Ancient Tools - The Stringline

[James Pallas]

I
There's also a string technique, but it's complicated by requiring one to find the two foci of the ellipse. That can be an elusive requirement if specific dimensions are wanted. Once the foci are determined, pins are placed at each of the foci and a string is looped around both with enough slack to reach to the intended width on one side or the intended length at one end. Then a pencil is inserted inside the loop and tensioned against the pins and translated around creating the ellipse.

i guess to me it's about simple shop work. There are many ways to make curved shapes that look okay but are not mathematically correct. I used the framing square method also that comes out closer to an ellipse. The string and pin method also. The easiest and most accurate is to CNC it and be done. I've used battens for curves also it works just fine. I guess I thought we were talking a simple string here.
Jim

[James Waldron]

Do you know Lou well?

Boat design and building has always fascinated me. My wife grew up with a 24ft glass "yacht", and loves ships, and I grew up with ski boats and diving, so for a long time we wanted to build our own sailboat and travel the world. When I looked more closely at the cost and long-term burden of owning a wooden clothesline, not to mention the instability of such a lifestyle, we changed our minds, but the romance of sailing ships has not left my heart.

I have deep respect for men that can build those beautiful and functional sculptures. Lou's may not go that far, but they look like a lot of fun. He gets his hands on some beautiful wood.

I don't know Lou personally. I've enjoyed his videos and learned a couple of good tricks from them. I don't work much in antiques or build wooden hulls, so our efforts are not entirely congruent. Actually, I don't build much of anything since I've retired, except a glass or plywood dinghy now and then.

[James Waldron]

i guess to me it's about simple shop work. There are many ways to make curved shapes that look okay but are not mathematically correct. I used the framing square method also that comes out closer to an ellipse. The string and pin method also. The easiest and most accurate is to CNC it and be done. I've used battens for curves also it works just fine. I guess I thought we were talking a simple string here.
Jim

Don't mean to be throwing rocks, Jim. Just a clarification. Sorry if it came across as a slam, sure didn't mean it that way.

The ellipse and the catenary (and other regular curves) have different uses and both are found in every art and craft, including ours. String lines can do a lot, but they do have their limitations (particularly on concave curves).

Next time you want to fair a short curve, up to say 18", try using a flexible steel rule as a batten and compare to your string technique. You may like the rule/batten, you may not.

Of course, with either technique, the radius of the curvature may make both a batten or string ineffective on a concave curve and the string may be useful on tight convex curves where the steel rule can't go.

Ultimately, a fair curve is often determined by the human eye if it is not a regular curve (ellipse, catenary, parabola, etc.). In boat building, regular curves are not often used and the batten is pinned in place to roughly define the curve, and then adjusted to suit the creator's eye. That is the technique for naval architects as well as loftsmen and shipwrights and boatwrights. That is the technique used even when the batten is an algorithm defined in software for digital computer design. The eye is far more refined and discriminating that any string or batten or algorithm.

In your shop, you choose. I would suggest, though, that you consider working toward developing your eye as the judge and select the tool that gets you closest to what pleases your eye with the least difficulty. In many cases of curves, the batten comes closer than the string line but it is still only a starting point. Even in "simple shop work" to use your expression, I've found it rewarding to expand my capabilities and to strive toward mastery of more and better techniques.

[Pat Barry]

In boat building, regular curves are not often used and the batten is pinned in place to roughly define the curve, and then adjusted to suit the creator's eye. That is the technique for naval architects as well as loftsmen and shipwrights and boatwrights... The eye is far more refined and discriminating that any string or batten or algorithm. .

Interesting thought, however, for a boat I suspect, what you didn't say, is that there is a preferred geometry based on some measure of performance and that's what the shipwright is aiming for - a curve of certain form and that form may or may not be a simple mathematical entity and through his experience - fairing the curve means it fits the mental image of what he feels performs the best..

[Mel Fulks]

Well, some of the curves that are highly successful in manufactured stock millwork and suffice for some doing custom work are ugly mistakes for others. The worst are the "two radius ellipses" drawn with the trammel points that should have been used to draw a REAL ellipse.....in those cases a piece of string could have done a better job.

[James Waldron]

Interesting thought, however, for a boat I suspect, what you didn't say, is that there is a preferred geometry based on some measure of performance and that's what the shipwright is aiming for - a curve of certain form and that form may or may not be a simple mathematical entity and through his experience - fairing the curve means it fits the mental image of what he feels performs the best..

Pat:

That takes us far off Stan's original course. That's probably okay, since "He's a sailor and used to hard ships." (from a very old shaggy dog tale)

The reality is that the shipwright strives to build the shape defined by the design process. He will work to fair the curves, but the overall shape of the curve is defined by the naval architect (or home diy designer in some cases).

That overall shape is determined by a large number of parameters, including hydrostatics, hydrodynamics, required load carrying capacity, stability, performance requirements, budget, and on and on. And of course, for pleasure craft, if the thing looks like a piece of crap it's going to be really hard to get SWMBO to allow the purchase.

