Hello Robert T. Page!
Sorry about the lateness of this reply.
You were absolutely spot -on with your reasons why a speedometer would not work well on a ship. The same reason apply more or less to using engine revolutions. Michael is correct. There is only one way - even up to the present day using sat. nav. - to determine a ship's average speed and incidentally, the true direction she made good during her travels and that is by establishing two correct, exact positions and measuring or calculating the distance between them then applying lapsed time.
JC
Sorry about the lateness of this reply.
You were absolutely spot -on with your reasons why a speedometer would not work well on a ship. The same reason apply more or less to using engine revolutions. Michael is correct. There is only one way - even up to the present day using sat. nav. - to determine a ship's average speed and incidentally, the true direction she made good during her travels and that is by establishing two correct, exact positions and measuring or calculating the distance between them then applying lapsed time.
JC