Freeboard Calculations Overview

Freeboard can be calculated for a wide variety of hull forms. The following freeboard limits are assessed:

• Type A

• Type B

• Type B-60

• Type B-100

• Timber and B+

The tabular freeboard and it's type are read from you dialog input according to the specific assignments defined by you (Type A, B ). Tabular freeboards at intermediate lengths are obtained by linear interpolation.

The tabular values are converted to basic values by applying the following corrections:

[1] Block coefficient (Cb) correction

Where the Cb value input exceeds 0.68.

[2] Special assignments

Reduction for B-60 and B-100 freeboards if selected by you.

[3] Hatch Correction

Increase for B+ freeboards if selected except where you have opted for timber freeboards, in which case no correction for hatched is admissible.

[4] Superstructures

Increase in any Type B ship of less than 100 meters in length where the total effective length of superstructure is less than 0.35L, where L is the freeboard length of the ship.

The steps 1 to 4 above give what is generally termed as the basic freeboard. Further corrections are as follows:

[5] Depth Correction

The freeboard is increased or reduced where the freeboard depth varies from the standard depth of L/15.

[6] Superstructures Corrections

The mean covered lengths of all superstructures (other than poop decks) and trunks are corrected, if the actual height differs from the standard height and/or the structure is inset from the ship's side. (The corrected length is called the effective length). Poop deck corrections are applied in the same way, except that the data input allows the mean covered length to be calculated where the deckhouses are inset.

Having obtained the total effective length of superstructures and trunks a tabular deduction is applied to the maximum permissible deduction for superstructures, according to the length of deck effectively covered and type of assignation required.

The value is taken from you input and for Type B ships, is corrected if the effective length of the forecastle is less than 0.07L. For ships less than 24 meters in length the maximum superstructure deduction is obtained by linear interpolation between 350 mm, the value at L=24 m, and 0 mm.

[7] Sheer Correction

The actual sheer values measured and corrected as indicated in the rules on measurement of sheer are included. If the poop or forecastle deck height is greater than the standard height, sheer credit is calculated.

The area under the actual sheer curve including any allowable credit, is calculated. The resultant virtual sheer is then compared with the standard sheer to assess whether any part in excess is in fact allowable or whether standard sheer ordinates are to be adopted.

[8] Other Corrections

Data values, input by you are added or subtracted from the basic freeboard to take account of deck thickness and position of the freeboard deckline in relation to the point of measurement.

The maximum draft which is the moulded depth plus sheer, plus forecastle height and stringer thickness, minus the required bow height, is calculated and also the maximum geometric draft, which is the freeboard depth minus the freeboard.

If the maximum draft is less than the maximum geometric draft, a bow height correction, equal to the difference between the two drafts, is added to the freeboard to give the final summer freeboard. The actual bow height is then the required minimum bow height minus the bow height correction. The maximum draft is therefore the summer draft.

According to the 1988 Protocol, all type 'B' freeboard, other than oil tankers, chemical tankers and gas carriers, shall have additional buoyancy in the fore end. In this case, the program will calculate the maximum allowable draft to give the minimum required projected area forward.