The first stage in converting a square plank into a round spar is to plane down the square edges to make the cross-section into that of a regular octagon. A spar gauge is a simple device which will mark two lines on the face of each side of a square plank (either straight or tapering), enabling the four corners of the plank to be planed down to these lines to produce the required octagonal shape. This page outlines the geometrical theory underlying the spar gauge and then gives guidance on construction.
Spar Gauge Geometry
The top sketch represents a cross-section of a square plank of width (w), before shaping into a spar. The first stage is to plane off the corners of the square to form a regular octagon (shown in black outline), with sides of identical width (x). The sides (x), in each corner of the square, form the hypoteneuse of four 45-90-45 triangles with the other sides (y) of the triangles being of equal length. Most importantly as regards a spar gauge, the ratio x : y will always be 1.41 : 1.
The lower sketch shows a spar gauge
consisting of a rectangular piece of wood with two
outer guides (represented by red circles) at each
end (placed either side of the spar), and two
pencils (also represented by red circles) fixed in
position either side of the centre point of the
gauge. The gauge is placed across the side of the
square spar and twisted in the direction of the
upper right and lower left red arrows so that the
guides are in contact with the sides of the spar.
Keeping the guides in contact with the sides, the
gauge is moved in the direction of the red (down)
arrow to scribe the two lines delineating the
corners of the regular octagon. The spacing between
the guides and the pencils simply makes use of
The gauge will correctly mark the position of the octagonal corners on the face of any square spar which will fit between the two outer guides, as long as the guides are both in contact with the spar sides as the gauge is moved down the spar.
The most useful derivative of the 1.41 : 1 ratio is 10 : 7.07 (shown in the sketch above), as this translates very neatly into 1¾" to 2½" with an overall distance of 6" between the inner faces of the guides - an ideally sized spar gauge for marking out Drascombe spars.
Having looked at the geometry of how a spar gauge works, this section gives a plan from which a spar gauge can be constructed.
A piece of wood (hardwood or softwood) with flat planed faces and measuring about 8" long, 2" wide and ¾" thick can be used to make the gauge. The holes for the pencils and outer guides are drilled along the centre line of the wood. Note that on the plan, the distances between the pencil and guide holes is from the centre of the pencil holes to the inside faces (NOT THE CENTRE), of the guide holes. The ¾" thickness of the gauge body is to make it easier to ensure that the guides and pencils are held at 90° to the upper & lower faces of the gauge and to this end the holes should be drilled as vertically and as true as possible.
The two outer guides can be made out of round wooden dowel (available in most hardware shops or from timber merchants). A diameter of 3/16" is suggested but can vary according to what is available. The length can be 2¾", the top ¾" being glued into the corresponding hole in the body of the gauge.
The two pencil holes have a distance between the centre points of 2½" with a suggested diameter of 5/16" which corresponds to the diameter of a typical pencil.
A Completed Spar Gauge
This spar gauge is based on the diagram above but the two outer guides are made out of 5/16" diameter aluminium tube, and the two pencils are contained in 3/8" aluminium tube.
Using a Spar Gauge
The Spar Gauge is placed across the plank and rotated sideways so that the two guides at each end are in contact with the plank sides. They must remain in contact as the gauge is moved along the length of the plank, with the two pencils marking the two lines as shown. Repeat for each of the four sides of the plank.
Last updated January 2006