Albert Barnes on Acts 28:11

And after three months. Probably they remained there so long, because there was no favourable opportunity for them to go to Rome. If they arrived there, as is commonly supposed, in October, they left for Rome in January.

In a ship of Alexandria. Acts 27:6.

Whose sign. Which was ornamented with an image of Castor and Pollux. It was common to place on the prow of the ship the image Of some person, or god, whose name the ship bore. This custom is still observed.

Castor and Pollux. These were two semi-deities. They were reputed to be twin brothers, sons of Jupiter and Leda, the wife of Tyndarus, king of Sparta. After their death, they are fabled to have been translated to heaven, and made constellations under the name of gemini, or the twins. They then received divine honours, and were called the sons of Jupiter. They were supposed to preside over sailors, and to be their protectors; hence it was not uncommon to place their image on ships. See authorities in Lempriere's Dictionary.