Pt 2:1 – The Numbers Game


Note: this is the first chapter of the second part of SailCraft. Here we start to look at the current state of design, rather than the history that lead us here. You may notice that this was written several years ago, and it’s getting a bit dated as the references to Moths may reveal – they were still in the early days of foiling. I’m not a dinghy designer, but since no one else was writing much about dinghy design it seemed that someone might as well step in and ask the experts about it. This section hasn’t been footnoted – all the information comes from my own discussions and correspondence with designers unless otherwise indicated. If there’s enough interest to put SailCraft out in e-book or hard copy form, I may start another round of interviews with the guys who know what they are talking about.

The magic numbers – like length, displacement, beam, and righting moment – and the ratios between them are real drivers of performance.

We tend to spend a lot of time (in my case, too much time) thinking about hull shapes and the way that they affect boats. When you get down to it, shape can determine how easy a boat is to sail, and it does make an obvious difference in the last “X” percent of performance. But many designers say that what really counts is not the lines, but the numbers. The most beautifully crafted Merlin Rocket will rarely beat even a badly shaped I-14 around the course. And in many classes, there’s an incredibly small performance difference between radically different shapes. In National 12s there’s a world of difference between the shape of a “Final Chapter” and an “Annie Apple”, yet on the racecourse they are similar in pace. The same applies in the 12’ Skiffs. “The Nash, Woof, and Nuplex hulls are totally different shapes” agrees 12’ Skiff champion Alex Vallings “but the performance difference is very small. They all have their moments.” So what are the numbers that count, and how do they relate to each other?

Length – the vital statistic

The longest dinghy? The Z-Jolle of Germany and Austria is up to 8.5m (28ft) long, but unballasted, capsizable and a dinghy by any measure. Pic from the class site.

In dinghies and skiffs, just as in yachts, more length equals more speed, upwind and around the course. It’s been recognized for a long time. Length, wrote Olympic gold medalist and 18 footer champion Peter Mander back in the ‘50s, is “not quite everything. Say ninety-nine percent”.

It’s obvious that waterline length is important at the top end of displacement speeds. But is it still important in the age of high performance skiffs that plane most of the way around the course? “Very; especially upwind and in waves – maybe not much when being blown down wind like a leaf behind a large asymmetric” says Phil Morrison. “Wavemaking still comes into it even for faster classes”.

Even Julian Bethwaite, a fan of a “less is more” approach, reckons that when it comes to length, more is more. “We’ve done out a whole bunch of research where we’ve worked out what is the ideal length for a two man boat, and it’s around 17’6”(5.3m). The ideal length for a three man boat is around 21’ (6.4m). And as time and technology progresses, that length will get longer not shorter. Things get lighter and stiffer, and as you’re able to build things at a lighter weight without greater mass, then the ideal length will get longer not shorter.”

Compare the International Europe and the Laser Radial. The Europe was designed as an international racing class, the Radial as a weekend funboat. The Europe has more hiking power, a narrower waterline, and is two-thirds the weight of the Radial. The Europe sets 8m2 of film sail on a carbon wing mast, while the Radial has just 5.7m2 of Dacron hanging off a mast made from drainpipes. The Europe has more sophisticated foils and gear, and costs about twice as much as a Radial. Every advantage lies with the Europe – except length. And the result? The Radial is normally considered faster in most conditions and all-round – mainly because of that extra one metre (3’) of length.

Length gives designers freedom; longer boats don’t have to be compromised by like waves, nosediving and the buoyancy to support the crew. It allows them to have a sleeker, lower-drag shape. The great length of an 18 Foot Skiff allows designers to create fine wave piercing bows that make them incredibly efficient – yet even 18 sailors would like a bit more length. Length allows boats to move faster before they enter the forced mode. The lesson is pretty clear, according to designers. For speed, go for the longest boat you can.

