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

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Blog entry by GnarlyErik posted 03-26-2017 09:04 PM 1808 reads 1 time favorited 3 comments Add to Favorites Watch
« 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:

-- "Never let your dogma be run over by your karma!"

3 comments so far

View shipwright's profile


8031 posts in 2879 days

#1 posted 03-27-2017 12:13 AM

Great topic, well written Erik.

The batten is the boss” is a mantra I had drilled into me early. My boatbuilding mentor would always check my lines and OK them when I was laying out sheerlines, rail top lines, planking lines, etc and would always “tweak” them just a little but I did occasionally I catch him moving a nail here and then there to balance and eventually moving them back to where I had had them. He would then say ” There that’s good now. On the three dimensional hull we always used nails above and below the battens.
... and we always checked the line by bending over and looking at it upside down.

So Erik, do bad lines in buildings, curbs, industrial design, etc. drive you crazy too?

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese!

View GnarlyErik's profile


312 posts in 2216 days

#2 posted 03-27-2017 01:57 AM

Amen Paul, on the mentoring and upside down looking. Regarding ‘standard’ architecture and civil/industrial design, etc., yes I too have problems with most of it. I studied architecture in college and worked with a large firm afterwards for a period. Most of the work was for government accounts (mostly state and local) and it was uniformly stifling. There was zero art, little imagination and very little grace to any of it. I suppose that is only to be expected with the committees which were the clients we answered to. It was a shock when I finally realized I could not wait to get away from it. To me the very most pleasing designs have an ‘organic’ feel, and I think that is why I have always loved boats. There is a term in architecture, ‘form follows function’ and most boats are proof of that, at least boats in the traditional sense.

Since you mentioned curbs, I recall one of my professors in college recommending never committing on sidewalk location on public buildings until after the buildings were occupied for a while (when possible). Then, note the foot traffic patterns the occupants were making in the landscaping. Hey, voila’! So THAT’S where the sidewalks should go, and how so organic is that! So sensible, but almost never possible due to the way bidding processes work.

-- "Never let your dogma be run over by your karma!"

View Ocelot's profile


2019 posts in 2719 days

#3 posted 02-08-2018 04:02 PM

This is a good reminder – and goes well with your bent lamination post from 3 years back.

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