Home | Articles | Links | Projects | Columns | Designs | Boat Index

jeff.jpg (3473 bytes)
Jeff Gilbert

Design Philosophy

(23' Catamaran)

Scarper Flo
(31' Gaff Cutter)

by Jeff Gilbert jgilbert@dynamite.com.au

December, 2000
Laws & Rules, & other Bull

Commonly held views and much quoted opinions often become laws by proxy. Never mind that a law should be universal; an idea that covers a "fair few" situations is fished out and purported to cover all.

A blatant example .I was scribbling away ineffectually at an unusually skinny cruising yacht hull the other day when a really nice chap I know looked at the drawing with the knowing eye of the truly inexperienced and said "Surely that boat will be too slow". I was wondering what it would be "too slow" for, but luckily before I could get too confused he reminded me that "I knew" that a monohull sailcraft was fastest at a length to breadth ratio of 3:1.

We were looking at a deck plan and by the time I had worked out how to respond, he was already onto the lack of winches and freeboard or some other glaring fault. The last point had obviously left me too defeated to even think of an excuse for overlooking the well-known truth that all yachts which are three times as long as their beam are faster than those that are not.

Rather than bother to explain that this is obviously ridiculous, even if talking about waterline measurements, lets look at where it may have come from, which is a bit tricky because it was a process not an event. I’m not sure, but my guess is this. Three to one produces an OK-looking cruising yacht which maximises the internal live-aboard accommodation space in a given length (read "given Marina cost") without unacceptable penalties in wet area and hence low-speed friction losses. A plodder that plods along pretty well whilst loaded to the gunwales with onions and Hammond Inness books. But every Clydesdale enjoys the odd gallop, and Cruising Chaps like to pile excess junk on the wharf and take the old girl out for a bash at the races, whether it be a short "twilight" race, or an ocean race for which such yachts are not specifically designed, but are nevertheless well suited and of course most welcome in making up a safe, experienced fleet. Such boats have no chance of Line Honours, but to add to the fun various Rules have been devised to enable them to compete at a level where by omitting sleep whilst caning their tub mercilessly, they can "win" on handicap. As such "Offshore Rule"s are honed by arguing long into the night, measuring and calculating, it naturally develops that the average sort of cruising yacht gets the best handicaps, and those whose Draughthorse can best impersonate Phar Lap win most often. The 3 to 1 ‘ers have their day!

Thus the handicap rule comes to favour the cruising style of sailboat, and whether or not it’s true, the one idea the Barely Informed cling to is that such boats have beam one third their length. Expensive Offshore Racers that are too small to win outright often attempt to cut their handicap losses by cleaving to the rule hull-design wise, whilst retaining all other properties that make their craft fast…superior rigs, light construction etc. So in fact the rule slows them down, but if they are cunning it slows them down less. And so there we have it, if we must generalise, 3 to 1 merely approximates the shape of many a happy plodder that may go faster than expected. It is unlikely to match a boat’s optimum ratio, which the designer only closes on after many tightening loops of a complex trial calculation spiral which involves simultaneous optimisation of length, beam, displacement, underwater form, windage, rig, sail materials, hull materials, expected seaway behaviour and on and on. A cunning designer may extract speed despite Racing Rules, but not because of them. As in motor racing, rules equalise by slowing.

Above all, bear in mind that no Rules have ever specified 3 to 1, its just a lot of boats roughly this shape have won on handicap, doing so because they’ve exceeded on their predicted speed. They are able to do this because THERE IS ROOM FOR IT, they are generally slow! The three to one notion has gradually developed. In post war times I doubt Messrs. Illingworth & Primrose ever paused at the drawing board to consider widening the beautiful Melusine II to sixteen feet four, thereby converting their 12 ton Greyhound to a 19 ton Tub. (I might add that upon completion this boat was cruised about Europe almost constantly, with no time allocated to concepts that have sadly become the de facto fathers of so called "cruising" design, viz Marina Living and Chartering. Perhaps to view some true blue water cruising yachts roll off the drawing board we now have to look to the construct of the Yachting World Magazine Design Contest 2000, which has an open brief for a fifty footer to undertake a three year cruise through wildly differing Oceans. The results should be marvellous, with even more interest being generated by Rob Humphreys opting to join the judging panel.)

Another example of ideas to rules concerns so-called Displacement and Planing hulls, that strange set-up where the noun displacement is used as an adjective to describe a type. We are happily told there are two types of hull, the displacement hull pushing its huge displacement wave out of the way and so limiting speed to a constant K times sqrt(WLL) where K varies between 1 and 1.75 or so depending on hull form (usually as defined by Prismatic Coefficient). Then we leap without pause to the planing boat, which climbs gaily out of its displacement hole by motoring powerfully up the slope of its "planing hump" and off into never-never land where it planes along on tippy toe, using fractions of guzzoline . "But it too has displacement," we protest. OK fine, lets pop it into the former formula and give it a coefficient of 3 or so. What’s that?…the displacement coefficient of a displacement hull whose waterline is gone anyway. Too bad, it’s a displacement boat dammit till it gets over the hump, then it’s a planing boat.

But there’s a problem with all this, a third (and hopefully not the last) type of hull. It has a narrow waterline to length ratio, too much weight to plane, but is so skinny it cuts rather than climbs its displacement wave, and proceeds at a rate which defies the coefficient times sqrt*(WLL) displacement boat speed model.

An example of this third type is the V-hulled cruising catamaran, which sails very fast despite its non-planing hull shape and weight. Another is the Cruising Power Cat, which is carries a full cruising load, yet attains much higher speeds at lower fuel consumption than a conventional "displacement" monohull of identical displacement. In Australia one of the more successful manufacturers has named this breed "displaning catamarans", and they are gaining an increasing share of the lucrative offshore cruising powerboat market. I prefer to call both power and sail versions "tracking cats" on account of their other great virtue, directional stability.

Once a sailing cat is loaded to the point where it won’t plane, it matters not much whether its hull sections are Veed or flattened rearward. If it will carry its load with a low hull beam it will defy the vices of displacement hulls, and WILL NOT fit our displacement theory. For this reason such designs are often criticised. As Bob Dylan said when making sense "Don’t criticise what you don’t understand". One of the best exponents of the type, the Wharram cruising cat, is probably the most criticised and misunderstood yacht on the planet. They are a moderately heavy but slim hulled minimalist catamaran - skinny hulls that neither plane nor shove a huge displacement wave. They are great for cruising as they are both directionally stable and seakindly. Outside of the thousands of happy owners, you wont see much written about them because people haven’t recognised their "type", preferring to complain they are not quite right because they don’t fit the inadequate displacement vs planing category division. The usual complaints are they are slow and not good to windward. This is quite ridiculous - if they don’t go to windward as well it’s because they are a beachable compromise -if you add a deep daggerboard like the cats the critics compare them with they will point with the best. Any lack of speed is purely to do with the modest and inexpensive home-buildable rigs. Again, shove a high aspect fully battened roachy main on one and see what happens. And if they wont support it, that’s purely in the moderate beam, which makes them possible to put in a berth without a gaggle of motors, gearboxes and thrusters.

Whatever, there are many types of boat, it just seems that our attempts to categorise them often leave us blinded to their virtues and to the available possibilities.

A Merry Xmas to all, and I hope everyone has a sail, and enjoys putting the finishing touches on (or starting!) their drawings for the magnificent Duckworks design contest. And if an entry is a wee bit late, I’m sure our kindly Editor will blame the post and not you. Honest!

May 2001 bring good things to you all.

Jeff, Dec 2000.


Home | Articles | Links | Projects | Columns | Designs | Boat Index