New answers tagged naval
After some more research I stand by my earlier comment in general the gun ports had minimal protection, relying on small size and being in the shadow of the gun. There is a large amount of negative evidence for this, for example in his books Warrior to Dreadnought, The Grand Fleet and Nelson to Vanguard D. K. Brown does not mention protection to turret gun ...
One factor not mentioned in the other answers is this: Those openings are very small relative to both the size of the vehicle and the accuracy at engagement ranges of the weapons trained against it. In other words, chances of the openings being hit are very small indeed, even if deliberately aimed for. Hence they're typically either unarmoured or armoured ...
Telling from your pictures of the Yamato and an Iowa-class, the holes were covered from the inside using steel, probably with less thickness than the turret armament. The combat environment of tanks and battleships is different, with battleships receiving more fire from higher elevation angles than tanks. This leaves mostly shrapnel from the deck as a threat ...
The Mayflower was a small-ish merchant ship (Dutch Fluyt) of the early 17th Century ~100 tons burden. With a crew of ~25-30 men and boys. Also all the models/diagrams seem to show stairs from the half deck to the upper deck and to the deck below that above the hold (and maybe down to the orlop deck or hold )
There are web sites that discuss the history of Channel ferries. Paddle steamers made their appearance in the early 1800s: Of course, every kind of boat could be used as a ferry, even small foot powered paddle boats. The 1827 scene below is from the United States, but the same kind of boats were used in the English channel:
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