Is there a downside for adding length? All else being equal, designers say, a longer boat is never slower. It has an advantage all the time except in fast downwind sailing, and even then it’s at no disadvantage compared to a shorter boat.  But while long boats may normally be faster, they are not necessarily more fun. Lots of sailors reckon short boats are more exciting. “The 18s are very fast, but it feels kinda like a big catamaran, like a locomotive, slow to accelerate (relatively) and on tracks as well” says US I-14 sailor Pete Mohler. “The 14s are astonishingly quick, you get the feeling that they will jump out from under you.”

As any sailor from the 12 foot development classes will tell you, short boats can be more fun. Pic from the Australian Cherub class site.

Form  Stability

A dinghy or skiff has two sorts of stability. One (form stability, sometimes known as inherent stability) is the basic stability that it shares with just about everything that floats. Naval architects can define it with terms like metacentric height and centre of buoyancy, but for dinghies it’s largely a matter of length and the effective waterline beam. The most stable shape of all (given equal displacement and beam) is normally a flat-bottomed scow (although they become less stable at high angles). Widely flared topsides can also contribute to form stability by creating more buoyancy out wide when the boat heels and they hit the water.

Form stability is a nice thing to have, especially for less expert sailors. It makes boats easy to sail, it allows for relaxing between races, and it keeps you upright and dry when a gybe or hoist goes wrong. But in a dinghy or skiff, form stability has almost nothing to do with the ability to carry sail. That vital factor comes from the crew’s righting moment.

Righting moment – the power number

Chesty Bond during the unlimited era in the 1980s, when the 18 Foot Skiffs measured as wide as 9m/30ft from wingtip to wingtip. Pic from

Righting moment is the definition of a boat’s ability to withstand heeling forces – in our case, the ability to stay upright under the press of sail. Ever since the 1880s, when America’s canoe sailors stopped laying back in their cockpits and started to hike over the side, it’s been clear that the leverage of the crew’s weight is the most important factor in a dinghy’s righting moment – the measure of its ability to carry sail. In fact, the crew’s leverage is one of the most important factors in a small boat’s entire performance.

As Julian Bethwaite says “at the end of the day, your power is limited by the amount of weight you put on the wire, and how far you displace it from the centerline, and it’s a very simple sum. You just can’t exceed it.” Americas Cup designer Andy Dovell agrees, “all this hull shape stuff is all fun and subtle, but the first and foremost thing in dinghies is where the crew is, relative to the centreline. Hiking beam – the widest point where a guy’s feet can stand, relative to the centreline – is absolutely the defining feature of a dinghy.”

The high performance sailing dinghy or skiff is almost the only craft that does not rely on form stability or weight for stability. “There’s no point in going wide in a dinghy hull, because they will be sailed flat. So you get no gain from form stability and centre of gravity when you heel, like you do in a yacht” notes Dovell. “The actual contribution of hull form stability or centre of gravity stability to the overall stability is very, very low. The only thing weight does in a dinghy is make it safer in an uncontrolled situation.”

The fact that hull stability is almost irrelevant makes it easy to calculate a dinghy’s righting moment. “The calculation is quite simple – the righting moment is the crew’s mass times the distance from the centerline, period. That’s how much sail force a dinghy can handle” says Dovell.

Even before it got onto foils, the Moth was cited by many of the designers I spoke to as one of the most influential boats in the world. Those who sailed the “seahugger” Moths broke new ground by sailing boats with hulls just one foot wide from wings over seven feet across, and no one did it better than Mark Thorpe, who not only designed and built the Hungry Tiger but also won multiple world titles on it. Pic from his boatbuilding business’ site, Thorpe Boats.

Some simple calculations show how important beam is in a dinghy or skiff. For our purposes, righting moment is calculated using the crew’s mass (varied according to normal crew weight in the class), taken at 915mm/3’ from their feet in trapeze and plank boats or 305mm/1’ from the gunwale in most hiking boats.

In an early narrow (1.76m/ 4’9”) International 14 like “Thunder & Lightning”, two 75kg/165lb crew hiking from the gunwale had a lever arm of    /3’5” and generated 1200 ft/lb of righting moment. When John Winter leapt onto the trapeze, he increased that to about 1500lb, allowing the rig to generate 25% more power (and according to some rules of thumb, the speed increase would be half that figure). If the same two crew were to hike from a modern boat of similar length but 510mm/20” wider like an RS 400, they’d generate the same righting moment without a trapeze, by using the extra beam. Add twin trapezes (like an International 14) and the righting moment jumps up to 1800 ft/lb. Add wings like those of a 49er, and the boat can generate twice the power of the early hikers.

