Frequently Asked Questions

Table of Contents


How does a steel boat float?

Steel, iron or aluminum boats float because the water displaced by the shape of the hull equals the weight of the metal hull and whatever is aboard. If you squeezed a baking pan into a cube of metal it would sink because it does not displace it own weight in the water. Left in the pans original shape, it would not only float but would float if a brick was added to the pan because the weight of  the two objects would  still be less than the maximum displacement of the pan.



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What is meant by stability on a boat?

A boat is designed so it is stable in the water or it will tip and capsize. The weights and free surface liquids must be contained, distributed and separated on a boat so their influence will not upset the balance.  If we use the baking pan again we can illustrate how this works.

BrickPan2 BrickPan3
If the brick is moved to one end of the pan the freeboard or amount of the pan above the water is dramatically reduced and the pan is in danger of swamping and sinking. If the brick is moved back and stood on end its gets even worse because more weight is at the end and it is higher. This will act as a lever in wave action and put the lip under.


panHeight1 panHeight2
Looking head-on to the pan in wave action the brick weight hasn't really shifted much and will not have much of an effect on stability. In this illustration the brick (weight) has been raised on a bracket. This places the weight farther  to the pans low side increasing the lean in wave action.
The momentum of the weight adds to the effect.

Free Surface

WaterPan WaterPan2
Did you ever try to carry a baking pan half full of liquid? It is very difficult because the wave action of the free surface liquid makes it spill from the pan as you walk and you can feel the weight of the water pulling from side to side. This is what could happen to a boat at sea with large partially full tanks. If you separated the liquid into smaller compartments similar to an ice cube tray the free surface is minimized or contained and it is easy to carry as there is virtually no weight shift. Small, full tanks actually help to stabilize a boat in wave action.

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What is the difference between a barrel and a drum?

The short answer is: A barrel is a measurement of 42 gallons and a drum is 55 gallons in the marine and oil industry.
Barrels were usually made of wood staves and depending on the liquid carried in them came in various standard sizes. Drums are made of steel and can be used for any type of liquid but in the marine field are usually used to deliver lubricating oils and/or anti-freeze.

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What is dwt?

Deadweight Tonnage. Sometimes termed deadweight carrying capacity, is the difference between the light and loaded displacements of a ship or barge. The Deadweight tonnage comprises the cargo, stores, ballast, fresh water, fuel oil, passengers, crew and their effects.

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What is Gross Tonnage?

Gross tonnage is a measure of the internal capacity of a ship, tug or barge. It is a cubic or space measurement of all areas of a vessel with some allowances or deductions for exempt spaces such as living quarters.

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What is Net Tonnage?

Net tonnage is a measure of the internal capacity of a ship, tug or barge's cargo space volume only.

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What is light weight tonnage?

Lightweight tonnage is the weight of the ship itself. The hull, machinery and all its fittings. Fuel, stores and crew are not included.

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What is the reason for the Plimsoll Mark (Load Line)?

Samuel Plimsoll, a British merchant and shipping reformer campaigned for this mark to prevent ships from being overloaded and lost at sea. His idea became law in Britain in 1875. The mark, a circle with a horizontal line through the center was painted on both sides of a ship. In order to keep their insurance a ship could not load deeper then this line. Until then a large number of  ships would sink every year as owners and captains overloaded their ships to stay competitive. An International Load Line was adopted by 54 nations in 1930 and amended in 1968.

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What do the letters stand for on the Load Line?

The letters on the horizontal line through the center of the circle stand for the classification society, i.e.; AB is the American Bureau of Shipping, LR would be for Lloyds Register of Shipping, etc.  The letters on the ladder lines to the side of the circle are as follows; TF is Tropical Fresh Water, F is Fresh Water, T is Tropical, S is Summer Season, W is Winter Season and WNA is Winter North Atlantic. Depending on what waters the ship is in at the time of loading, it cannot load above the corresponding mark.


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    R. Mattsson