when working in salt water, with liners of gun metal, galvanic action
ensues, and extensive corrosion takes place in the iron at the ends of
the brass liners, more especially if they are faced up at right angles
to the shaft. Some engineers have the uncovered part of the shaft
between the liners, inside the tube, protected against the sea water
by winding over it tarred line. As this may give out and cause some
trouble, by stopping the water space, I have not adopted it, and shall
be pleased to have the experience of any seagoing engineer on this
important matter. A groove round the shaft is formed, due to this
action, and in some cases the shaft has broken inside the stern tube,
breaking not only it, but tearing open the hull, resulting in the
foundering of the vessel. Steel has been used for screw shafts, but
has not been found so suitable, as it corrodes more rapidly in the
presence of salt water and gun metal than iron, and unless protected
by a solid liner for the most part of its length, a mechanical feat
which has not yet been achieved in ordinary construction, as this
liner would require to be 20 ft. long. I find it exceedingly difficult
to get a liner of only 7 ft. long in one piece, and the majority of 6
ft. liners are fitted _in two pieces_. The joint of the two liners is
rarely _watertight_, and many shafts have been destroyed by this
method of fitting these liners.

I trust that engine builders will make a step further in the fitting
of these liners on these shafts, as it is against the interest of the
_shipowner_ to keep ships in dry dock from such causes as defective
liners, and I think it will be only a matter of time when the screw
shaft will be completely protected from sea water, at least inside the
stern tube; and when this is done, I would have no hesitation in using
steel for screw shafts. Though an easier forging than a crank shaft,
these shafts are often liable to flaws of a very serious character,