exhaust stroke. These engines are designed with a separate oiling and mechanical system to operate the valves. This feature limits direct leakage of fuel, but also adds additional weight to the engine that puts them at a disadvantage as a lightweight outboard engine.

For this report, oil and gasoline inputs are calculated for standard (crankcase scavenging) two-stroke engines ranging in size from 16-175 horsepower (20-230 kW), that are fueled by a mixture of oil and gasoline. The population of engines used in recreational vessels is changing dramatically. In 1996 the EPA produced estimates on the potential impact on this changing population. However, four-stroke engines and direct-injection two-stroke engines were not included in this analysis because their populations are not adequately known. Population of other recreational vessels such as diesel outboard and inboard engines is known (EPA population model). However, aqueous fuel emissions studies have focused exclusively on gasoline engines and to our knowledge aqueous diesel emissions are not reported.

Overall, oil and gas inputs from two-stroke outboard motors are estimated to be between 0.6 to 2.5 million gallons per year (average 1.6 million gallons) or between 2,200 and 9,000 tonnes (average 5,300 tonnes) per year for coastal waters of the United States.¹³

Worldwide estimates of discharge of petroleum hydrocarbons by two-stroke engines were not estimated because of a lack of population databases. The number of two-stroke engines operational in non-North American waters, however is likely extremely large, and it is felt that this is a major source of petroleum hydrocarbons to the world’s oceans.

Similar to the evaluation of tank vessel spills, the U.S. Coast Guard database was used to estimate spills from non-tank vessels in U.S. waters. During the ten-year period from 1990 to 1999, there were 1,745 spills from non-tank vessels of at least 100 gallon size. Of the recorded spills, 45 were greater than 10,000 gallons (34 tonnes) in size, and these accounted for 53 percent of the total spill volume. The average annual spillage from non-tank commercial ships was 910 tonnes per year, about 23 percent of the spillage from tank vessels.

Because comprehensive spill databases for Canadian and Mexican waters were not available, spill volumes for these waters were estimated by adjusting the U.S. figures by the relative movements of dry cargo. On this basis, the calculated oil spillage from non-tank vessels into North American waters is 1,100 tonnes per year. This figure was applied as the minimum estimate. Recognizing the completeness of the U.S. data set, the spill quantities were increased by just 5 percent to 1,200 tonnes to obtain the best estimate, and by 20 percent to 1,400 tonnes to obtain the maximum estimate.¹⁴

International spill data were obtained from the Environmental Research Consulting database and includes information gleaned from the International Maritime Organization, ITOPF, and other national and regional agencies. The international data are not collected consistently and do not include spills under 10,000 gallons (34 tonnes) in size, and are therefore regarded as underestimates. The international spill quantities were increased by 25 percent to obtain the minimum estimate, by an additional 10 percent to obtain the best estimate, and further increased by 25 percent to obtain the maximum estimate.¹⁵ The international and North American figures were then combined to obtain the worldwide estimates of spillage from tankers. The best estimate is 7,100 tonnes; the minimum estimate is 6,500 tonnes per year and the maximum is 8,800 tonnes per year.

The NRC 1985 report used data from the International Tanker Owners Pollution Federation Ltd. (ITOPF) to estimate the quantity of oil entering the marine environment from tanker accidents. In the NRC 1985 report, the spillage from tankers was averaged over the 6-year period from 1974 to 1979, establishing a best estimate of 20,000 tonnes per year. In the IMO 1990 report, the spillage from non-tankers was not available, and a best estimate of 7,000 tonnes was made by assuming that the volume of non-tanker spills equaled 6 percent of tanker spillage. In U.S. waters, spillage from non-tankers during the period 1990-1999 was 57 percent of the spillage recorded during the period 1980-1989. It is likely that the IMO 1990 report underestimated the spills from non-tankers, and that spills from non-tankers have declined worldwide during the 1990s.

The NRC 1985 study assumed a typical motor ship with a 20,000 HP propulsion plant generated 15 gallons of bilge oil

Front Matter (R1-R12)

Executive Summary (1-4)

1. Introduction (5-18)

2. Understanding the Risk (19-62)

3. Input of Oil to the Sea (63-88)

4. Behavior and Fate of Oil (89-118)

5. Biological Effects of Oil Releases (119-158)

References (159-182)

Appendix A: Committee and Staff Biographies (183-188)

Appendix B: Definitions and Conversions (189-190)

Appendix C: National Seepage of Crude Oil into the Marine Enviornment (191-192)

Appendix D: Oil and Gas Extraction (193-202)

Appendix E: Inputs of Petroleum Hydrocarbons into the Ocean Due to Transportation Activities (203-218)

Appendix F: Inputs into the Sea from Recreational Marine Vessels (219-220)

Appendix G: Spills from Coastal Facilities (221-222)

Appendix H: Atmospheric Deposition and Air-Sea Exchange of Petroleum Hydrocarbons to the Ocean (223-232)

Appendix I: Estimating Land-Based Sources of Oil in the Sea (233-252)

Appendix J: Methods Used to Estimate PAH Loadings to the Marine Environment (253-254)

Appendix K: Regulatory Framework (255-258)

Index (259-265)