At small angles of inclination, a vessel’s stability is indicated by her GM or metacentric height. But when a vessel encounters conditions that cause her to roll, heave, pitch, sway and yaw excessively, for example in heavy or rough seas, she needs reserve buoyancy to counter the effects of these forces. Reserve buoyancy is the volume of enclosed spaces above the waterline and is measured by freeboard. Generally, more reserve buoyancy is required in the tempestuous North Atlantic in winter than in the tropics.
In the relatively tranquil seas defined as the “Summer Zone,” less reserve buoyancy is needed to offset the environment than in waters described as “Winter” or “Winter North Atlantic,” when storms and heavy seas are more common. Loadlines, designed to indicate the appropriate amount of freeboard, vary depending on the season or the service. If a vessel is loading in a tropical zone, passing through a summer zone and headed for a winter zone, she will have to meet the freeboard requirements for each of those zones upon entry. Before leaving the dock, officers must determine the initial freeboard necessary to meet the minimum requirements of each zone.
Another factor must be considered when loading in the fresh or “fresher” water of a port. Fresh water, which has a specific gravity of 1.00, is less buoyant than salt water (specific gravity 1.025 or 1.026). Dock water, while rarely fresh, is still less buoyant than the salt water for which the vessel is headed. Because of this density difference, a vessel may submerge her marks in port (have less freeboard) for by the time she enters salt water, buoyancy will have raised her higher in the water. Fresh Water Allowance (FWA) is the number of inches by which the mean draft changes when a ship passes from salt water to fresh water, or vice-versa, when the ship is loaded to the Summer displacement. The hydrometer measures the specific gravity of the water.
Here two typical USCG examination questions ask for the minimum freeboard required to meet the requirements of each zone. In solving these problems, it is easiest to calculate the minimum freeboard required at departure to meet the requirements of each zone. The largest freeboard will be the minimum required at departure.
|You are loading in a port subject to the tropical load line mark and bound for a port subject to the winter load line mark. You will enter the summer zone after steaming one day, and you will enter the winter zone after a total of eleven days. You will consume 33 tons of fuel, water, and stores per day. The hydrometer reading at the loading pier is 1.004, and the average TPI is 46. What is the minimum freeboard required at the start of the voyage? (See table below.)|
|A. 85 inches|
|B. 80 inches|
|C. 78 inches|
|D. 82 inches|
|Freeboard from Deck Line||Load Line|
|Tropical||81 inches (T)||7 inches above (S)|
|Summer||88 inches (S)||*|
|Winter||95 inches (W)||7 inches below (S)|
|Fresh Water Allowance||6 inches|
The first step is to calculate the dock water allowance. The ship is not loading in fresh water (SG 1.000) so she cannot submerge her marks the full FWA of 6 inches. Instead the following formula is used to determine how much she can submerge her marks, in this case, in slightly brackish water with a specific gravity of 1.004.
|Dock Water Allowance (DWA) =||Fresh Water Allowance (FWA) x||SG SW (1.025) – SG DW (1.004)|
|SG SW (1.025) – SG FW (1.000|
In this case, the FWA (6 inches) x .021/.025 = 5.04 inches. 5.04 inches can be subtracted from the required freeboard for each zone.
The next step is to calculate the amount the vessel will come up in the water due to consumption of fuel, water and stores enroute to the zone.It takes the vessel one day to reach the Summer Zone. She consumes 33 tons of fuel, water and stores per day. If the TPI (tons per inch immersion) is 46, then the vessel will rise 33/46 or 0.7 inches getting to the Summer Zone. It takes her eleven days to get from the dock (Tropical) to the Winter Zone. 11 days at 33 tons of “burn-off” per day amounts to 363 tons. 363 divided by the TPI of 46 equals another 7.9 inches she’ll pop up in the water by the time she reaches Winter.
Tropical Summer Winter
81.00 inches 88.00 inches 95.00 inches
- 5.04 inches -5.04 inches -5.04 inches
-0.72 inches -7.89 inches
75.96 inches 82.24 inches 82.07 inches
While the vessel could sail from tropical with a freeboard of 75.96 inches, by the time she reached the subsequent zones she would not have sufficient freeboard to meet the summer or the winter zone requirements.
To meet the load line requirements, the vessel would have to sail with a minimum freeboard of 82.07 inches of freeboard. The answer is (D).
|You are loading in a port subject to the winter load line mark and bound for a port subject to the tropical load line mark. You will enter the summer zone after steaming four days, and you will enter the tropical zone after a total of twelve days. You will consume 39 tons of fuel, water, and stores per day. The hydrometer reading at the loading pier is 1.025, and the average TPI is 49. What is the minimum freeboard required at the start of the voyage?|
|A. 90 inches|
|B. 87 inches|
|C. 80 inches|
|D. 77 inches|
|Freeboard from Deck Line||Load Line|
|Tropical||76 inches (T)||7 inches above (S)|
|Summer||83 inches (S)||*|
|Winter||90 inches (W)||7 inches below (S)|
|Fresh Water Allowance||10 inches|
In this problem, the voyage begins in a winter zone, will pass through summer on the way to tropical. The DWA = 10 inches x 0/0.25 = 0. The water is loading in salt water and therefore no allowance can be made for increasing buoyancy as she heads out to sea.
She’ll burn 4 days x 39 tons of consumables/day enroute to the summer zone. Given a TPI of 49, she could submerge her marks 4 x 39/49 or 3.18 inches to meet the summer requirements.
She’ll burn 12 days x 39 tons of consumables on her way from the winter zone to the tropical zone. 12 x 39 divided by the TPI of 49 means she could submerge her marks 9.55 to be be in compliance with the tropical requirements.
|90.00 inches||83.00 inches||76.00 inches|
|- 0.00 inches||-0.00 inches||-0.00 inches|
|-3.18 inches||-9.55 inches|
|90.00 inches||79.82 inches||66.45 inches|
But she cannot leave the dock with anything less than the 90 inches of freeboard required for winter. The answer is (A) 90 inches.
A final note: Remember these questions are concerned with the vessel’s freeboard (which is above the waterline) and not her draft (below the waterline).