I've done a bit more research on penteconters and, in particular, their dual purpose use in carrying people and cargo as well as as warships. It is thought a penteconter may have been able to carry fifty passengers on top of its crew or, crucially, 30-50 cubic metres of cargo. So our Herodotan penteconters might have carried 12-20 cubic metres of stores with 30 marines. If this was grain, that's 8-14 tonnes ( less if using amphorae).
Interesting. As mentioned, if the pentekonters' marines are debarked onto the beach before any cargo is loaded for the day, they can take more cargo (at least 30 people's worth, perhaps doubling the given estimate)
and have a 30-strong landing party on shore to help with unloading. Or, if the corn-ships had minimal crews, the marines could be placed on board for the duration of the transfer to provide manpower to shift amphorae. Or some of both. Given the extensive experience of maritime peoples of the East Med, I would assume they would operate as efficiently and effectively as possible.
I do not know why Roy sees a problem with this happening on a large scale. It is not really a 'just in time' system so much as a 'take a day out' system. The only time during the march to Thermopylae when the fleet and army were out of touch for several days was when the army was marching through Macedonia, to meet the fleet at Thermum (near Thermopylae). Is there any good reason why the fleet would not have arrived before the army, unloaded a week's supplies at leisure (even taking a few days if it desired, there being no great urgency prior to the army's arrival) and then waited for the army while the empty corn-ships, perhaps with empty amphorae, went back to Asia Minor for a refill?
The fleet of triakonters and pentekonters would presumably service only as many corn-ships as were needed to deliver supplies for that instalment, working a few ships until they were empty before starting on the next. Cargo transfer might involve any number of techniques from using yardarms as cranes to cable transfer systems to simply handing amphorae down from the corn-ship to the pentekonter. We can actually put some basic figures forward for the overall process.
If we look at a system where two pentekonters service a single corn ship, one of them coud put its marines on board and the other its marines on the beach. This gives balanced loading and unloading parties and if we rate the cargo of a corn ship at 50 tons (probably an underestimate) then at, say, 15 tons per pentekonter (perhaps a bit less for the sake of optimising time and effort) it only takes four pentekonter-loads to clear one corn-ship, i.e. two trips by each pentekonter, then it is on to the next corn ship. Say one hour to empty each ship. This involves 30 men on the ship plus any crew moving 50 tons in one hour - about 1.7 tons per man per half-hour , the other half of that time being spent in transit between ship and shore. Hence each man is moving 33 hundredweight in 30 minutes, about one hundredweight (or amphora) per minute per man involved, with 50 men in the pentekonter helping to receive and stow the loads. Such figures are speculative but if reasonable give us a rule of thumb by which two pentekonters empty a small corn ship and deliver its load to the beaches in about an hour.
With, say, 600 pentekonters it would not take long to unload 300 corn ships, beach size permitting. Beach size would in fact appear to be the limiting factor with regard to unloading capacity. With one pentekonter loading at the corn ship and the other unloading at the beach only half the pentekonters involved need ot use the beach at any one time, and if the loading/unloading cycles of alternate ships are offset this can be halved again. Having 200 pentekonters taking three times as long would empty the same number of corn ships. This is still only three hours.
Yes, a squall would cause problems but squalls are not common in the western Aegean in the spring and summer. The prevailing northerly winds were known and everyone would have taken them into account for cross-Aegean journeys. Beach-landing was well practised (unlike WW2) because everyone was used to beaching ships; also well-practised was loading and unloading of vessels inside and outside ports. The main consideration for large-scale activity is large-scale organisation, and the Persians were not strangers to large-scale maritime expeditions (
vide Marathon).