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post #17 of Old 02-01-2013 Thread Starter
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Re: Sailors rescued 900 miles off shore..

Originally Posted by mad_machine View Post
Winter on the North Atlantic is one of the more unforgiving places to be. If you look carefully as most commercial ships, you will notice the draft numpers for and aft? Used to be (not sure if it still is) that the bottom most number would be labeled "WNA" as in... Winter North Atlantic.. Loading to this line would put the ship at her shallowest depth and giver her the most freeboard to survive the waves from the storms that churn this part of the ocean up during the winter.
Plimsoll Line

Standard load line marks
Load Line Mark and Lines and Timber Load Line Mark and Lines for power driven merchant vessels
Load Line Mark and Lines for commercial sailing vessels

The original "Plimsoll Mark" was a circle with a horizontal line through it to show the maximum draft of a ship. Additional marks have been added over the years, allowing for different water densities and expected sea conditions.

Letters may also appear to the sides of the mark indicating the classification society that has surveyed the vessel's load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register, GL for Germanischer Lloyd, BV for Bureau Veritas, IR for the Indian Register of Shipping, RI for the Registro Italiano Navale, NK for Nippon Kaiji Kyokai, and NV for Det Norske Veritas. These letters should be approximately 115 millimetres in height and 75 millimetres in width.[6] The Load Line Length is referred to during and following load line calculations.

The letters on the Load line marks have the following meanings:

TF – Tropical Fresh Water
F – Fresh Water
T – Tropical Seawater
S – Summer Temperate Seawater
W – Winter Temperate Seawater
WNA – Winter North Atlantic

Fresh water is considered to have a density of 1000 kg/m³ and sea water 1025 kg/m³. Fresh water marks make allowance for the fact that the ship will float deeper in fresh water than salt water. A ship loaded to her Fresh Water mark in fresh water will float at her Summer Mark once she has passed into sea water. Similarly if loaded to her Tropical Fresh water mark she will float at her Tropical Mark once she passes in to sea water.

The Summer load line is the primary load line and it is from this mark that all other marks are derived. The position of the summer load line is calculated from the Load Line Rules and depends on many factors such as length of ship, type of ship, type and number of superstructures, amount of sheer, bow height and so on. The horizontal line through the circle of the Plimsoll mark is at the same level as the summer load line.

The Winter load line is one forty-eighth of the summer load draft below the summer load line.

The Tropical load line is one forty-eighth of the summer load draft above the summer load line.
The Fresh Water load line is an amount equal to \tfrac{\triangle}{40T} centimetres above the summer load line where \triangle is the displacement in metric tonnes at the summer load draft and T is the metric tonnes per centimetre immersion at that draft.
In any case where \triangle cannot be ascertained the fresh water load line is at the same level as the tropical load line.
The position of the Tropical Fresh load line relative to the tropical load line is found in the same way as the fresh water load line is to the summer load line.
The Winter North Atlantic load line is used by vessels not exceeding 100 metres in length when in certain areas of the North Atlantic Ocean during the winter period. When assigned it is 50 millimetres below the winter mark.[7]
Timber load line marks

Certain vessels are assigned Timber Freeboards but before these can be assigned certain additional conditions have to be met. One of these conditions is that the vessel must have a forecastle of at least 0.07 the length of the vessel and of not less than standard height, which is 1.8 metres for a vessel 75 metres or less in length and 2.3 metres for a vessel 125 metres or more in length with intermediate heights for intermediate lengths. A poop or raised quarter deck is also required if the length is less than 100 metres. The letter L prefixes the load line marks to indicate a timber load line.[7] Except for the Timber Winter North Atlantic freeboard the other freeboards are less than the standard freeboards. This allows these ships to carry additional timber as deck cargo, but with the facility to jettison this cargo.

The letters on the Timber Load line marks have the following meanings:

LTF – Timber Tropical Fresh Water
LF – Timber Fresh Water
LT – Timber Tropical Seawater
LS – Timber Summer Seawater
LW – Timber Winter Seawater
LWNA –Timber Winter North Atlantic

The Summer Timber load line is arrived at from the appropriate tables in the Load Line Rules.[8]

The Winter Timber load line is one thirty-sixth of the Summer Timber load draft below the Summer Timber load line.

The Tropical Timber load line is one forty-eighth of the Summer Timber load draft above the Summer timber load line.

The Timber Fresh and the Tropical Timber Fresh load lines are calculated in a similar way to the Fresh Water and Tropical Fresh water load lines except that the displacement used in the formula is that of the vessel at her Summer Timber load draft. If this cannot be ascertained then these marks will be one forty-eighth of the Timber Summer draft above the Timber Summer and Timber Tropical marks respectively.[7]

The Timber Winter North Atlantic load line is at the same level as the Winter North Atlantic load line

Last edited by aa3jy; 02-01-2013 at 01:55 PM.
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