November / December 2020

The Spar Gauge

Variations and cautions in creating a useful boatbuilding tool
Spar Gauge

The author’s spar gauge, based on a type long in use at Mystic Seaport Museum, makes accurate marks for
eight-siding a spar. This type of gauge is more complicated to build than others but easier to use, scribes lines all the way to the end of a spar, and works with spars of a comparatively wide variety of diameters.

Few articles written about making a boat’s spars fail to include instructions for building a gauge for creating accurate facets as an intermediate step between squaring and rounding. Usually called a spar gauge or an eight-siding gauge, this valuable tool can be used to help shape a mast, boom, gaff, oar, boathook, or any other round-sectioned shaft. Its particular forté is its ability to take into account the spar’s taper, making accurate guidelines that would be much more time-consuming with any other method of marking.

The important word here is accuracy. Any measuring or guiding tool should be made as accurately as possible, otherwise its errors will be reproduced (or at least will have to be accounted for) each time it is used. This article shows how to avoid potential sources of error and presents three different configurations of the gauge itself.

A common mistake while learning to shape spars is to do any rounding of the stock before as many accurate facets as possible are made. You will start with a tapered, four-sided spar that is square in cross-section. A spar gauge will be used to mark lines to guide the next step, which is to shape the spar to octagonal, and the lines will be accurate at any point along the spar’s length. With a spar 3" or less in diameter, 16-siding can be achieved with sufficient accuracy by eye, but spars larger than this will benefit from correctly scribed lines for 16-siding, followed by 32-siding and even 64-siding by eye. Accurate scribing for 64-siding is an unrealistic goal but the longer you resist reaching for the sandpaper, the easier sanding will be and the closer to perfectly round the finished spar will be.

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