Generating Planshape Curves

I was checking out “swied”'s thread on his HWS Project and was interested by some planshape generation discussion there.

I thought I would share the method I have just used on a HWS project of my own which has worked well for me.

Most uninterrupted planshape curves (exluding the first and last 12 inches of the board) can very nicely approximated by a large radius (normally somewhere between 4 and 6 metres for a shortboard).

So, for example, if I want to generate a curve for a 6’8" (2032mm) board with a 13" (330.2mm) nose, a 20" (508mm) widepoint 1" (25.4mm) behind centre, and a 15" (381mm) tail. I do the following.

Using a point on the stringer 12" back from the nose as the zero point with the x direction down the stringer and the y direction at 90 degrees to this we end up generating the following coordinates.

1st point coordinates (x,y)=(0,330.2/2)=(0,165.1)

2nd point coordinates (x,y)=(2032/2+25.4,508/2)=(1041.4,254)

3rd point coordinates(x,y)=(2032-304.8,381/2)=(1727.2,190.5)

Plug these coordinates into:

http://home.att.net/~srschmitt/script_circle_solver.html#top

This particular example returns an answer of a radius of 4891mm (with a centre point of X934 and Y-4636).

Now mark your template with a centreline and measure and mark your three points above onto the template.

Put a free standing vise on the floor or ground, pull your tape measure out to approximately your radius (4891mm in this case) and put it in the vise at this dimension. Tape a pencil to the other end of the tape measure, position your template at the correct location which the pencil will scribe through your 3 marked points (this can take a little trial and error) and then scribe your curve. The Circle Solver program coordinates of the centre point of your circle can help cut down the trial and error of positioning your template to get the pencil passing through all 3 points.

Now you only have to complete the curve for the last and first 12 inches of the planshape, which can almost be done with steady hand and good eye, or french curves, etc.

Sorry if I have made this sound more complicated than it is as it’s not nearly so complicated as I’ve made it sound. Hopefully it will make some sense to someone.

Cheers

Rohan

In Epoxy 101 Shaping I showed how outlines were traditionally created. This has been the method used since probably ancient Hawaiians on boards and it’s also the method used in lofting curves for most other products for centuries. I’m not sure this new method is better but it’s nice to see someone is thinking this way.

Hi Greg,

I haven’t seen Epoxy 101 Shaping, but I assume you’re talking about lofting the curves with flexible battens. This traditional method is, as you have said, an excellent method for easily producing good curves. I came up with the method I described here because I had trouble finding inexpensive uniformally flexible batten material. And with the HWS I was building I also didn’t want to waste material and go to the trouble of making a template with fixing posts for the battens, etc when I could scribe the planshape outline directly onto the top and bottom skins.

Cheers

Rohan

Quote:
... Tape a pencil to the other end of the tape measure, position your template at the correct location which the pencil will scribe through your 3 marked points (this can take a little trial and error) and then scribe your curve. The Circle Solver program coordinates of the centre point of your circle can help cut down the trial and error of positioning your template to get the pencil passing through all 3 points.

An easy way to find the center point of your circle so that your arc will pass through each of your three points is to position the end of your tape/string at each point and then draw a short arc near where you think the center of the circle would be. The three arcs will intercept and that point will be the center of the circle. you actually only need to arcs to find the center, though the third arc will indicate how acuractly you found the center. If all three arcs intercept at a single point then you have the center. If they form a little triangle then the center point is actually contained within the triangle some where. The larger the triangle the less accurate your arcs were.

Thanks 4est.

A Draftsman in a former or current life I presume.

Or a first rate geometer. Reminds me of high school geometry, where you were given a compass and an initial line segment on the chalkboard, and you had to find a perpendicular and 45 degree line sengments that intersect it in the center…

Using small arc segments (pieces of a circle) you can model damn near any curve, it just depends on how many arc segments you want to use…

You can also draw ellipses using pencil and string, you just need to know the positions of the two foci if you don’t have a batten…

JSS

FYI

Home depot

1/8 x 3/8 fir strip 8ft long

less than 5 bucks

choose a good one with straight grain

and do a bit of shaving on the ends

or use and old fishin pole with the eyelets removed then you can do the nose and tail

Here’s how I do mine…

Measurements in CM

Hey Hicksy,

I do almost the same thing, just in Excel…I’m not good enough with my hands to use only 11 points so I use 27, loaded more towards the nose and tail, as that’s generally where changes happen more quickly…

JSS