We study the velocity of bubble walls in the electroweak phase transition .
For several extensions of the Standard Model , we estimate the friction and calculate the wall velocity , taking into account the hydrodynamics .
We find that deflagrations are generally more likely than detonations .
Nevertheless , for models with extra bosons , which give a strongly first-order phase transition , the deflagration velocity is in general quite high , 0.1 \lesssim v _ { w } \lesssim 0.6 .
Therefore , such phase transitions may produce an important signal of gravitational waves .
On the other hand , models with extra fermions which are strongly coupled to the Higgs boson may provide a strongly first-order phase transition and small velocities , 10 ^ { -2 } \lesssim v _ { w } \lesssim 10 ^ { -1 } , as required by electroweak baryogenesis .