Catamaran Stability

Home Articles How We Design Catamaran Stability
By James Wharram and Hanneke Boon

Foreword (James Wharram, 2004)

It is 50 years since I designed my (and Britain's) first offshore Sailing Catamaran. The accepted opinion at the time, expressed in Yacht Magazines, was that the offshore catamaran would break up in high sea waves, that their motion on the high seas would be so violent as to render the crew helpless and that the double canoe/catamaran could not sail to windward.

Well, the voyage of Eric de Bisschop of France who sailed his 38ft. KAIMILOA half way around the world in 1937/39 and his two by him inspired 'Sailing Sons', Rudy Choy of Hawaii in the Pacific and James Wharram in the Atlantic proved these 'theorists' wrong.

What is interesting on looking back is that no critic at the time mentioned 'capsizing' as a possible fate of the historic offshore double canoe/catamaran. The reason is that early in the 1950s, the wartime experience of hundreds of men who had survived, sometimes for weeks, in small open boats, in rubber life rafts, cork Carley rafts or even floating wooden hatch covers, encountering severe storms with big waves without capsizing, was in seafaring circles common knowledge.

That you could capsize at sea on a form stable sailing ship (which is what a catamaran is) through having too high a mast and too much sail area was at the time also common knowledge amongst seamen, as it affected all commercial sailing ship design. There were many people around in the 1950s who still had knowledge and practical experience of such ships. Their knowledge certainly influenced the mast heights and sail areas of my first seagoing catamaran designs, as did the writings on Form Stable ships by Howard Chapelle, the great American naval architect.

In the late 1950s, the Prout Brothers were developing a 16ft. racing day boat catamaran. It was fast and outsailed all monohull racing, dinghies of the time. Like racing sailing dinghies, without skilled handling, they capsized frequently. Still, with the attendance of the patrolling Race Guard Boats no one died.

Offshore catamarans began to first develop in the 1960s. From the beginning, there were some designers, like myself, who saw them as Form Stable boats following traditional Form Stable Stability values i.e. boats which 'looked after their crew'.

There were also designers (not many), drunk on the speeds of day racing catamarans, who used the sail areas, mast heights and stability values of the day racing catamarans on Offshore Cruising Catamarans, i.e. at all times the Crew looked after the stability of the boat.

Unfortunately, designers of these low stability catamarans have nearly always tended to imply in yacht magazines, to the public, that they are more skilled designers "Because their boats sail faster"?

Equally unfortunate was that by 1976 many of these low stability catamarans were publicly capsizing, when their trained crew got tired or, particularly, when sold to unsuspecting monohull sailors. Suggestions in England and America were made to 'ban offshore multihulls'. Hanneke Boon and I wrote our first article on cruising catamaran stability ("The Stable Multihull") in 1977, and things settled down again.

However, around the late 1980s another group of young designers from racing background or using racing catamaran concepts moved into Cruising Catamaran design and, once again, capsizes with deaths occurred. So, again we wrote in another article on our observations on safe stability for Cruising Catamarans.

This article was first published in 'Practical Boat Owner' (UK) in August 1991 and since then in several other countries. The I.S.O. has recently also published formulas for calculating catamaran stability as part of the Recreational Craft Directive. On examination, their formula is the same as the one published by us in 1991 with a slightly smaller safety margin for Dynamic Stability (70% to our 60%). So far, the I.S.O. has not yet given recommendations as to what is a 'safe' stability for offshore sailing.


In November 1989, the British Multihull Club, M.O.C.R.A., had an International Symposium on multihull design to celebrate its 20th anniversary.

During the lunch break, one very pregnant lady asked me: "Why don't they discuss capsizing? That is what I want to know about. I do not like heeling monohulls, but I do not fancy swimming with my baby out of an upside down catamaran."

Unfortunately, what I had to tell the pregnant lady is that they never seriously discuss capsizes at Multihull Symposia except in a self-congratulatory way, saying that an "upside-down catamaran floats as against a monohull that sinks". Ignored are hypothermia, broken limbs, lost crewmembers and mothers frantically trying to find their children to say nothing of at least half the value of the boat, i.e. the interior destroyed by the inrushing water.

It is a very emotive subject between designers and their followers because it touches not just on sales and profits, but also on masculine subjective attitudes like high tech., low tech., modern, traditional, taking risks, being cautious...etc. Symposium organizers realize that free discussion would lead to uproar. For the monohull sailor wishing to buy a cruising catamaran to suit his/her family's needs, Multihull Symposia and Multihull Magazines so far have given no real information.

Fortunately, the formulas that most catamaran designers use nowadays to calculate stability are not all that difficult to understand, and with them a prospective owner with a little background knowledge on sailing ships in general, can make his/her own decision as to whether the cruising catamaran they desire has the stability to be safe for their intended usage.

