sinks like a stone

[gontofe]

If, for whatever reason (bombproofing?), I wanted to build a boat out of concrete, how big would it have to be in order to displace enough water to stay afloat?

[PhilLew]

I don't know the ratio of weight to displacement but it has already been done! there was a company in Granton near Edinburgh that used to make concrete boats, I think they were a subsiduary of Pioneer Concrete :-)

[PhilLew]

Try WWW.FERROBOATS.COM for more info

[sddsddean]

Any size you want. We hired a ferroconcrete cruiser on the River Shannon about 30 years ago. It was about 40 foot long and 10 feet beam. The advantage was, that when Mum put it on the rocks (I'm saying nothing about women drivers!) and put a 18"dia hole in the bottom, it was repaired at the nearest slipway by simply knocking up a bit of concrete and slapping it on! It got us back to the hire base(about 3 days away) no problem and Mum was briefly famous up and down the river!! She also liked to go to The Boat Show where manufacturers would advertise their concrete boats as unsinkable. She would say they weren't, 'cos she had almost sunk one!!