Construct Curves to Smoothly Connect Lines at Any Angle

One of the steps when building my son's Optimist dinghy (similar to this 'ible) was to attach "knees" in all corners. Here the task was to find/construct a curve which neatly "connects" side and front edges. Now what can be done to acomplish this:

grab all round plates, pots, coffee and corned beef cans you can find and look if one of their diameters is roughly what you need
construct a curve which exactly does what you want

I went for (2) by the following steps ...

Step 1: Mark Edges

First step is to mark the edges which should be connected by a curve.

In my case I chose to do this "in situ", but the method will be the same if you do this on a sheet of paper. What needs to be found are

the angle at which two lines intersect
start and endpoints of the connecting curve on these two lines

Working "in situ" has the advantage that the two lines can be found easily by holding a ruler against the edges and just extending the edge line. The curve enpoints were chosen to lie on the very edges of the plates.

Step 2: Divide

Next is to divide the lines between curve endpoints and intersection into equal pieces. My lines were measured to (roughly ;-) 100 and 130mm so I chose to divide them in 10 equal spaces of 10 and 13mm, respectively.

As you can see on the 2nd picture, the 10th divider of the vertical line isn't "exactly" meeting the intersection with the other edge line due to the fact that the measure was 103mm - a difference I chose to neglect - the error is small enough in practice.

Step 3: Draw Tangents

Last step is to draw the curve tangents in the following way

You start with a tangent from 1st division mark after endpoint on 1st edge line to the first mark from intersection on the 2nd edge line
You advance from mark to mark until you have connected last mark before intersection on 1st edge line to last mark before endpoint on 2nd edge line

E voila - you have just created a set of tangents along which you can cut / file / sand to obtain a nice curve connecting two edges at any desired angle.

Step 4: For the Curious - Background

For those of you who are curious: mathematically speaking this curve is not part of a circle but a parabolic curve.

Let's compare this parabolic line with a circle

case 1: edges are of equal length

circle center can be found by intersecting perpendicular lines through the edge endpoints
circle removes slightly less material
circle radius equals edge length only for 90° angle

case 2: edges are of different length

circle can be found by intersecting perpendicular lines at distances equal to the length of the shorter edge
circle meets the shorter edge but not the longer edge

note: pictures in this section show