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, owing to the contraction of the _mass_ of metal forming the coupling; the outside cooling first tears the center open, and when there is not much metal to turn off the face of the coupling, it is sometimes undiscovered. Having observed several of these cavities, some only when the _last cut_ was being taken off, I have considered it advisable to have holes bored in the end and center of each coupling, as far through as the thickness of the flange; when the shafts are of large size, this is sure to find these flaws out. Another flaw, which has in many cases proved serious when allowed to extend, is situated immediately abaft the gun metal liner, in front of the propeller. This may be induced by corrosion, caused by the presence of sea water, gun metal, and iron, assisted by the rotation of the shaft. It may also be caused under heavy strain, owing to the over-finishing of the shaft at this part under the steam hammer. The forgemen, in these days of competition and low prices, are instructed to so finish that there won't be much weight to turn off when completing the shaft in the lathe. This is effected by the use of half-round blocks under the hammer, at a lower temperature than the rest of the forging is done, along with the use of a little water flung on from time to time; and it is remarkable how near a forging is in truth when centered in the lathe, and how little there is to come off. The effect of this manipulation is to form a hard ring of close grain about one inch thick from the circumference of the shaft inward. The metal in this ring is much harder than that in the rest of the shaft, and takes all the strain the inner section gives; consequently, when strain is brought on, either in heavy weather or should the propeller strike any object at sea or in the Suez canal, a fracture is caused at the circumference. This, assisted by slight corrosion, has in my experience led in the course of four months to a screw shaft being seriously crippled.