Navigation Help:
Nautical Mile is 1,852 meters or 1.15miles
1’ of latitude equals crossing 1NM.
One Degree of Latitude equals 60NM/sea miles
A Cable equals 1/10 of a Nautical mile
A nautical mile is just over 6,000 feet or 15% longer than a statute mile.
Error West, Compass Best, Error East Compass Least
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50/90 Rule: Speed of tidal stream
Hours before/after slack 0 1 2 3 4 5 6
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% of Max. rate 0% 50% 90% 100% 90% 50% 0%
Rule of Twelfths: Divide tidal range by 12
Height of the water
From low to high + high to low in hours,
1/12 2/12 3/12 3/12 2/12 1/12
Ferry Glide:
Tide running at half of paddling speed make allowance of 45⁰ to stay on course.
Rule of Thirds: Drift
Hours before/after slack 1st. 2nd, 3rd. 4th. 5th. 6th.
Rate of drift (thirds of max) 1/3 2/3 3/3 3/3 2/3 1/3
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Sea State Scale
The WMO sea state code largely adopts the ‘wind sea’ definition of the Douglas Sea Scale.
WMO Sea State Code |
Wave Height (meters) |
Characteristics |
0 | 0 | Calm (glassy) |
1 | 0 to 0.1 | Calm (rippled) |
2 | 0.1 to 0.5 | Smooth (wavelets) |
3 | 0.5 to 1.25 | Slight |
4 | 1.25 to 2.5 | Moderate |
5 | 2.5 to 4 | Rough |
6 | 4 to 6 | Very rough |
7 | 6 to 9 | High |
8 | 9 to 14 | Very high |
9 | Over 14 | Phenomenal |
A statute mile is 5,280 feet in length.
A nautical mile is 6,076.11549… feet in length.
statute miles x .87 = nautical miles
nautical miles x 1.15 = statute miles
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As a rule of thumb, roughly 7 nautical miles equals 8 statute miles. You can convert nautical to statute by multiplying nautical miles by 8 and dividing the product by seven. To reverse the conversion, statute miles times 7, then divide by 8.
A Nautical Mile is 1/60th of a degree or one minute of latitude. Be sure you know what distance measurement is being used on the charts you are working with. There are four common measures of distance used on charts:
- Nautical miles are used on ocean and coastal waters.
- Statute miles are used for inland areas such as the Intracoastal Waterway and the Great Lakes.
- Yards are often used to define distances of a mile or less.
- Meters are being seen increasingly on U.S. charts and are used almost exclusively on Canadian and other charts of the world.
Credit Ed Lawter