Created nov. '01
Dutch ship Twee Anthony's at Batavia roads, 1844 (aquarelle by J. Spin)In the beginning of the 19th century the need was felt to for a method to classify the different forces in wind strength.
In 1805 Sir Francis Beaufort, commander of the Woolwich, a man-o'-war of 44 guns, deviced a wind force scale, whereby he was led by influence the wind forces had on his ship. He made a division of 13 windforces, based on the sail settings of a full rigged ship in different wind conditions. His scale was based on windforce, not on windspeed; he looked at the behavior of his ship, not at the wind itself.
By 1838 the Beaufort wind force scale was made mandatory for log entries in all ships of the Royal Navy.The original Beaufort scale stayed in use for 100 years. In 1905 Sir George Simpson adapted the scale to the modern time of steamships.
In 1921 the International Metrological Comity approached Sir Simpson to develop a new Beaufortscale, which could be accepted by all nations. Simpson included indications which were better to understands for the shore-based part of mankind, such as ascending smoke, leaves rustling and the swishing of trees. He also added wind speeds to the scale, where after the wind force could be measured (with an anemometer).In 1927 captain Petersen remodelled the Beaufort scale again to the needs of the sailor. By the lack of trees at sea, Petersen developed an estimation method based on the effect of the wind at waves at open sea.
Below you will find the original scale as Sir Beaufort redacted it in 1805 with the addition of Petersen's definition. The starting point of the Beaufort scale is a full rigged ship with single top- and topgallant sails, on a course clean full.
The
| Bf. | Knots | ||||
| Calm | All (Courses might be clewed up, to prevend chafing) | Rolling | Mirror like sea, only swell | ||
| Light Air | All sail set, incl. studdingsails | Just sufficient to give steerage way | Ripples with the appearance of scales | ||
| Light Breeze | All sail set, incl. studdingsails | Clean full, would go in smooth water from 1 to 2 knots | Small wavelets; crests of glassy apprearance, not breaking | ||
| Gentle Breeze | All sail set, incl. studdingsails | Clean full, would go in smooth water from 3 to 4 knots | Large wavelets; crests begin to break; scattered whitecaps | ||
| Moderate Breeze | All sail set, incl. studdingsails | Clean full, would go in smooth water from 5 to 6 knots | Small waves, becoming longer; numerous whitecaps | ||
| Fresh Breeze | Royal breeze | Favourible condition at running course | Matige golven van aanmerkelijk groter lengte. Overal zijn witte schuimkoppen te zien. Hier en daar opwaaiend schuim. | ||
| Strong Breeze | Topgallant breeze, single reefed topsails | Light sails have to be taken in | Larger waves forming; whitecaps everywhere; more spray | ||
| Near Gale | Double reefed topsails | Light sails have to be taken in, also on running courses | Sea heaps up; white foam from breaking waves begins to blow in streaks | ||
| (Fresh) Gale | Triple-reefed topsails | More sail has to be taken in | Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown into wellmarked streaks | ||
| Strong Gale | Close-reefed topsails and courses | Still more sail has to be taken in | High waves; seas begins to roll; dense streaks of foam; spray may reduce visibility | ||
| Whole Gale | Close-reefed main topsail and reefed forecourse | Running with close reefed square sails, or hove to under storm staysails | Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility is reduced | ||
| Storm | Storm staysails only | Running or hove to under storm staysails | Exceptionally high waves; sea covered with white foam patches; visibility still more reduced | ||
| Hurricane | Sails blow out of their leeches | Or that which no canvas could withstand | Air filled with foam; sea completely white with driving spray; visibility still more reduced |
As stated before, the sail settings mentioned are for a full rigged ship on a clean full course.
A ship on a running course can set more canvas then a ship close hauled. This has to do with the difference between the true wind and the apparent wind. When a ship lies at anchor, on deck one feels the true wind blowing. If the wind force is 5 Bf, one experience a wind of 20 knots, for example.
When there is a true calm and the ship is tugged (or sailing on auxiliary power), the wind experienced on deck is the wind generated by the ship's speed.
When a ship is sailing close hauled to the wind one experience a wind that is a resultant of the wind generated by the ship's speed and the true wind. This resultant is larger then the true wind.
When a ship is sailing with a groundspeed of 7 knots, the head wind felt on deck would be 7 knots if there were no other wind. With a true wind of 20 knots on starboards bow the apparent wind experienced on deck is 25 knots and is coming in more forward then the true wind. So one experience a force 6 while it only blows a force 5 Bf. The royals have to be taken in and a single reef might be needed in the topsails.
When a ship is running free, so she is sailing partly in the direction of the wind, the combination of the ship's speed and the true wind will result in an apparent wind which is smaller then the true wind and still coming in more forward.
The true wind in this example is still 20 knots, the ship's speed still 7 knots. The apparent wind however will be only 14 knots, which equals a force 4 Bf. This means that the reef can be shaken out, the royals and studdingsails can be set. If the ship's speed is increasing, the apparent wind will decrease furthermore in force.
