One thing that interests me are keel lengths for Dutch ships

They only seem to be scarcely documented, but there are some keel lengths known for Dutch warships. They seem known for more ships built from the 1680's and beyond. Ron van Maanen has some of them in his long document:

Adm   Ship                Date Length    Keel Length  Length to Keel
A     Argo                1772 149ft-9in 140ft-2in    1.0688
N     Arnhem              1693 145ft     123-1/2ft    1.174
A     Beemster Koe        1686 138ft     115ft        1.2
N     Beschermer          1690 170ft     146-1/4ft    1.1624

Sources:

1. Ron van Maanen, unpublished manuscript "Dutch Warships 1600-1800", undated but circa 1992