John Welsford's website

John Welsford's designs

From the Drawing Board
(occasional ramblings of a Small Craft Designer)

by JohnWelsford

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 compartment.

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 information.

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.