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. It was finally 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) 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 ("blueprint") for a historic backstaff made by Will Garner in 1734 held at the National Maritime Museum, Greenwich (NMM),(2) which was used for the dimensions of my example.

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 legal piece of Brazilian rosewood was fortunate to find a plank at our local hardwoods supplier which had been legally imported before the 1990s and stored in musical instrument makers shop . then found that boxwood 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 age 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 threaded into tapped holes in the rosewood frame and boxwood arcs.

In the original drawing 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 then 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 staff 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) strut to give a 15° angle, as shown on the drawing. I then 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. 

I next marked the lower strut to give a 30° angle, but since I had decided to use stainless steel (ss) screws to hold it together rather than the classic mortise and tenon joints, I did not cut it yet.