John T wrote:What where the average capacity of these barges?
bulk loads of 100-400 tons?
What do a infantery company weights?
If you want to use a barge with such a light load you'd like to weight her down with ballast and sand and steel plate is pretty good ballast.
I see two tradeoffs,
Center of gravity to aviod that the barge becomes top heavy,
so you have to load 200 tons on sand low and add 50 tons of steel plate on the top and then fill the space between with your infantery company.
And draught, the lighter the barge the closer to land it comes before getting stuck.
There appears to be some real confusion of terms here, which is typical when talking about seagoing vessels. First you have to understand the parameters.
'Tonnage’ is actually a number of different things in respect to shipping:
Short Ton equals 2,000 pounds, Long Ton equals 2,240 pounds, Metric Ton equals 2,204.6 pounds. All are measures of weight.
Gross Register Tonnage (GRT) is a measure of the entire interior space in a ship as measure by Register Tons (100 cubic feet or 2.83 cubic meters). It is not actually a measure of cargo capacity; it is a measure of volume.
Dead Weight Tonnage (DWT) is a ship's total carrying capacity including the ship's gear, supplies and crew, in long tons, i.e., 2,240 pounds (1.016 metric or 1.12 short tons). It is a measure of weight and not of volume. The Dead Weight Effective Lift of a cargo vessel is expressed in long tons and is typically less than 80 percent of its Dead Weight Tonnage.
A Measurement or Ships Ton is 40 cubic feet of space (1.13 cubic meters), conversion of measurement tons to weight reflects the density of the cargo.
Net Tonnage is the entire useful cargo capacity of a ship expressed as register tons.
1,000 GRT = approximately 1,500 DWT = approximately 1,200 Long Tons of Dead Weight Effective Lift = approximately 1,775 measurement tons.
The vessels in question were the Prähme – un-powered barges, over 2,000 of which were confiscated in Belgium, Holland, France, and Germany for use in the operation. There were generally two types the Spitsen or pinnace (typically 38.5 x 5.05 x 2.3 meters, 360 DWT or about 240 GRT) and the larger Kempenaars (or French Campinois, typically 50.0 x 6.6 x 2.5 meters, 620 DWT or about 413 GRT).
We may roughly calculate the DW effective lift of these vessels as about 288 and 496 long tons respectively. However, that works very well for high-efficiency cargoes like grain, coal and lumber, which the vessels were designed for and less well for military cargoes, which tend to be highly inefficient.
Military cargoes include lots of people, which are extremely low-density. For one thing you can't pack them like sardines in a tin, it makes them cranky. Typically for any trip more than a few hours long at least a measurement ton must be allowed per man, so that 125-kilo man (with his equipment) is roughly equal to one-half a 'ton' (GRT). Or to put it another way, your 125-kilo man is taking up the space that typically could be occupied by 665 kilos of more efficient cargo. BTW, horses are worse and more cantankerous to boot.
Now a tank is more efficient, we could estimate that a 20 metric ton tank probably takes up about 20 cubic meters of space, so 1 metric ton per roughly 0.88 measurement tons. But then we have the opposite problem, the hold deck has to be capable of supporting that weight.
Military cargoes in an assault also need to be highly accessible. They can't be packed in as tightly as possible for efficiencies sake. Typically, the US Army found that in regular shipping from the US to Britain, about 70 to 80 percent of a ships usable capacity (it's 'full and down' capacity where the maximum hold space used equals the maximum allowed weight) was normal and that assault shipping was even less, approaching half the ships normal capacity.
So overall we could expect that the Spitsen could probably accommodate the men and equipment of an infantry company, without the vehicles, horses, and other baggage of the Troß. The Kempenaars would easily accomodate an infantry company, possibly including the Troß. Either could accomodate motor vehicles, probably 5 or so in the Spitsen and 7 or 8 in the Kempenaar. One efficiency would be that they at least could be embarked loaded with cargoes on the order of 2-5 tons per vehicle would be about average. Perhaps 2 or 3 armored vehicles could be loaded per Spitsen and myeb double that for the Kempenaar. Finally, probably two or more Spitsen would be required for a typical 4-gun artillery battery with its crew, prime movers and other vehicles. It is possible, but unlikely, that an entire battery could fit into a Kempenaar.
Eventually 1,336 Spitsen and 982 Kempenaars were collected, but as of 17 September only 1,277 were at the designated 'invasion' ports (Dunkirk, Ostende, Rotterdam, Calais, Antwerp, Boulogne, and Le Havre) while another 698 were being prepared or were on the way. That indicates that 343 more were collected sometime after 17 September.