| 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
Designer.
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