ON THE COMPARATIVE RESISTANCES OF LONG SHIPS OF SEVERAL TYPES.*

By W. FROUDE, Esq., M.A., F.R.S., Vice-President.

[Read at the Seventeenth Session of the Institution of Naval Architects, 7th April, 1876; the Right Hon. LORD HAMPTON, G.C.B., D.C.L., President, in the Chair.]

THE trial of the model of Mr. Denny's ship Merkara, referred to in the Paper I have already read, furnished materials for extending and giving practical completeness to a comparison which our series of experiments had already led us to institute between several types of form.

The comparison is interesting, as showing the relation between length and resistance with two types of form-(1) That form in which a straight parallel-sided middle body is interpolated between two ends of greater or less fineness; (2) That in which the whole length of the ship is utilised in fineness of form; the results being worked out for the Merkara and three other ships of the same displacement, but of different form and proportions. The lines of these three other forms selected are those which in our series of experiments have been found to give, on the whole, the best results within ordinary available limits of speed. The displacement in each case is 3,980 tons, which was that of the Merkara on her trial. For shortness I shall call the four models A, B, C, and D; A being the Merkara. A and D represent No. 1 type, but with different proportions and different degrees of fineness; B and C represent type No 2. They have different proportions but the same degree of fineness. The ends of D beyond the parallel middle body have the same fineness as the ends of B and C; indeed all these forms have what may be called the same entrance and the same run, for though their dimensions as well as their proportions of length to beam differ, the cross-sections on which their lines are based are throughout the same, the longitudinal spacing being in each case made proportional to the total length they occupy, or as it may be termed having different degrees of expansion. The Table on next page gives the leading particulars of the four-ships.

* Received 4th April, 1876.

The length as given does not include the screw aperture, taken to be 9 feet. In A the entrance and run are each 3.87 beams; the total length is 9.67 beams; the draught is 436 of the beam. In D the entrance and run are each 2:08 beams; the total length is 6.25 beams; the draught is 392 of the beam. In B and C the total length is 7.82 beams and 6.26 beams respectively; in both the draught is 392 of the beam.

These explanations are a sufficient introduction to the consideration of the several Figs. 1, 2, 3, and 4 (Plate XII.), which give the lines of the several ships; and Fig. 5 (Plate XIII.), which gives their respective curves of resistance, that is to say, curves in which the abscissæ represent speed in knots; and the ordinates resistance in pounds. In studying Fig. 5, perhaps the circumstance which first deserves notice is, that at the maximum speed included in the steam trials of the Merkara there is so extremely small a difference between the resistance of these four ships, differing so considerably as they do in form and proportions. Making the comparison at 12 knots, the greatest of the resistances, that of D, is 21,000 pounds; the smallest, that of C, is 18,700 pounds; the difference being only 2,400 pounds, or scarcely over 10 per cent. An interesting light under which to view this amount of difference is that thrown on it by the fact that it is only half of the thrust equivalent to the constant or initial friction of the Merkara's engines, a circumstance which indicates how easily a difference in the performance of the ship may be masked by a difference in the performance of the engines. If in comparing the ships at this speed we omit D, as possessing considerably less sharp lines of entrance and run than either of the other three, the differences between the resistances are still smaller, the 20,000 pounds of the Merkara exceeding the 18,700 of C by only 1,400 pounds, or 7.5 per cent.; and even at 13 knots, which just exceeds the highest speed included in the Merkara steam trials, though the resistance of D has become rather more in excess, yet that of the Merkara and B are identical, and that of C, the smallest of the three, is still only 1,400 pounds below theirs. At 13 knots the resistance of D is beginning to diverge rapidly into excess, and the Merkara's is also beginning to diverge in the same

direction, though less rapidly. At still higher speeds the forms C and B, which have no parallel middle body, show a growing superiority to the Merkara, though the longest of them is only as long as that ship, and is nearly 8 feet broader while C, the shorter of the two, is 51 feet shorter; and at the same time 12 feet broader than the Merkara. Up to 16 knots the resistance of C is less than that of D, though C is 4 feet the broader and 24 feet the shorter.

The results of the comparison are interesting in their relation to a proposition which I have elsewhere insisted on—namely, that at very low speeds (speeds low as compared with the length of the ship) a ship's resistance, if her form be fairly fine, consists practically of nothing beyond surface friction; and it is worth while to notice how far the proposition is verified in the case before us. The model of the Merkara was tied down to a speed lower than we used formerly to include; so that at the lowest speed which her curve of resistance includes I have not data for the verification of the proposition with the other forms; but it is the fact that in the Merkara's case at from 5 knots to 8 knots the surface friction is about 92 per cent. of the ship's entire resistance. And referring to Fig. 5 (Plate XIII.), we see that although at 9 knots D, with a less skin, has a greater resistance than the Merkara, yet the line of her resistance-curve is converging on the Merkara's, and might be expected to cross it at a still lower speed; B, however, which has a skin area rather larger than the Merkara's, has, nevertheless, rather the smaller resistance of the two; but C, which has the smallest skin area of all, has also the smallest resistance.

