SUSAN BATSFORD, GRAPHICS EDITOR, TWITTER @SBATS1/QMI AGENCY

DISCOVERYDISCOVERY

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How do ships float?

Cruise ships are typically constructed with extra-strength steel and double hulls (a hull within a hull).

At 9:30 p.m. on Friday,Jan. 13, the cruiseship Costa Concordiastruck rocks 300metres off the islandof Giglio, near thecoast of Italy, rippinga hole in the hull. Thecaptain had deviatedfrom the deep waterroute usually taken bylarge ships to travelcloser to shore.

When a boattakes on enoughwater it willbecome tooheavy to remainbuoyant.

The wash from a ship closeto shore has no where to goand may have reboundedon the hull, causing theship to roll.

Fresh water hasa density of

1,000 kg/cubic metre.Ocean salt water has an

average density of1027 kg/cubic metre.The weight of sea waterdepends on a number of

variables, including the tempera-ture, the amount of salt (salinity)

and whatever other foreignitems may be present.

How

muc

h does water weigh?

The hull (body of the ship below themain deck) is typically very wide and

has a deep base line, or bottom.

railing deck

deck

deckframes(ribs)

stringerscargo hold

keel plate outer bottom

inner bottom

Round-bottom displacement hull

Rectanglar with rounded edges to dissipate drag.Allows large, heavy ships to move smoothly.Extremely stable. Passengers rarely feel anyrocking or side-to-side movement.These hulls move fluidly, but the resistance of thewater makes them extremely slow.

Vertical watertight dividers known asbulkheads are installed throughout theinterior of the hull. These dividers keepdamaged ships afloat by containing incomingwater into a compartment or compartments,preventing the whole ship from flooding.

Why did the Concordia sink?

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Tonnage: 114,500 GTDisplacement: 51,387 tonnes

Costa ConcordiaLength: 290.2mWidth: 35.5m

How do ships float?The Greek Mathematician and inventorArchimedes lived during the 3rd centuryB.C.According to history he was in thebath one day when he discovered theprinciple of buoyancy which is thereason why huge Greek ships weighingthousands of pounds could float onwater. He noticed that as he loweredhimself into the bath, the waterdisplaced by his body overflowed thesides and he realised that there was arelationship between his weight and thevolume of water displaced.

A ship will float whenthe weight of the waterit displaces equals theweight of the ship andanything will float if it isshaped to displace itsown weight of waterbefore it reaches thepoint where it willsubmerge.

Ship weight: 1,000 kg

Displaced water:1,000 kg

These objects are the same size (occupysame volume of space), but havedifferent densities.Bowling ball will sink. Balloon will float.The bowling ball weighs more than theweight of the water it displaces.

The bowling ball has a low surface areato weight ratio.

Different materials have different densities but can haveequal volume and therefore will have a different buoyancy.

A boat is essentially a hollow shell filled with air,meaning that it has a large surface area to weight ratio.Therefore, the boat will still be well above the surfacewhen it has displaced its equivalent weight in water.

Archimedes’ discovery

The Buoyancy PrincipleArchimedes’ buoyancy principle:

Bowling ball vs. balloon

A good portion of the interior of any boat is air. Theaverage density of a boat — the combination of thesteel and the air — is very light compared to theaverage density of water. So very little of the boatactually has to submerge into the water before ithas displaced the weight of the boat.