ever, because the rules are different, any attempt to draw equivalencies is somewhat subjective, and this can place serious restrictions on ships that could conceivably operate in regions governed by different national regulations. For example, the American Bureau of Shipping (ABS) has constructed a recent comparison among selected classification societies shown in Table 6.3 that defines new categories and variations in correspondences. It is also important to note that the International Association of Classification Societies (IACS) has been working on a standard range of ice classifications shown as PC 1 through PC 7. It is anticipated that these will be adopted in the near future and should be helpful in describing the capabilities of ships classified by those societies that belong to IACS.

While there are differences, the essential approach taken in the more recently revised rules is to specify maximum design loads based on ship-ice interaction models that have been calibrated with full-scale measurements. The design loads depend on displacement and power and are applied to different structural elements according to a pressure-area relationship. Scantlings are determined using elastoplastic criteria that permit stresses in excess of yield so that some permanent hull deformation is acceptable. Compared to traditional ship structural design methods, this leads to a combination of thinner plate, bigger frames, and larger frame spacing.

As noted earlier, the U.S. Coast Guard has generally used the thickness of ice that can be broken continuously at 3 knots as a measure of icebreaker performance; but it was too difficult to extract this information for all icebreakers in the world fleet, and this simple, loosely defined rule-of-thumb cannot be matched consistently to the various ice classification schemes. In seeking a general definition of a polar icebreaker, one authority has developed a listing that includes the following parameters:

• Having sailed in significant sea ice in either the Arctic or the Antarctic,

• Ice strengthening sufficient for polar ice, and most significantly,

• Installed power of at least 10,000 horsepower

Historically, ships with lower power levels have successfully operated in polar regions, but as demonstrated by the evolution of U.S. Coast Guard designs in the last 40 years, mass and velocity are the key factors in breaking heavy ice. Propelling heavy ships and/or developing higher speeds requires considerable power. Thus, while information has been obtained and could be provided on as many as 60 icebreaking ships, many of these are not believed truly suited for polar icebreaking.

Table 6.4, which draws heavily on data provided by L.W. Brigham (personal communication, October 2000), provides a listing of the current inventory of polar icebreakers organized by country of ownership. Baltic icebreakers have also been included, although it is often a matter of opinion rather than fact which ships have some polar capability. This presentation was chosen to highlight both the fleet size and the key data for various nations having interest in the polar regions.

The world fleet of icebreakers with greater than 10,000 horsepower is 50. Russia has the largest fleet. Finland, Canada, and Sweden each operate six to seven icebreakers. The Unites States has four ships, and six other countries have one to three ships. Only Russia has used nuclear propulsion plants (seven ships). Only Russia and the United States operate ships with propulsion greater than 30,000 horsepower. Most icebreakers operate primarily in the Baltic Sea area. Russia is notable for its emphasis on icebreaker tourism.

In continuous running mode, icebreakers break ice by weight. As an icebreaker is propelled forward, it moves up onto the ice, and the weight of the hull breaks the ice. The traditional icebreaker bow is in the form of a spoon that fa-