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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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The three-sided plot combining boat movement, tide and resulting track
All dead reckoning problems reduce to one triangle whose sides are the water track, the tidal stream vector and the ground track. Any two known sides determine the third. Constructed for the EP, the tide vector is added to the end of the water track; constructed for course to steer, the tide vector is laid first from the start position. All three vectors must cover the same time period, and all directions must be in degrees true.
Read definition →How fast you are really progressing towards the wind or a mark
VMG is the component of boat speed resolved along the direction of interest, so it equals boat speed multiplied by the cosine of the angle between heading and that direction. Sailing at six knots at 45 degrees to the true wind yields about 4.2 knots of upwind VMG. Pinching raises the angle but kills speed, so the best upwind and downwind angles are those which maximise VMG, taken from the boat's polar.
Read definition →The highest latitude point reached on a great circle track
Every great circle has two vertices, in opposite hemispheres and 180 degrees of longitude apart. At the vertex the track runs due east or due west, so the course is exactly 090 or 270 degrees true. Vertex latitude follows from Napier's rules as cos(latitude of vertex) equals cos(departure latitude) multiplied by sin(initial course). Checking the vertex before departure shows immediately whether ice or storm limits will be breached.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.