately a 3 year turn around, not just for the builders but also for the sailors.

So what I am saying is that to switch from one sailmaker to another in the 49er, it can only happen in

a) an Olympic year, (and even that needs some pretty careful planning) and

b) the builders need to know about 3 years out, builders in this case don’t only mean the boatbuilders but it also requires the old sailmaker to retire workers and stock and the new sailmaker to build up stock and capacity, and it requires the boatbuilders to forecast far enough ahead to have adequate stock to satisfy the Olympic sailors till the end of the Olympics where the change is to take place.      Nb1

An unforeseen cost is that it is near impossible to get an existing hi-end sailor to test the new sails, so inevitably it becomes a live test.

I will give you an example!   We are just about to introduce sails into the 49er class that will better handle the increased downhaul loads that the 49er sailors are now subjecting the mainsails to.    We produced 10 suits of trial sails so that they could be tested.    These sails are off the same patterns, same size, on the same masts but have altered panels in the luff to better distribute the loads.    Everyone knows they are coming but no one wanted to use them because they were different.

So once again, we have a live test, and that’s expensive on the sailors!

So first thing we do is ask the sailors.

The next bit, there has never been a push towards open sailmaking in the 49er or 29er class and the attached excel spreadsheet probably explains why.


Oct-16 Source Currency if not $USD $USD Nb1 NB2 Nb3
Sail Type Web Cost Battens etc Assc costs Sail Cost Area Cost/sq m
49er Mainsail £1470 $1,475 $163 $100 $1,212 16.2 $74.81 Nb4
49er Jib £710 $713 $80 $100 $533 5.7 $93.43 Nb4
49er Spin £1,580 $1,586 $100 $1,486 31 $47.93 Nb4
FX Mainsail £995 $999 $100 $100 $799 13.8 $57.87 Nb4
FX Jib £575 $577 $60 $100 $417 5.4 $77.24 Nb4
FX Spin £1,095 $1,099 $100 $999 25.1 $39.80 Nb4
29er Mainsail £706 $708 $75 $60 $573 8.64 $66.38 Nb4
29er Jib £380 $381 $40 $60 $281 3.76 $74.83 Nb4
29er Spin £730 $733 $60 $673 16 $42.04 Nb4
470 Mainsail $1,162 $1,162 9.45 $122.96 Nb5
470 Jib $598 $598 3.59 $166.57 Nb5
470 Spin $805 $805 12.16 $66.20 Nb5
420 Mainsail $797 $797 7.4 $107.65 Nb5
420 Jib $469 $469 2.8 $167.41 Nb5
420 Spin $601 $601 9 $66.74 Nb5
Nacra Mainsail $1,907 $1,907 15 $127.14 No info, 2014 Pr
Nacra Jib $701 $701 5 $140.19 No info, 2014 Pr
Nacra Spin $1,115 $1,115 19 $58.70 No info, 2014 Pr
Laser Mainsail $604 $15 $589 7.1 $82.96 LP Web Price
Laser Radial Mainsail $604 Does not make sense $15 $589 5.76 $102.26 LP Web Price
Finn Mainsail $1,610 $70 $1,540 10.2 $150.98 Nb5
Optimist Mainsail $540 $15 $15 $510 3.3 $154.55 Nb5
RSX Mainsail € 996 $888 $888 9.6 $92.50 Pryde Web price
Nb1 = 9er sails are sold complete (see Nb4) in a bag, with full set of batten & a repair kits.  Their cost is as per Ovington web page
By way of example of the above, a set of 49er battens = £175.95 + Sail Bag £15 + Reapir Kit £25.    Total =  £215.95
£ 215.95 = $USD 287.11  Less 18% Vat Less 2,5% duty less 5% Logistic  = $USD 163.02
Nb2, Where a Class association fee is included in the price of the sail and not added afterwards
Nb3 = Sail Area as per ISAF page unless obviously wrong, in which case via the class rules, see
Nb4 = Ovington web page, less VAT (18.5%) less 2.5% Duty & less 5% Logistics
Nb5 = North Sail, North America site, see

I can’t claim credit for this format, it was started in 2000 by Tim Coventry of Laser fame, but I update it every 2 years. The rationale is if I go to any of the big sailmakers and ask for a price of a mass produced suit of sails, first question is what’s the area, and second question is how many battens does it have, third question is what’s it made from. They then give you a rough price! If you get past that point, who is doing the digitisation (as in whose design) and who is managing distribution comes into play.

In the attached spreadsheet we actually go to quite extraordinary lengths to get the numbers right. It’s important. We go to the Ovington page, as they have fully priced 9ers sails and we know the cost of importation, freight (often sea), duty and VAT.

We also can go into exactly the same page and find out the price of battens and sail bags and the repair kit. We also know how much the class association button costs and we can end up with a highly reliable cost figure.

Just so this is clear, a 29er mainsail on the Ovington site is £ 706. In that price is VAT, duty and logistics (freight and handling). If you remove all of those and convert it to $USD you end up at $USD 708 for a 29er mainsail in a bag, with battens and a repair kit with the class fee paid. If you now remove the cost of all those items, you end up with a cost of $USD 573 for a 29er main.

And then we go to say the LP Laser site and get the price of a Laser sail.

We specifically pick the North site for the cost of the other “notable classes” because those sails are made in the same facility as the FX sails are made, namely Sri Lanka.

Compare a 470 mainsail and a 29er mainsail because they are the closest in area, and these prices are as of September 2016.     But the empirical evidence is plain, and probably the reason no 49er or 29er sailors want to go to open sail making.

