Freeboard and Windage
|By James Wharram (2003)|
|The modern catamaran is a remarkable craft. It is the direct descendant of two of the most ancient water craft used by Man, the Canoe and the Raft (see fig. 1,(a) and (b).)|
|The Canoe and the Raft are the first 'People Movers' and antedate the use of the wheel and the horse by thousands and perhaps 100s of thousands of years. Many people think that they are Pacific Craft, but evidence exists that Palaeolithic Man all over the world used the fast Canoe and the stable Raft. By simple logic two fast canoeforms joined into a raft shape for stability gives a third type of craft that combines the characteristics of both canoe and raft, namely the speed of the canoe and the broad beam and stability of the raft. This 'concept' modern yachtsmen call the 'Catamaran'.
Sometime in the last 10,000 years, with the end of the ice age and the flooding of the Indonesian and the Philippine land mass, canoeform craft (which include single and double outriggers) were fanning out from these areas across the Pacific and Indian Ocean in exploring voyages.
The double canoe in fig. 1c is not a fancy drawing. It is based on a lifetime of study and sailing. It is in fact one of my new Ethnic Study design. There are several enquiries from people who wish to build this one and similar designs.
By the mid 1950s, for reasons outside this article, the Double Canoe concept had become designated by Westerners as a primitive craft, unseaworthy, with a violent motion, unable to sail to windward and liable to break up on the high seas!!!?? But there were some individuals who thought differently and were prepared to test out their ideas.
I am one of 3 pioneers: Eric de Bisschop of France, Rudy Choy of Hawaii and James Wharram of England, who, by 1955-6 took the at that time described as 'unseaworthy, primitive craft with no windward ability' onto the oceans and proved the critics wrong. This gives me an overall perspective of the present cruising catamaran scene wider than most.
45 years ago, as a young dreamer wishing to be an ocean sailor, I saw the ancient, traditional sailing boat, the double canoe (the word catamaran hadn't been coined yet), using the minimum of material and labour, as 'the' boat to get me ocean sailing. That is how I became a catamaran pioneer. (My pioneering voyages are described in my book 'Two Girls Two Catamarans' )
I know now, that at the time I had only realized a fraction of the possibilities of the catamaran concept, for nowadays, if you wish to race around the world at maximum speed to please some big business sponsor, you design a 120ft. catamaran that is remarkably close in basic hull design aspects and proportions to the ancient double canoe (Fig.1 (c) ).
If you wish to impress people with your ability to provide sophisticated luxury accommodation on a boat with a reasonable sailing performance you can quite confidently commission a catamaran design. Indeed, it is the possibility of large accommodation, spread out over the wide deck area between the hulls of the catamaran, which has attracted the ever increasing number of modern sailors to invest their savings in owning a catamaran.
Many people will be very surprised to know that this aspect of the wide, usable deck of a modern catamaran comes from the 'Raft' part of its ancestral development. (See Fig. 1 (b) )
I must advise readers not be frightened of the word 'Raft'. Modern replica raft voyages, from Tor Heyerdahl's 45ft. balsa raft 'KON TIKI' of 1947, which sailed from Peru to the Tuamotus, to Tim Severin's bamboo raft, 'Hsu Fu', voyage in 1993 from Vietnam into the North Pacific, give ample testimony of the raft's sea kindly motion and deck space for accommodation. So seaworthy is the raft form that all modern yachtsmen are advised to carry miniature (inflatable) rafts if for some reason their parent craft sinks. Raft features enable shore style accommodation on catamarans that can still sail reasonably well.
For 11 years, I have been sailing the world on a 19m catamaran, the SPIRIT OF GAIA (see article by Mark Smaalders 'Cruising Helmsman' - Aug. 2002 , which in much of its hull design is similar to fig.1c and could have existed several thousand years ago. I love this boat. She fought her way North from New Zealand to Fiji in one of those long lived tropical convergence gales in winds 70 degrees off the bow and wind speeds on the 'clock' of 45-50 knots. She sailed 600 miles North up the Red Sea against the dreaded Red Sea North winds of 30-35 knots, smashing through vicious short steep seas. In mainly light winds - hardly a ripple on the water - she dream drifted across the Indian Ocean making an average of 100 nautical miles a day.
Last Christmas, 2002, deciding I had enough of the Fake "Yo Ho Ho, it is Christmas time" I went with my family out to Corfu, Greece, to spend 2 weeks aboard GAIA tied up in the Marina, for a rest. When the sun shone it was wonderful and warm, but in the cold rain I reflected on the comfort defects of a traditional proportioned Double Canoe.
Behind me, on the Marina pontoon was a modern 12 metre, large accommodation, cruising catamaran, designed 10 years ago by an Antipodian designer, modified slightly by its Dutch builder and owner Wim (as Wim is a former catamaran racing enthusiast, his modifications added to the design, not detract from it). (See Fig 2).
This catamaran can be described as one of the 'Leader' designs, in the developing modern cruising, charter catamaran market of France, South Africa, America etc. of the last 10 years.
On rainy days Wim and his wife Marion invited the crew of GAIA to enjoy the closed, modern house-style comfort of their boat. Not only the crew of GAIA, we had several parties on board when Wim and his wife were hosting 9-10 yachtsmen.
