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Terminology

In this section I'll try and define some of the main terms that you may come across in discussion of a ship design.

General

Load Waterline - The waterline at which a vessel floats at in its Full Load condition (as defined below under Weight Categories).  It is measured from the point where the water surface intersects the bow to where the water surface intersects aft most point of the stern or transom.

Design Waterline - The waterline at which a vessel floats at in its design condition. Typically a design condition is stated as either Full Load, Half Load, or some other typical operating condition.  Right now for design purposes I have assumed all calculations (such as powering estimates and fuel consumption) are to be based on a ship's Full Load Displacement, and as such a vessel's Design Waterline and Load Waterline are currently assumed to be the same.  As I understand it, however, in some navies things like powering and fuel consumption may instead be determined at a half load or part load condition.  As such, eventually I may revise the calculations to allow designing ships for other loading conditions.

Dimensions

Length -

Length Overall (Loa) - In general on a ship the distance from the forward most point of a ship to its aft most point is called Length Overall .

Length on Design Waterline (Lwl) - In general this is the distance from the point where the bow of the ship crosses the Design Waterline to where the stern or transom crosses that waterline.

Length Between Perpendiculars (Lbp) - One issue that occurs in ship design, is that although you try and make your best estimates of weights early on, as the design and construction progresses, the overall displacement of the ship may begin to very from the design estimates as the actual weights of the individual components become better known. Typically margins are included in the weight estimate to allow for this, but sometimes there are late design modifications or unforseen events that may exceed these margins.  As such, when finally delivered a ship's full load waterline may be different from its initial design estimate. To address this issue, it is typical to define a location at the bow of the ship (usually where it crosses the initial design waterline and define this as the Forward Perpendicular.  Similarly (for naval vessels) it is typical to define a location at the stern or transom of the ship (usually where it crosses the initial design waterline and define this as the Aft Perpendicular.  These two locations are then set as reference points on the vessel, and typically are not moved (even if the final full load displacement of the ship changes).  [Side Note: On single screw merchant ships sometimes the aft perpendicular is instead defined as the rudder post (since this is an easily identifiable location on the vessel, but for our purposes we will use the transom location defined above as being more typical of naval vessel.]  As such, for our purposes LBP and Lwl are considered to be the same.

L Def

Beam -

Beam Overall (Boa) - In general on a ship this is the distance across the widest part of the ship.  On some modern vessels with significant flare, this can be appreciably greater than the width of the ship at its design waterline.

Beam on Waterline (Bwl) - In general on a ship this is measured as the maximum distance across the ship where it intersects the Design Waterline.

Depth -

Mean Depth (Dm) - In general on a ship this is the distance from the base of the keel to the main deck height amidships.  Because some vessels have camber in the main deck (to help ensure that water will drain from the deck) the depth at centerline may be a little higher than the depth at side.

Average Depth (Da) - On some ships, such as those with extended focsle decks, the term Average Depth  is used as an indication of what the equivalent depth of the hull would be if the focsle deck were extended along the full length of the ship, but reduced in height so that the total volume enclosed within it was the same as the shorter, full height space.

Draft -

Mean Draft (Tm) - In general on a ship this is the distance from the base of the keel to the height of the Lwl  amidships.  This measure of Draft does not include the impact of any appendages that extend below the bottom of the keel or the influences of vessel trim.  It is basically a measure of the Draft of the bare hull of the vessel.  It is important to note that the letter T  is used to denote Draft while D is used to denote Depth.  I believe that the letter T derives from German the word for Draft  which is Tiefgang.

Navigational Draft (Tnav) - On some ships, appendages such as Sonar Domes and propellors can extend a fair distance below the ship's keel.  Additionally, if the the loading of the ship were such that the ship was operating with trim, the bow or stern of the vessel could be lower in the water than the keel at midships.  As such, Navigational Draft  is a measure of the Draft of a vessel taking into consideration any appendages that extend below the keel of the ship and any impacts of vessel trim.  It basically represents the depth of water at which the lowest point of the ship would touch bottom when the ship is at rest in calm water.  Operationally, because of the impacts of wind, waves, and tides as well as certain shallow water flow effects that can cause a ship to sink lower in the water as the ship's speed increases, for navigational purposes you really need a certain additional margin of distance between a ship's navigational draft and the local water depth to operate safely in shallow waters.


