Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 05/08/2013 12:37:46
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As ships, such as Victory, fired their canons, the ship must have got lighter. I wonder how they compensated for this ?
The ship would now rise above the intended water line ? Did they take in water as ballast after firing many canon balls ? Or maybe the loss of weight did not make much difference ?
Best Regards, Nicholas "Nicky" Oelofse (Johannesburg)
Asked by Nicky Oelofse
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Some math may help you answer your own question.
If you mean the HMS Victory commanded by Lord Nelson at Trafalgar, you can go to HMS Victory in Wikipedia, and using values given there, compute its displacement in pounds and compute the weight of cannonballs if all its guns were fired (to make this simple, let's ignore the weight of the gunpowder used). Then compare the two values.
Tell us what you get for numbers. The most exciting part of this is — do you think you can answer your own question?
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I would say it depends on how one built. If we take Vasa, that sunk in a harbor under light winds, her construction, width relative height, and weight, should have made her a very dangerous platform for shooting, even though the full weight of one sides cannonade only weighted 267 kg.
"What made her arguably the most powerful warship of the time was the combined weight of shot that could be fired from the cannon of one side: 588 pounds (267 kg). This was the largest concentration of artillery in a single warship at the time, and it was not until the 1630s that a ship with more firepower was built. This large amount of naval artillery was placed on a ship that was quite small relative to the armament carried. By comparison USS Constitution, a famous Napoleonic era frigate built 169 years after Vasa, had roughly the same firepower, but was over 700 tonnes heavier"
"Vasa was one of the earliest examples of a warship with two full gun decks, and was built when the theoretic principles of shipbuilding were still poorly understood. The safety margins at the time were also far below anything that would be acceptable today. Combined with the fact that 17th century warships were built with intentionally high superstructures (to be used as firing platforms), this made the Vasa a risky undertaking."
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For the USS Constitution, I computed the ratio of a broadside throw weight to the ship's displacement to be about 1/6,250.
Maybe you could do the same for the Vasa and Nicky could do it for the HMS Victory. Remember that a broadside is half of the cannons. What's convenient is that, at those times in history, ship's cannons were rated by their throw weight (32-pounders, 24-pounders, etc), and Wikipedia has all the necessary info, so it's easy to compute.
On a somewhat related point, I have read that the US Navy determined that when their largest battleships fired a broadside, it moved the monstrous ship sideways 10 feet!
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So, in essence, the fraction of the ship's overall weight that is accounted for by ammunition is extremely small and so the change in displacement following a battle will be negligible.