Artos which part do you disagree? The one with the 30%? Or the one with the missing escorts. As OTL the USN was a substantial part of the escorts how does the UK make up for this? And 30% is a pretty standard figure.
Well, to some degree all of it.
a) a convoy sytem cuts transport by 30% compared to single riders.
This is true, or at least the best estimate. However, IOTL prior to the use of convoys that Admiralty policy of diverting shipping around areas where raiders were known to be operating, and the need for shipping to follow zigzag courses also cut the rate of transport. Thus shipping was already operating at a reduced rate. When Convoys were eventually implemented it was found that the rate of reduction was not much greater than it had been previously.
That additional shipping simply wasn‘t there.
This one is a little trickier to determine, since it is difficult to find an accurate assessment of what ships of what tonnage were employed where. However, it is notable that rationing was only instituted in WW1 in 1918. It implies some level of margin in the shipping requirement.
I had a list that I posted in another thread asking similar questions that I now have a hard time finding that detailed the number of British ships (though not tonnage) that was employed at different tasks in 1917. If I find it I will put it up.
b) you would also need the escorts which the RN did not have.
In March 1917 the Admiralty calculated that they would need 75 destroyers assigned to escort duty to make convoys work. They felt they could spare 43. This was part of their argument for not implementing convoys. However, once they did end up implementing them it was found that their math had been off for several reasons:
1) Their initial figures were based on the total amount of shipping going through the ports, including lighter traffic going up the coasts. This accounted for a huge portion of the daily sailings. In reality less than half of those sailings would have needed to be in convoys. I am unsure of this error was discovered before or after March 1917, and thus whether it contributed to the 75 destroyers figure, but it is indicative of the difference between expectation and reality when it came to convoy escort.
2) Trans Atlantic convoys could be done without the use of destroyers. Armoured Cruisers, sloops, armed merchant cruisers and pre-dreadnoughts could all be used with basically no loss in protection. Even in the more dangerous inner waters sloops ended up taking a lot of the burden off destroyers
3) The required ratio of escorts to merchant vessels turned out to be much lower than expected. The first trans-Atlantic convoy in May had nine escorts (8 destroyers and an armoured cruiser) to 12 merchant ships. By June they had 20 ships per convoy, 26 in September and 36 in October.
The 30 destroyers that the US sent were certainly very helpful. However, partially this was because it allowed the GF to retain more of its destroyer screen (including screens for the newly arrived American Battleship squadron), and allowed them to use destroyers (who were certainly better at the job) in more areas. It was not because it was impossible to implement without American escorts.