A lone Wellington yachtsman was showing signs of hypothermia after a snapped mast left him stranded nearly 2 nautical miles from shore this evening.

Maritime police are lauding the vigilance of a York Bay resident after he called police for help after noticing the sailor in trouble near Matiu/Somes Island about 5pm today.

The lone yachtsman was intending to sail from Lowry Bay to Matiu/Somes Island and home again when his mast snapped nearly 2 nautical miles from shore, police said.

The resident had been watching the vessel for about 10 minutes before realising there was a problem.

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Senior launch master Constable Paul Curd said the early call prevented a serious incident occurring, as the sailor was already showing signs of hypothermia when they arrived.

"With the summer water sports season and water safety week approaching it is very reassuring to know that members of the public are helping to keep local boaties safe," he said.

"We are very grateful for his call."

The cold and wet sailor was delivered safely to shore with his damaged vessel by the Police launch Lady Elizabeth IV.