In discussing vessels of this period it is common for the name and rig to be given followed by a tonnage and maybe some reference to the number crew and possibly the guns/cannons carried. Little reference, if any is given to the vessels’ physical dimensions.
This note tries to address this in order that the reader will more readily understand and appreciate the conditions and privations the crew faced in undertaking voyages to the American Continent and other far flung destinations in vessels not much larger than an average yacht.
To help rationalize vessel size of our era it is useful to have a brief history/knowledge of ‘tonnage’.
From the Norman Conquest the ‘tonnage’ referred to in the description of a ship is most certainly a reference to the number of large casks of wine the ship was able to carry. These casks were known [and still are] as ‘tuns’, containing 256 gallons of wine weighing approximately one imperial ton. Although it is possible that a tun of wine could be loaded and unloaded onto a vessel of this era, it is thought that the more easily handled barrel of 32 gallons capacity and the standard measure for other goods such as flour, water, salt beef, dried/smoked fish, cereals etc would have been used. 8 standard barrels equal 1 tun
The measurement of all ships from 1694
A formula for this was adopted, for tax purposes: the function of length of keel x breadth x depth [to main deck], all divided by 100.
This tax replaced the earlier [1303 and 1347] tax systems based on the number of tuns able that could be carried.
Ships for purposes other than the carriage of goods i.e. fishing vessels and naval ships etc, were also then required to contribute toward dock and harbor dues and probably the exchequer.
This new tax, based on the physical dimensions of a ship is known as ‘TONNAGE’ and was the basis for the future development of current internationally recognised Gross and Net Tons from 1854.
Conclusion– It is almost certain that until 1695 the ‘tons’ referred to in the descriptions of ships of our era is the number of casks/tuns the ship was designed or able to carry. Since the weight of a ‘tun’ is about 1 imperial ton it also defines the cargo weight the ship can carry too.
We are grateful to the Manitoba Museum in Winnipeg for data taken from the full-sized replica of the Nonsuch. (The ketch built as merchant vessel in 1650 and used for the first trading voyage to Hudson’s Bay for what was to become the Hudson’s Bay Company in 1668). Although built in North Devon to mark the tri-centenary of the Hudson Bay Company it is now on display in this Winnipeg museum.
In reviewing the limited data available, it was evident that a number of dimensional ratios for ships of different size were common. This is not unusual, particularly since it had been stated in contemporary literature that when referring to the Nonsuch, ships of this type can be regarded as ocean going ships and would probably remained so for 100 or so years’.
The Nonsuch was described as ‘a ketch of 42 tons/tuns and built as a merchant ship in Wivenhoe, Essex in 1650. She was sold to the Navy in 1654. After some 13 years  she was bought and converted back to a merchant ship for the voyage to Canada.
The ‘Nonsuch’ had a keel length (Lk) of 37’ 0’’, an overall length [Loa] of 50’ 0’’, a beam [B] of 15’ 0’’, a head height in the hold of 7’ 0’’ which would give a Hull Depth [D] of 11’ or 12’ depending on structure. The displacement was approximately 75 tons.
The following dimensional ratios are therefore given as a guide to estimating the approximate size of ships of our era and interest.
Keel/length = 0.75 overall length
Length overall/beam = 3.5
Depth of Hull = 0.8 of beam
Draught = 0.5 depth of hull
Cargo weight = 0.6 x displacement (i.e. total weight of ship, including cargo, stores, arms and ammunition, masts, sails and rigging, ballast, crew etc.)
Cargo weight relates almost directly to the keel length i.e. 40’ keel length = 40 tons cargo.
Thus, knowing the keel length, the overall length is easily calculated i.e Overall length = keel length/0.75 or keel length x 4/3.
Having established an over-all length we are able to get a feel for the vessel size and the difficulties facing sailors of our era in making trans-Atlantic crossings