The 1:10 model and what photos I have seen of the real ship show the upper gundeck ports to be slightly smaller than the lower gundeck ports. Is this actually the case? Given the guns were the same 24-pounders on both decks, why would the port sizes be different?
The lower gundeck ports are about 75-80 cm wide and 79-84 cm high at the outer opening, while the upper gundeck ports (and the last port on each side on the lower gundeck) are 73-75 cm square. It is not a big difference, but noticeable even from a distance. It is not possible to say with any certainty why it was done this way, although it was already an established tradition in ships with multiple gundecks to have graduated ports. The difference may be purely aesthetic - the profusion of sculptures and the money spent on them indicate that aesthetic considerations were often important in warship design. If you draw out a side elevation of a multi-decked ships and make all the ports the same size, it looks odd.
It is possible that the different port sizes reflect the complicated armament history of the ship. At least seven different armament plans for Vasa survive, all of them made before the ship sailed. Most of the plans from 1626 and 1628 provide for a unitary armament on the gundecks, with two full decks of 24-pounders. The surviving plans from 1627, as well as one from 1626, stipulate a mixed armament of 24-pounders on the lower gundeck and 12-pounders on the upper gundeck. The ports were probably cut and finished during 1627, so it may be that the armament plans being discussed at the admiralty influenced the construction of the ports.
One practical consequence of the port size was the salvage methods in the 1660s. The lower parts are large enough to allow a gun to be pulled out through them, but the smaller ports of the upper gundeck are just enough smaller to make this impractical, and so the salvors ripped up the upper deck and got the guns out through the beams. The two guns at the after end of the lower gundeck, which have smaller ports, were two of the three guns still left on the ship. There is evidence that the salvors tried to remove these but could not.
Thanks for the information, Fred. Interesting that aesthetics could be a consideration. I would have thought there would be structural integrity issues with large ports, and that the shipwrights would have wanted as small a port as possible for a given size of gun.
Port size does affect structural strength, but the difference in this case may not be significant. We are still not sure exactly how the port openings affect strength and stiffness, but we are about to find out! We are building a section of ship side, about 4x3 m, for stress testing in order to figure some of these questions out.