PRINCIPLES OF STABILITY
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Ranger Hope © 2008, contains edited material courtesy of A.N.T.A publication’s.
1. Explain Archimedes’ principle.
2. Explain the meaning of the term ‘displacement’.
3. Explain the meaning of the term ‘deadweight’.
4. Explain the meaning of the term ‘Fresh Water Allowance’.
5. Explain the meaning of the term ‘Tonnes per Centimetre Immersion’.
6. Explain the meaning of the term ‘Trim.
7. Explain the meaning of the term ‘Reserve Bouyancy’.
8. Why does the draft of a vessel change when it sails from fresh water into salt water?
9. The summer loadline mark should not be submerged
a) in a Tropical zone
b) in a Summer zone
c) in Port
10. A seasonal zone is one that
a) is associated with good weather all the year round
b) is associated with bad weather all the year round
c) changes its name at different times of the year
11. A vessel loaded to full draft in a winter zone will have a waterline
a) at the top of the winter loadline mark
b) at the bottom of the winter loadline mark
c) at the top of the plimsoll mark.
12. The letters on either side of the loadline disc indicate
a) the name of the vessel
b) the initials of the vessel’s owner
c) the initials of the survey authority
13. Draft is the distance from
a) the keel to the waterline
b) the keel to the freeboard deck
c) the keel to the highest point on the vessel.
14. The specific gravity of a substance is the ratio between its density and
a) the density of steel
b) the density of fresh water
c) the density of salt water
1. Archimedes’ Principle states that when a body is wholly or partially immersed in a fluid it appears to suffer a loss in mass equal to the mass of the fluid it displaces.
2. Displacement is the weight of a floating vessel. It equals the weight of the water ‘displaced’ by the vessel.
3. The difference between load displacement and light displacement is called deadweight. Things such as fuel, fresh water, crew, gear, cargo, fish, etc., are all items of deadweight.
4. In sea water, the underwater portion of the hull will be smaller, that is the vessel will not sink as far as it will in fresh water, and the draft in sea water will be less than the draught in fresh water. The difference between the two drafts is called the fresh water allowance (FWA).
5. The amount of weight which will sink the vessel 1 cm deeper in the water, that is, the weight which will increase the draft by 1 cm is called the tonnes per centimetre immersion (TPC).
6. The difference between the draft aft and the draft forward, is called the trim.
Trim = Draft Aft - Draft Forward
7. The amount of freeboard which a vessel has, is a measure of the amount of buoyancy which is left above the water line, to support the vessel in case of bad weather or damage, etc. This buoyancy is referred to as reserve buoyancy. Every vessel is designed to operate with a certain freeboard which provides for safety of vessel and crew.
8. Imagine a vessel, floating first in sea water and then in fresh water. It will need to displace more cubic metres of fresh water to balance its weight, than it would in sea water, because each cubic metre of sea water balances more weight than each cubic metre of fresh water. The number of cubic metres displaced determines the size of the underwater portion of the hull.
In sea water, the underwater portion of the hull will be smaller, that is the vessel will not sink as far as it will in fresh water, and the draft in sea water will be less than the draught in fresh water.