Curves of the fairer kind

Fair curves

When I was taking the Westlawn course, well worth the money by the way, all the work was done by hand as we have been discussing in the posts until now. Much cursing and erasing was needed to get the curves to fall fair. The cause of all that is the limitation of the medium. A 1/16^th pencil line on your 1/2”=1' drawing is an inch and a half in the real world hence the need for full size lofting when building.

I just want to take a moment here to dwell on the need to actually build some of the boats you're designing. A sound knowledge of how boats go together makes you a better designer. I spend a lot of time building the boat I'm designing in my head. Imagining all the joints, frames, beams and ceiling which would all surround the interior design and really determine how that design would come together.

Back to the drawing, a minute error in transferring a half breadth or height above the waterline causes a cascade of errors in the curves and you'll get a flat spot or a hard spot which has to be smoothed out by subtly changing the lines drawing. All that can be avoided to a great extent by not doing drawings by hand.

When I was about two thirds of the way through the Westlawn course I bought a new computer, it came with a bunch of bundled software one of which was a 1985 or so version of AutoCadLT. I began exploring lines drawings using this program and rapidly learned that the accuracy of the drawings increased exponentially. And with accuracy came better lines.

I used that ancient version of AutoCad for the next ten or more years. Then along came Windows 7 and that program would no longer work. So I began to hunt around for another CAD program within my budget, AutoCad being stratospheric with respect to the budget at hand. I continued to use AutoCad on an old laptop until I discovered QCad. I have been very happy with this program, primarily because it is easy to master the basics and secondarily because it allows for the importation of AutoCad drawings and lastly because it was free.

Why am I enamored by CAD? Because you can do the basic things, like drawing the grid, very quickly and accurately. You will recall in the third post we constructed the grid and I spent some time talking about having to change the spacing because 13' was not easily divisible into ten spaces. With a CAD program you don't need to worry about that. You draw your waterline and then a perpendicular line at one end, tell the program that you want a circle of 1.3 feet radius, apply that circle at the intersection of the two lines and copy the vertical line to the intersection of the circle and the waterline and tell the program that you want 10 copies. Bingo. It takes longer to tell you how to do it than to actually do it. And erasing the circle is just a key click, no muss, no fuss.

Curves are dead easy, you establish the point at the end of the bow, the low point of your sheer and the end point of the transom, click on the curve and connect the three points, voila! a fair curve.

Transferring points from plan and profile to the sections drawing is done the same way as as I described in the last post only now there is little or no room for error. A line drawn from a single point, such as the intersection of station five and the sheer in plan can be drawn accurately perpendicular to the centreline of your transfer drawing, creating an accurate intersection with the 45 degree line and an accurate line from that intersection to the baseline of the sections drawing. Again erasing all these lines takes just a key click and if you erase the wrong line you don't have to draw it again you just undo the mistake.

In the last post I ended by saying that we would discus why the curves fell fair on the first go. It's because I used my CAD program to draw the lines and two fair curves in plan and profile will naturally produce fair curves in the sections.

From here on in I will be using the CAD program to develop our other drawings in this design, however everything I tell you can be done by hand just more slowly.

Next week – what are those other drawings?