Stepping on board Wim's catamaran by the stern steps - the freeboard is too high to step up on to the hull off the pontoon - we, the guests, were welcomed in the stern cockpit of a spacious 8 square metres. This cockpit has an all-enclosing weather proof awning, removable, or reduced to a bimimi, in the summer.
The deck cabin, through the wide patio doors, covered an area of 12 square metres. In all there was a covered weatherproof deck space of 2O square metres, with standing headroom throughout. This covered deck space looked even bigger because on either side of this permanent deck cabin was the space of the open hulls of another 1.5 metres on each side (one stepped down to the hull floor level), giving a total visible, well lit space of 7 metres wide! With the large windows giving a wide view (moored in Gouvia Marina this view is superb), the interior with its cabin furniture and fittings in high quality Dutch taste, this modern catamaran is a superb living boat.
Wim built it, and is happy with it, for this reason, but as an experienced multihull racer and having seen my GAIA streaking across the horizon, he was most apologetic about his sailing home's sailing abilities. I think he was too critical, but it is a fact that, once you have experienced the sparkle, the quick surge of acceleration of boats with the design parameters of the ancient Pacific Double Canoe/Racing style modern catamaran, you are spoilt for sailing other boats.
There are two important design parameters which slow down or speed up a sailing boat: Wave drag and Windage (windage on all courses but running dead before the wind).
Most people are aware that a monohull yacht (apart from recent skimming racing designs) has a limiting speed of between vWLL x 1.3 - 1.5, due to the built up of wave drag resistance.
What is not generally realised is that Multihulls also suffer from wave drag resistance. In the 1960s I first observed that catamaran designs with different cross sections (flat bottoms, V-eed and semi circular hulls), all pulled visible drag waves at speed if they had an individual waterline length/beam ratio of 8:1, i.e. a limiting speed factor. (For interest, my 63ft. SPIRIT OF GAIA (see article by Mark Smaalders, Cruising Helsman - August 2002) has a WL length/beam ratio of 17:1.)
Ancient Pacific catamarans, as in Fig.1c, had waterline length/beam ratios ranging from 12:1 to 20:1. Modern racing catamarans also have length/beam ratios of 12:1 to 20:1!! Wim's 12 metre catamaran hulls, for accommodation and load carrying purposes, have a waterline length/beam ratio of 9.5:1, only slightly better than the wave dragging 8:1 hulls I observed in the 60s! My studies have shown that the average modern accommodation cruising catamaran hulls has a WLL/Beam ratio of between 8:1 and 10:1 (most designers never give this vital figure).
Windage: The freeboard/windage height of Wim's boat - of the hull with the deck cabin included - is like most modern cruising catamarans about 20% of its overall length.
Historically, the last time Western non-engined sailing ships used such a high freeboard/windage height were the Spanish Armada ships of the 16th century. (See Fig 3). The English ships of that time with a lower windage height sailed circles around the Armada Ships.
Over the centuries, the windage height of non-engined Western sailing craft became lower and lower, by the end of the commercial sailing era it was around 8%. Traditional monohull sailing yachts (both shallow and deep drafted) have a windage height of 11-12% their overall length, so do modern monohull racing yachts. On the ancient Pacific sailing boats' (and Viking era ships!) windage height ranges from 8% to as little as 6% of the overall length.
The windage height of 20% of the modern luxury cruising sailing catamaran does seem to be excessive for good sailing performance. Maybe that is the reason why powerful engines always play a prominent part in their advertised specifications.
It is amazing that one of the oldest ship types in the history of Man has been able to be adapted to fit the needs of modern, stiff bodied, Urban Man in its provision of bedroom privacy, number of toilets/showers, kitchen facilities and spacious lounge/dining area (an accommodation equal to, to use a modern term, a 3 or 4 star hotel) and can still sail.
Some designers have tried to make the modern high freeboard, beamy-hulled, catamaran sail fast. They empty the hulls of all weight, put a large sail area, with high centre of effort, on a tall mast, fit deep dagger boards in the hulls and in smooth water, these craft can 'skim' along.
Unfortunately, it is not a custom for designers or salesmen to give the stability values of these craft. Briefly, most of such craft have a stability close to a racing dinghy or day racing catamaran. Sailed by untrained, unsuspecting sailors, these craft also capsize like a dinghy, for the 'raft stability' has been compromised.
This is not the way to go. To get speed we have to look back at established elements of general yacht design, using the freeboard/windage height of around 11-12% of fast monohull yachts and the fast catamaran waterline length/beam ratio of a minimum of 12:1. This does mean that you do not have full standing headroom in the hulls until you reach a boat size of 36ft. (10 metres). That is nothing new in yacht design, it was always considered the norm in traditional shallow draft sailing yachts, which after all a multihull is. Standing headroom in a centre deck cabin with adequate centre deck clearance from the sea will always compromise speed potential.
Anyone can quote theories, the acid test is, can they be made to work. All my designs over the years have been designed with traditional windage heights and slim hulls. To illustrate my latest design thoughts I want to focus on our ISLANDER 55 design (see Fig. 4, following), the first of which is being built in Surabaya in our professional yard, PT PAL. This design has an hull waterline length/beam ratio of 12:1. The visual feeling of space is achieved by flaring out the gently curved V-eed hulls, getting greatest width at shoulder height. Flare with overhangs give easy, gradual lift going through rough seas, prevent pitch poling and with the lower traditional freeboard keep the decks dry.
|High freeboard of modern cats.||Standing on a box to see over the cabin.|