To maintain a constant speed, the engines take over automatically when the wind dies down, and they stop completely when the wind is of sufficient strength when blowing in the right direction.
tə meɪntˈeɪn ɐ kˈɑːnstənt spˈiːd, ðɪ ˈɛndʒɪnz tˈeɪk ˌoʊvɚɹ ˌɔːɾəmˈæɾɪkli wɛn ðə wˈɪnd dˈaɪz dˈaʊn, ænd ðeɪ stˈɑːp kəmplˈiːtli wɛn ðə wˈɪnd ɪz ʌv səfˈɪʃənt stɹˈɛŋθ wɛn blˈoʊɪŋ ɪnðə ɹˈaɪt dᵻɹˈɛkʃən.