If you have forgotten most of your mathematics since you left school and, like most of us, hate to admit it, do not be frightened of the word formula. Calculators now do most of the work, and everyone knows or has some bright adolescent only too eager to use and demonstrate his/her latest calculator acquisition. However, the calculator only produces figures. To relate these to our needs, we do need to know some sailing ship history.

Polynesian Origins

The historic catamaran is the workboat of the Polynesian Pacific. Archaeological excavations, legend and early Western observers have shown that they had been in use hundreds of years - perhaps thousands - for fishing, coastal trade and ocean exploration, a background usage similar to that of the Chinese junk types and our own traditional Western sailing boats (before the development of the modern ballast keel yachts). Catamarans have exactly the same stability behavior as Junks and the traditional Western Sailboat.

Joshua Slocum's SPRAY is a typical example of a workboat of the late 18th and early 19th century. (See Fig.1)

Joshua Slocum's SPRAY illustration
Figure 1

What kept the SPRAY and traditional sailing ships from capsizing under the pressure of the wind on the beam were the wide hull beam, flattish bottom shape (i.e. "form stability"), and a selection of heavy rocks (i.e. "ballast stability"). In addition, the masts were kept short to lower the heeling moment of the sails.

Extra sail area for light winds was achieved by spreading the base of the sails out by means of bowsprits and bumpkins rather than raising the sails higher on a longer mast, creating a greater heeling/capsizing force.

According to Chapelle, in his book "The Search for Speed under Sail", if traditional sailing ships heeled much more than 55º, then they were in trouble. The loose rock ballast, about 10% of the total displacement, could break loose. A complete capsize would then occur and the boat would remain upside down. Capsizing, until the advent of the modern ballast keel yacht, was the theoretical possibility of ALL seagoing sailing vessels. Designers/ Boatbuilders have been able to design boats stable enough to stay well away from the possibility of capsizing for at least 3000 years. (ULU BURUN SHIP -Nat. Geographic Magazine, Dec. 87)

Racing developed the modern ballast keel yacht. To sail closer to the wind the rigs got higher. To balance that, the ballast changed from rocks to heavy iron (this became cheaper with industrialization), and finally, to be able to use even higher masts, the ballast changed to the heavier lead and moved from inside the hull to the outside in a deeper keel. As a side result, and not intended by design, the modern self-righting yacht was born.

Those who observed this development towards self-righting yachts did not regard it as a total blessing. They commented on how these "new" yachts plunged and rolled, which made sailing very uncomfortable and caused seasickness.

Even so the modern ballast keel yacht is still a relatively broad-beamed vessel, i.e. with a waterline length about 3 times longer than its beam - in technical terms, a length/beam ratio of 3:1.

Beamy hulls of 3:1 have to push a lot of water around them when sailing. This produces the well-known drag waves. (See Fig.1) and limits the maximum possible speed to approx. 1.4 x √WLL (in feet). Thus with a waterline length of 25 feet, your average speed will be about 5 - 6 knots.

The catamaran's unique speed potential, greater than that of the equivalent size monohull has arisen because it developed out of two ancestral boat types of the Pacific. Around the Pacific Ocean of antiquity there were various maritime peoples. Some used large paddling canoes up to 60 feet long for coastal trading, fishing, and whale hunting. (See Fig.2) Their long slim hulls with length/beam ratios of 12:1 to 20:1 allowed the water to part and run around them without creating drag waves at √WLL. They could reach speeds as high as 2 or 3 times the √WLL. So a canoe of 25 foot waterline length could reach speeds of 10 knots and over.

Joshua Slocum's SPRAY illustration
Figure 2

Hard paddling men with their food and water add up to weight. Even the toughest men can only paddle for a few hours.

Other Pacific maritime people had sailing rafts. Thor Heyerdahl's Kon Tiki expedition of 1947 used a modern replica of this type of craft. 45 ft. long, 18 ft. wide, rigged with a squaresail, manoeuvred by daggerboards, it could sail sufficiently against the wind to be a true sailing craft. It carried a crew of six in basic, though surprising comfort across the Pacific. (See Fig.2b)

It was not a speedy craft, but by its beam and weight, it was practically impossible to capsize and thus had stability, an essential part of seaworthiness.

Long ago some genius in the Pacific joined two fast, easily driven canoe hulls into a beamy raft shape, giving a new type of sailing craft with the stability of the broad beam raft and the high-through-the-water speed potential of the single canoe. (See Fig.2c)

Fig.2c is an approximation of a traditional Polynesian sailing craft and how it developed from its two ancestral types. It has a raft-like deck platform that could house people, and ample room to move around. From early European explorers' descriptions, the crew sailed with families, friends, lovers, singers and dancers in one joyous group from island to island - a marvellous way of life.

