The festival attracts a wide-ranging audience from all over New England and the Northeast. On the last weekend in September, the streets of Portsmouth ring with the music of the sea. Singers and musicians wander the sidewalks downtown, and the sounds of sea shanties and forebitters waft through the doorways of nearly a dozen venues all within a block or two of Market Square. The festival runs from 10 a.m. Saturday through Sunday afternoon, with a concert on Saturday evening.There is a Maritime Songwriting Competition on Saturday morning at 10:00; the winner will be invited to perform the winning song at the Saturday night concert.The grand finale of the festival is the Chantey Blast, a rollicking “all hands in” public sing in Market Square at 5:15 p.m. Sunday, with each of the weekend’s performers leading a song.Sea shanties, the rhythmic work songs that helped sailors coordinate their efforts while raising or lowering sails…