The barges were much more survivable than many would have us believe. Peter Schenk's book on the German plans and efforts around Sealion “Invasion of England 1940” was translated in 1990 and makes the following observation.
All barges had to meet following naval requirements…
Able to handle open water up to sea state 2 [Significant wave height of 1.4 feet or 0.4 meters], which was the basic English channel sea state.
Able to land on beaches with slope of 1 degree
Able to transport a 25 ton tank
Able to use all Dutch Belgian and French canals.
However the barges exceeded these figures , here’s a quote from Schenk “Invasion of England 1940” Translated 1990, pp 70
"For the first criteria it was calculated that the barges would need a freeboard of at least 2 m and would have to be in a good state of repair. As it turned out , the barges were more seaworthy than expected, shipping little water during exercise in winds of force 4 to 5 and coping well with waves. Even at wind forces of 6 to 8 only two barges reported damage to external bow doors during one exercise with the 17th Infantry Division."
Force 4 winds are 20-30kph and 1 meter waves, while Force 5 is 30-40kph winds and 2 meter waves. Force 6-8 gale force winds are 60-75kph and waves of 3-5meters. Storm studies from the channel show waves topping 2 meters, but such storms happen once per month in the spring and fall and twice a month in the winter. They are all exceedingly rare in the summer months and rarely last more than 4 days duration. At most this is one storm day out of every 6, suggesting such storms would only impose a delay on landings by several days.
http://en.wikipedia.org/wiki/Beaufort_scale
Schenk notes there were 1336 x type A1 "Péniche" barges @ 39 meters long 5 m ; wide & 2.3m high , with a capacity of 360 tonnes. In addition there were 982 larger "Kampinen" type A2 barges @ 50 meters long 6.6m wide & 2.5m high and a capacity of 620 tonnes [able to carry 4 tanks] .
Most barges were towed and about ~ 1/3 of these barges were already motorized with 6-7 knts speed, however it was reckoned that in some cases 30% of the barges were so old and worn as to be unusable, while the rest were damaged due to untrained crews. If this is applied across the board the 2400 barges would net 1600 usable barges of which maybe ¼ were motorized.
The historical conversion task was massive and between August and early September 2400 barges and 400 motorboats were assemble. By early September about 1269 barges were converted and by September 24th about 1552 converted towed landing barges had arrived at the embarkation points, including another ~ 330 in reserve. When the task was completed some months later the total had swollen to 1939 converted landing barges.
[P Schenk “Invasion of England 1940” , pp 65-114].
By early September at least 225 motorized landing barges had been acquired , while power units were added to another 25 towed barges. In October the Heer and Luftwaffe erected another 50-100 engineering rafts , each employing 2-3 surplus 500-750hp engines from a total of 2000 such surplus engines. These modified Sieble/Herbert Rafts could cruise across the channel at ~ 6knts and dash to the shore at ~ 10knts. Contrary to perception they apparently were quite seaworthy in coastal waters and ‘the channel’.
[P Schenk “Invasion of England 1940” , pp 115-129].
The ~ 800 remaining unconverted barges were instead modified into supply barges and employed to warehouse fuel oil , diesel & gasoline as well as water and other supplies. Most of these barges were powered barges that had ‘broken down’ and treated as towed barges. Their total load capacity was around 225,000 tons while the gross transport tankerage was 130,000 tons in 39 tankers and 75 ‘lighters/barges’ . The expected daily consumption was thought to be about 10-12,000 tons at full strength, so on at most only 1/10th of the tankers [ 4 tankers and 7 lighters] had to cross the channel each day to feed the two armies logistical needs. [ “The Invasion of England 1940” , Peter Schenk, pp 174-175].
The barge sortie rate only envisaged the use of 2/3 of the barges on the first sortie, while a mere 400 barges were needed for each of the following 7 sorties to sustain the offensive and transport follow on waves. Since turnaround time was at best 4 days, at most 100 barges would need to make the crossing each day to sustain campaign at full strength.[ “The Invasion of England 1940” , Peter Schenk, pp 232-235].