 

Weight Classification Systems (please see the Background page for more details)

SWBS - The US Navy's Ship Work Breakdown Structure.  It is a method for categorizing the weights of all components of a vessel into groups based on the function .

ESWBS - The US Navy's Extended Ship Work Breakdown StructureESWBS  is an extension of the previously used Ship Work Breakdown Structure (SWBS) and for our purposes they are pretty much the same.

BSCI - The US Navy's Bureau of Ships Consolidated Index.  Prior to the mid 1960s (or so) this system  was used for categorizing weights.  Although this system is in general similar to the SWBS/ESWBS system, there are some differences between how some weights are categorized.

NES - A system used in the UK based on their Naval Engineering Standards (NES).  As with BSCI  the NES system is in general similar to the SWBS/ESWBS system, however there are some differences between how some weights are categorized.

Weight Categories (please see the Background page for more details)

Group 100 - In all the weight classification methods noted above, group 100  generally refers to all components of the ship relating to the basic structure of the vessel.  It includes items such as:

  • hull plating,
  • deck structure, and
  • bulkheads, etc.
However, it does not typically include items such as:
  • paint,
  • insulation,
  • furniture, or
  • handling gear (like cranes, windlasses, etc)
I may also refer to this as SWBS100  or w100.   In general I had intended both group 100 and SWBS 100 were meant to refer to the category and w100 is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v100 will refer to the vertical height of the center of the weight of the group 100 items and c100 will refer to the estimated cost of those items.

Group 200 - In all the weight classification methods noted above, group 200 generally refers to all components of the ship relating to the vessel's propulsion system.  It includes items such as:

  • main propulsion engines,
  • gearing,
  • shafting,
  • propellors, and
  • fuel oil & lube oil service systems, etc.
However, it does not typically include items such as:
  • air conditioning & ventilation,
  • the ship's distilling plant, or
  • the ship's fuel oil and lube oil transfer systems (ie the system used to transfer the fuel and lube oil from their storage tanks to their ready service tanks)

I may also refer to this as SWBS200  or w200.   In general I had intended both group 200  and SWBS 200 were meant to refer to the category and w200 is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v200 will refer to the vertical height of the center of the weight of the group 200 items and c200 will refer to the estimated cost of those items.

Group 300 - In all the weight classification methods noted above, group 300  generally refers to all components of the ship relating to the vessel's electrical generation and distribution systems.  It includes items such as:

  • the ship service generators,
  • the emergency generator,
  • lighting distribution (cabling),
  • power distribution (cabling), and
  • lighting fixtures, etc.
However, it does not typically include items such as:
  • electronics (like radars, sonars, satellite communications, & radars, etc), or
  • controls, etc

I may also refer to this as SWBS300 or w300.  In general I had intended both group 300  and SWBS 300 were meant to refer to the category and w300 is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v300 will refer to the vertical height of the center of the weight of the group 300  items and c300 will refer to the estimated cost of those items.

Group 400 - In all the weight classification methods noted above, group 400 generally refers to all components of the ship relating to the vessel's Command, Communications, Computers, Controls, Intelligence, Surveillance, & Radars (C4ISR) systems.  It includes items such as:

  • the navigation system,
  • internal communications,
  • external communications,
  • the degaussing system, and
  • combat related electronics (like radars, sonars, etc)
However, there are differences between the different systems relating to stuff like machinery control.  In the NES based system machinery control is included, but in the SWBS/ESWBS systems (and I beleive also the BSCI system) machinery control is part of group 200.

I may also refer to this as SWBS400 or w400.  In general I had intended both group 400  and SWBS 400  were meant to refer to the category and w400  is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v400 will refer to the vertical height of the center of the weight of the group 400 items and c400 will refer to the estimated cost of those items.

Group 500 - In all the weight classification methods noted above, group 500 generally refers to all components of the ship relating to the vessel's auxiliary systems.  It includes items such as:

  • heating,
  • ventilation,
  • air conditioning,
  • ballasting systems (but not the actual ballast water), and
  • fuel and lube oil transfer systems,
  • the distilling plant, and
  • fire fighting systems, etc.

