From the Drawing Board
Occasional ramblings from
a Small Craft Designer

A design puzzle.

I mentioned in passing a couple of times recently that reduction of pitching was for me a major consideration in designing a particular boat, and commented in a bit about the International Americas Cup Class boats that they have similar issues. I had, very soon after a question from a keen student of boat design as to how this might be achieved.

But first, I should explain why the reduction of fore and aft pitching is high on my list of priorities in design.

A boat has a natural frequency of pitch and roll, the weights of hull, rig, and keel rock back and forth just like a pendulum. With a pendulum a series of tiny pushes at the right time will cause the pendulum to swing through a larger and larger arc, and so with a boat, a series of small pushes, timed just so, can be enough to eventually capsize a large commercial vessel let alone a small boat.

In pitching, the risk of capsize is not great, but rowing boats and sailing craft are propelled by vanishingly small amounts of horsepower, and the considerable loss of energy in pitching will stop a boat dead in its tracks. Most of us who sail small craft will recall trying to make progress in waves that were "just the wrong shape", the boat bucking once, twice, and stopping on the third one, wallowing for a moment and then sluggishly moving forward again to repeat the process.

What is happening there is that the wave's time interval, or spacing is just right to reinforce the boat's natural interval of pitch, or pendulum action, each hit by the wave rocking the boat further until all the energy driving the boat is expended in the rocking back and forth, the boat stopping or slowing until the interval changes, the boats motion stabilises and she starts forward again only to hit the critical frequency and so on.

It doesn't help that the motion does awful things to the airflow around the sails, or so upsets the rowers stroke that the driving power is much diminished which further reduces the boats progress. Its times like these that stamp collecting would seem a fascinating and attractive hobby.

Wave spacing and even prevailing wave shapes can be broadly predicted. I for instance, designing Huffboat, knew that I would be using her mostly on shallow lakes, where the lower density water and short fetch for the wind to build the waves means short, steep and sloppy waves. Awful places lakes in bad weather, darn near every lake has regular users who will wisely nod their head, sucking their breath through their teeth and tell you that "it can get awfully rough out there in no time at all". And they're generally right, not big, but rough. So Huffboat was drawn for that sort of environment.

Americas Cuppers ( I know I go on about them, but we've the Louis Vuitton Americas Cup Challenger series round three starting tomorrow about 2 hours drive from where I am, and they are amazing boats regardless of what you think of the money and hype that surrounds the whole thing). Americas Cuppers are very very location specific. Aucklands Hauraki Gulf has a predictable set of wave patterns for each wind direction, and with a boat that is afflicted with a 20 tonne weight on the end of a 4 meter long lever underneath, as close to a pendulum weight as you will get plus a rig that goes on up forever plus a little bit, the tendency to hobby horse has to be designed out or that team may as well not even bother coming.

The third example is the Mini Transat class boats, 20ft 6in long, 10 ft wide, half a ton of lead 6ft down and a 40 ft mast up there with heavy terylene sails, these boats could be the worst rocking horses imaginable so in designing for that event I went to the world hydrographic survey, and to the world meteorology office and got a whole bunch of information on wave patterns sizes and shapes that prevailed on the course from France across the Bay of Biscay to the Canaries and across the Atlantic to Guadeloupe. I have to say that I did not understand some of the information but from what I did fathom out managed to produce a boat which (comparatively speaking) rode like a well sprung Limo in the longer spaced ocean waves with their gentler sloped sides and flatter peaks . She was a total disaster in a harbour chop though.

So having established that there is a need to match the boat to its predominant environment in this respect, how to go about it?

Not easy to explain. I have drawings of boats that are/were known disasters, around 1965-1970 the fashionable shape, driven by the IOR handicap rule produced some shockers. Pot bellied and very fine ended at both ends some of these things were amongst the worst handling boats ever inflicted on the yacht racing world, and excessive pitching was one of the features of their shape.

They provide some clues.

Other clues came from towed barges, now there has been more money spent in researching the efficiency of towed barges than ever went toward our recreational craft and they have shapes far more effective than their rusty topsides would suggest.

