Boat Building

[Monkzie]

Why do they not build boats out of cork? Or balsa wood?

[Mudman]

Monkzie. Get a cork. Put it in a bowl of water, Make a few 'ripples' with your hand and try to imagine yourself (scaled down), standing on the cork...

It's a 'stability' issue... Though the cost plays a part also.. A 'conctrete' barge would cost much less than a 'balsawood' vessel (of the same strength).

[Narolines]

In some parts of the world they do. The Indians of Brazil used (probably still do use) balsa boats and rafts as regular river transport - in fact the word "balsa" actually means "float" or "raft".

[ll_billym]

And military boats need to be able to take a few shells before sinking.

[gen2]

Buoyancy in a boat comes from two sources.

1) The buoyancy of the material itself.
Balsa contains a lot of air spaces so initially it has a high level of buoyancy that is suitable for raft building. Unfortunately, these air spaces are interlinked or only weakly separated so fairly soon the wood becomes waterlogged and sinks. Read Thor Heyerdahl's account of the Kon-Tiki expedition for a description of this. He also describes how the wood easily chaffs away with constant movement.

2) The buoyancy from the shape displacing water.
A rigid watertight hull is needed for a conventional boat shape. Whatever material is used has to be strong enough to withstand wave action and support internal structures without leaking. Neither cork nor balsa could do this other than in the short term.

[stanleyman]

They don't need to, a steel or even heavier concrete ship has tremendous displacement and bouyancy. My 25 mtr steel barge needs 40 tonnes of concrete ballast just to keep the prop in the water. A ship or boat with air/water tight compartments cannot sink, apart from the Titanic of course!

[stevie21]

They've got to be the weakest most easily bent/broken woods that I can think of.
Waves etc. would break the boats up.

[Ralph]

It's mainly a stiffness thing. Cork or balsa may be strong enough to use as a boat building material but it wouldn't be stiff enough. You would be naturally limited to a very very short boat because anything bigger would bend and warp out of shape.

Balsa is used as a core material in composite boat building, where glass fibres wetted out with resin are laminated on either side of it. In this case, the balsa is used because it is very light (compared to the glass) and is used predominantly to move the glass skins apart, making a very stiff and fairly light panel.