From en.wikipedia.org:
[Ship cargo space volume]
NET TONNAGE ( NT, N.T. or NT) is a dimensionless index calculated from the
total moulded volume of the ship's cargo spaces by using a mathematical
formula. Defined in _The International Convention on Tonnage Measurement of
Ships_ that was adopted by the International Maritime Organization in 1969,
the net tonnage replaced the earlier net register tonnage (NRT) which denoted
the volume of the ship's revenue-earning spaces in "register tons", units of
volume equal to [100].[1] Net tonnage is used to calculate the port duties and
should not be taken as less than 30 per cent of the ship's gross tonnage.[2]
Net tonnage is not a measure of the weight of the ship or its cargo, and
should not be confused with terms such as deadweight tonnage or displacement.
Also, unlike the net register tonnage, the net tonnage is unitless and thus
can not be defined as "tons" or "net tons".
** History
The International Convention on Tonnage Measurement of Ships, 1969 was adopted
by IMO in 1969.[3] Ships built before that date were given 12 years to migrate
from their existing tonnage to the use of NT and GT.<ref name="imo1982"/> The
phase-in period was provided to allow ships time to adjust economically, since
tonnage is the basis for satisfying manning regulations and safety rules.<ref
name="imo1982"/> Tonnage is also the basis for calculating registration fees
and port dues.<ref name="imo1982"/> One of the Convention's goals was to
ensure that the new units "did not differ too greatly" from the traditional
GRT and NRT units.<ref name="imo1982"/>
Both NT and GT are obtained by measuring the ship's volume and then applying a
mathematical formula.<ref name="imo1982"/> Net tonnage is based on "the
moulded volume of all cargo spaces of the ship" while gross tonnage is based
on "the moulded volume of all enclosed spaces of the ship".<ref
name="imo1982"/> In addition, a ship's net tonnage is constrained to be no
less than 30% of her gross tonnage.<ref name="imo1982"/>
** Calculation
*** Choice of draft value
The net tonnage calculation is based on a number of factors, one of which is
the moulded draft _d_. The choice of the value to use for _d_ can be
complicated. For ships subject to the International Convention on Load Lines,
the Summer Load Line draft is used, with the exception of cases where that is
a timber load line. For passenger ships, the draft used is the deepest
subdivision load line assigned in accordance with the International Convention
for the Safety of Life at Sea. Otherwise, if a ship has been assigned a load
line by its national government, the draft for that summer load line is used.
If the ship has no load line, instead, a maximum draft assigned by its
national government, that value is used, if it has been assigned a maximum.
Finally, for a ship to which none of the above applies, the value of _d_ is
taken as 75 per cent of the moulded depth amidships.
*** 12 or fewer passengers
The Net tonnage calculation is defined in Regulation 4 of Annex 1 of _The
International Convention on Tonnage Measurement of Ships, 1969 (see
http://www.admiraltylawguide.com/conven/tonnage1969.html) _. It is based on
three main variables:
- _V<sub>c</sub>_ , the total volume of the ship's cargo spaces in cubic meters (m³),
- _d_ , the ship's moulded draft amidships in meters, and
- _D_ , the ship's moulded depth amidships in metres
The first step in calculating NT is to find the value known as
_K<sub>2</sub>_, a multiplier based on _V<sub>c</sub>_. It is obtained by
using the following formula:
<math> K_2 = 0.2 + 0.02 \times \log_{10}(V_c) </math>
And then these three values are used to calculate NT using this formula:
<math> NT = K_2 \times V_c \times (\tfrac {4d}{3D})^2 </math>
Where the factor <math>(\tfrac {4d}{3D})^2</math> will not exceed 1, the term
<math>K_2 \times V_c \times (\tfrac {4d}{3D})^2</math> will not be less than
0.25 GT, and the final value of NT shall not be taken as less than 0.30 GT.
*** 13 or more passengers
When calculating NT for ships certified to carry 13 or more passengers, an
additional term is used in the NT formula. It is based on three additional
variables:
- _GT_ , the ship's gross tonnage ,
- _N<sub>1</sub>_ , number of passengers in cabins with not more than 8 berths, and
- _N<sub>2</sub>_ , number of other passengers,
First, a multiplier K<sub>3</sub>, based on the ship's gross tonnage is found,
<math> K_3 = \frac{1.25 \times ( GT + 10000 )}{10000} </math>
Then the net tonnage is calculated:
<math> NT = K_2 \times V_c \times (\tfrac {4d}{3D})^2 + K_3 \times ( N_1 +
\frac {N_2}{10} ) </math>
Where the factor <math>(\tfrac {4d}{3D})^2</math> shall not be taken as
greater than unity, the term <math>K_2 \times V_c \times (\tfrac
{4d}{3D})^2</math> will not be less than 0.25 GT, and the final value of NT
shall not be taken as less than 0.30 GT.
The difference between the cases of 12 of fewer passengers and 13 or more
passengers is due to a restriction given in the net tonnage definition that
states "...when N<sub>1</sub> + N<sub>2</sub> is less than 13, N<sub>1</sub>
and N<sub>2</sub> shall be taken as zero."
** Usage
In the United States, net tonnage is used to determine eligibility for
registering boats with the federal government. Vessels with net tonnage of
five or more are eligible for federal registration and not required to display
state registration numbers. Most vessels longer than [25] have net tonnage of
five or more.[4]
** Notes
[Transport]
[Reflist]
** References
- {{ cite web
|url= http://www.admiraltylawguide.com/conven/tonnage1969.html |title=
International Convention on Tonnage Measurement of Ships, 1969 |access-date=
2008-03-23 |date=1969-06-23 }}
- {{ cite web
|url= http://www.imo.org/Conventions/contents.asp?topic_id=259&doc_id=685
|archive-url=
http://arquivo.pt/wayback/20090707134036/http://www.imo.org/Conventions/contents.asp?doc_id=685&topic_id=259
|url-status= dead |archive-date= 2009-07-07 |title=International Convention on
Tonnage Measurement of Ships, 1969 |access-date= 2008-03-23
|author=International Maritime Organization |author-link= International
Maritime Organization |year= 2002 |publisher= International Maritime
Organization }}
- [last=Hayler ] .
- [last=Singh ]
- {{ cite book
| last = Turpin
| first = Edward A.
|author2=McEwen, William A.
| title = Merchant Marine Officers' Handbook
| edition = 4th
| year = 1980
| publisher =Cornell Maritime Press
| location = Centreville, MD
| isbn = 0-87033-056-X
}}
[Ship measurements]
[DEFAULTSORT:Net Tonnage] Category:Shipbuilding Category:Nautical terminology
Category:Ship measurements