Efficient Crab Claws

Modern wind tunnel tests, as done by Tony Marchaj, of Southampton University, have shown that the Polynesian sail shapes were highly efficient to windward. With efficient sails, a hull form that allowed the boats to sail faster than the maximum speed of 1.4 x √WLL of Western ships and enough raft stability to be uncapsizable, (i.e. the sails would rip before the ships could capsize), the Polynesian catamaran was a remarkable sailing craft and worthy of being developed as a modern pleasure sailing craft.

Though to-day's yachtsman increasingly accepts the concept of the double-hulled ship, he/she places modern urban attitudes on the catamaran. These are: 1) to get the maximum speed potential out of the catamaran form. (Faster is always equated with being better, no matter what the cost.) 2) to alter the hull form to get as close to modern urban style accommodation needs as possible, which was described in the recent RYA (Royal Yachting Association) 'Competent Crew Handbook': "The typical modern cruising yacht has....interior design principles....much in common with a caravan".

The quest for speed

It is Demand 1) which creates most controversy for, in order to reach the maximum speed potential of a catamaran, you have to carry a large sail area, which reduces its inherent stability to the point, where with the average cruising crew, it is in danger of capsizing well before the average monohull suffers a knockdown.

With a sense of realism any would-be catamaran owner, once he/she knows how to calculate stability, can make his/her own decision when viewing a cruising catamaran design, whether they want maximum speed or maximum stability. As the formula will show, you cannot have both at the same time. Fig.3 shows how to calculate catamaran stability. Fig.4 and Fig.5 are helpful to learn how to determine the position of the center of Effort and the Center of Lateral Resistance.

How to calculate stability
Figure 3

In 1976, catamarans built using this stability formula were capsizing all over the world at mean wind speeds a lot lower than the wind speed the formula predicted.

In an Article called "The Stable Multihull", published in 1977, Hanneke Boon and I demonstrated that the given formula was a static formula for static state conditions.

However, wind is a turbulent, gusty, dynamic force. Gusts can be as much as 40% to 60% greater than the mean wind speed, so the static formula has to have built in a safety factor for dynamic, natural state wind conditions to allow sailing craft to absorb the extra wind gusts without immediately capsizing in the manner of a dinghy.

Since 1977, this dynamic formula concept, after much initial argument, has been accepted. It has now been generally agreed amongst designers, that taking 60% (x 0.6) of the Static stability allows for a suitable safety factor. So, the Dynamic Stability (i.e. maximum mean wind speed it is safe to sail in before reducing sail) is found as follows:

Static Stability x 0.6 = Dynamic Stability

At the M.O.C.R.A. symposium were the designers of two 34-35 ft. catamarans about to be placed on the market. We will use them as examples of two opposing design attitudes towards speed and catamaran stability. Their dimensions, obtained from yacht magazines and brochures, are given in Fig.6a.

Lever diagram
Figure 3a
Finding the centre of effort
Figure 4
Finding the centre of effort
Figure 4
Centre of pressure
Figure 5
Comparing stability of catamarans
Figure 6

The first noticeable points from Fig.6a are that catamaran B has a wider beam than catamaran A, but carries 33% more sail and has a much lighter construction weight.

If you asked the opinion of the designer of catamaran A with reference to design B, he would say that he has been designing and building catamarans for thirty years, that his sail area to weight ratio to beam etc. had evolved to provide the maximum stability, Which adds up to sailing safety.

The designer of catamaran B, a more recent designer in the cruising catamaran field, would point out, that his design had much more beam (which is a feature of catamaran design over the last ten years) and. thus has the stability to carry the extra sail area.

You, the would-be catamaran purchaser, without the aid of the given formula would be at a loss to know:

  1. The Static Stability of either design, which can be described as the "Moment of Truth" when the boat is on the edge of capsizing i.e. when the windward hull lifts out of the water.
  2. The Dynamic Stability, when it is safe to sail along with all sails up and have sufficient reserve stability to meet safely any wind gusts that lie under that lovely white cloud or just along the coast where a narrow, scenic valley opens to the sea and down which the wind unexpectedly gusts.

Fig.6b shows the working out of the Static and Dynamic stability of both designs, using their lightly loaded weights, sometimes described as racing trim. From the formula we can see that catamaran B has less stability in spite of its wider beam than catamaran A. So what!

These figures must be related to the real world of sailing. To do this I will use a book by the sailing meteorologist, Alan Watts, called "Instant Weather Forecasting" (published by Adlard Coles).

