John Welsford's website
John Welsford's designs
From the Drawing
(occasional ramblings of a Small Craft
So how to do it?
In this trade, as in every other walk of life, some days
are better than others. And there are always the types who, after you have
expended much energy and sweat to make a day a good one, will come up
looking envious and say something like "oh, aren't you lucky"! Killing
them would be much too kind!
But there are ways of reducing the stress, and one of the biggest
stressors to a boat designer is the question of how close the design will
perform to specification. I mean, when I first started I was pleased if
the thing floated right way up, and even more pleased if she sailed bow
first but as other peoples time and money became a consideration I felt
that I needed to do better than that.
So how to do it?
Prediction of performance is not easy in terms of whether or not she will
win races, after all there are lots of other designers trying to get their
boat to the finish line ahead of you, but "performance" is not necessarily
about speed. Performance is better defined as "suitability for the boats
intended use". A lightship, designed to sit at anchor all of its life, to
survive everything that the elements send its way, and to keep its crew in
some sort of livable comfort will not have speed as a part of its
specification. Whereas the fleet of Americas Cuppers currently sailing in
Auckland near here ( the challenger series starts in 2 about months) are
designed to fit the rule and beat the others to the exclusion of all else
so you can see that the definition of "performance" would be very
different for the two different boats.
I design quite a wide range of craft. Power, sail and rowing, displacement
and planing hulls, rough weather and flat water. It is not possible to
build up a serious body of experience on everything, so like most
designers I have favourite areas of interest, in my case I am best known
for performance cruising sailing dinghies and dayboats, and fixed seat
rowing boats. But of late I have been involved in a four man ocean going
rowing racer intended for a 1500 mile race, a 13.6 meter ( 44 ft) very
long range displacement launch ( for my own use) and a 30 ft flat out
harbour racing yacht among other things. A wide variety of boats needing
very different approaches to size, shape and weight.
Theories developed for one design type will not work with others,
practical structures are a different thing and not so much of a worry but
to try and get boat types as diverse as the above to perform appropriately
is not that easy.
My approach used to be to look at boats that perform well in the type of
use I wanted, and slightly accentuate the features that I thought
contributed most to the desired characteristics, then hope for a good day
on launching day. Luck? I needed it and must admit to having been pretty
lucky most of the time.
Today? Lets use an example.
My own project
is still in the design stages, I have the shape worked out and most of the
layout, but before I even put a pen to paper I knew a lot about the boat.
First, I wrote a performance brief, I established that she was to be a
long range motor cruising launch. Range under power, minimum 3000 nautical
miles at about 150 miles a day, ( 6 1/4 knots) , fuel consumption to be as
low as possible, accommodation to be of a standard that would allow Denny,
Brendan and I to live and work on board for several years, a layout that
although suited for cold waters and cooler climates would be liveable in
the tropics as well. We also needed to have enough private corners aboard
so we each of us could have a little space when needed. The boat should
have a slow roll and pitch period, exceptional directional stability, the
ability to carry a heavy load without greatly increasing the fuel
consumption, and particularly good access to all services and the engine
John's own: Emerald City
Given all of the above, I went to my library. Robert P
Beebe's excellent book "Cruising under Power" has several sets of tables
and graphs which enabled me to establish that a slim hull with a 40 ft
waterline, a Prismatic Coefficient (P/Cf a measure of the fullness of the
ends of the boat) of 0.53, a Displacement to length ratio of about 280,
and a Center of Gravity (C/G) around 48% aft of the stem would give me the
performance I needed. Cross checking with tables in Dave Gerr's "The
Nature of Boats" confirmed this as did the figures in Jay Benford's "
Small Ships", each book providing a few insights as well as the basic
From this information I drew a sketch on graph paper, the spacings of the
graphs enabling me to draw to scale freehand. I had the w/l length and an
idea of the beam, draft and distribution of weights so I could work out a
tentative accommodation layout and styling from this.
With the length and beam , the center of gravity and the displacement (
that comes from the waterline length and the desired D/L ratio) plus the
P/Cf and the waterplane loading ( a figure that gives an idea of the rate
that a wave going under the boat will accelerate her vertically, too light
and she will shake your teeth out, too heavy and she will be awash half
the time) I could calculate the underwater cross sectional area at the
maximum beam point, and then a curve of areas to which all of the other
cross sections should conform to. This gave me the wetted skin area,
frictional and wave making resistance curves, from those came
horsepower/speed curves and from that a fuel consumption curve. The
prediction was for a consumption of 2.2liters an hour at 6.8knots. Right
on the button. Whew!
And at this stage I had not drawn anything much other than a couple of
freehand sketches plus some odd looking doodles on tracing paper. ( And no
computer other than my integrating planimeter and some of the calculation
programs in my hand held.) I could go to a customer and say that yes the
design brief was workable and could be achieved.
So rather than draw a shape and analyze it to see if it will perform as
advertised, which is what I used to do for a while, you can see that I am
researching a set of statistical factors consistent with the performance
envelope that the design brief requires, then draw a shape consistent with
those . There is much much more than the basics I've mentioned above, roll
inertia, same for pitch, directional stability, trim change with roll and
on and on and on. But the upshot of all this work is that I can, with
reasonable certainty, predict the performance of a new design of a type
which I may not have drawn before.
It makes launching days a lot more relaxed.
John Welsford Small Craft Design.