A more practical aspect of the comparison may be made in relation to higher speed; and even in reference to a speed as high as 12 knots it will be seen how valuable a diminished area of skin may be when the effect of fouling is taken into consideration—a circumstance of especial importance to a ship of war which may be under the necessity of keeping the sea for long consecutive periods. I am not able to say how much fouling exactly is to be anticipated thus under any given circumstances, nor again can I say. exactly how much extra resistance a given amount of fouling will produce. But our experiments on surface friction show that the substitution of a surface of ordinary unbleached calico for one of clean paint produces just a double frictional resistance, and a foul bottom must often be no less obstructive. For simplicity of calculation I shall suppose a degree of fouling by which the resistance is exactly doubled, and it will be seen that the effect of this is to improve greatly the relative positions of the shorter ships. The construction of the Table sufficiently explains itself.

It will be seen that under these circumstances D, which in the comparison as it stands with the clean skin is the worst of the four, having a resistance exceeding that

of the mean of the two long ships in the ratio of nearly 11 to 1, has become as good as the best of the two, now that the skin resistance is doubled, and under the same circumstances the superiority of C has been notably increased.

The superiority of the shorter ships in respect of handiness need hardly be referred to. Moreover, I cannot but believe that the weight of hull constructionally necessary to the strength of the ship must be considerably less for the shorter than for the longer ship. Mr. Denny indeed is not, I believe, prepared to admit this; but the question raised is one which I think must be capable of something like a general and almost demonstrative solution, and it is one which ought to be treated on general mechanical principles, and not decided by mere reference to the existing Rules of Lloyd's. These however are I trust undergoing revision in very capable hands.

Nevertheless, it must be admitted that in view of the importance of large carrying power combined with limited draught-a limitation which the Suez Canal has done much to emphasize-and I may add, in view of the practical sufficiency of what may be called "moderate speed," the prevailing tendency to great length, including a long parallel middle body, is a fair result of "natural selection ;" and this form if rationally treated, is perhaps, under the conditions indicated, the best adapted for commercial success; though where deep draught is unobjectionable a shortened form with no parallel middle would be, as I have shown, unquestionably superior; or were it an object to obtain very high speed, without notable increase of resistance, parallelism of middle body would even with the longer form be inadmissible.

The logic of the circumstances shapes itself thus:-Large displacement means large dimensions, somehow or somewhere; but the limitation of draught forbids enlargement of dimension except in the direction of length, since increased ratio of breadth to depth would involve an objectionably raised metacentre and objectionable increase of skin; greatly extended length has therefore, for mercantile purposes, become essential to large carrying power. Now, with a very long ship, if the ends are so far fined as in

effect to limit the resistance to surface friction, the parallelism of the remainder clearly assigns a valuably increased carrying power to the ship as a whole; or, what comes to the same thing, secures a given carrying power with less total skin and therefore less resistance at moderate speed. What I contend against is, not the parallel middle body per se, but the mistaken idea which to most minds forms the basis of its justification-the idea, namely, that to lengthen a ship by merely introducing a parallel middle involves no material increase of resistance, the supposition being that the middle thus added will follow unobstructed through the opening made in the water by the full-sized ends.

In conclusion, I must remark that the performance of Mr. Denny's ship is somewhat better than previous experiments had led me to expect it would be; for I had not expected to find that, after taking account of the resistance due to surface friction, her residuary resistance would be barely less at 12 or 13 knots than that of B, a ship of the same length and certainly of finer lines. Whether this rather unexpected goodness lies in some speciality in the lines of the entrance and run, or in the form of the midship section, or whether it merely indicates that at a speed so moderate compared with the length of the ship's entrance and run as 12 knots it matters little what the lines are, provided they do not fall short of a certain standard of fineness,-is an important question which I shall not feel has been decisively answered till a series of experiments bearing on the point shall have been completed. Some additional steps in the series have been suggested by the present examination.

DISCUSSION.

Mr. WILLIAM DENNY: I hope you will excuse my making a remark or two. As the Merkara is a ship built by my firm, I may be able to say something that will elucidate the reason for her being made in a rather peculiar form. I think Mr. Froude has very well put the war requisites. The war requisites are, primarily, a short length of side requiring armour, and a small amount of skin to be fouled. But our requisites in mercantile naval architecture are different; and I would especially call attention to one of them which is being more and more felt every day. Yesterday I referred to the unfitness of great merchant steamers-especially of high speed and as built now-for carrying heavy armaments, because they possessed so slight a margin of stability; and I wish that this fact should be kept very carefully in mind. Now some advantages of the Merkara's lines—if I may presume to point them out to you are that, so far as Mr. Froude has shown you, they have fulfilled all the requirements of speed; but they have greater advantages than these so far as stability is concerned. In the Merkara we tried to get

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