Something else that’s quite striking is just how powerful a dinghy can be. We often tend to dinghies as light and tippy, but in fact, a dinghy (scaled up) generates much more power than a yacht. An International 14 scaled up to 6.5m/21’ generates about 20% more stability, proportionately, than a 6.5m(22 ft) Mini Transat yacht which has both water ballast and a canting keel.

The skinny on beam

The ultimate high-performance dinghy and skiff would have the widest possible overall beam, and the narrowest possible waterline beam. The reasoning is obvious – a wide overall beam allows for more righting moment, and a narrow waterline beam creates a low drag hull.

In older boats, overall beam and waterline beam were linked. Some classes, like 505s and Merlin Rockets, had wide hull flare above the waterline, but there were limits because of wave impact on the wider topsides and water scooping over the lee gunwale. The Northern hemisphere’s move to skiff-type boats broke the connection between overall and waterline beam, and classes like the International 14 have modified their rules to allow narrower hulls. At the same time in the South Pacific, the swing away from the flat planing hulls has also put the emphasis onto a narrower, slimmer hull.

Learning from Moths

The International Moth may have been the most influential class in the trend to minimise waterline beam. Long before it went to foils, many of the top skiff designers looked to it as the shape of the future. Over the last decade, the Moths have dropped their waterline beam from about 48cm/19in to 30cm/12in. The change has transformed the tiny 11 footer from a boat that could pace a Laser, to a boat that can beat a top-class Flying Dutchman. No class has followed the Moth route but they have gone part-way. “The Moth lead the push towards narrower hulls, and proved beyond a shadow of a doubt that they worked” says Michael Nash, who helped lead Australian dinghies and skiff down the narrow path. The “seahugging” pre-foil Moth lived in a special zone where wavemaking resistance becomes almost negligible, an area open only to boats that are about 10 times as long as they are wide. But even in the more conventional boats, the advantages of narrow beam are obvious; less form drag, less wavemaking drag, less wave drag, and less wetted surface.

So what are the limits to narrow waterline beam? The obvious problem is how to keep the whole deal upright. The skinniest dinghies ever were Moths around 200mm wide, but they were just too unstable to handle. They also ran headlong into a basic problem of geometry. The narrower a boat is, the deeper it will float, and eventually so much of the hull sides are immersed that the wetted surface becomes excessive. But good sailors can handle boats with just a touch more beam. The “standard” narrow Moths are demanding, but they are also universally accepted to be easier to sail than their beamier predecessors because they track so much better. Assymetric Canoes prove that even a singlehander can balance a 1m wide boat well enough to handle three sails including a 22.5 m2 spinnaker.

Crewed boats face the problem that they have to support more weight as well as more sail. Murray Burns and Dovell explored the narrow theme in an Australian 16 foot skiff, before that class went one design. “We went just a little too close to the edge with the 16 we did for Trevor Barnabas” explains Andy Dovell. “We did a joint decision to keep the thing round bilged, and it was a narrow, narrow boat. Probably the difficulty in handling the boat was the narrowness and the fact that it was round bilged. But, he would be so far in front he’d capsize, get up and still be winning the boat race.”

Alex Vallings is an expert designer and sailor, but even the experts get it wrong sometimes. Pic from the 12 Foot Skiff class site.

Alex Vallings, a national champion in both 12 Foot Skiffs and R Class, has the narrowest boat in the 12s. His “Nuplex” has a waterline beam of 77 cm/2.5ft. When powered up and moving fast, “Nuplex” is an excellent performer. But 12 Foot Skiff crews have to stay well aft to stop their boats from nosediving, so the narrow boat tends to drag its stern. At low speeds – when tacking or in light airs or waves – Nuplex can “bog down” while the beamier boats keep moving at a more consistent pace, and she is forced to sail higher angles downwind to keep the speed up. After his first season in the boat, Vallings chopped into the topsides and widened it. The weak spots improved, but he plans to widen it again. Vallings believes that 12s could cut their waterline beam down dramatically if they were allowed hydrofoil rudders to lift the stern, but the need to have enough form stability to support the big rigs dictates a minimum waterline beam of about 500 mm. That would still leave the skiff much wider than that “magic” ratio of 10 to 1.

