Quote:
Originally Posted by sarabande
I'm sorry , but I cannot agree with that. Very rough figures:
Boat in slings has its mass supported by about 60 sq ft of webbing (two slings, say 18" wide, each with a contact area of 20 ft)
Boat in water has an underwater surface area of 50 x 6 ft each side, so say 600 sq ft.
So loading in slings is 30tons/60sqft = 0.5 tons/sqft
and afloat is 30tons/600sqft = 0.05tons/sqft.
A bit simplistic but given the description of the vessel's condition, I can understand why the yard abandoned the attempt to lift her out.
FWIW sheathing in ferro will add about 9600 lbs weight to the vessel (re-inforced concrete at 150lbs/cuft, and he will need about 1000sqft coverage at 2" depth). What that will do to the waterline and the metacentric height is looking a bit sad.
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I agree that my comparison with a boat supported by a bow/stern wave is a bit simplistic, but to be honest, so is your calculation! Most of the weight on the strops is under the keel, and, depending on hull shape, beam etc the strops are being deflected out to the sides and have nothing like the loading on the keel. I have no idea how you really calculate that loading or if there is actually any point. What we do know is that a wooden boat in good nick shows no signs of stress/distortion/damage when lifted, and the ones that do are a bit ropey!
I would guestimate that a half ton per sq/ft is possibly comparable to a large boat, say 20/30 tonner dropping off a wave into a trough while say, beam reaching in a strong breeze??
The reality is, unless its a houseboat, most want a strong vessel. We went aground at the end of last year on The Shrape, just off Cowes. No problem, politley told the lifeboat that came to rescue us that we were absolutley fine and waited for the tide. The point of this tale is that although we went aground in fine weather, when the wash from the deep sea tug hit us and lifted the boat about 3 feet,dropped us on the sand about 3 times, the boat was strong and did not leak a bit. No need for heavy weather to test the boat, and no need to worry either (well I did at the time!) You could try and calculate 18 tons dropping 3 feet onto high density sand at a speed of xxmph, with a keel width of 6" what would be the loading per sq/ft? Tricky and may I say almost impossible to do acurately!
If we had been in a delicate hull that had been sheathed or just kept for calm conditions we would have been in bits.
Remember how quickly the Maria Asumpta broke up when she touched the rocks?
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