Walking along the beach at the end of the Proteus episode, Stephen sees “Moving through the air high spars of a threemaster, her sails brailed up on the crosstrees, homing, upstream, silently moving, a silent ship” (3.503-05). That night, in Eumaeus, Stephen and Bloom meet one of the threemaster’s sailors, unsilent able-bodied seaman D. B. Murphy, who tells them, “We come up this morning eleven o’clock. The threemaster Rosevean from Bridgwater with bricks” (16.450-51).
In “Ulysses” Annotated, 3.504-05, Don Gifford notes that the “Shipping News” from the Freeman’s Journal for June 16, 1904, identifies Rosevean as a schooner, adds some information about the port of Bridgwater and its brick industry, and glosses the religious sense of “crosstrees.” In Conversations with Joyce (1934), Frank Budgen elicits a more detailed gloss from Joyce himself:
I stopped at the door as I was about to leave.
“You know, Joyce,” I said, “when Stephen sees that three-masted schooner’s sails brailed up to her crosstrees.”
“Yes,” he said. “What about it?”
“Only this. I sailed on schooners of that sort once and the only word we ever used for the spars to which the sails are bent was ‘yards.’ ‘Crosstrees’ were the lighter spars fixed near the lower masthead. Their function was to give purchase to the topmost standing rigging.”
Joyce thought for a moment.
“Thank you for pointing it out,” he said. “There’s no sort of criticism I more value than that. But the word ‘crosstrees’ is essential. It comes in later on and I can’t change it. After all, a yard is also a crosstree for the onlooking landlubber.”
And crosstree does recur in the pattern in that episode where Stephen discusses Shakespeare with some Dublin scholars. “Who, put upon by his fiends, stripped and whipped, was nailed like bat to barndoor, starved on crosstree.”
(“James Joyce’s ‘Ulysses’: A Casebook,” ed. Derek Attridge [Oxford University Press, 2004], p. 262)
I now add this anonymous painting of the actually two-masted Rosevean, cleaned in post-processing and with an inserted arrow pointing to the fore crosstrees. The original is in Bridgwater’s Blake Museum, https://artuk.org/discover/artworks/the-rosevean-of-bridgwater-39761.