I'm glad to see that you are reading a book about sailing in olden times, but - as you have already realised, given the many similar queries you have recently submitted - such topics have their own particular vocabulary and style of writing which can be very hard to translate into "landlubbers'" terms. Even for those of us who have a fair understanding of these matters, it can be difficult to work out the meaning of particular passages without access to much more of the context than you have been providing.
In this particular case, the writer is referring either to large (immense sweeps) or small and intermittent (swelters) changes of the wind and water (waves/current?) which may also be reflected in movements of the sails.
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