Why an Allied fleet swap punishes the Soviet Union and Britain

What happens if the major Allied powers swap their navies during the Second World War?

The Soviet Union cannot sustain the US fleet, Britain lacks enough ships, and the United States can adapt the Royal Navy despite an imperfect doctrinal fit.

The scenario gives the US Navy to the Soviet Union, the Royal Navy to the United States, and the smaller Soviet and French fleets to Britain. Supporting the much larger American fleet is judged beyond Soviet capacity, with collapse predicted under the hypothetical's conditions. Britain would be left with too few ships for its commitments. The United States could adapt the Royal Navy's capable ships to American service, although the designs would not perfectly fit its normal operations.