Download Boating Tip #2: Nautical Conversion Factors (PDF format, 60KB)

Visibility

Distance to the horizon in nautical miles = 1.17 x (square root of your eye height)
Distance at which an object becomes visible
1.17 x (square root of your eye height) + 1.17 x (square root of the height of the object)

Atmospheric Pressure

Atmospheric pressure at sea level = 14.7 psi For each foot of water depth, pressure increases 0.445 psi Pressure doubles every 33 feet
Example:
Calculate pressure at a depth of 66′
66′ x 0.445 psi / ft = 29.37 + 14.7 = 44.1 = 3 atmospheres

For each 1,000 feet in altitude, pressure decreases 1 foot of head or 0.433 psi, so at 2,000 feet above sea level, pressure would be 14.7 – 2 x 0.433 = 13.83

Barometric pressure

Average at sea level: 29.92 inches = 1013.2 millibars (range 950 mb – 1035 mb)
Inches of mercury x 33.86 = Millibars
Millibar / 33.86 = Inches of mercury

Length

Statute mile = 5,280 feet
Nautical mile = 6,076 feet, 1 minute of latitude, 1/60 degree, 880 fathoms, 7.5 cables
League varies, but in U.S., Great Britain, France and Spain, 1 league = 6,075 yards, 18,225 feet, 5,555 meters, 3 nautical miles

Fathom = 6 feet, 1.83 meters

Meter = 3.281 feet, 0.547 fathoms

Cable = 120 fathoms, 720 feet

Speed conversion formulas

Nautical miles per hour (Knots) x 1.151 = Statute miles per hour
Statute miles per hour x 0.869 = Knots

Nautical miles per hour (Knots) x 1.852 = Kilometers per hour
Kilometers per hour x 0.540 = Nautical miles per hour (Knots)

Statute miles per hour x 1.609 = Kilometers per hour
Kilometers per hour x 0.621 = Statute miles per hour

Nautical miles per hour (Knots) x 1.687 = Feet per second
Feet per second x 0.5925 = Nautical miles per hour (Knots)

Statute miles per hour x 1.467 = Feet per second
Feet per second x 0.682 = Statute miles per hour

Distance / Speed / Time Conversions

Distance = Speed x Time
Speed = Distance / Time
Time = Distance / Speed

60 D ST

Distance in knots
Speed in nautical miles per hour
Time in minutes

Speed = (60 x Distance) / Time
Distance = (Speed x Time) / 60
Time = (60 x Distance) / Speed

Temperature Conversion

Fahrenheit to Celsius (F° – 32) / 1.8 or F° – 32 x 5/9
Celsius to Fahrenheit (C° x 1.8) + 32 or C° x 9/5 + 32

Time

Day = 24 hours, 1,440 minutes, and 86,400 seconds

Time zones were standardized in 1883, with the prime meridian set at Greenwich, England. The earth was divided into 24 international time zones at each 15° of longitude. Each zone is one hour apart. Zones are sometimes altered due to geographic and political boundaries. Some states have modified their time zone boundaries, for example, Alaska, Texas, Michigan, Florida, and Indiana. Some countries, like China, have chosen to use only one time zone.

Time zones W of the prime meridian are “earlier” than Greenwich Mean Time (GMT). Greenwich Mean Time is also referred to as Universal Time (UTC). Times zones E of the prime meridian are “later” than GMT.

The International Date Line is an imaginary line directly opposite Greenwich at 180°. When traveling west, advance one day when you cross the International Date Line.

Daylight Savings Time is applied in many parts of the world to add an additional hour of daylight during parts of the year. Most countries observe Daylight Savings Time during their Summer months, but a few countries add an extra hour during their Winter months instead. In the U.S., Arizona, Hawaii and parts of Indiana do not observe Daylight Savings Time. In most of the United States, remember to:

Spring forward 1 hour the first Sunday in April
Fall back 1 hour the last Sunday in October

Note: There is no “J” or Juliet time zone. J is used to describe the current time for the observer.

Resistance to Capsizing Formula

Divide the boat’s gross tonnage in pounds by the constant 64, then take the cubed root of that number.
Divide the beam of the boat in feet and tenths of a foot by the number derived above.
If the result is less than 2, the boat is relatively safe from capsize

Example:
Boat displaces 13 Gross Tons = 26,000 pounds
26,000 pounds / 64 = 406.25
Cube root of 406.25 = 7.406
Beam = 12′ 11″ = 12.916′
12.916′ / 7.406 = 1.74
1.74 is < 2

Boat Displacement : Length Ratio Formula

Divide the LWL by 100, then cube the result
Divide the resulting number by the boat’s displacement in gross tons

Example:
34.9′ / 100 = 0.349
(0.349)3 = 0.0425
13 Gross Tons / 0.0425 = 305.9

> 380 Very heavy displacement boat
320 – 380 Heavy displacement boat
250 – 320 Medium displacement boat
120 – 250 Light displacement boat
50 – 120 Very light displacement boat
< 50 Ultra light displacement boat

Maximum Hull Speed

1.34 x the square root of the LWL (waterline length)

Larger displacement sailboats can usually travel faster than smaller boats because hull speed depends on the length of the boat’s waterline and the volume of water displaced. Resistance increases as speed increases. At optimum speed the bow and stern wakes combine to form a single wake or wave. Theoretically, a sailboat cannot travel faster than the wave created, and wave speed is 1.34 times the square root of the distance between wave crests. Wave length increases proportionally to wave height. So the higher the wave created, the greater the distance between crests, the faster the wave travels. As sailboat speed increases, a greater volume of water has to be displaced, the bow wave increases in height, the distance between crests increases, and boat speed increases. It is only possible to exceed hull speed, if the sailboat, because of its design (some fin keel and centerboard designs), is capable of planing along the water surface, or by sailing downwind and surfing down the front of a wave.

Beaufort Scale for Measuring Wind Speed