Just about every open class sail making is +$100/m²

Just about every single source class sail is sub-$100/m²

In the defence of the open sailmakers there are reasons why they are more expensive, and they include different designs for different masts, different demands from crews, who want different style sails (because they can) and the demand by those crews to use materials that are often expensive (again because the crews demand it) whereas with single source you simply don’t have those options.

While still on this spreadsheet there are other factors that have not been taken into account.

For instance, the number of sails, often there are 3 types of Finn mains, because you can.

When it comes to longevity of the sails, we demand that a sail be good for racing for at least 3 events in a worst case scenario, so for instance a 29er spinnaker is made from top end DP SCN 300 cloth silicon impregnated and we are just about to up-grade (over the next 2 years) to an even higher grade cloth (that will be introduced into the 49er this year.)    Its nominally 3/4oz and by definition will last significantly longer than ½ oz polyester used in other classes.   Again, if you are sailing say a 470 and can use ½ oz, why wouldn’t you!

49er sails are under higher load than the FX, bigger boys, pretty simple so the corner patches/piping are bigger and made of “sticky back”.    It’s expensive hence the increase in cost per m² over say an FX.

We could quite possibly cut back the size of the reinforcing in the 29er main and jib, but I think that is a false economy.

So if the decision is made to stay with single source sail making, we start to look at different sailmakers, and I personally run a test every 2 years.

That test comes in many different forms.

Presently I am in the process of developing some new products so I had need to seek some pricing and one of the sails I was seeking was near identical in size to a 29er mainsail.  So I spec-ed a sail near identical in size to the 29er, to 2 of the big sail making companies in the world.    Same number of battens, similar material, same potential numbers to be produced, as far as possible the same product.     They came in very similar price to Pryde (the current 49er and 29er sailmaker) but without battens, bags or sail kit!

29er 2013 Worlds11

More often, I get approached with a request to consider a new sailmaker to produce for one of the classes.   Presently exactly this is happening with 29er sails!  So in this instance we simply allow that process to run through to completion, that is yet to happen, and it will be interesting to get the results.

If the numbers are “interesting” we get into the ability to supply, distribution, warranty, stocking, materials and then the ubiquitous testing process.

Above I have highlighted Nb1 as the unforeseen costs of making a change in sail making.   The late Ian Bruce used to tell me that it cost about 7% to switch suppliers.

It’s probably true, but if we took that line then we would never switch, and if someone does come up with a new and better idea, and it’s impressive, then we will look at it.

Bottom line, I’m always looking!   Nothing remains the same, and I do miss the excitement of unbridled sail and mast development.

But until the sailors tell me that they are happy to pay open source sail making prices, I can’t justify the additional cost, particularly as the rules and complexity of the class would increase dramatically for little or no benefit.

But Norths are regularly approaching me, and they used to produce for us, Quantum have also made some overtures and if it ever did happen, we have got extremely good at developing a system whereby the sailors make the decisions and I just get to rubber stamp a change.

Julian Bethwaite
4 September 2016


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


Why rope vs wire on trapezes?

I have had 3 salutary experiences.

a) was my own, where as an 18 or 19 year old, got caught under an 18teen, I got out, shaken and stirred and I can still re-live it to this day.

b) not so long ago friend of mine got caught under an 18teen again, only reason he is alive is that the other 2 crew dived and blew air into his mouth while untangling him, ended up in hospital for a few weeks.

And c) Magnus Graver, sailing my 49er in Helsinki got trapped.  Alexandro from Spain, cut the rig out and got him up, long stay in hospital and he now runs the Swedish Sailing program.  Meet him regularly, last time in Lausanne this year. Very lucky boy.

Paddy Boyd who was then the Gen Sec of Canadian Yachting but obviously Irish told me once that more people die from drowning in Ireland riding horses than die from sailing related drownings world-wide, so the risk is small, but there is a risk, and we should do everything we can to mitigate it.

If you use rope, you need to anchor it.  The best “commercially” available rope anchor is the Blue Wave T-ball/Eye and it’s heavier than a simple T-ball.  And if you do the sums on a 29er the all up weight difference is less than 100gms.

Against that the simple fact is that well maintained wire does not pig-tail and most importantly it does not float.  Spectra floats and pig-tails. People float.

So why would you put floating people in the same region as floating, pig-tailing rope, all to save say 50gm and have a significant increase in windage to-boot?

We have made quite a few changes, very often against public sentiment.

I remember the donut on the end of the spinnaker pole, which was decried and opposed.  But if those people had not persevered and had not been successful then today there would be many FX girls with significant life changing scars on their upper bodies after last year.

No-one is decrying the donuts now!

Same goes for the shock cord spinnaker take up which is a mandatory fit-out in the 29er and 49er.  I may not have saved a single life, but I for one am not willing to remove it to find out!  I am not about to approve any spec change to allow rope on 29er trapeze wires and it would be a very brave ISAF or I29erCA to force the change.

Julian Bethwaite

29er Carbon Rig – status update

Status update – 29er Carbon Rig June 2016
by Julian Bethwaite

29er carbon rig


Rig size, why the taller mast?

Many, many classes have switched to carbon tubes.  In most cases all they have done is switch from alloy to carbon and there has been a weight reduction in the spar and also, in 99.99% of the cases, there has been a reduction in crew weight.

More …  Rig Size June 2016

So what is an assembly model?

In the case of the existing mast, all the alloy comes from Fletcher Aluminium in Hamilton, New Zealand.  We tried UK and Canadian base extruders bu the NZ alloy was deemed superior and many years ago we standardised on NZ alloy, of a very specific grade and a very specific temper.

More …  Assembly Model June 2016