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Effect of weight on boat speed
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NickA View Drop Down
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    Posted: 16 Nov 20 at 9:00pm
I mostly sail my super light 3000 single handed. Not just the reduced weight that gets it planing early, also the fact that the crew weight is all on a trapeze. Low weight but lots of righting moment.

Hence why my club has me sailing a boat designed at a Py of 1032 (and currently languishing at 1064) off 978.

On the other hand .. F5 or more and I'm screwed.
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Posts: 1760 		Post Options Post Options   Quote maxibuddah Quote  Post ReplyReply Direct Link To This Post Posted: 06 Nov 20 at 11:06am
Originally posted by Rupert

It's also fairly clear that movable ballast has a different effect on boat speed than fixed, or Phantom sailors and water ballast wouldn't exist.

Phantom sailors only exist to ensure Ginsters remains in business.

Moveable ballast is to improve the righting movement against the rig though isn't it? Simple levers surely? 
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 06 Nov 20 at 10:14am
Originally posted by A2Z

Good grief. How can a STEM education site get it so wrong?

Dead Dead Dead

It’s very simple, the boat will sink until the weight of the boat divided by its submerged volume equals the density of the water it is sat in.  

It is indeed very simple. A floating object displaces it's weight of water a sunk object displaces it's volume of water. for an object to float it's volume must be greater than that of the water it displaces.

Density is just a measure of how much an object weighs relative to it's volume. Wood floats 'cos 1 cc of it weighs 0.7g so it only has to displace 0.7cc of fresh water. Lead sinks 'cos 1cc of it weighs 11+g but can only displace 1cc of water.

Ice floats 'cos 1cc of it weighs around 0.9g (or put another way 1g of it takes up slightly more volume than 1g of water, that's why your pipes burst when they freeze).


Edited by Sam.Spoons - 06 Nov 20 at 10:15am
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 06 Nov 20 at 8:07am
When Brunel built the Great Britain, people said she would sink because iron doesn't float. I had hoped we had a better understanding nowadays, but reading this thread I'm not sure we do, entirely.

It's also fairly clear that movable ballast has a different effect on boat speed than fixed, or Phantom sailors and water ballast wouldn't exist.
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Posts: 3420 		Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 7:31pm
https://courses.lumenlearning.com/physics/chapter/11-7-archimedes-principle/
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 7:22pm
Good grief. How can a STEM education site get it so wrong?

It’s very simple, the boat will sink until the weight of the boat divided by its submerged volume equals the density of the water it is sat in.  
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Posts: 3420 		Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 7:05pm
I think they mean, the amount of air (buoyancy) to weight of object, to make it float, ie you will need more air (buoyancy) for lead to float than wood.
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 6:58pm
That's wrong too..... It says 

How does buoyancy relate to density?

If a block of wood measuring one cubic centimeter (1 cm x 1 cm x 1 cm) is placed in a container of water, the amount of water displaced will equal the weight of the block of wood. But what about if a block of the same size is made of lead? Lead has a much higher density than wood. If a one cubic centimeter block of lead is placed in a container of water, the amount of water displaced will equal the weight of the block of lead.


The amount of water displaced by the block of lead (weighing 11.34g) will be exactly 1cc and will weigh 1g, so it will equal the volume of the block of lead but not it's weight  Wacko
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Posts: 3420 		Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 6:49pm
More specific to why ships float
https://letstalkscience.ca/educational-resources/stem-in-context/why-do-ships-float
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Posts: 3400 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 20 at 6:48pm
Yup, that's pretty much what I was saying.
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