primarily for that [transportation] purpose,” 543 U.S., at 495; that a “watercraft need not be in motion to qualify as a vessel,” ibid.; and that a structure may qualify as a vessel even if attached—but not “permanently” attached—to the land or ocean floor. Id., at 493–494. We did not take these statements, however, as implying a universal set of sufficient conditions for application of the definition. Rather, they say, and they mean, that the statutory definition may (or may not) apply—not that it automatically must apply—where a structure has some other primary purpose, where it is stationary at relevant times, and where it is attached—but not permanently attached—to land.
After all, a washtub is normally not a “vessel” though it does not have water transportation as its primary purpose, it may be stationary much of the time, and it might be attached—but not permanently attached—to land. More to the point, water transportation was not the primary purpose of either Stewart’s dredge or Evansville’s wharfboat; neither structure was “in motion” at relevant times; and both were sometimes attached (though not permanently attached) to the ocean bottom or to land. Nonetheless Stewart’s dredge fell within the statute’s definition while Evansville’s wharfboat did not.
The basic difference, we believe, is that the dredge was regularly, but not primarily, used (and designed in part to be used) to transport workers and equipment over water while the wharfboat was not designed (to any practical degree) to serve a transportation function and did not do so. Compare Cope v. Vallette Dry Dock Co., 119 U.S. 625 (1887) (floating drydock not a “vessel” because permanently fixed to wharf), with Jerome B. Grubart, Inc. v. Great Lakes Dredge & Dock Co., 513 U.S. 527, 535 (1995) (barge sometimes attached to river bottom to use as a work platform remains a “vessel” when “at other times it was used for transportation”). See also ibid. (citing Great Lakes
