Page 258 - Introduction to Naval Architecture

PROPULSION 243

Figure 9.21 Measured mile

accurate enough for speed trial purposes. Indeed the speed trial is
often used to calibrate the log.
Traditionally a ship has been taken to a measured mile for speed trials
although nowadays use can be made of accurate position fixing systems
which are available in many areas. The measured mile, Figure 9.21,
comprises a number of posts set up on land at known distances apart.
These distances are not necessarily exacdy one nautical mile but it
simplifies analysis if they are. The posts are in parallel pairs clearly
visible from the sea. There may be two pairs as in the figure, or three
pairs to give a double reading on each run. By noting the time the ship
takes to transit between adjacent pairs of posts, the speed relative to
land is obtained. For accuracy a number of precautions are needed:

(1) The ship must be travelling at right angles to the line of posts.
(2) The ship must have reached a steady speed for the power used by
the time it passes the line of the first pair of posts.
(3) The depth of water must be adequate to avoid the speed being
affected due to squat and trim.
(4) A clear day with little or no wind and calm seas is needed.
(5) The ship must be newly out of dock, with a clean bottom. If this
condition is not met some allowance may be needed for the
increased resistance due to time out of dock.