At every stage in the process, every curve is evaluated and adjusted to be fair. That starts on the drawing board (or in the computer, more and more often), then on the lofting floor Lofting is the process of scaling the design to full scale, a process many of us use for our furniture builds, etc. For a boat it's a bigger and rather more complex requirement, and lot of techniques not common to furniture building. Then on to the shipwright for construction.

At the scale of a drawing board, fairing is limited by scaling factors. It's just not possible to adjust a drafting batten by 0.001" and yet that 0.001" at a scale that may be 1/4" / foot, converts to 48 times as much, i.e., 0.048". Occasionally, the drafting batten may be off by as much as 1/8" and when expanded on the lofting floor, that expands to 6". Most unfairness falls between these two extremes. The role of the loftsman is to correct these to provide a fair curve for the shipwright. (An ancillary role is to detect and correct any substantive design errors which may occasionally creep in. Lofting is a very important skilled trade.)

The job of the shipwright is to then take his measurements (with a story pole, not a tape measure) from the lofted drawing and build the framework for the construction of the defined hull shape. There are quite a few ways that is done that I won't bore everyone with. The important thing to note is that working a full scale to the lofted design and a story pole, errors are apt to creep into the process. (Cutting changing bevels on the edge of a plank with a band saw is a bit of an art, but an imprecise one, to cite just one illustrative example.) So the battens are used to correct the lines in three dimensions and to eliminate errors in the construction techniques employed. Fine judgement is required when a curve is unfair: do we cut down the high spots or do we shim up the low spots? (And when we do the same curve on the other side of the hull, will we remember the original choice or do it the other way this time? Hence the result that boats are often not quite symmetrical and perform better on one tack than on the other.

Designing, lofting and building boats are quite rewarding trades. Doing them well requires the accumulation of a considerable knowledge base and a diverse skill set. In the last analysis, like any other trade or craft, there are very good practitioners, some not so good and many somewhere along the path to very good. Much like the denizens of this forum.

[allen long]

Thank you Stewie for the video link. I was having trouble imagining the knot trick just from the text.

Great thread everyone. Greatly appreciate all the useful information.

Many Kind Regards . . . Allen

[allen long]

These are just things I have done in the past. A quick way to make a half pattern for an ellipse. Mark an x y axis on you pattern decide how wide you want your half pattern. Mark half of that on your pattern. Decide how long you want your pattern, mark that on the other axis. Put the piece verticle in your vise. Hold your string at the two longer marks. Let it hang down to touch the width mark. Now carefully mark along the string and you have it.
Jim

James, can you post pictures for those of us who have difficulty envisioning what you are describing? (and by "us" I mean "me")

Many Kind Regards . . . . Allen

[Damien Braun]

There's also a string technique, but it's complicated by requiring one to find the two foci of the ellipse.

Finding foci is not that hard knowing that (major axis)² = (minor axis)² + (foci)²; a² = b² + c² on the drawing
The string part is more unusual

ellipse.png

[steven c newman]

I used to drive to pins. Then made a loop in the string, pencil into a loop. Stretch the string, with the pencil, out to the edge of the board. Then trace the pattern by hold the pencil and allowing the loop to work around the two pins. Have to keep the pencil vertical the entire way around.

[Alan Schwabacher]

Finding foci is not that hard knowing that (major axis)² = (minor axis)² + (foci)²; a² = b² + c² on the drawing

Putting this another way, if you know the length (long axis) and the width (short axis) of an ellipse you want to draw, you can easily determine the distance between the foci (nails) to draw it with a string and nails.

1 Draw two perpendicular lines, and mark a first point on one line the short axis of the ellipse from where they cross.
2 Set a pair of dividers to the desired long axis of the ellipse.
3 Use the dividers set on the first point to mark a second point on the other line.
4 The distance between the where the two lines cross and the second point you marked is the distance between the foci (the two nails you wrap the string around).

This construction does exactly the same thing as the algebra above.

[David Bassett]

Putting this another way, ....

Very well described.

On the Woodwright Shop, broadcast this weekend locally, Roy Underhill had a couple guys doing an inlay project where they laid out ellipses. One they did with a layout tool, (made by George Wilson when he was at Williamsburg,) and the other they did as you describe. I don't layout enough ellipses, nor have the skill, to make a custom tool, but the string method really seemed easy to use if you can remember the details, I didn't. Thanks!

[James Pallas]

James, can e nerve damageyou post pictures for those of us who have difficulty envisioning what you are describing? (and by "us" I mean "me")

Many Kind Regards . . . . Allen

Here it is really quickly Allen.
i just used a string to measure and a square to put a width line in. The pictures should explain. Don't pay much attention to the pencil lines as I have nerve damage that keeps me from free handing a pencil. I put the template against an elliptical frame for reference. Start from the last picture first. This is not as good as can be just a how to.
Jim

[Jay Aubuchon]

They used to make small wood blocks for masons to tie the line and act as a standoff for the line.

Like these line blocks?

[Rush Paul]

http://www.finehomebuilding.com/2014...ies-with-a-tug

A second thank you for the video link - it was very helpful to view.

Stan, this has been an excellent thread - thanks for sharing this information!