Carry this sailing navigation super tool for safety!

by John Jamieson on 5 Jul 2013 In my mind, nothing beats simple and fast when it comes to plotting on a small sailboat. After all, you are short handed, on a heel, and want to spend minimum time in the cabin (if that's where you are plotting).

Enter the tool I use and have used for over two decades. And it comes not from the marine world--but that of the aviation world. I wouldn't sail without one or two aboard. Read on to discover this 'gotta-have-it' navigation magic tool!

I navigated big ships and small boats in the US Coast Guard in the days before push-button navigation. And navigation was anything but a relaxed environment as it often will be when sailing. Fixes in inland piloting waters were taken and plotted every one to three minutes, set and drift of current determined at each fix, course and speed to maintain track computed, and a recommendation given to the conning officer for new course, time to turn to next course, and so forth.

With ships drawing 12 to 17 feet (draft), there's not a lot of room for error. We needed fast, accurate, 'works-every-time' tools. You may have read my recommendation to use a drafting compass (also called a 'plotting compass') for most practical chart work in navigation. Matter of fact, I rarely ever use dividers except in the planning stages of a trip.

The same goes for parallel rules. I believe they are a challenge to use on any but the most stable of platforms. Take a bit of roll or pitch and they will often slip and slide, which can result in navigation errors of several degrees.

Do I use parallel rules? You bet--they make a great planning tool (before you depart), serve as a good straight edge, and are a superb backup tool in case your other tools break or fail. But they will never be my primary plotting tool.

Enter the Weem's plotter. This tool was first used by aircraft navigators. The Weems looks like a clear rectangular shaped protractor with a small bar on rollers attached to the bottom. The wheels enable you to roll the protractor across any flat surface.

An engraved half circle with a cross-hair has been placed in the center. Degrees on the outside of the half circle are for directions from north through east to south.

Degrees on the inside of the half circle are for directions from south through west and back up to north. Quarter circles are engraved at each end to use with parallels of latitude if desired (rarely used, but nice to have).

The top edge has convenient distance scales that can be used on three common coastal and harbor chart scales : 1:80,000; 1:40,000 and 1:20,000.

Follow these easy steps along with the illustrations to learn how to use the Weems plotter. Five minutes of practice and you will be a pro with this super navigation tool for sailing!

1. Draw the course line with the Weems onto your nautical chart. Or, you can skip this step if you want to plot a specific pre-determined course or bearing (illustrations). Go to step 2.

2. Roll the Weems over to any convenient vertical line nearby (use a meridian of longitude or other vertical line).

3. Line up the center cross-hair of the half circle with the line.

4. Read the direction where the vertical line crosses the half circle. Notice that each direction will be a reciprocal of the other. Take care to read the direction you are going.

Look at the illustrations and note that the vertical line intersects the Weem's half circle at 110° and 290° (the reciprocal). If you are sailing to the southeast your course will be 110°. If you are sailing to the northwest your course will be 290°. Use this same logic when taking bearings to prominent objects ashore or afloat.

5. You can also use the Weems plotter to determine the direction of a pre-plotted course line. Lay one part of the straight edge along the line. Slide the Weems over to any convenient vertical line. Read the instrument as described in the steps above.

How to Use the Weems Plotter with a Compass Rose
You may prefer to continue to use compass roses. Use the Weems and you will slice 50% or more off your plotting time compared to parallel rules. Line up the long edge of your Weems with a plotted course line. Slide the Weems over to the closest compass rose. Read the direction indicated on the compass rose (use the same cautions described earlier to be sure you read the correct direction).

Or, to start with the compass rose and find direction, line up the straight edge of the Weems with the center cross hair on the compass rose. Read the direction on the compass rose. Roll the Weems over to the departure point, draw the new course line and label with the direction.

What are the drawbacks?
The Weems was not built in a robust fashion. The plastic protractor part may seem a bit flimsy. And, you want to store it out of direct sunlight to prevent warping. I made a simple case for mine out of scrap cardboard. Cut two rectangular pieces two inches larger than the instrument all around. Use packing or wrapping tape on the long edges and one short edge. You can now insert the Weems into the case to protect it when packing for a trip or aboard your boat.

Boost your sailing navigation accuracy with the power of the Weems plotting protractor. This will give you the confidence to navigate your small sailboat faster and easier--wherever in the world you choose to sail or cruise!

John Jamieson (Captain John) with 25+ years of experience shows you the no-nonsense cruising skills you need for safer sailing worldwide. Visit his website at website! Sign up for the Free, highly
popular weekly 'Captain John's Sailing Tip-of-the-Week'. Discover how you can gain instant access to hundreds of sailing articles, videos, and e-Books!