I may also refer to this as SWBS500 or w500.  In general I had intended both group 500 and SWBS 500  were meant to refer to the category and w500  is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v500 will refer to the vertical height of the center of the weight of the group 500 items and c500  will refer to the estimated cost of those items.

Group 600 - In all the weight classification methods noted above, group 600 generally refers to all components of the ship relating to the vessel's outfit and furnishings.   It includes items such as:

  • paint,
  • floor coverings,
  • office and living space outfit,
  • workshops,
  • the galley, and
  • laundry, etc.
In the NES based system it also includes items such as:
  • anchoring and mooring systems,
  • ship's boats, and
  • replenishment at sea systems
However, in the SWBS/ESWBS system these items are considered part of group 500.

I may also refer to this as SWBS600 or w600.  In general I had intended both group 600 and SWBS 600  were meant to refer to the category and w600  is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v600 will refer to the vertical height of the center of the weight of the group 600 items and c600 will refer to the estimated cost of those items.

Group 700 - In all the weight classification methods noted above, group 700  generally refers to all components of the ship relating to the vessel's weapon systems.  It includes items such as:

  • guns,
  • missile launchers,
  • torpedo launchers, and
  • small arms & pyrotechnics, etc
However, it does not typically include items such as:
  • the ammunition for the ship's guns, or
  • the missiles for the missile launchers, etc

I may also refer to this as SWBS700 or w700.   In general I had intended both group 700 and SWBS 700 were meant to refer to the category and w700 is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly v700 will refer to the vertical height of the center of the weight of the group 700 items and c700 will refer to the estimated cost of those items.

Group F - Group F generally refers to all components of the ship relating to load items.  (In the NES based system I believe that these are instead referred to as Group 800).  This category includes items such as:

  • crew and their effects (Group F10),
  • ammunition (part of Group F20),
  • provisions & stores (Group F30),
  • fuel and other petroleum based liquids (Group F40),
  • other non-petroleum based liquids including drinking water and hydraulic fluids (Group F50),
  • cargo (Group F60), and
  • aircraft/helocopters (part of Group F20) & their associated fuel (part of Group F40), etc.
In the NES based system it also includes items such as:
  • some repair parts, and
  • some operating fluids (ie the stuff that is normally in a piece of equipment that is required to make it operate), etc
However, in the SWBS/ESWBS system these items are considered part of other weight groups.

I may also refer to this as SWBSFlds or wFlds.  In general I had intended both group F  and SWBS Flds  were meant to refer to the category and wFlds  is intended to actually refer to the weight of the items in this category. Eventually I intend to go back and try to ensure that I have been consistent, as I get the time.  Similarly vFlds will refer to the vertical height of the center of the weight of the group F  items and cFlds will refer to the estimated cost of those items.

Group M - Group M generally refers to all components of the ship relating to margins.  This category includes items such as:

  • margins to account for design uncertainty,
  • margins to account for growth during building,
  • margins to account for uncertainties in the weights of equipment that will be supplid by the government, and
  • margins to account for potential contract modifications during constuction of the vessel, etc
As I understand it, potential growth of the vessel in service is actually considered an allowance for growth and not necessarily a margin, and in US practice is called a Service Life Allowance.   However, in the SWBS/ESWBS system Service Life Allowance is included within Group M weights.

In the SWBS/ESWBS system these are categorized as:
  • M11 - Design and Building Margin
  • M21 - Preliminary Design Margin
  • M22 - Contract Design Margin
  • M23 - Construction Mod Margin
  • M24 - Government Furnished Material Margin
  • M25 - Service Life Allowance

Light Ship Weight - In general, groups 100  to 700  add up to give the vessel's basic light ship weight, though for early stage designs a number of margins (from Group M) are often added to address the uncertainty in early stage design numbers.  Also, if permanent ballast is required it is often added in as well.

