Davis Quadrant

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 (it had been brought to the US before its use became restricted, and held in storage for years) and then a friend brought me some boxwood planks from England as I couldn't find any in the US. 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.

As a first step in this project I took the original rosewood and boxwood stock and flattened it with an eight-inch Craftman jointer, followed by bringing all pieces to a common thickness of 0.67" using a Craftsman 12 inch planer. I then used a table saw to cut strips of rosewood to give  a width of 0.73" after planing both sides with my jointer. 

I laid out both arcs on a single piece of  15" x 4" x 0.73"  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 of190 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.