In practice Wehrmacht logistics through out the war , were never run at full strength or anything like capacity. This rarely slowed offensives since combat doctrine and tactics were more important. While lack of logistics did crimp operations and slow tempo, it didn’t prevent operations even when the delivery rate fell below 50% of capacity, as was the case when the Germans moved further and further into Russia in 1941. It was always a combination of effects that terminated campaigns against the Germans, only part of which was logistics.
50 Naval gun ships
http://www.nzcoastalshipping.com/dutchcoasters.html
Its reported roughly 100 “Coaster” vessels were also employed in the Sea lion plan. Coasters were miniature freighters of 100-400 tons able to haul large loads from small coastal ports to larger ports for mass shipment overseas on larger Freighters. They have limited range and facilities but large cargo space and hoist. Plan was to convert ~ 50 coasters each with a pair of light flak guns and to mount one 6” gun on each of twenty heavier coasters , while three 3” gun were to be mounted on each of 27 smaller coasters.
These boats had sand added to ballasts to increase stability and provide limited below water protection . In addition concrete was added to the wheelhouse and each gun mount had thin armored shield . This provided small arms resistance and splinter protection all round. In addition each ship was degaussed against magnetic mines.
Historically only 5 of 20 heavy 6” gun coasters were converted and 27 lighter 3” gun coasters were converted. But another 200 guns/howitzers were made available for usage on such improvised gun ships. Although the ships were quite small they had high free boards allowing operations in ‘sea state 6’ or 3-4m waves and 40-50kph winds. [P Schenk “Invasion of England 1940” , pp 46-48].
Pioniersturmboot 39
It was planned to convert and deploy hundreds of fishing boats to land assault troops with each wave. The plan was to mount them on ramps on either side of the hundreds of fishing boats, Trawlers ,Coasters and Minesweepers. These boats were able to haul a 50-75 troops plus light arms and deploy them ashore @ 25kph [15knts] through a pair of assault boats launched and deposit and recovered through ramps mounted on the side of the boat. It was shown through trial and error that these fishing boats could launch the assault boats quite well traveling at ½ speed [IE 5-6 knts]. The assault boats were the Pioniersturmboot 39 , of which about 500 out of planned 1500 had been delivered in time for Sealion. By December about 800 had been assembles for this task, when the effort was halted.
[P Schenk “Invasion of England 1940” , pp 48-58].
Each invasion group had a leader boat 10knts speed, plus two tugs to tow one powered and one un powered barge. At a prearranged point the barges would be detached and the powered barge would tow the un powered barge into shore. Most barges had a light flak gun mounted amid ship , although hundreds mounted either 3" howitzers or Pak guns. While useless at hitting ships [3 near misses on 100 test shots @ 600-1000m range], they were thought to be very important in contributing fire support to landing troops, while vulnerable on the beaches.
While the build up was rushed massive and impressive one is left with the impression that given enough time the cross channel invasion would certainly have worked. There is clear evidence that the Germans had been experimenting with amphibious assaults since 1925 and plans for such an invasion of UK had been in the works since 1938 ...but Hitler would hear nothing of these developments due to his believe in England as an ally.
Finally the plan didn't envisage the need to attain air supremacy over England prior to any invasion as is commonly reported, instead all that was required was air superiority over the channel, which was achieved in September 1940. [P Schenk “Invasion of England 1940” , pp 246].At the end of the day the decision to go or not to go rested with Hitler himself and he could not throw his belief that the Brits would cave with draw from the war and allow him free hand in the east. He played each service branch off against each other since it served his purpose to put the pressure on the UK to fold. Schenk notes the following in conclusion pp 357-358.
"If conditions had been right , the German air superiority over southern England should have sufficed for a German landing operation. However, Germany had still hoped to bomb Britain into submission,".....
"In the autumn of 1940 the navy had the chance to end the conflict with Britain with one lightning combined arms operation. While it was able to amass a hugh transport fleet in a Herculean effort , the navy considered it impossible to protect. Ansel contradicts this notion, regarding it conceivable that a British attack on the fleet could have been thwarted given sufficient measures on the part of the navy and Luftwaffe. if all the factors are taken into consideration-Luftwaffe attacks on the Royal navy, mine barriers , coastal artillery and the deployment of the German navy in its entirety- then Ansel could be right. Sealion was cancelled primarily for political and not military reasons".