2. Flotation

Question 1

Explain Archimedes Priniciple?

Answer 1

“any body, completely or partially, submerged in a fluid (liquid or gas) at rest is acted upon by an upward or buoyant force, which is equal to the weight of the fluid displaced by the body.”

Question 2

Archimedes Law of Floatation:

Answer 2

Based on Archimedes principal;

“for an object to float, it must displace a weight of water equal to its own weight.”

When a body is placed in a liquid, the immersed portion of the body will displace its own volume of the liquid. If the body displaces its own weight of the liquid before it displaces its own total volume then it will float in that liquid, otherwise it will sink.

See example attched: what happens to the draft and water level when throwing an anchor out while in a basin?

Question 3

Reserve Bouyancy

Answer 3

See image

Question 4

Explain TPC (TONNES PER CENTIMETRE IMMERSION)

Answer 4

If weights are loaded or unloaded, the draught will change.

The TPC is the number of tonnes needed to change the draft by one centimetre.

It is calculated by the Naval Architects and given in the Stability Data.

It will vary with draft and trim.

TPC = WPA x RD/100

When calculating TPC the RD is always 1.025 i.e. SW

You will either be given TPC in the question or you will extract the value from

the Hydrostatic Tables.

Question 5

1. Change in draft(d) (due to change in Relative Density) calculation =
2. Change in draft(d) (due to weight loaded/discharged) TPC calculation =

Answer 5

1. New draft(d) / Old Draft(d) = Old Denisty(RD) / New Density(RD)
2. TPC = WPA x RD / 100 = cm
3. 1 Change in draft = Weight loaded/discharged / TPC = cm

TPC (Tonnes Per Centimetre) immersion always given or calculated for Salt Water only. Read in hydrostatic tables.

Question 6

Explain FRESH WATER ALLOWANCE (FWA)

Answer 6

The Summer Load Displacement is the weight of the vessel when she is loaded to her Summer Loadline in SW.

If she is floating in FW with the same displacement (weight) she will have a deeper draft.

However, this would mean that her Summer Loadline would be submerged and it would look like the master was breaking the Loadline Regulations and loading the vessel too heavily. For cargo vessels this would mean that loading in FW ports would mean carrying less cargo.

To allow a vessel to load to her correct SW displacement in FW the Naval Architect calculates an allowance called the Fresh Water Allowance (FWA).

It is calculated as Summer Load Displacement divided by four times the TPC.

FWA = 𝑺𝒖𝒎𝒎𝒆𝒓 𝑳𝒐𝒂𝒅 𝑫𝒊𝒔𝒑𝒍𝒂𝒄𝒆𝒎𝒆𝒏𝒕 = mm

𝟒 𝐓𝐏𝐂

or

FWA = 1/48 x Summer load draft = mm

Question 7

Explain DOCK WATER ALLOWANCE (DWA)

Answer 7

A vessel will change draft moving from SW to FW. In fact floating in water of any density between that of SW and FW would result in a change in draft.

Water of density between SW and FW is known as Dock Water (DW).

To allow a vessel to load to her correct SW displacement in ports where the density is less than that of salt water but more than freshwater, the loadline may be submerged by a calculated amount.

This is the proportion of the total FWA depending on the difference between the density of the DW and the density of SW.

This is called the Dock Water Allowance and is represented by the formula:

DWA = 𝑭𝑾𝑨(𝟏𝟎𝟐𝟓 – 𝑫𝒐𝒄𝒌 𝑾𝒂𝒕𝒆𝒓 𝑫𝒆𝒏𝒔𝒊𝒕𝒚(DWD) = mm

𝟐𝟓

Question 8

What vessels are required to carry an ILLC, (International Load Line Certificate)? Who issues the certificate and how long is the certificate valid for?

Answer 8

Vessel’s greater than 24m in length and on international passages! The certificate is issued by a classification society in behalf of the MCA and is valid for 5 years.

Question 9

Master Formula for calculating displacement/W =

Answer 9

Displacement/W = L x B x d x Cb x RD = tons