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Old 08-09-2015, 18:34   #1
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Confusion like a Newfoundland fogbank

When I try and compare multihull boats to one another I really believe that weight is something that I use to judge the boats by. I know underwater appendages, bow fineness, flatness of the outruns, width of hulls, on and on are all important for speed. How much rig does she carry? How much sail area and whatever for headsails?

But I seem to be getting confused lately about true weight of a boat (weight of everything carried on the boat + weight of the boat itself) versus the displacement of the boat.

I understand the idea of something the shape of a brick being placed into a bucket of water and a certain defined (and measureable) amount of water is 'displaced' by that brick. That water that is displaced is pushed out of the bucket by the size and weight of the brick. That water that was displaced has a certain weight. So, please educate me...I beg you...how does displacement correlate to the weight of the boat?

If the brick is hollow and made of plastic versus a brick of the same dimensions is made of gold...versus a brick that is same dimensions with sand inside of it....help me...
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Old 08-09-2015, 19:39   #2
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Re: Confusion like a Newfoundland fogbank

The buoyant force on an object is upward and equal to the weight of the volume of water displaced by the object. A boat with weight W will sink into the water until the volume of water displaced, or occupied by the hull, has an equivalent weight W. As weight is added to the boat; crew, equipment, provisions; the boat will sink lower into the water until the weight of the additional water displaced is equal to the weight added to the boat. In other words, 10,000 lbs of boat will displace 10,000 pounds of water. A brick likewise will experience an upward buoyant force equal to the weight of an equivalent volume of water when it is submerged. But since a brick is more dense than water, its weight is greater than the buoyant force and it will sink to the bottom. If the brick landed on a scale on the bottom, the scale would read the weight of the brick minus the buoyant force.
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Old 08-09-2015, 19:52   #3
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Re: Confusion like a Newfoundland fogbank

If you're in newfoundland, don't get a crappy boat.

These turd burglers will tell you you need some stupidly large boat to sail from here to there, not true.



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Old 08-09-2015, 22:46   #4
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Re: Confusion like a Newfoundland fogbank

The displacement of the boat is exactly equal to its weight. Displacement refers only to the weight of the water which the boat displaces because it was historically far easier to measure displaced water than a boat's weight. By definition, the water and the boat weight the same.

What you're mistaking is the empty vs. laden displacement. The more a boat weights, the more water it will displace, and the lower it will sit in the water. This increases the wetted surface drag of the boat and slows it.

Boats are always advertised using their unladen weights, because there's no way to know what kind of additional equipment an owner will put on.

It's generally assumed that the extra weight will be similar for any boats (i.e., your weight, provisions, and equipment will all be the same no matter which boat you buy).
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Old 09-09-2015, 08:41   #5
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Re: Confusion like a Newfoundland fogbank

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Originally Posted by FamilyVan View Post
If you're in newfoundland, don't get a crappy boat.

These turd burglers will tell you you need some stupidly large boat to sail from here to there, not true.



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I dont understand this denegrating remark? The size of a vessel was not even mentioned.
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Old 09-09-2015, 08:50   #6
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Re: Confusion like a Newfoundland fogbank

Intended as a joke, re reading it, it isn't that funny, I was thinking about a different thread when I wrote it.

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Old 09-09-2015, 09:49   #7
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Re: Confusion like a Newfoundland fogbank

There we go. .civilized dicourse.Nice to see.
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Old 09-09-2015, 10:21   #8
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Re: Confusion like a Newfoundland fogbank

thanks guys for the info on displacement. much appreciated.
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Old 09-09-2015, 10:34   #9
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Re: Confusion like a Newfoundland fogbank

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Originally Posted by alansmith View Post
If the brick is hollow and made of plastic versus a brick of the same dimensions is made of gold...versus a brick that is same dimensions with sand inside of it....help me...
I'd say buoyancy is a pretty important concept to get your head around if you are going boating. Get out your rubber ducky and hit the tub.

Since I'm not sure of the relative density of sand vs gold vs "plastic" I'll use an easier example: A bowling ball and a basketball. Imagine they are the same size. That means the are the same "volume", they take up the same amount of space. Lift them up. That bowling ball is a lot heavier than the basket ball, yeah? Its more dense.

Throw the basketball in the tub. Look at the water level in the tub. It didn't change much, right? And the basketball is almost sitting on the surface of the water. It didn't displace, or relocate, very much water. It took very little water to equal the weight of the basketball. It didn't have much ability (weight) to lift the water.

Now place the bowling ball in the tub. Check out how much the water level in the tub rose, now that the bowling ball is sitting at the bottom of the tub. The bowling ball displaced a volume of water equal to the bowling ball's volume, and that still didn't equal the weight of the bowling ball. Its more dense than water, so it sank.

From visualizing this, it should be clear that it is the density of the object itself, and not the material its made of, that determines whether something floats. A piece of glass sinks, its a denser material than water. But an empty beer bottle floats, because the density of the bottle itself (its mass divided by the space the bottle takes up) is low. Splash around and fill that bottle with water though, and it'll sink, just like a swamped vessel. Once full of water, the density of the bottle (the mass divided by the space it takes up) has increased.
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Old 13-09-2015, 15:12   #10
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Confusion like a Newfoundland fogbank

I believe what everyone is discussing is called "Archimedes Principle". As I recall this Ancient Greek philosopher came up with it to determine the purity of gold. Anyway a quick web search should show you more than you will ever want to know.


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