Instead of boiling them try this way.
Lift the top shell off and clean away gills and mustard.
If you insert thumb in vent( ) twist sideways it should lift the top off.
Put some sweet chilli and lemon juice in stomach cavity, and parcel wrap in aluminium foil, place on BBQ right way up and cook for 10/15 minutes or so.
Eat when cool enough,
I havent boiled a crab since someone put me on to that one.
Paul