Originally Posted by tac foley
With the best will in the world, a fair wind and following sea and all going well in the engine room, a Dunkirk-style 'small ship' might just be making 10kn. However, the prevailing wind in the English Channel does not favour smaller boats heading Easterly, nor do the many currents that abound in such a nautical bottleneck help in straight-line navigation of any kind.
With a sailing distance of between 230 and 260 miles, depending on many other factors, this may have taken up to 36 hours of non-stop sailing to achieve.
This is from my next-door neighbour, who is a yottie with an auxiliary yacht based in Kingsbridge, Devonshire.
I have not yet seen the film - but my guess is that it will be as accurate historically as ‘Braveheart’ or the one where the US navy captured the Enigma machine from the UBoat.
Small boats in the ‘30s generally had displacement hulls – they had to push the water out of the way when going forward. This creates a bow-wave in front and a stern-wave (drag) at the back, creating a hollow in the middle into which the boat ‘sinks’. As a result the maximum theoretical speed a displacement hull can attain is roughly 1.2 x √LWL (length water line). Thus a 30ft motorboat with a LWL of 25ft could motor at 6 knots, so a distance of c250miles would take about 40 hours. That is the time of a bit more than six tides, (3 ebbing, 3 flowing) and as they flow in /out the Channel they more or less cancel out. At this time of the year the wind generally is south-westerly in the Channel, so it would be a help if coming from any port on the Western Approach, and a hindrance on the return voyage.
(who will never forget his first time crossing the shipping lanes in the Channel, no Luftwaffe but lots of fast big ships with no brakes)