Nicky Oelofse asked:
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)
Dave - HMS Victory, I've looked it up and she carried about 27 tonnes of ammunition. So, if you fired all that off, she'd be be 27 tonnes lighter which seems quite a lot. But she was a ship who, I think her displacement was actually several thousand tonnes. So, it’s quite a small proportion of the weight of the ship. And also, that's quite a small effect compared to other things which she was carrying. She'd be carrying maybe even 3 or 400 tonnes of water and 50 tonnes of beef, another 50 tonnes of pork. So, she'ds probably carrying 500 to 600 tonnes of stuff in her hold.
Chris - They're not firing the pork.
Dave - No, they're eating that. So, the difference from when she started off on a long voyage and when she finished, whether or not you'd been firing any pork out of the canons, it would be far, far greater than whether she'd had a battle or not.
Chris - I tell you what though, I know that Rolls Royce with whom we’ve done some work to do with their jet engines and how they work. They do experiments where they use chickens which they fire into engines to test bird strike. And so, they do use sometimes frozen chickens because occasionally, if you get a bird that's been very, very cold and it suddenly gets sucked up into an engine then it’s a really good model of what will happen if an engine ingests a bird. So, I suppose you could argue that sometimes frozen things could be quite good weapons because it demolishes a jet engine when it goes into it. The thing just falls to pieces.
Dave - I can quite imagine, but yeah. Now back to the original question... I think certainly, when they were very, very empty, they would take on ballast and lumps of rock and things, but the amount of weight in the cannonballs was really quite small compared to the – it would just float slightly higher and it should probably sail a bit faster and no one would really notice.
Some math may help you answer your own question.
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.
For the USS Constitution, I computed the ratio of a broadside throw weight to the ship's displacement to be about 1/6,250.
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. chris, Fri, 19th Jul 2013