How to Make a Marine Triangle Protractor for Easy Course Plotting

This short instructable shows you: (a) How to make a nautical Triangle Protractor and (b) How to use it to navigate in marine environments.

These protractors are surprisingly expensive, at about US$20-$30, from sellers such as Weems & Platt and West Marine (in the USA). So I decided to design my own, laser etch and cut, and fill in the etched pattern with acrylic ink. I added two useful tools to the Protractor Triangle: A nomogram to calculate distance, speed or time; and a reminder of how to convert from magnetic to true north. I also wrote up a short instruction manual for those who are not aware of the quick and accurate way to navigate with these Triangle Protractors.

1. You will need access to a laser cutter that can cut acrylic.
2. Acrylic or plexiglass; I used 1 mm thick clear acrylic but ~3 mm thick acrylic should also work well.
3. Water-based Black or dark acrylic paint such as water-based craft paint or poster paint to fill in the etched marks.
4. Rubbing alcohol to remove the excess paint.

I have attached an SVG file in a format and line colors that are recognized by most laser cutters. The black colored text will be raster etched, the blue lines will be etched, and the red lines will be cut through.

1. Import the attached SVG into your laser cutters software and make sure that the line colors are associated with the required action, raster, line and cut.
2. Important: Make sure that the laser setting is for a reverse etch or 'mirror design' as shown in the screen shot from the excellent 'K40 Whisperer' software (Advanced Setting --> Mirror Design).
3. Place and adjust the acrylic sheet and start the etching and cutting based on parameters for 1 or 3 mm thick acrylic.

1. Wash and rinse the etched acrylic triangle with diluted dish-washing detergent. I used Dawn. Make sure there are no soap residues left.
2. If you are using (dry) black poster paint, moisten a small rag with water, pick up some of dry poster paint onto the moist rag and rub it onto the etched acrylic triangle making sure that the lines and marking are filled in with the paint. Do not worry about excess paint on the surface. If you are using wet paint, just drop a few blobs onto the acrylic triangle and use a moist rag or a rubber squeegee to spread the paint. A few drops of water will help the paint enter the etched marks.
3. Let the paint dry.
4. Moisten soft tissue or cloth piece with 70% isopropanol and wipe the surface clean. The etched lines and marks will hold the paint but the paint will come off the top surface.
5. Optional: You may seal the painted marks with an acrylic water based varnish.

Note: I tried doing multicolor fills, as shown in the image, but not worth the effort.

I have attached a short 'instruction manual' but using the Protractor Triangle is much more elegant and accurate than the usual methods of parallel rulers and referencing that to the compass rose on the chart.

There are very good youtubes on the use of Protractor Triangle for navigation. My favorite is from the Maryland School of Sailing - which actually inspired me to design and make this triangle! https://youtu.be/IN0O6XIU8J8