One of the problems facing yacht owners is the difference between piracy and robbery. Apparently, that has been solved in the US Courts a long time ago.
"...the case cited as precedent, United States v. Smith, 18 U.S. 5 Wheat. 153 (1820) which wends its merry way to a determination that (at p. 161,162):
There is scarcely a writer on the law of nations, who does not allude to piracy as a crime of a settled and determinate nature, and whatever may be the diversity of definitions in other respects, all writers concur in holding that robbery or forcible depredations upon the sea, animo furandi, is piracy. The same doctrine is held by all the great writers on maritime law in terms that admit of no reasonable doubt.
We have therefore no hesitation in declaring that piracy, by the law of nations, is robbery upon the sea, and that it is sufficiently and constitutionally defined by the fifth section of the act of 1819."
The above was taken from Eagle Speak - one of the blogs I subscribe to. Their blog tends to follow the larger Somali/Gulf of Aden and related piracy acts against merchant seaman and ships. Great information and resource.
So, to my non-lawyer mind, an act of robbery at sea is piracy, and attempted act of robbery at sea (unable to board) would be an attempted act of piracy.