The gross tonnage of a vessel includes its entire closed-in capacity; the net tonnage is, approximately, a vessel's capacity available for cargo and passengers.
The gross tonnage, as has been explained, is obtained by measuring the contents of the closed-in spaces above and below the upper deck and by dividing the contents thus obtained by 100, if in cubic feet, or by 2.83, if in cubic meters.
The net tonnage is ascertained by deducting from the spaces included within the gross tonnage the spaces required for the crew, for the navigation of the ship and for propelling power.
A vessel ton, net or gross, is 100 cubic feet, or 2.83 cubic meters.
A vessel's net tonnage is its earning capacity in units of 100 cubic feet or 2.83 cubic meters.
These definitions state what gross and net tonnage theoretically are and what they would be in practice, if uniform and exact measurement rules were applied in the same manner in all countries.
As a matter of fact, however, the rules for the measurement of vessels to determine their gross and net tonnage are not the same, and the interpretation and application of the rules are not uniform.
Different Measurements of Net Tonnage
Vessels of identical construction may vary widely as to gross tonnage, and especially as to net tonnage, when measured and registered in different countries. It is to be hoped that this may not always be the case.
It is highly desirable that there should be uniform practice the world over as regards the vessel spaces to be included in gross tonnage, and as regards the spaces to be deducted therefrom in calculating net tonnage.
If international unity in vessel measurement and tonnage is ever secured, it will presumably be brought about by the general adoption by all countries of a set of measurement rules based upon the two principles that must underlie any logical or consistent code of rules; i. e., that gross tonnage shall include the vessel's entire closed-in capacity, and that net tonnage shall include the actual space available for the stowage of cargo and the accommodation of passengers; or, stated differently, that gross tonnage shall represent the entire closed-in volume, and net tonnage the actual earning capacity.
These principles were the basis of the measurement rules formulated for the Suez Canal Co. by the International Tonnage Commission at Constantinople in 1873, and have since been maintained by the company in interpreting and enforcing the rules.
The Panama measurement rules recommended in this report are believed to carry out, even more fully than the Suez rules do, the principles that gross tonnage shall equal the entire closed-in volume of vessels and net tonnage the actual earning capacity.
The Suez rules and those recommended for Panama, though varying in certain details, are not so dissimilar as to make their unification seem especially difficult.
Should future events harmonize these two sets of rules so that either a Suez or a Panama tonnage certificate will be accepted at either of the canals, a strong reason, and possibly a controlling reason, will exist for the complete international unification of vessel measurement and tonnage.
The problems to be dealt with in bringing about common rules and common practice are indicated by the discussion of gross tonnage in the preceding chapter and of net tonnage in the following pages.
The spaces deducted from those included in gross tonnage to ascertain a vessel's net tonnage are of two general categories: (1) Those required for propelling power, and (2) those required to house other machinery, to accommodate the ship's personnel, and, in general, the non-exempted spaces, other than those occupied by the engines and fuel, required for the navigation of the ship.
The deducted spaces of the second category may be either in the hull of the vessel below the uppermost full-length deck or may be in "permanently covered and closed-in spaces" on or above that deck.
Space Deducted for Propelling Power
The space deducted for propelling power—the space occupied by the engine room, fuel, and the shaft tunnel—is determined by the application of one or more of three rules:-
- The British Board of Trade percentage rule provides that the deduction for propelling power shall be 32 per cent of the gross tonnage in the case of vessels with screw propellers, when the space occupied by engine and boiler rooms is more than 13 and less than 20 per cent of the gross tonnage of the vessel; and shall be 37 per cent of the gross tonnage in the case of vessels with paddle wheels when the space occupied by the engine and boiler rooms is more than 20 and less than 30 per cent of the gross tonnage.
To screw-propelled vessels whose actual engine-room space does not come between 13 and 20 per cent of the space included within gross tonnage, and to vessels propelled with paddle wheels whose engine-room space does not come between 20 and 30 per cent of the space included in gross tonnage, the following rule is applied to determine the tonnage to be deducted for propelling power.
- The Danube rule stipulates that the deduction for propelling power shall be the actual volume or space occupied by the engine and boiler rooms, with the addition of 75 per cent in the case of screw-propelled vessels and with the addition of 50 per cent in the case of vessels propelled by paddle wheels. This is the rule enforced by the Suez Canal Co. for all vessels having movable coal bunkers. In the case of vessels having fixed blinkers the Suez rules allow vessel owners to choose between the Danube rule and the following rule.
- The actual-volume, or the so-called German, rule provides that the deduction for propelling power shall, in the case of vessels with fixed bunkers, be the tonnage of the space actually occupied by the engine and boiler rooms and fixed bunkers. This rule originated in England and not in Germany, and does not now prevail in Germany. It was, however, in force for a while in Germany and came to be called the German rule in contrast to the British or percentage rule.
In all cases the engine and boiler rooms include the spaces actually occupied by the engines and boilers and by the shaft tunnel or tunnels when the vessel is screw-propelled and the spaces framed-in around the funnels and within the light and air casings above the engine and boiler rooms.
It will be explained later, however, that the measurement rules prevailing in different countries treat the light and air and funnel spaces above the engine rooms differently, some rules including more of those spaces than do other rules.
The history and a critical discussion of the rules concerning the deduction of propelling power spaces are presented in Chapter IX of this report.
Comparison of British, Suez, German, and American Measurement Rules
An analysis and comparison of the British, Suez, German, and American measurement rules as regards the provisions concerning the deductions for propelling power and the deductions allowed for the housings and spaces required for machinery other than propelling power, and for officers' and crew's quarters—i. e., the spaces occupied by the men, stores, appliances, facilities, and machinery required for the operation and navigation of the ship—will reveal the factors to be considered in formulating rules for the measurement of vessels using the Panama Canal, and will show to what extent the four sets of rules compared adhere to, and to what extent they disregard, the principle that net tonnage should include and accurately express the actual earning capacity of merchant vessels.
The Panama rules will thus be framed with knowledge of the main provisions of existing rules, and may be formulated with a view to avoiding those provisions of the existing rules that do not tally with the principle that net tonnage should accurately state the merchant vessel's earning capacity.
Johnson, Emory Richard, Measurement of Vessels for the Panama Canal, Volume 2, Government Printing Office, Washington, DC 1913, Pages 73-74