Full Load Displacement - a vessel's full load displacement at delivery is equal to its light ship weight plus its loads (such as crew & their effects, fuel, stores, and munitions, etc).  Since it is realized that over the life of a vessel it will probably grow in weight, due to added systems, ship alterations, and other various additions, typically a service life allowance is also included to reflect the allowable expected growth of the vessel.  As such, the full load displacement of the vessel when new (at delivery) plus its service life allowance would be equal to the vessel's full load displacement at the end of its service life

Finally, I believe that for non-US vessels some of the terminology will be different (ie that may not call all the weights, margins, and allowances exactly the same) but in general I believe that the overall concepts are similar.

Propulsion Plant Configurations

In general, right now I have categorized propulsion plant types into the following groups:
  • Steam plants,
  • Diesel plants,
  • Combined Diesel or Gas Turbine (CODOG) plants,
  • Combined Gas Turbine or Gas Turbine (COGOG) plants, and
  • Combined Gas Turbine and Gas Turbine (COGAG) plants,
For Steam Plants - right now I have not made any attempt to separate out higher pressure steam plants from lower pressure plants.  In the future, however, I may look into this to see if the data I have shows any appreciable differences.

For Diesel Plants - right now I have not made any significant attempt to separate out higher speed engines from lower speed engines, though I have separated out some limited data for Fast Attack Craft, because they appear to be a little lighter than the other plants.  Additionally I haven't yet tried to separate out those plants that may have more than one engine geared together to a single propeller shaft from those which appear to have only a single engine per shaft.  In the future, I hope to look more closely at the data for all diesel plants to see if the data show any appreciable differences.

For Diesel or Gas Turbine plants - these are propulsion plants where the vessel is fitted with diesel engines sized to allow the vessel to attain its cruise speed, but which also have gas turbines installed for operating at higher speeds.  The way these plants are configured the diesel engines do not run when the vessel is operating at high speed due to the complexity of the gearing that would be required to allow the vessels to operate either with only the diesels at lower speeds, or with both the diesels and gas turbines together at higher speeds.  In this type plant high-speed operations are done solely on the Gas Turbines.  Hence these type plants are referred to as diesel OR gas turbine plants.

Recent advances in gear design and control system technology, however, have made it less difficult to design a plant where the diesels and gas turbines both can operate together at high speeds.  (I belive that the German F124 and the US Coast Guard's National Security Cutter are both configured this way).  Such a plant would be referred to as a Diesel AND Gas Turbine plant.

Similar to the Diesel or Gas Turbine plants, a Gas Turbine or Gas Turbine plant is a plant where the vessel is fitted with small gas turbines sized to allow the vessel to attain its cruise speed, but which also have larger gas turbines installed for operating at higher speeds.  The way these plants are configured the small gas turbines do not run when the vessel is operating at high speed.  I believe that several classes of Royal Navy vessels from the 1970s (such as the Type 21, Type 22, and Type 42) all have such type plants.

For Gas Turbine and Gas Turbine plants, these are vessels where the main propulsion plant consists of two or more gas turbines geared together such that lower speed operations can be conducted on only one engine, but for higher speed operations the other gas turbines are also clutched in.  I believe that the US Navy's FFG7, DD963, CG47/51, and DDG51 class vessels are configured this way.

Hullform Definition

Basic Coefficients - There are a number of coefficients and other parameters that can be calculated for a vessel to help give an simple defintion to its hull shape and form, or other characteristics.  These coefficients and parameters are often used in early stage resistance and powering estimates, seakeeping estimates, and stability calcs.  Some of these more common coefficients and parameters are defined below.

Block Coefficient - A vessel's Block Coefficient or Cb is one of the most basic coefficients that is used to help define and categorize a vessel's hullform shape.  Very simply a vessel's Block Coefficient  is the ratio of the volume contained within hull below the waterline as compared to the volume of a block whose length is equal to the vessel's Length between perpendiculars, and whose width is equal to the vessel's waterline beam, and whose height is equal to the vessel's mean hull draft.  As such Block Coefficient  is a handy term for relating the relative fullness of a ship's underwater hullform and is sometimes used in equations for estimating resistance, seakeeping and stability, etc.  Very full ships, like bulk carriers or large tankers can have block coefficients approaching 0.90 whereas faster vessels, including modern naval surface combatants can have Block Coefficients as low as 0.45, or so (as shown in a Figure on the
Initial Hullform Definition  page).

