In reading about warships of the early modern era, it's difficult to compare sizes because older ships tend to be measured in tons (or tuns) of burden (or burthen), compared to modern vessels being measured in tons (or tonnes) of displacement.

The former is a measurement of volume, the latter of weight, but for a floating vessel they should in theory be equivalent. Except that burden may be calculated by the simple dimensions of a ship, not its actual volume.

Searching Wikipedia for famous ships, I've found a few listing both burden and displacement:

Ship             Burden Displacement Ratio
---------------- ------ ------------ -----
Santa Maria         108     165 tons  1.53
HMS Victory       2,142   3,500 tons  1.63
USS Constitution  1,576   2,200 tons  1.40
HMS Orlando       3,740   5,493 tons  1.47
HMS Mersey        3,733   5,643 tons  1.51
Mayflower II        180     242 tons  1.34

These are fairly consistent; if the answer is "1.5 is usually fine," I can live with that. However, I have not yet found an example for a carrack (the Vasa is listed with displacement but not burden; the Mars either has a typo in one measure or the other, or uses a different measurement system). Being relatively heavily-built compared to these others, should I estimate the displacement of the Henry Grace à Dieu (1,000 tons burden) as, say, 1,700 tons?

  • It would seem a century later the builders assumed that the burthen was ~3/5 of the displacement [see here],(en.wikipedia.org/wiki/Builder%27s_Old_Measurement) but this is not necessarily appropriate to a ship with high castles as these probably shift the balance between displacement and burthen.. – Conrad Turner Nov 9 '15 at 9:55
  • @goldilocks: No; displacement is only the total interior volume of the vessel, so is an invariant measure for the vessel and has no relationship to the weight of the ship beyond any approximation based on the cube-square law. Loaded and unloaded displacement vary by an approximate measure of the interior volume required for operating purposes: rigging, fuel, crew, etc. – Pieter Geerkens Nov 10 '15 at 0:19
  • @PieterGeerkens Point taken...but in the case of the unarmored sinking ship I am approximately correct ;) – goldilocks Nov 10 '15 at 0:56
  • Note that all the figures you give for displacement are actually estimates based on individual historian's calculations and guesses, and tell more about the accuracy of the available historical records than actual vessels. No-one actually calculated displacement as a measure of ship size until late in the 19th century. For gaming purposes (a guess at your motivation) a figure of 1.5 would easily be as accurate as anything else. Remember that merchant vessels were deliberately designed during this period to maximize capacity for minimum burden, to minimize tax per voyage.. – Pieter Geerkens Nov 10 '15 at 2:25
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    @PieterGeerkens not all of these are estimates - Victory and Constitution are still around to measure, as is the (replica) Mayflower II, which is probably a fairly good copy. (The replica Santa Maria is something of a guess, though.) – Andrew Nov 10 '15 at 15:55

The simple answer is that there is no direct or meaningful way of converting burthen tonnage to displacement.

As the article you linked to notes, burthen tonnage was a rough calculation based on the length and beam of a vessel. This calculation varied over time and from country to country. When it was devised, it was intended as a measure of a ship's comparative carrying capacity for the purpose of taxation (at a time when warships were simply merchant ships with more guns). So comparing burthen tonnages would tell you which ship was larger only if you are comparing ships from the same country and era.

The burthen calculation is obviously an approximation and takes no account of the ship's actual hull form or design. It's therefore possible for two ships which have the same length and beam dimensions (but with very different hull forms and loadings) to have the same burthen tonnage but different displacement tonnages. Similarly, two ships of the same displacement (but different hull forms) could have very different burthen tonnage measurements (and length/beam measurements).

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  • True, but I'd hope that vessels of the same general class would have similar ratios. Unfortunately I've not yet found enough examples to confirm or deny this - I have found a caravel, a first-rate, a heavy frigate, a screw frigate, and a fluyt. – user4139 Nov 9 '15 at 13:22
  • You may find that similar ships (i.e. ones for the same purpose with similar hull forms) built around the same time have similar bm to displacement ratios. However, changes to ship design and construction over the years mean that that ratio may vary even with later examples of the same type. For example, the shift from tumblehome to wall sided ships meant that for the same beam and length, the later ships had greater capacity and a corresponding increase in displacement, which isn't reflected in the burthen tonnage calculation. – Steve Bird Nov 9 '15 at 13:52

The Mars link at Threedecks.org does not contain an error, but shows Displacement and burthen in Lasts (not tons BM). Lasts are a completely different unit of measure of volume utilized extensively in Scandinavia and the low countries and uses a different calculation to tons.

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  • This is a commentary on the question, not an answer to the question; I think the information is valuable, but SE conventions discourage commentary in answers. Would it be possible to rewrite this as an answer to the question? – Mark C. Wallace Sep 12 '16 at 12:18
  • Might be better just to convert this to a comment. – Steve Bird Sep 12 '16 at 12:26
  • OK, didn't know that and don't know how to convert the response to a comment? – Cy_ Sep 14 '16 at 7:57

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