Small Dutch ships armed with 18pdrs

I still am rather mystified about a small Rotterdam ship armed with 18pdrs. The ship in question is the small Rotterdam, built in 1639, which I compare with the considerably larger Gelderland (built in 1634). Both of their lower tiers were armed with 18pdrs. I really don't understand how that could be for the Rotterdam, unless they were drakes or chambered guns, as I would have thought that normal iron guns would be too heavy. Too great an armament weight was a sign that something was amiss with the Aemelia and Brederode dimensions. It turned out that they were listed in Maas feet, but many people treated them as if they were measured in Amsterdam feet. People like Ab Hoving and Jan Glete had long recognized this issue, and had made the adjustment. The dimensions for the Rotterdam are: 116ft x 27ft x 11ft (in Maas feet: 106ft x 25ft x 9.5ft) The dimensions for the Gelderland are: 128.5ft x 30.5ft x 13.5ft (in Maas feet: 118ft x 28ft x 12.5ft) The Rotterdam's armament (30 guns) was: brass guns: 2-chambered 24pdr, 2-chambered 4pdr iron guns: 18-18pdr, 8-8pdr The Gelderland's armament (40 guns) was: brass guns: 2-light 18pdr, 4-light 12pdr, 2-chambered 12pdr, 2-chambered 6pdr, 2-chambered 4pdr iron guns: 16-18pdr, 6-12pdr, 6-6pdr The Rotterdam, in particular, seems to be anomolous. I estimate the Rotterdam's displacement to have been 486.5 tons. Her broadside was 222 lbs. The armament weight was at most 48.1 tons. I am guessing that the 18pdrs were drakes or chambered, and that would reduce the weight down from the maximum of 8.59% of displacement. I use that measure rather than percentage of burden, as Frank Fox had suggested that it was a better measure than "gross tonnage". I estimate the Gelderland's displacement as 741.4 tons. Her broadside weight was 224 lbs, little more than that of the Rotterdam. The armament weight was at most 45.32 tons, which would be 6.11% of the displacement, which is in line with other 40-gun ships. If I use my estimates for brass chambered and drakes, derived from what I learned from Nico Brinck, then the Rotterdam's armament weight drops to 26.84 tons, which is 5.52% of displacement. Of course, this is all guesswork, but it still explains a lot.