Weems & Plath Marine Navigation Compute-A-Course Multi-Purpose Plotting Tool

I use this for sea kayak navigation, and it is excellent! I was alone, in unknown waters, in Casco Bay, Maine for 5 days. With this, a compass, and a chart, I was able to reach every destination I aimed for. It works great.The plastic is somewhat flexible. So it easily conforms to the curved deck of my kayak.It has 3 moving plastic parts.Part 1: Baseplate and True North compass rose. It is rounded on top, and flat on the bottom. The red grid lines are on this part. There is also a true north compass rose on this part. The true bearings are in a smaller font, and they are in the smaller compass rose. The vertical grid lines point to true north and true south. The horizontal grid lines point to true east and true west. At the bottom of this part there is a "ruler" to measure. It has scales for 1:40,000 and 1:80,000. FYI, I don't use that. I use this approximation:Scale 1:10,000 Your thumb to index finger = 1 mileScale 1:20,000 Your thumb to index finger = 2 milesScale 1:30,000 Your thumb to index finger = 3 milesScale 1:40,000 Your thumb to index finger = 4 milesScale 1:50,000 Your thumb to index finger = 5 milesand so on. But the scale would definitely be more accurate.Part 2: Magnetic North Compass Rose. This part has the magnetic compass rose. This magnetic compass rose is in a bigger font, and it is further out. The magnetic bearings are at the outer edge of the device. That makes it easier to read the magnetic bearings, than to read the true bearings. That helps you avoid mistakes. It is round. It does not have any grid lines. It rotates with respect to the baseplate. So you can set any declination angle or variance. There is a line for each degree.Part 3: Arrow. This is a thin strip of plastic with an arrow. It rotates to any setting. So you point it from your current location to your destination. The tip of the arrow crosses the magnetic bearing. If you want the true bearing, you can look at the smaller-font bearings, which are on the inner bearing circle. There is a line for each degree.MY CUSTOMIZATION:Let's say that you put the center on your current location. Then you look at your destination, and it is beyond the reach of the device and its arrow. That's a bit of a problem. So I added a string.There is a little hole in the middle. You can mark your position on the chart, or map, through that hole. I put a string in that hole. The end of the string at the bottom side of the device needed a small knot, so it wouldn't be a bulgy thing between the device and the chart. So I flared out the threads of the line. Then I burned it. That worked great. The burned end is wider than the hole. So it stays in place. At the top end of the string, I put a knot. I made the string as long as the diagonal dimension of a chart, so it can reach any point on the chart. If the burned end ever comes out, I can untie the knotted end, put it back, and tie the knot again.I keep the device, and the string, in it's original plastic bag. I keep them under the chart, under the bungee cords, on the deck of my kayak. It's always there when I need it.There were other similar devices that were cheaper. But they did not give you true and magnetic bearings. With this device, you don't have to add or subtract in your head. It tells you the true and magnetic bearings. The grid lines make it easy to align with longitude or lattitude lines. This device is well worth the price!

Would be OK for standard charts

I teach basic and advanced navigation classes for the CG Auxiliary in Maine and use the device on my own boat where folded charts are the norm. Having what is essentially a portable compass rose is a necessity for plotting on a smaller boat. I’ve set the magnetic variation and locked it in with a couple of pieces of clear tape, so I know my magnetic bearings are correct without checking every time I use the device.When I show the Compute-a-Course to my students who’ve been using a roller plotter or parallel rules to work through the plotting exercises, they immediately want one. I hate to tell them the price, however.The only reason I didn’t give it 5 stars is the price. The W&P nautical slide rule is a much more substantial product and seems far costlier and precise to produce, yet sells for far less.

This gadget is great for sailors, since we tack back and forth, and change course a lot. If one is in unknown waters, a roller plotter is too cumbersome and time consuming. Just put this on a Rastor chart, and you have a good idea of what heading to take.

Great for what I need which is quick reciprocals when I set up race courses. The print is easy to read without my glasses.

This is an excellent aid, especially for use with 1:80,000 or 1:40,000 charts where the scale mates up with these charts for direct measurements.

This is a very good addition to my navigation arsenal, especially when your chart is folded in such a way that you rose is not completely legible

I am used to the chartroom on a ship so was pleasantly suprised at how well this tool works on the trail.