The ex-England loose forward had scored as Hull finally ended their 13-game losing run at champions Wigan Warriors last week and he was at it again today as Lee Radford’s side quickly added a second.
Westerman grabbed a scrappy opener in the 15th minute, fumbling a pass in the constant rain but kicking on before the ball hit the ground and winning the 25m race to the loose ball.
Hull trailed 8-6 at the break but the 29-year-old barged over from close range early in the second period as his side - with Albert Kelly back in action for the first time since last summer - reclaimed the lead and they never looked back in what was a highly-scrappy contest.
Depleted Huddersfield, whose sole try came via Darnell McIntosh’s 28th minute effort, remain bottom of Super League having lost all four games so far and they again made far too many errors in the poor conditions here.
That said, they were hit by further injuries both before and during today’s game.
Stand-off Lee Gaskell - due to start at centre with Joe Wardle missing - did not make it through the warm-up so 18th man Jake Wardle was drafted in.
However, they also lost forward Adam Walne towards the end of the first half with a shoulder injury, Hull seeing Jake Connor limp out of action as well.
England star Connor, who faced his former club at full-back with Jamie Shaul injured, suffered a knee problem when dealing with a kick in the final play of the first period.
He had to be helped off the field but, miraculously, returned for the start of the second half with his knee heavily bandaged and a little later than everyone else.
However, within seconds, Connor had signalled to the bench he could not carry on and centre Josh Griffin was switched to the No1 role.
It mattered not; Westerman - with his fourth try in his last three games - got them moving and they scored again almost immediately.
Veteran full-back Scott Grix, recalled for Giants with Akuila Uate one of their seven injured players, made a mess of Marc Sneyd’s high kick to concede a drop-out and, from there, Sneyd sent Fijian winger Ratu Naulago in for his third try in two games.
Grix then failed to deal with another Sneyd kick and this time Griffin proffited, the former Huddersfield centre adding his second try at the death, Sneyd adding a penalty in between.
Young half-back Oliver Russell kewpt trying for Huddersfield in his first game this season, converting McIntosh’s try and adding a penalty, but Simon Woolford’s side fell away in that second period.
Huddersfield Giants: Grix; McGilvary, Turner, Jake Wardle, McIntosh; Russell, Frawley; Ta’ai, Leeming, Matagi, Murphy, Mellor, Lawrence. Substitutes: O’Brien, Roche, English, Walne.
Hull FC: Connor; Naulago, Tuimavave, Griffin, Faraimo; Kelly, Sneyd; Paea, Houghton, Matongo, Minichiello, Manu, Westerman. Substitutes: Thompson, Lane, Ellis, Hadley.
Referee: Scott Mikalauskas (Leigh)