The method
What the checker computes, and where the rules come from
Plotroom works every problem on a flat plane in nautical miles, which is exact enough on a chart this size. The rules are the standard ones of coastal piloting.
1. Compass conversions
True = compass + deviation + variation, with easterly values added and westerly values subtracted (T, V, M, D, C in order). Deviation belongs to the steering compass on a given heading; a hand-bearing compass used away from the boat's iron is taken to have none.
2. Time, speed, distance
60 × D = S × T, with D in nautical miles, S in knots and T in minutes.
3. Dead reckoning
From a known position, each leg is laid off along the true course for speed × time. A DR position ignores current and leeway by definition; the difference between a DR and a later fix is what skill 5 measures.
4. Two-bearing fix
Each bearing is converted to true, and its line of position is drawn from the object along the reciprocal. Two lines that cross at 30° to 150° give a usable fix; the problems here use 50° to 130°.
5. Set and drift
Set is the true direction from the DR position to the fix taken at the same time (where the water carried you TO). Drift is that distance divided by the time since the last fix, in knots.
6. Course to steer
The current triangle: the current vector for one hour, then the boat's speed through the water swung from its end to meet the track. The swung line gives the heading; the length along the track gives speed over the ground.
7. Doubling the angle on the bow
If the relative bearing of an object doubles, the boat, its earlier position and the object form an isosceles triangle, so the distance off at the second bearing equals the distance run between the bearings. The distance off when abeam is that distance × sin(second angle). It assumes a steady course and speed and no current.
8. Light rising
Distance to the horizon d = 1.17 × √h nautical miles for a height h in feet. A light's geographic range for an observer is the sum of the two horizon distances; it can be seen out to the lesser of its geographic and luminous range. On a clear night Plotroom takes the light's listed nominal range as its luminous range.
The 1.17 coefficient is the one in Bowditch's distance-of-visibility table, in Chapman Piloting and in the U.S. Coast Guard Light List's geographic range table. Some texts use 1.14 or 1.144 (about 2.5 % less); the problems say which one to use.
Before you take a boat out
Plotroom is a place to practise the arithmetic and the plotting. For real passages use current official charts (NOAA charts in U.S. waters) and take a NASBLA-approved boating course or a U.S. Coast Guard Auxiliary piloting course. A course teaches the parts a checker cannot: reading a real chart's notes and corrections, choosing safe marks, and keeping a plot on a moving boat.
Sources
- National Geospatial-Intelligence Agency, Pub. No. 9, The American Practical Navigator (Bowditch): piloting, dead reckoning, current sailing, distance of the horizon.
- U.S. Coast Guard, Light List, introduction: nominal, luminous and geographic range.
- Chapman Piloting & Seamanship: compass conversion and piloting conventions.
No passage from these works is copied here; the summaries above are written for this site. The practice chart is invented and is not a nautical chart.