How much inherent buoyancy required to keep a boat afloat...

[m chappelow]

Think I would try get some of the inflatable sausage buoyancy floats as was said earlier in the thread that some of the old sailing dinghys use There was a few kicking around the Gare yard a few years back after we scrapped a few old dinghys

couple of old Atlantic self righting bags stuffed under the seats .

as your in Whitby though no one seine nets anymore you could try and get a few of the very large Dahn bouys the type seine netters used to use for marking the anchor they are about 6 ft in diameter but you could perhaps deflate them stuff them into a void and re inflate
If your working on the 3 tons thing suppose you have to visualise 3 cubic metres and see how much stuff you would need volume wise .

Are you thinking of going for hole in the hull and boat full of water scenario or just a good knee high swamping from a bad wave .
thing to remember is that whatever voids or the like your going to fill if it does sink especially with a steel hull will the deck joints take the upward forces of the bouyancy especially if its made from wood or grp.

[chewy]

The deck joints won't be a problem.
Was thinking of a hole in the hull.
Even if we don't have 3m3 We can use whatever we have as something is better than nothing.

[m chappelow]

If anyones intrested as we are on about bouyancy , just found some photos from about 18 years ago when we used Drums to lift the clubs old concrete slipway as i mentioned earlier in the thread,
biggest raft we made was using 20 drums that lifted near 5 tons ,

last pic taken the same weekend is of an old plywood sailing boat that was completely rotten ,
we tried to save the sinking boat after the owner had bought it cheap and given it a coat of paint whilst working up north ,
He was hoping to sail it home down the east coast from sunderland to southend distance of about 240 miles,
he only got about 15 miles before he ran it aground at Redcar knocking a keel out of place
Redcar lifeboat towed the sinking boat for him into Teesmouth to our slip where we lashed some drums along side to try keep it float whilst we got it onto a borrowed trailer,,that rotten we burned it the next morning and the owner caught the train home .
it was that badly rotten in places you could push your finger through the hull like wet cardboard .