Ferries, especially the small short haul ones are hugely affected by the conditions that prevail on their runs and can be very closely designed to suit them. There are some interesting lessons here. Easily accessed too, you can spend half a day riding back and forth hanging over the stern and watching the wake ( " research Dear, research").

Todays fishing boats suffer from the fact that fuel is comparatively cheap, and are trending toward a depressing uniformity in shape and design but if you go back 75 years and have a really close look at the regional types and then analyse the areas in which they worked you can find a lot of clues as to how a craft may be designed to suit a particular set of conditions. I find books such as Eric McKee's "Working Craft of Great Britain" a huge help, it has a map showing the homes of those designs which have evolved to suit that place and the conditions that prevail there and studying the differences between the environment and the boats within each area is very helpful.

It is not hard to estimate that natural pendulum action and from that the time interval of the boats natural motion in pitch and in roll, and so from that it is possible to design shapes that modify or damp that motion. In doing so it is then necessary to add to the equation the information as to wave spacing, or impulse interval. If you have the two coinciding you have a reinforcing couple that could see the pendulum accelerated, if well apart, the motion is dampened.

In reducing pitching in small sailing craft, the shapes that are working for me in light displacement boats have very long fine entries, with strongly flared topsides above those fine entries. The boats have a firm bilge a little further aft than midships and a very full shape aft which is compensated for when heeled by a slightly shorter and steeper run than has been the trend of late. They still plane freely and the angle of the run helps keep the fine bow up when heading off downwind, they steer well when heeled in spite of the powerful stern quarters and fine bows, and oddly enough have little or no apparent forced mode when making the transition from displacement speed to planing speeds.

Be aware that this shape has some traps, not enough flare above the bow and she will be both wet and hard on the helm, not enough rocker aft and she won't steer when heeled, too fine forward and she wont lift at all, take the maximum waterline beam too far aft and she wont steer at all on any point of sail. Get it right though and you have a very soft motion in a head sea, much softer than you would expect for a lightweight boat, the pitching is damped by the broad and comparatively flat underbody aft, while her bow will penetrate a wave quite a way before the flare above lifts the boat bodily rather than just lifting her head up, and the firm bilge continues that lifting action as the boat rides forward through the wave, the rocker in the aft section stops the stern from being lifted too sharply as the wave exits the boats body and that firm midship section still supports the boat slowing the downward pitch of the bow.

Now to get all that, and still have a shape that conforms with the requirements for prismatic coefficient, waterflow angles, centres of bouyancy, gravity lateral plane and so on, one that does not change trim too rapidly or worse still in the wrong direction as she heels, fits the aesthetic and ergonomic requirements and the multitude of other considerations that go with drawing a hull shape that will fit a particular combination of usage and environment, is a balancing act, one that only experience can bring. I've not found even a hint of how to do this in text books, so am working from first principles and experiments. I suspect that like almost all textbooks they are all out of date before they are written and that this is a fairly new art.

Heavier boats are harder, they are a heavier "pendulum", and have a motion that is not as easily damped. But it is possible to look at the amount of water that has to be displaced as the boat goes bow down, then rocks back to go stern down. The more water she has to push and pull in and out as this happens and the further the boats centre of bouyancy moves as she changes trim fore and aft, the more damping of motion there is going on, the Americas Cuppers rely in part upon a very flat overhanging stern to do this .

In a bigger boat weight distribution will make a difference as well, those IOR racers, when out of date as competitors usually went cruising and when the previously unoccupied ends of the boats were filled with cans of antifouling, fishing gear and tanks of outboard motor fuel some of them became uncontrollable.

Again, the period of the pendulum can be altered, and with experience a hull can be drawn that will have a pendulum period different to the impulse interval. You need figure out the one that you can't alter ( impulse interval = wave spacing x pointing angles x speed ) before you know if the other one is right or wrong, its a fascinating exercise in research and theory. Find the bad examples and the things that make them worse and listen to what they tell you.

Now, next time you are engaged in debate about boat design, you will have several new terms and phrases with which to impress and confuse the conversation. And next time you are designing a boat you'll have a whole new can of worms with which to confuse yourself. Have fun.

John Welsford