Repair and Maintenance of a Drascombe Lugger

Obtaining Profile Dimensions for a Gaff

When making a new spar, after deciding on the general dimensions, it is necessary to obtain detailed measurements of the profile and shape. These dimensions are then drawn on to the square or rectangular wooden blank from which the spar will be made. The shaping of the spar can then proceed.

This page describes how the detailed profile measurements for a replacement gaff yard were obtained using the 'Projection Method'.

See also Obtaining/Making a New Gaff describing how the gaff was constructed using the measurements obtained by the method described on this page.

Obtaining the profile by the 'Projection Method'

Knowing the general dimensions of the gaff, the next problem was to obtain a series of measurements to achieve the 'double-taper' flattened profile.

Guidance notes for plotting the projection

The animated diagram (above) illustrates how the dimensions of the gaff were plotted. The notes that follow give additional information for anyone wishing to undertake a similar exercise in constructing a spar with a curved profile.

The plotting was done on millimetre graph paper at a scale of 2 mm on the graph paper equivalent to 1 mm actual size.
First, using a compass, draw a quarter circle representing a radius of 50 mm (actual radius 100 mm on the graph paper).
Draw a horizontal line across the circle at 38 mm (76 mm on the graph paper), from the baseline.
Where the horizontal line intersects the circle, draw a vertical line down to the baseline.
Divide the baseline between the centre of the circle and the vertical line into ten equal parts. If you are not sure how to do this then Select this LINK
From each of the ten points plotted on the baseline, draw a vertical line upwards passing through the circle.
At each point of intersection between the vertical lines and the circle, record the height (in millimetres), from the baseline. These readings represent the intermediate points of the curve between the maximum depth of 50 mm in the centre of the gaff, and the minimum depth of 38 mm at the end of the gaff.
The final construction is shown below.

Each of the vertical lines defining the ten equal divisions ( 0 - 10 ) has a value where it intersects the curve of the circle. The ten divisions can now be elongated and projected to represent the half length of the gaff. Instead of a circle, the values define a new flattened curve shown in the diagram below.

Finally, this projection can be extended to the other half of the gaff to give all the measurements required for the full length. This is shown in the sketch below.

We now have all the detailed measurements required to construct the gaff. See also Obtaining/Making a New Gaff describing how the gaff was constructed using the measurements obtained by the method described on this page.