Few people have better credentials on this issue than Emmett (“The Professor”) Lazich. Lazich was a world Moth champ designer/skipper before he moved to 49ers, where he become a world-class competitor and coach to the 2000 Gold Medallists. His modified GP18 18 Foot Skiffs may have been the fastest non-foiling dinghy ever. Yet Emmett believes that the super-slim shape is not suitable for crewed skiffs. He feels that the seahugging Moth (like catamarans) had a top speed of around 18 knots. That’s super fast for an 11 footer with just 8m of sail, but it’s well down on the top speed of 12 Foot Skiffs. The problem, Lazich believes, is not the Moth’s lack of power, but the fact that it is too narrow to plane efficiently. So Lazich believes that the super-slim Moth route is a great one for boats with small rigs, but not for overpowered types that are capable of higher ultimate speeds.

Handling Skinny Boats

So are narrow boats really harder to handle? They obviously have less innate form stability than beamier shapes, but lighter modern rigs have helped to compensate. The 12 Foot Skiffs, for example, have dropped their waterline beam by a couple of feet over the last couple of decades, but in the same time the weight of their 9m long masts have dropped to just 7kg (rigged). “When a modern 12 heels, you can save it because of the carbon mast” notes 12 Footer designer Jim Walsh.

Technology apart, it seems that top-class crews may find narrow boats easier to sail because they are easily driven and tend to handle well at high speeds. Good crews have the skill to keep the boat up at low speeds when tacking, gybing or doing spinnaker work. On the other hand, the less expert crews (who often have trouble at low speeds around the “corners”) may struggle with narrow boats. The reduction in low-speed stability means that there is less room for error in low-speed moves like tacks and gybes. Some say that the move to narrower boats has opened up the gap between the experts (who can sail the boats upright and on the designed lines) and the average sailors (who let the boats heel and drag their wings or fall onto the beamier sections above the waterline). The underlying issue is of course that the issues with handling a narrow boat may drive sailors out of a class – or out of the sport.

A slender Patterson Cherub. Pic from the Bloodaxe Boats site.

One of the gurus of the narrow boats is Andy Paterson, a designer/builder from England’s Isle of Wight. He created the first successful narrow skiff Moth and the most successful narrow Cherub. “My narrow Cherub design does feel Moth-like in its lack of inertia. It is very unstable, and will fall over in no wind with or without crew. There is a little initial stability, but any slight heel will increase it. Once sailing at any speed it is OK, and very responsive and light to steer.”

“The hardest bit of the narrow boat is when displacement sailing in stronger winds, ie when tacking, or big changes in direction or going slowly when getting the kite up or down in planing conditions. They (the narrow Cherubs) are much harder than the old boats, but not as hard to learn as a Moth. It’s possible for a competent sailor to sail a Cherub (slowly perhaps), but not many can sail a Moth for more than a minute or so for the first time without capsizing.”

Fellow UK Cherub designer David Roe notes that “the narrower boats are easier to sail when planing. They are less stable laterally in displacement conditions, but that was never much of an issue – people sail Moths after all. The down side of making boats narrower is that the amount of water moved during pitching (added mass) goes down, so pitching in displacement sailing does increase. It is noticeable that as the boats have become narrower that nosediving/pitchpoling has definitely become less of a problem… provided the crew stays back!”


Author: cthom249

A former sailing journalist and magazine editor, I was lucky enough to grow up in Sydney, one of the world's sailing hotspots and to win national and state championships in classes like J/24s, Windsurfer One Designs, offshore racers, Laser Radial open, Windsurfer OD Masters, Raceboard Masters and Laser Radial Masters, to get into the placings in a few other classes, and do a few Sydney to Hobarts.

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