Bucketeers Glossary

People who work in shipbucket scale, often get a lot of comments containing words not commonly found in the english language or even words meaning different things when applied to ships and ship design. This page is created by Acelanceloet to provide a reference for such words. This article will provide an explanation of words describing ships dimensions (both real and relative), ship shapes, ship stability etc. It is allowed and even encouraged to improve, add to and expand these explanations. Definitions too complicated to fit in a few sentences deserve their own articles in the future, which will be linked from this page.

Dimensions

Length over all

Abbreviated as L_OA

The length from extreme bow to extreme stern of the hull of a ship.

Length on Waterline

Abbreviated as L_WL

The length of a ship on the design waterline.

Length between perpendiculars

Abbreviated as L_PP

The length of a ship between fore perpendicular and aft perpendicular

Aft Perpendicular

Abbreviated as A.P.

Depending on the ships design, either

• The aftmost side of the rudder post
• The centerline of the rudder stock if there is no rudder post

ships with unusual stern shapes normally don't use the "perpendicular" description. If they do though, it either follows a 'rule length' or is the aft end of the design waterline.

Fore Perpendicular

Abbreviated as F.P.

The point where the bow of a ship enters the water when at design draft, fore end of the design waterline

Beam

Abbreviated as B

The greatest width of a ship, measured in the inner shell.

Largest Beam

Abbreviated as B_MAX

The greatest width of a ship, including shell thickness.

Depth

Abbreviated as D

The distance between the top of the keel to the top of the deck beam at side of the uppermost continuous deck, measured amidships. (= 0,5*L_WL)

Draft

also written as draugth

Abbreviated as T

The greatest distance between keel and waterline at a load condition. If just the draft is given, this is design draft.

Draft over all

Abbreviated as T_OA

The distance between the waterline and the lowest point of the ship.

Displacement

Abbreviated as Δ

The volume of water a ship "displaces" when it's total weight is in the water. Equal to Volume * density of the water. Displacement in meters is equal to the ships weight in metric tons.

Equal to Volume when the ship is in fresh water (density = 1t/m³)

Volume

Abbreviated as ∇

The volume of a ship below the design waterline.

Coefficients

These coefficients are ways to make a ships particulars dimensionless, so they can be compared between ships of different shape and size.

Block Coefficient

Abbreviated as C_b

Coefficient describing the ratio of volume a ship occupies from a block of it's main underwater dimensions, thus describing the 'fullness' of a ship. Full ships such as oil tankers have a high block coefficient, sailboats a low.

Equal to Volume divided by L_WL * B * T

Midship Coefficient

Abbreviated as C_m or C_x

Coefficient describing the ratio of cross sectional area of a slice of a ship (at midship C_m and at the largest section for C_x) occupies of a rectangle having the same width and depth as this underwater section of the hull. This defines the fullness of the midships body, a high midship coefficient describes a boxy (cargo ship) hull, while a low midship coefficient means an very cut away midsection.

Equal to the cross section area divided by B * T

Prismatic Coefficient

Abbreviated as C_p

Coefficient describing the ratio of volume a ship occupies from a prism of the same length as the ship and the cross section equal to the cross sectional area of the largest underwater section of the hull (midship section). This coefficient is used to evaluate the distribution of volume in the underbody, a low prismatic coefficient indicates fine ends, a high prismatic coefficient indicates full bow and stern. Planing hulls tend to have high prismatic coefficients, efficient displacement hulls tend to have a low prismatic coefficient.

Equal to Volume divided by L_WL * B * T * C_m

Can also be found by dividing the block coefficient by the Midship coefficient.

Waterplane Coefficient

Abbreviated as C_w

Coefficient describing the ratio of area a ship occupies from a rectangle of it's main waterline dimensions, thus describing the 'fullness' of a ships waterplane. A low waterplane coefficient indicates fine ends while a high waterplane coefficient indicates fuller ends. A high waterplane coefficient improves stability as well as handling behaviour in (relatively) rough conditions, while a low waterplane coefficient might result in a higher wavemaking resistance.

Equal to Waterplane area divided by L_WL * B

Special numbers and dimensions

Froude number

Abbreviated as Fn

The Froude Number is a dimensionless number based on the speed-length ratio. In maritime engineering the Froude number is used to compare different ships or even models of ships with each other. If you have 2 ships of different size but the same shape, you can scale the speeds and wave patterns of this ship by comparing the Froude numbers: if a ship is to go twice the speed of another, the ships length/wavelength would be multiplied by 4.

The Froude number says a lot about a ships length compared to the wavelength created by its speed, and is because of that an important parameter in ship resistance. For comparison see https://en.wikipedia.org/wiki/Froude_number#/media/File:Froude_numbers_and_waves.png

The Froude number is calculated by dividing the speed (in m/s) by the square root of the gravitational constant (9,81 on earth) times the ships/bodies length or the water depth (in meters), whichever is lower.

Fn = v/√(g * L_WL) or Fn = v/√(g * water depth)

Reynolds number

Abbreviated as Re

Hull speed

The hull speed is the speed at which the wavelength of the wave produced by a ship is exactly as long as the ships waterline. This is the maximum speed a ship can efficiently reach in displacement mode. This is because at this speed, the stern is no longer supported by the second top of the bow wave, so the ship starts to sail up it's own wave. While it is possible to go faster then this without planing, foiling or other non-displacement modes, the standard power calculations for displacement ships are no longer valid above this speed. Wavemaking resistance will increase exponentially.

Hull speed in knots can be calculated by taking 1,34 times the square root of the the waterline length (L_WL) in feet or alternatively by taking 2,43 times the square root of the the waterline length (L_WL) in meters.

example: a hull with an L_WL of 225 feet ( 68,58 meters ) has an hull speed of 20,1 knots

Natural speed

The natural speed can be used to compare the relative 'fastness' of a ship. Alternative to the more scientifically defined Hull speed

Natural speed in knots is the square root of the waterline length (L_WL) in feet or alternatively by taking 1,81 times the square root of the the waterline length (L_WL) in meters.

example: a hull with an L_WL of 225 feet ( 68,58 meters ) has an natural speed of 15 knots

Metacentric height

Righting arm