Tiger Woods & Lindsey Vonn -- The Topless Boat Party

Tiger Woods lost the shirt off his back ... and this time his ex-wife Elin Nordegren had nothing to do with it.

The 37-year-old golf star aired out his man nipples and soaked up the sun by going shirtless on his yacht in Palm Beach on Tuesday.

While his fancy schmancy boat is called Solitude, Tiger spent the afternoon lounging and fishing with his two adorable kids and his sexy bikini-clad girlfriend Lindsey Vonn yesterday.