On pages 10 and 11, Alan Watts describes the behavior of dinghies and deep keel monohulls in various wind strengths. (I have extracted these details for wind forces 3 to 6.). See Fig.7.

Comparison of dinghies and deep keel monohulls in varying wind strengths
Figure 7

Catamaran A with a Dynamic stability of 18.2 knots (Fig.6) needs to be carefully sailed or reefed by the middle of force 5 (remember this is a lightly loaded catamaran). Alan Watts describes the deep keel monohull in a force 5 wind as "Craft's way somewhat impeded by seaway. Genoas near their limit. Yachts approach maximum speed.'

Force 5 is a well-known wind state that to the average yachtsman draws attention to itself by strong audiovisual signals of waves and wind, which leads him naturally to take particular care in sailing, changing headsails or reefing.

Therefore, if a monohull yachtsman handling Catamaran A is slow at reefing in force 5 and is hit by a strong wind gust, he would have approx. a 60% stability safety margin to absorb his slowness and the gust, for his windward hull would not begin to lift from the water (the Moment of Truth) until 30.5 knots of wind hits the sails (i.e. a force 7 gust).

Similar sail handling

Conclusion, the same sail handling habits of the monohull cruising sailor can easily be applied to the lightly loaded catamaran A, without fear of immediate capsize. Catamaran B with a Dynamic stability of 13.3 knots (lightly loaded) will need to be carefully handled, aware of wind gusts, or reefed, to preserve its 60% stability safety margin in the middle of force 4 (11-16 knots). THIS IS AT ONE FORCE lower than monohulls or Catamaran A.

Watts describes force 4 for monohulls as: "Best general working breeze for all craft, genoas at optimum". With the necessity to reef to preserve the 60% safety margin, this description does not apply to Catamaran B. However, his description of force 4 for dinghies does, for he writes: "Dinghy crews lie out...", i.e. are attentive to stability to prevent capsizing. As dinghies, so Catamaran B.

Catamaran B continues to echo dinghy-handling characteristics at increasing wind strengths. Catamaran B's Static stability, i.e. hull lifting point, lies on the borderline between force 5 and 6 (22.5 knots). Watts' dinghy handling descriptions for force 5 and 6 are as follows: Force 5: "Dinghies ease sheets in gusts...some capsizes." Force 6: "Dinghies overpowered when carrying full sail. Many capsizes." Conclusion: Lightly loaded Catamaran B, above wind force 4 can only be sailed safely by skilled dinghy type sailing techniques or should be reefed at force 4.

Why not reef?

The enthusiasts for the Catamaran B type argue that for general family cruising, you can reef Catamaran B and give it the same monohull type stability as Catamaran A. At other times, with a trained crew and all sails up, you have the benefits of fast exciting sailing.

This is true, and Fig.8 shows the use of the formula to see how much sail you would have to reef down to give Catamaran B the same stability as Catamaran A with all sail up. This is a sail reduction from 750 sq.ft, to 511 sq.ft.

Reefing formula
Figure 8

Providing that reefing was a rigidly applied rule when there was not a fully experienced dinghy technique skipper standing by the sheets or helm, it would be effective.

If you feel a little conspicuous, sailing reefed in a Force 4 breeze, you can carry more load to stabilize Catamaran B. Again, the formula (see Fig.9) shows that if the boat weight is increased to 11787 lbs with extra stores and equipment (in fact, its full cruising payload), your stability would again equal that of Catamaran A in its lightly loaded condition.

Weight and stability formula
Figure 9

However, if Catamaran A increases its payload to the designed maximum (approx. 11050 lbs) its Dynamic stability goes up too, and it would require a gale gust of 34 knots, to lift one hull out of the water. This conforms to the wind stabilities of traditional sailing craft throughout the ages. A cruising catamaran designed to these principles gives no stability problems to the average yachtsman and his family, enjoying its broad decked upright sailing.

Further Reading:

  • Philosophy behind the Ethnic Approach

    An overview of James Wharram Design's core design principles and how new designs are created to reflect these principles.

  • Heavy Weather Sailing on Wharrams

    How the design of Wharram catamarans is influenced by the experiences of many ocean storms.

  • The Wharram Wingsail Rig

    First designed in the early 1980s for the Tiki 21, the unique Wharram Wingsail Rig is a simple, aerodynamically clean sail rig. It is now available for use on all Wharram designs as sloop, or schooner rig. This article gives all the details of what makes it work.

  • Freeboard and Windage

    Freeboard affects the windage and hence the windward sailing ability of a catamaran. This article written for an Australian yacht magazine in 2003 looks at the origins of multihulls and what makes them work.

  • Design Discussion

    James compares Gaia’s design features with those of the charter Pahi 52. Spirit of Gaia’s Wingsail Rig is tested by other catamaran sailors.