Page 357 - Ship Construction.DJ Eyres 6Ed

Ch31-H8070.fm Page 346 Wednesday, October 18, 2006 7:06 AM
346 Ship Construction
(e) Sheer Correction The area under the actual sheer curve is compared
with the area under a standard parabolic sheer curve, the aft ordinate of
which (S ) is given by 25(L/3 + 10) mm and the forward ordinate (S ) by
A
F
2S mm. Where a poop or fo’c’sle is of greater than standard height, an
A
addition to the sheer of the freeboard deck may be made.
The correction for deficiency or excess of sheer is the difference between
the actual sheer and the standard sheer multiplied by (0.75 – S/2L) where S
--- ใช้เพื่อการศึกษาเท่านั้น---
is the total mean enclosed length of superstructure. Where the sheer is less
than standard the correction is added to the freeboard. If the sheer is in
excess of standard a deduction may be made from the freeboard if the
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
superstructure covers 0.1L abaft and 0.1L forward of amidships. No deduc-
tion for excess sheer may be made if no superstructure covers amidships.
Where superstructure covers less than 0.1L abaft and forward of amidships
the deduction is obtained by linear interpolation. The maximum deduction
for excess sheer is limited to 125 mm per 100 m of length.
If the above corrections are made to the basic freeboard, the final calcu-
lated freeboard will correspond to the maximum geometric summer draft
for the vessel. The final freeboard may, however, be increased if the bow
height is insufficient, or the owners request the assignment of freeboards
corresponding to a draft which is less than the maximum possible. Bow
ึ
height is defined as the vertical distance at the forward perpendicular between
the water-line corresponding to the assigned summer freeboard and the top
of the exposed deck at the side. This height should not be less than the
values quoted in the Convention.

MINIMUM FREEBOARDS The minimum freeboard in the summer zone
is the freeboard described above; however, it may not be less than 50 mm.
In the tropical and winter zones the minimum freeboard is obtained by
deducting and adding respectively 1/48th of the summer moulded draft.
The tropical freeboard is, however, also limited to a minimum of 50 mm.
The freeboard for a ship of not more than 100m length in the winter North
Atlantic zone is the winter freeboard plus 50 mm. For other ships, the winter
North Atlantic freeboard is the winter freeboard. The minimum freeboard
in fresh water is obtained by deducting from the summer or tropical free-
board the quantity:
Displacement in SW
------------------------------------------------- mm
4 × TPC

where TPC is the tonnes per cm immersion at the water-line, and displace-
ment is in tonnes.

TIMBER FREEBOARDS If a vessel is carrying timber on the exposed
decks it is considered that the deck cargo affords additional buoyancy and