The TugSafe International Addendum provides guidance for applicability of international conventions on towing vessels required to comply with 46 CFR Subchapter M that operate internationally. This job aid was designed with the assumption that a vessel holding a valid international certificate is equipped in accordance with the same, and the equipment is in serviceable condition. Requirements are based on length, build date and a tonnage measurement system.  NVIC 11-93 Change 3 provides Coast Guard guidance on applicability of tonnage measurement systems.

Describe the vessel:
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 Build Date

The keel laid date or the date the vessel was substantially altered, whichever is later.

 Convention Length

Either 96 percent of the length on a waterline at 85% of the least molded depth measured from the top of the flat keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline, whichever is greater. In vessels designed with a rake of keel, this length is measured on a waterline parallel to the design waterline.

 Convention Measurement System

A U.S. tonnage measurement system that is based on and complies with the requirements of the International Convention on Tonnage Measurement of Ships, 1969. Gross tonnages assigned under this system are referred to in terms of gross tonnage, ITC (GT ITC) or gross tonnage (GT). This system uses total molded volume as its basis. This system is not available for measurement of vessels with an overall length less than 24 meters (79 feet). Requirements for measurement under this system are detailed in 46 CFR 69, Subpart B.


Gross Register Tons. This refers to the gross tonnage assigned under the regulatory measurement system.


Gross Tonnage, International Tonnage Convention. This refers to the gross tonnage assigned under the convention measurement system.

 Keel Laid

Refers to the date a vessel's keel was laid or the vessel was at a similar stage of construction.

 Overall Length

The horizontal distance of the hull between the foremost part of the stem and the aftermost part of the stern, excluding fittings and attachments.

 Regulatory Measurement System

A U.S. tonnage measurement system with a historical basis that requires that all closed-in spaces on a vessel be measured and treated according to their use. Gross tonnages assigned under this system are referred to in terms of gross register tons (GRT). Prior to 1983, the regulatory measurement system was the only U.S. tonnage measurement system available.

 Substantially Altered

Refers to the date a substantial alteration on a vessel was commenced. A vessel is considered to have been substantially altered if it undergoes alterations or modifications which result in an increase or decrease of more than 5% in either the regulatory measurement system gross tonnage (if assigned) or the convention measurement system gross tonnage (whether assigned or not). For the purposes of this job aid, there is no difference between a vessel for which the keel was laid on a particular date and a vessel that was substantially altered on that same date. In most cases, addition or removal of temporary deck equipment does not involve "alterations or modifications" to a vessel, and therefore would have no bearing on whether or not a vessel has been substantially altered.