The Portuguese already have very good ships with +10% hit points due to their civilization bonus. Carrack becomes very handy for Portuguese players in open water maps where naval combat is commonplace. It should be treated as a Dock technology and it is recommended to research Careening first due to the gold cost. When combined with Careening, Carrack provides a substantial boost to a Portuguese player's anti-ship defense making the Portuguese navy arguably the most resilient in the game.
Historically, the Carrack was a type of ship developed by the Genoese in the 15th century before being adopted by the Spanish and Portuguese for oceanic travel. Because of this, it is rather odd for this technology to fall into the Portuguese tech tree rather than the Italians'. However, it could possibly mean that the Portuguese are simply incorporating the beneficial traits of the Carrack into their naval vessels to make them sturdier.