I've been watching the crews lifting wrecked boats out of the water at the marina where I live here in Kemah, Texas and the question came up, just how heavy are these boats? Is the displacment the same as the weight the crane is actually lifting?
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Displacement weight vs total weight
- Thread starter Blues Cat
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- Aug 16, 2007
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our travellift has a gauge for each strap so you can get the weight balanced as close as possible. We have just lifted a "certain manufacturer's" 50 footer, and it weight about 10,000lbs more than advertised. We checked with a moisture meter, and the boat is very wet.
You have to ask the Travelift operator when the last time the scales were calibrated. The weights they give are often accurate to within a couple thousand pounds either way. A small boat in a big lift will be less accurate than a big boat in that same lift - if the lift is rated for 50 tons and the boat is only 5 tons, the weight reading will not be as accurate as a boat that weighs 30 tons.
Brian
Brian
Displacement is the weight of the water that the boat displaces when it is floating in the water.
If a boat has a 25 ton displacement, then that is the weight of the water that was where the boat hull now is, up the the waterline.
Dry weight is the weight of the boat like new, no water, oil, fuel or supplies on board.
Displacement is theoretically the same as the actual boat weight. The more weight you add to the boat, the lower it sinks in the water,
the more water it displaces, the higher the displacement weight.
The boat will be lighter if weighed when in water than on land due to the force of the water pushing on the bottom. Bouyancy.
The displacement weight will change if freshwater and saltwater. It will also change in hotter water or colder water due to
density of the water.
A boat will float lower in hot salt water than cold salt water due to the density of the water changes with temperature.
Same with freshwater temperatures.
If a boat has a 25 ton displacement, then that is the weight of the water that was where the boat hull now is, up the the waterline.
Dry weight is the weight of the boat like new, no water, oil, fuel or supplies on board.
Displacement is theoretically the same as the actual boat weight. The more weight you add to the boat, the lower it sinks in the water,
the more water it displaces, the higher the displacement weight.
The boat will be lighter if weighed when in water than on land due to the force of the water pushing on the bottom. Bouyancy.
The displacement weight will change if freshwater and saltwater. It will also change in hotter water or colder water due to
density of the water.
A boat will float lower in hot salt water than cold salt water due to the density of the water changes with temperature.
Same with freshwater temperatures.
quote:
The displacement weight will change if freshwater and saltwater.
not quite, only the volume will change not the displacement which as you indicate ins measures in tons or pounds
That is correct. That's what I get when a Coonass tries to sound intelligent.....
I meant that the boat will float at different levels depending on different water conditions.
Like salt to fresh and tropical to artic temps.
On commercial vessels, there is a mark that shows the max. draft that a vessel can have in order to
safely operate in most conditions.
There are different level markings for different water qualities.
I meant that the boat will float at different levels depending on different water conditions.
Like salt to fresh and tropical to artic temps.
On commercial vessels, there is a mark that shows the max. draft that a vessel can have in order to
safely operate in most conditions.
There are different level markings for different water qualities.
quote:
Originally posted by Blues Cat
I would think if a boat had taken on water and needed to be hoisted out, then it would weight more than the published displacement weight....
El Sea
That is correct. They are lifting a lot more weight than dry or displacement weight.
Those cranes they are using will lift a lot more than you think. Some just pull the boat to the surface then pump it out before lifting.
I think displacement weight for recreational boats is close to dry weight. Not loaded weight.
On commercial boats, there is a line on the side for gross tonnage. This is the max that vessel can be loaded.
Those cranes they are using will lift a lot more than you think. Some just pull the boat to the surface then pump it out before lifting.
I think displacement weight for recreational boats is close to dry weight. Not loaded weight.
On commercial boats, there is a line on the side for gross tonnage. This is the max that vessel can be loaded.
Let's back up a step to the original question. I think what was asked is the difference between displacement and weight. The correct answer--none--was given. But the reason is not intuitively obvious. Displacement literally is the amount of water an object moves aside, or displaces. If an object displaces more water than it weighs, it floats. If it displaces less, it sinks. That is why boats made of concrete or steel or other heavy things float. Salt water weighs 64 lbs per square foot. Shape a hull to displace at least one foot for every 64 lbs it weighs, it will float. Except in fresh water, which weighs 62.2 lbs per square foot. That's why boats sit lower in fresh water than in salt--they need to displace a higher volume of fresh water to offset their weight. If you weighed the amount of water displaced by a floating boat, it would always equal the weight of the vessel.
Don't get started on published vs actual displacement. Differences are always hotly contested.
Brett
Don't get started on published vs actual displacement. Differences are always hotly contested.
Brett
If we all look at the original question, it asked if displacement weight is the same as the weight of the sunk boat being lifted.
It is not. The boat is full of water, sunk, it weighs a lot more.
The boat us usually lifted to the waterline and pumped out before being put on a barge or truck.
That is what they did with most of the boats around here after Hurricane Rita.
Published weights and displacement weights will never be the same because you have to put supplies and other necessities on the boat
in order to take her out and have fun.
It is not. The boat is full of water, sunk, it weighs a lot more.
The boat us usually lifted to the waterline and pumped out before being put on a barge or truck.
That is what they did with most of the boats around here after Hurricane Rita.
Published weights and displacement weights will never be the same because you have to put supplies and other necessities on the boat
in order to take her out and have fun.
Incidently the "mark" which is a circle with a line thru it is called a Plimsoll mark: This is summer load line in salt water. There are also the following marks--the fresh being 2 upper marks, usually toward the circle: TF tropical freshwater mark; F freshwater mark; T tropical load line; S summer load line; W winter load line; WNA winter load line, North Atlantic salt water. The salinity of various oceans varies.
Even after the boat has been pumped, it will be heavier than the stated weights, because all furniture and many areas are saturated with water which is not removed in the pumping. Most wooden and glass boats take on water into the hull structure after being in water for some time--and again, this makes them heavier. Metal boats do not absorb water.
Even after the boat has been pumped, it will be heavier than the stated weights, because all furniture and many areas are saturated with water which is not removed in the pumping. Most wooden and glass boats take on water into the hull structure after being in water for some time--and again, this makes them heavier. Metal boats do not absorb water.