# Formula for GT to derive displacement

Discussion in 'Stability' started by Laranjo123, Aug 20, 2011.

1. ### Laranjo123Previous Member

I'd like to know is there any formula of GT to derive the displacement of the ship?..Exampl if GT is given how do i get the displacement?..LBP B d area also given

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### TackwiseMember

GT or Gross Tonnage is related to the complete volume of the ship. That is not only the ships displacement, but everything above the waterline as well, deckhouse, funnels etc. It is therefore not possible to accuratly get the displacement from the GT.

To illustrate:
Two vessels may have the same GT, but different displacement. for instance one vessel may have a small deckhouse and a large displacement, as the other vessel has a smaller displacement and a larger deckhouse.

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### Steve WSenior Member

So thats the way it works, i did not know that. Why would one need to know gross tonnage?
Steve.

4. ### Laranjo123Previous Member

thx mr.tackwise

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### TackwiseMember

Gross tonnage was, and still is in many ports, the size for which harbour fees are calculated! (for the commercial vessels) When they devised the GT, it was hoped it would describe a vessels carrying capacity, to a certain extend.

Vessel owners were smart enough to change the ship design to reduce its GT while allowing it to carry more cargo. For instance a vessel could be designed for a small GT volume, which meant less cargo within the hold. The vessel would however be designed for an enormous deck carrying capacity!! This is one of the reasons the container vessels have 'evolved' into their current design. I believe (not certain) that harbour fees for container vessels are no longer based on GT.

As no better dimension is as of yet available, GT is also used as an arbitrary boundary for several regulations. If you keep statistics on vessels you will often see large clusters of vessels below a certain GT. Crossing those GT boundaries will often lead to more costs.

Playing with the GT design of a vessel can be fun!!

6. ### Laranjo123Previous Member

again you're right. Mr. Lacson one of the shipyard owner here in philippines told me that they would have lowered the ship's GT to lessen the amount and the price as well. ( ship's owner/company would ask the engineer to lower which made me laugh and surprised haha.) i guess ship's are too big to talk about and have them as a topic. Thx guess this forum helps me a lot.

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### jehardimanSenior Member

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### ABoatGuyMember

As well as paying for passages through canals. The Panama and Suez both have there own versions of tonnage rules.

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