HOW MUCH DOES YOUR BOAT WEIGH?
A rather personal question, yes? All kidding aside, the Coast Guard Deck Officer license structure is based on capacity (Master, Mate, Operator), propulsion (steam, motor, aux. sail), tonnage, and the waters you are authorized to sail on. However it's tonnage or "Gross Register Tons" that separates the many of the levels within the deck license scheme; all the way from the MASTER 25 Gross Tons to the MASTER Any Gross Tons and everything in between!
But what is gross tons? In its most basic definition, a "gross ton" is a volume measurement that represents 100 cubic feet. It DOES NOT represent how much the vessel weighs or how much water the hull displaces (Displacement tonnage). Gross Tons is a fairly good representation of the overall size of a vessel or how much internal volume it has. There is a basic formula for calculating the Gross Tonnage of your boat. It is based on the overall length, multiplied by the breadth, multiplied by the "depth" of the hull. That sum is then divided by 100 and then multiplied by a hull coefficient. For most vessels it's .67. Depth is not referring to the Draft. Depth is measured from the main deck to the top of the keel. Think of it as trying to calculate how much "stuff" your boat can hold. So, for example. A 40 foot vessel with a 10 foot beam and 4 foot depth would come in at about 10.7 Gross Tons.
You may wonder why the Coast Guard uses this form of measurement in it's licensing scheme. Well there are many reasons. For one, the regulations pertaining to the inspection of vessels is tied to Gross Tons. Also, there are historical rules pertaining to tariffs and other fees a vessel would pay based on how much cargo they could carry. Often that is calculated by the Net Tonnage, which is Gross Tons minus certain spaces on a vessel. In the world of big ships pilot fees in many ports are based on the vessels draft AND gross tonnage.
For better or for worse, Gross Tons is here to stay and to complicate issues even further, the International Tonnage Convention (ITC) provides another way to measure Gross Tons. This system is even more basic than the U. S. System and more acurately captures a vessels true size and volume. Newer vessels being built will most likely be admeasured under the ITC system and the Coast Guard is still wrestling with how deal with both systems.
Under the U. S. domestic tonnage system, vessels are measured (or "admeasured") by a Naval Architect using formulas that are allowed by regulation. There are several formulas used under this system, all of which have to be atested to by the Coast Guard or a Third Party classification society. They will use their "vodoo" calculators and do their best to keep the Gross Tonnage to it's lowest possible value. By exempting many spaces on a vessel, they are able to design and document vessels at a tonnage that exempts them from many rules. The final approved tonnage is what is listed on the vessel's Certificate of Documentation (COD), issued by the Coast Guard. On foreign vessels this would be found on the tonnage certificate. This is why you may see many large U. S. vessels, several decks in height, under 100 gross tons. Under the ITC system, they would not be allowed to do this and their "true" tonnage would be assigned to the vessel.
The point of all of this is to understand that Gross Tonnage is a volume measurement AND it may not make sense to you when you are looking at certain vessels and knowing what their Gross Tonnage listed at. Like many parts of the regulatory process, it's not simple and often makes no sense. Those often effected by this are mariners who are trying to gain experience to upgrade their license. Many have been caught by the "vodoo" calculators and not been able to upgrade, even though their practical experience certainly deserves it!!
Be advised of what your vessels Gross Tonnage is and understand there are many complicated and historic rules that went behind the determination of just how much it "weighs"!!