To keep communications open it is not necessary to secure every
friendly ship traversing the line against attacks by the enemy.
All that is necessary is to restrict the enemy's activity so
far that he can inflict only such a moderate percentage of loss
on the friendly vessels that, as a whole, they will not cease
to run.
Keeping communications open will not secure a friendly place
against every form of attack. It will, however, secure a place
against attacks with large forces sustained for a considerable
length of time. If he can make attacks of this latter kind, it
is clear that the enemy controls the communications and that
we have failed to keep them open.
If communications are open for the passage of vessels of the
friendly mercantile marine, it follows that the relatively much
smaller number of supply-vessels can traverse the line.
As regards supply-vessels, a percentage of loss caused by the
enemy must be allowed for. If we put this at 10 per cent.--which,
taken absolutely, is probably sufficient--it means that _on_the_
_average_ out of ten supply-vessels sent we expect nine to reach
their destination.
We cannot, however, arrange that an equal loss will fall on every
group of ten vessels. Two such groups may arrive intact, whilst
a third may lose three vessels. Yet the 10 per cent. average
would be maintained.
This condition has to be allowed for. Investigations some years
ago led to the conclusion that it would be prudent to send five
carriers for every four wanted.
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