Shop Tips & Tricks #20: Curves, Fair Curves and the Lack of Them

 Blog entry by GnarlyErik posted 03-26-2017 09:04 PM 2411 reads 1 time favorited 3 comments
 « Part 19: The Art of Middles of Symmetries Part 20 of Shop Tips & Tricks series Part 21: Jacking sideways, or "Walking the Jack." »

”My curves are not crazy.”
~ Henri Matisse

“There are no straight lines in nature.” was a truism in the School of Architecture when I attended college. This is not strictly true for the human eye at least, when you consider crystalline structures and such things. Perhaps there it still even holds true in the sub-atomic world. But, it is true in almost all of the visible natural world. There is something about curves we all seem to like. Men in particular certainly seem to favor a curvaceous woman over a ‘linear’ one!

While a straight-line, angular composition is often attractive, an introduction of curves can make a spectacular difference in its perception. Think of the arch over a door or window, or a curved wall. The high school where I live has circular buildings mixed in with rectangular ones which make the complex as a whole far more interesting to me.

The dictionary provides two definitions for the noun ‘curve’ of the kind I speak of. One describes a line in one plane, and two dimensions:

1. “A line that deviates from straightness in a smooth, continuous fashion.”

Another definition describes a ‘surface in planarity’, which in effect means a curve in three dimensions, such as a boat hull, airplane or automobile:

2. “A surface that deviates from planarity in a smooth, continuous fashion”.

Curves can be easy to form, as in a circle, or part of a circle which can be described with a compass, or they can be more difficult as in a curved line which continuously changes the radii of its curvature. Note that BOTH definitions above demand that a curve should ‘deviate in a continuous fashion.’ This means curves should not have ‘bumps’, ‘knots’, ‘jerks’ or ‘elbows’ in them.

A continuously ‘smooth’ curve is called a ‘fair’ curve, and checking or making a curve continuously smooth is called ‘fairing’ the curve, and is the heart of this little article. Any boatbuilder is familiar with ‘fairing curves’, since there are very few if any straight lines in most boats’ hulls.

That is unless you consider the angular, upside-down/inside-out monstrosities the U.S. Navy is now having built, the hulls of which were apparently designed by an amalgamated group of the people who designed the Rubik’s Cube, Legos and some of our present day linear ‘modern art’. Yes, I know the reason behind all the angularity, but really? And what happens when the inevitable ‘what-ifs’ occur? How is any crew going to see anything, or even stand on deck for that matter? On her maiden voyage to the West Coast, this vessel broke down twice and had to be towed into port in Panama once. I’m still wondering how and where in the dickens the tow-line was made fast and how they steered this pig.

But back to my main point: A boatbuilder derives fair curves by eye, striving to avoid ‘jerks’ and ‘elbows’. Some do it by eye alone during the process of building the boat, but the majority by far do so with the aid of ‘fairing battens’. Most traditional boatbuilders make their own battens, using straight grained and predictable woods such as white pine. Besides being straight grained, fairing battens must avoid all knots, which are apt to ‘jerk’ the batten in use, or cause it to break. And, battens must lay flat in both directions. Many battens are square in cross-section, but sometimes a more rectangular shape is needed, particularly where the curves become more pronounced. Many fairing battens are tapered at the ends so they can handle tighter curves better, such as in the ends of boat hulls. Most boatbuilders have a variety of different sized fairing battens hanging on the walls of their shops to handle any type curve. Sometimes they are thirty or more feet long. I am out of that industry now, but boatbuilders today can and do buy fiberglass and plastic battens in various sizes too, which can be very expensive. These are called ‘pultruded’ battens. What an awful name! Sailmakers use them too, and theirs can be quite long, sometimes over 100 feet. Mega-yacht builders often use specially-built metal battens in their operations as well.

Fairing battens are used both in the original layout of ‘lofting’ as it’s called, and in checking the fairness of the work as it progresses. And always, the ultimate aim is to make sure all the curves are ‘fair’ and without bumps in all directions!

My fairing needs are modest now, but I still make use of a variety of battens in laying out various things. I can always make a longer one if needed. And even though straight-grained woods are hard to come by in long lengths nowadays, you can make longer battens if you’re willing to spend the time to make 16 to 1 scarf joins to mate two or more pieces to obtain what you need. Always make the scarf join before giving the batten its final shape. This is so the final planing to size goes through the join in a continuous line on all four sides.

Here are a few of my small fairing battens, and the little clips devised to hold them in place in lieu of nails. Note the various sizes of the battens, and the tapered ends on some. It is surprising how often I use these battens. The clips are made from scraps, work well and allow adjusting the curvature easily.

I sometimes use draftsmen’s lead ‘ducks’ or ‘whales’ for very small or tight curves with smaller battens. The whales weigh around 4-5 pounds apiece, and have felt glued to their bottoms for use on my drafting table, although they’re used in the shop too. The pictures show examples of how they are used.

Here is an another discussion about battens and fairing you may also find interesting:

http://www.everything2.com/title/batten