Why a iron sink in water but large ship float in water?

iron doesn't sink in water - if put aboard boat!

let's assume mass of iron 5kg.

because density of (fresh) water 1000 kg/m³, 5 kilogrammes of downward force, in order float (any material), must displace 5 litres of water. if not, sink. iron therefore displace 5 litres of water when 'afloat', aboard boat, own volume (which little on half litre - iron compact: 'dense') of water when submerged, overboard. in case, clearly, sink.

long can devise way of making material (the iron) displace volume of water equal in mass itself, can float it.

in other words, either put iron onto boat, displace 5 kilogrammes more water, or reshape iron object has, @ least, 5 litres of volume - , float.
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edit / defence against (clearly) paranoid spreader of myth: poster still unable distinguish between hydrostatics , dynamics. stationary vessel @ rest can only, quaintly put "displace more water weight weight of ship", means of unseen hand of god or fingers of little pixies.

ie: magic.

[hot tip ... don't need 'reserve buoyancy' float. not stay afloat! (not sink). in other words, don't need hydrodynamics answer question. (the cause of myth).

forge 5kg iron sealed, 5 litre 'bottle' , float. in 'not sink'.

why?

because density of (fresh) water 1000 kg/m³ ... density of 5kg / 5 litre iron bottle is.]

;)

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