Freeboard calculation

(At the ship design stage)

Introduction

Calculation of the freeboard is one of the basic and primaries that are to be done at the initial ship design stage, where it is not absolutely necesary to be quite exact. An approximate and quick calculation is therefore very suitable. After a brief concepts reminder I enclose a form where main ship dimensions are to be input by the user and freeboard results are displayed.

 Definitions

I recollect here some of the basic concepts relating to freeboard. A complete definitions and explanations list can be found here, International Convention On Load Lines, 1966 This Convention not only defines the calculations to be done but also specifies some stability, strength and construction requirements that the ships must comply with in order to deserve being assigned the minimum freeboards legally possible. The freeboard calculation that is presented here applies to merchant ships (except very small ones), but not to ships of war, fishing vessels and pleasure yachts. The freeboard is the distance measured vertically downwards amidships from the upper edge of the deck line to the upper edge of the related load line. There is a basic freeboard, that is named "summer freeboard (SF)", and is the required in normal navigating conditions, and it is the one calculated in the form shown below. This SF is what is controlled by the ship designer during the dimensioning process. But in special conditions and/or special ship types different freeboards apply and should be taken into account as the case may be. These special freeboards are those applicable in:

• winter
• winter in North Atlantic
• tropical area
• fresh water
• fresh water in tropical area
• with timber deck cargo
• sailing ships
• dredgers

Ship types

In connection with freeboard there are 3 ship types: Type A: Is designed to carry only liquid cargoes in bulk, has a high integrity of the exposed deck with only small access openings, has low permeability of loaded cargo compartments. These ships are to withstand the flooding of their compartments, according to rules. This mainly refers to tankers. Type B: All ships not complying with the provisions regarding type 'A'. This mainly refers to dry cargo vessels. Type B-60: Ships of Type B that comply with flooding resistance as Type A, as detailed in the rules, can get a freeboard intermediate between those of A and B. This mainly refers to large bulkcarriers. Also it has been considered the case of Type B ships with a timber deck cargo.

Input data and Results

• Number of input data has been reduced to a minimum, in order to simplify calculations. Only 4 data are obligatory: Length, Breadth, Depth, Block coefficient at design draft. Remaining, sufixed with an *, are optional, and taken equal to zero by defect, except draft that is estimated by the program.
• Maximum allowed length Lpp is 400 m and minimum 24 m.
• % Sheer curve, means the percentage of Lpp that is affected by the Upper deck sheer.
• % Sheer, means the percentage of actual sheer as compared with the rule standard one.
• For the 3 ship types, plus with timber deck cargo, the freeboard and draft are calculated in meters. Also minimum bow height required is displayed.

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Freeboard calculation Form

Length Lpp (m)
Breadth (m)
Depth (m)
Block coefficient @design draft
Draft design (m)*
Forecastle length (m)*
Poop length (m)*
% Sheer curve *
% Sheer *

Results	Freeboard(m)	Draft(m)
Type A
Type B
Type B-60
Timber deck cargo
Minimum bow height(m)