THAMES
TONS
[(L-B) x B x 0.5B]94
114 CLASSIC BOAT AUGUST 2016Sternpost
I
n a nautical context, the little word ‘ton’ has
probably caused more confusion than any other. The
problem is that the word has two quite distinct uses;
one is related to weight, and one is an indication of the
volume (or capacity) of a vessel.
Ton as a unit of weight we are all familiar with. In
the UK one ton is equal to 2,240lb; sometimes referred
to as a long ton or imperial ton in order to distinguish
it from short tons of only 2,000lb. Short tons are fairly
common in the USA but rarely encountered elsewhere;
if the type of ton is not actually specifi ed, you have to
take an educated guess, based on context and origin,
which type is being used; if you have no other
information to guide you, it’s odds-on that it will be a
‘long’ ton of 2,240lb. Of course, we also have a metric
tonne (the spelling distinguishes this version) which is
1,000kg; and if you do the conversion you will see that
a metric tonne is equivalent to 2,204.6lb, so is slightly
less than an imperial ton. Metric tonnes and imperial
tons are close enough that the difference may not be
signifi cant, depending on context. Only tons and
tonnes that relate to actual weight (displacement) can
be converted from metric to imperial or into lesser
units (Americans often quote displacement in pounds,
for example, probably to avoid any confusion over
which sort of ton is being referred to; and in Europe
kilogrammes are often used).
The other use of the word is the one that causes the
confusion, where it indicates a vessel’s volume or
capacity. It stems from the origin of the word; tun, a
type of barrel. From early times authorities wanted
to quantify the potential capacity of ships
involved in trade for customs purposes.
Calculating the cubic volume of a hull,
especially of wooden ships with bulky
structures, was complex. It was more
pragmatic to look for a formula that
expressed the potential cargo capacity from
easily measured dimensions such as length
and beam. The result was a fi gure expressed in
tons, but was actually related to the capacity; a
ship was said to be so many tons, giving itscalculated nominal capacity in terms of number of
standard size barrels. This wasn’t necessarily the actual
capacity of the vessel; merely a consistent way of
comparing hulls; it evolved eventually to give us gross,
net and register tonnages (and all the variations); all
volumetric approximations of the capacity of a hull,
not of weight. The use of these relates almost
exclusively to registration or licensing, and tells you
nothing about how much a yacht (or ship) weighs.
Until 1854 yachts were measured by the same
method as commercial craft, but eventually the Royal
Thames Yacht Club decided to refi ne the method of
measurement, specifi cally for yachts, and so for the
following season they adopted this formula:
[(L-B) x B x 0.5B] / 94 where L is length on deck
‘from stem to sternpost’ and B is maximum beam; and
the result was said to be so-many ‘Thames tons’.
In fact the formula yields a result in cubic feet; so it
is still an approximation of the cubic capacity (or
volume) of the hull, and not related to the actual
weight (or displacement). It was adopted by the YRA
in 1876 and its use became widespread.
For well over a century yachts were commonly said
to be so-many (Thames) tons; but it was so common
that the ‘Thames’ was often implied or assumed and
not spelt out. Especially with classic and traditional
craft you still come across references to yachts being so
many tons, and this nearly always relates to their
Thames ‘tonnage’. For example, Britannia is often
quoted as being 221 tons; this is her Thames tonnage;
her displacement (weight) was more like 160 tons.
With certain hull shapes the two may be the same;
Jolie Brise has by chance a Thames tonnage about
the same as her displacement, a rare coincidence.
As these tonnages are not weights, they
cannot be converted to different units. It is
nonsensical to refer to Thames tonnes, for
example, as we did in CB335 relating to the
8-ton (Thames) cutter Witch. Like many aspects
of yacht history, this muddled picture has simply
evolved over time, and it’s not surprising it catches
us all out from time to time.Ton, tonne or tun?
Theo Rye clears it up once and for all
where L is length on deck ‘from
stem to sternpost’ and B is
maximum beam; and the result
was said to be so-many
‘Thames tons’1
1 1
BRITISH
US
TONNE(LONG or IMPERIAL TON)2,240lb
1,000kg
(2,204.6lb) 2,000lb(METRIC TONNE) (SHORT TON)TON
TON TUN1