A ship's Block Coefficient can be easily visualized as shown in the figure below [Add Figure] where the volume of the ship's hull below the waterline is shown in Red while the block that would bound the ship's hull below the waterline is shown in Light Blue.   If you assume a standard density for salt water of 35 cubic feet per Long Ton (or 1 cubic meter per 1.025 metric tonnes) then:
  • Cb = Displacement * Density / (Lbp * Bwl * Tm)

However, you need to make sure that you are using Lbp, Bwl, and Tm, and not Loa, Boa, or Tnav otherwise you can get incorrect results.

Midships Area Coefficient - A vessel's Midships Area Coefficient or Cm is another basic coefficient, however whereas a ship's Block Coefficient  is a three dimensional term (based on length, beam, and draft) a ship's Midships Area Coefficient is two dimensional, being related only to a vessel's waterline beam and mean draft.  In general if you were to cut a ship in half at its mid-length, as shown in the figure below [Add Figure] then its Midships Area Coefficient would be equal to the ratio of the cross sectional area of the hull below the waterline at that section (shown in the Figure below in Red) divided by a box whose width is equal to the vessel's waterline beam and whose height is equal to the vessel's mean draft (as shown below in Light Blue).

For vessel's with a high Block Coefficient, it is not uncommon for the hull cross section at its midlength to have flat sides and a flat bottom, with a simple radius at the turn of the bilge (as shown for section (A) in the Figure below) [Add Figure].  Such a section shape is described as having "no deadrise".  Sometimes, however, even for ship's with a relatively full Block Coefficient, the the hull bottom may have a modest slope to it (as shown for section (B) in the Figure below).  This type section shape is typically described of as having "modest deadrise".  Finally, for ship's with relatvely low Block Coefficients it is not uncommon for the hull bottom to have a greater slope to it (as shown for section (C) in the Figure below) which is typically described of as having a "high deadrise".

For conventional type vessels, mid-length is relatively close to the vessel's section of maximum area, and sometimes the term Midships Area Coefficient and Maximum Sectional Area Coefficient are used kind of interchangeably, though in reality they can be a little different from each other.  For our purposes, a ship's Maximum Sectional Area Coefficient  is really of more importance to us, as it is more handy for use in defining hull shape, etc.  As such that is what I intend to use in my calculations, though for alot of the ships I have collected data on its not fully clear if what they have reported is Midships Area Coefficient or Maximum Sectional Area Coefficient and occassionally I know I will inadvertantly refer to Midships Area Coefficient when I really mean Maximim Sectional Area Coefficient.  As I noted above though, for the most part for conventional type vessels the two terms should be relatively similar, and hopefully it won't cause too much confusion.


Prismatic Coefficient - A vessel's Prismatic Coefficient or Cp is coefficient related to both a ship's Block Coefficient and its Maximum Sectional Coefficient.   If you were to take a ship's Maximum Sectional Coefficient and stretch it along the length of the hull, as shown in Light Blue  in the Figure below [Add Figure] you would have a prism whose volume is equal to Lbp * Bwl * Tm * Cm.

The vessel'sPrismatic Coefficient is then equal to the ratio of the volume of the hull below the waterline (shown in the Figure below in Red) divided by the volume of this prism (as shown below in Light Blue).  Mathematically, a vessel's Prismatic Coefficient  is equal to its Block Coefficient divided by its Maximum Sectional Coefficient or in other words;

  • Cp = Cb/Cm
Prismatic Coefficient is another means of measuring the relative fullness of a vessel's underwater hullform, and is used in a number of different resistance estimation techniques.  Specifically, there is a theory that relates a vessel's Prismatic Coefficient to relative peaks and troughs (or humps and hollows) in its resistance curve, as shown graphically on the Initial Hullform Definition  page.


Waterplane Area Coefficient -

Displacement Length Ratio -

Waterplane I nertia Coefficient -

Vertical Prismatic Coefficient -

Other Hullform Parameters

Half Entrance Angle -

Longitudinal Center of Buoyancy Location -

Wetted Surface Area -

Transom Area Coefficient -

Transom Beam Coefficient -

Transom Draft Coefficient -

Cubic Number -

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