The Davis Quadrant or Backstaff

The Davis  Quadrant or Backstaff was one of the most important and popular navigation instruments of the 17th and early 18th centuries. Invented by Captain John Davis in the 1890's, it became the most popular navigational instrument for for finding the latitude at sea. It was later displaced over a number of decades with the development of the Octant or Reflecting Quadrant (the immediate predecessor of the Sextant), beginning around 1730 (1),though they were still being made and advertised in America as late as the 1790's. (2) Apart from its importance as a navigational instrument, I also enjoy its appearance as an art object. I first started thinking about making a backstaff  over 15  years ago. I was able to get a copy of plans (a "blueprint") for a historic backstaff made by Will Garner in 1734, now held at the National Maritime Museum, Greenwich (NMM),(3) which was used for the dimensions of my example. I was able to see the original of this backstaff when my wife and I visited the Ships, Clocks and Stars: The Quest for the Longitude exhibit commemorating the tercentenary of the first Longitude Act of 1714 at the National Maritime Museum, Greenwich. While we were there, one of the exhibit's curators, Richard Dunn, kindly took my wife and I on a tour of the museum stores where I was able to make detailed observations and take photographs of Garner's 1737 backstaff.

The American examples I was familiar with when I first conceived this project were identified as being made with a Brazilian rosewood frame and boxwood arcs, so I started looking for these woods. (I have since found examples of many other woods, which would have simplified my search!) I was fortunate to find a piece of Brazilian rosewood at our local hardwoods supplier (Almquist Lumber) which had been legally imported before the 1990s and stored in a musical instrument makers shop . I then found that boxwood in pieces large enough to make the arcs was equally difficult to acquire in the US (I even contacted Roy Underhill of "The Woodwrights Shop" on PBS who replied that he was unaware of American sources). I finally got a couple of small boxwood planks when a colleague went on sabbatical in England and bought them for me at a lumber store there. I held the wood in storage for over a decade before I finally became inspired to actually begin work on it in 2024. Unfortunately my aged eyesight and arthritis have reduced my confidence in my ability to do precision joinery, particularly with hard woods such as rosewood and boxwood, so I modified my design into a 'modern hack', replacing mortise and tenon joints with stainless steel cap-screws (mostly recovered from a 1980's vintage German research IR spectrometer) threaded into tapped holes in the rosewood frame and boxwood arcs.

In the blueprint of the Garner backstaff all frame members have a rectangular cross-section of 16 mm x 18 mm with lengths of up to 645 mm. Since most of my tools and measures are in marked in inches I converted all measurements. As a result I initially cut about six feet of 3/4" square rosewood sticks from the original plank I started with, then I planed them to a rectangular cross-section of 0.71" x 0.625 (5/8)". I simultaneously planed the boxwood boards to 16 mm (5/8" = 0.625") to match the rosewood frame. 

I laid out both arcs on a single piece of  15" x 4" x 5/8"  boxwood using a beam compass carefully set with a meter stick. Following the NMM plans I laid out the large (25°) arc with an outer radius of 605.5 mm (23.82") and an inner radius of 572.5 mm (22.54"). I was then able to fit the small arc inside the larger, with an outer radius of 190 mm (7.48") and an inner radius of 170 mm (6.69"). Both arcs were then cut on a bandsaw with a 3/8" blade. I then used my Stanley Victor No 20 adjustable circular plane to smooth and clean up the outer and inner surfaces of the arcs as shown in the images below.

Using the dimensions on the NMM plan, I cut the four rosewood frame pieces slightly oversized, allowing enough excess to later cut them to the precise angles and curves. I first trimmed the far end of the main (upper) limb to give a 15° angle, as shown on the drawing. At 1 1/2" from the far end I scribed a line across the front face of the staff and then bisected it to create a datum point that would serve later to scribe all of the arcs that would create the scales on both the large and small graduated arcs.

I next measured the distances to the four holes in the main limb that would be used to assemble the quadrant, two from the upper surface (at 5 1/2" and 12 1/4") and two from the lower surface (2 1/8" and 7 1/8"). Using a punch to mark the center of each hole, I then used a 13/32" brad center wood drill to drill 1/4" deep counter sunk holes to accommodate the ss cap-screw caps at the appropriate locations, two from above and two from below as noted before. Before the initial, alignment assembly discussed below, I marked and cut the lower limb to give a 30° angle on the far where it would be fitted against the upper limb. In order to ensure proper alignment of the screw holes, I clamped the cross-pieces and arcs, butted and optimally aligned to the limb before carefully drilling 3/64" (specified hole size for tapping M6-1 threads) through the center of  each counter-sunk hole and into the appropriate cross piece or arc before tapping the target holes with standard M6-1 metric machinist taps. (Be careful to drill deep enough to tap the threads without going too deep and penetrating the limb surface). While the pieces were aligned I also marked the arcs and other limb and cross pieces for where I would need to drill additional holes to complete the assembly. I then used the drill press to first drill counter-sunk holes for the cap screw heads and then 3/64" holes, both while the pieces were held to give perpendicular holes to the outer surfaces. 

I later came back and drilled out the body holes in the frame pieces and arcs to 1/4". In actuality, I partially assembled the lower limb and the small arc to the upper limb, then aligned and marked the angles and curves to be cut/shaped at the ends of the limbs etc. I then drilled the final holes based on how everything fit.

After graduating both arcs, I disassembled the cross-staff to trim the far end of the main limb to make the post mount for the horizon vane. I used a Delta™ tenoning jig on my table saw, set to 15° to create 3/32" shoulders on either side. I then set the jig vertically to create 3/32" shoulders to match top and bottom, approximately matching the appropriate edges from the 15° cuts, finally matching the 15° angles with a chisel to create the post mount for the horizon vane. 

Backstaff Vanes