How is DWT in container ships achived?

[karamss]

Hi there, in martime terms DWT includes the containers weight as well. What i don't understand is how is it achieved? All ships specifications of container capacty has higher weight than the DWT.

 

[anorlunda]

See https://en.wikipedia.org/wiki/Deadweight_tonnage

Even in that article, they mention multiple definitions for DWT. Those definitions can get very complicated and specific, just like the Plimsol Line and the other lines painted on the side.

It may be hard to find someone here at PF who knows the exact answer. But, be patient a few more days and see what other answers you might get.

 

[karamss]

See https://en.wikipedia.org/wiki/Deadweight_tonnage

Even in that article, they mention multiple definitions for DWT. Those definitions can get very complicated and specific, just like the Plimsol Line and the other lines painted on the side.

It may be hard to find someone here at PF who knows the exact answer. But, be patient a few more days and see what other answers you might get.

Hi thanks for the reply anorlunda!

I checked the wiki and also checked a more reliable source which has the same difinition that i provided.

http://www.themaritimesite.com/a-guide-to-understanding-ship-weight-and-tonnage-measurements/#

provided that one container can hold around 30tons. the capacty is always higher than the DWT by a lot.

https://www.marineinsight.com/know-more/10-worlds-biggest-container-ships-2017/amp/#comment-1659253

Its ok I am just curious how they do it

 

[JBA]

Just as a note, the TEU is the total volume (in containers), not the cargo weight, that the ship can transport, so it cannot be compared to the DWT of the cargo vessel.

 

[karamss]

Just as a note, the TEU is the total volume (in containers), not the cargo weight, that the ship can transport, so it cannot be compared to the DWT of the cargo vessel.

thanks a lot but then what's the point of the teu if i cannot know the real weight a ship can transport?

 

[Tom.G]

thanks a lot but then what's the point of the teu if i cannot know the real weight a ship can transport?

Maybe someone is shipping a load of Teddy Bears for the kids; low weight but high volume. Versus a shipload of machinery parts.

Cheers,
Tom

 

[karamss]

Maybe someone is shipping a load of Teddy Bears for the kids; low weight but high volume. Versus a shipload of machinery parts.

Cheers,
Tom

i take my previous comment back this makes sense. thanks a lot Tom!

 

[dragoneyes001]

the container cranes at the ports also log the weight of each container as they load ships prioritizing the placement for the most stable stacks and lowering the center of gravity. what the limits and procedures to meet the ships limits are you'd probably need to ask the port authority for a lead to who

 

[JBA]

They really need to know the weights before the containers get to the cranes for priority loading or they wouldn't know how select correctly. Maybe all incoming vehicles carrying containers are weighed as they enter the port.

Apart from that, it doesn't make any sense to me that they don't have some standard weight because the published DWT is not changed at every ship loading. I was thinking that it is easier for tankers; but, then again it depends upon the density of the liquid they carrying.

 

[dragoneyes001]

They actually do pre weigh the containers but they also stack them on shore in order before they load the ships.

 

[Ketch22]

All ships specifications of container capacty has higher weight than the DWT.

The DWT is a rating not a limit. How they get to that point is a complicated process. I would recommend getting a little more understanding of Metacentric height or GM.

The heal angle of a ship and the righting moment developed at that angle is related to the height of the center of Gravity above the center of Buoyancy (basic definition of GM). As a ship settles into the water the submerged portion changes shape which affects a change in the geometric center of the submerged portion or Center of Buoyancy. Each container adds weight to the overall weight, it also adds to relational changes in the balance or Center of Gravity. This height difference also affects the rate that the forces develop and decrease. This is codified on a ship by what is called "Cross Curves of Stability." These are a multi axis graph that shows the righting moment developed at a particular GM and a determined total weight.
As a ship is loaded the overall stability is calculated and the final load point is chosen for a stability determined to be acceptable. This usually takes form as a prescribed ultimate stability. This is remember a calculation on the total load and the physical change in GM. It is possible by careful loading of a ship to adjust the
GM and thus to keep the ultimate stability within the rating.

Check out cross curves of stability and look a little further into how they reflect the action of a ship at sea. I think you will find more of your answer. The short answer of how DWT is determined is that this is the maximum weight under fabricated conditions that stability is acceptable. What I am reading into your question is " How can they routinely exceed the limit?" The answer is that with proper, calculated loading, the stability can be preserved to a weight beyond the fabrication.