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of the two other vessels. As an example, it will be seen from Table I that if there is a 20-knot head wind blowing then the relative speed of ship to wind will be 380 knots; the resistance in pounds will be 7450 pounds, and the percentage of this to the naked resistance of the cargo vessel at 10 knots will be 27.7. It will be noticed that the effect of wind is to increase the total resist- ance of the ship by a greater per- centage in the case of the cargo vessel than in either of the other two cases. This is principally due to the fact that the resistance of the cargo vessel is small compared to the sur- face exposed to the wind. For in- stance, the thwartship area above the load water-line of the cargo vessel in case (1) is 1685 square feet as against 5270 square feet in the case of the liner, and 2100 square feet in the case of the cross-Channel steamer. The corresponding resistances are: 27,000 pounds for the cargo vessel at 10-knots; 189,000 pounds for the liner, and 45,000 pounds for the cross- TABLE Il Air Resistance Due to Wind of Velocity VRelatively to Ship (3) Cross-Channel Steamer ' Resistance, Lb. 455 1,024 1,820 2,840 4,090 5,575 7,280 11,380 16,380 tion of Engineers and Shipbuilders in Scotland. This is:— P normal=K At” V** at 10 degrees K=0.55 15 degrees K=0.78 From this formula the pressure on the rudder of this vessel with 10 degrees of helm would represent an increase of 4.35 per cent to the naked resistance at 10 knots, and with 15 degrees helm an increase of 9.35 per cent. The ratio of area of rud- TABLE II Air Resistance Due to Wind of Velocity V Relatively to Ship (2) Liner Resistance, Lb. Channel steamer at 20 knots. More- over the superstructures in the liner and cross-Channel steamer mask each other much better than in the case of the cargo vessel. ‘ If the wind is on the bow it is not uncommon for quite large angles of helm to be required to keep the vessel on her course. The following estimates have been made from the results of model experiments on rudders, and the estimates give the added resistance due to helm angles of 10 degrees and 15 degrees. The figures are based principally on Messrs. Baker and Bot- tomley’s papers on rudders behind single-screw and twin-screw vessels, and on Mr. Denny’s paper on spade rudders. In the case of the cargo vessel, the formula used in calculating the nor- mal pressure on the rudder is the one given by Mr. Bottomley in the paper he read this year before the Institu- Air Resistance Naked Re- sistance at 20 knots 0.2 Per cent 5 0.8 1.8 3.2 5.1 7.3 0.0 3.1 0.4 9.4 0.0 BPNNRee der for this ship to length multiplied by. draft’ is as: 1:70. In the case of the liner the rudder pressures were calculated from the results of Mr. Bottomley’s 1924 paper to the Institution of Engineers and Shipbuilders in Scotland. These are for a vessel of 0.76 prismatic co-efficient, and it is probable that the actual pressures for this liner would be higher as she was a good deal finer than Mr. Bottomley’s form. One of Mr. Bottomley’s future papers. will probably clear this up. The calculated — results show that at 10 degrees helm the naked resistance at 20 knots is increased by 1.6 per cent and at 15 degrees by 3.7 per cent. In the case of the cross-Channel steamer the estimates of rudder re sistance were made from Mr. Maurice Denny’s paper on Spade Rudders (Trans. I. N. A., 1920). At 10 degrees helm the resistance is increased by MARINE REVIEW—May, 1927 Air Resistance Per cent Naked Re- sistance at 20 knots 0.25 ra o SCS Heh D im bo re oo Ee bo ON whee 2 per cent and at 15 degrees by 3 per cent. : It will be seen, then, that the in- crease of resistance due to direct wind effect and rudder is most serious in the case of the cargo vessel. It is not always realized to what an extent a wind can upset measured- mile trial results. An example may best illustrate this. Referring to Table I, which gives the wind pressure re- sults for the cargo vessel, it will be seen that if the vessel meets a 20-knot wind when she is herself doing 10- knots, the resistance due to a relative wind speed of 30 knots will be in- creased by 27.7 per cent. On the re- turn run the relative wind speed will be 10 knots following, and the re sistance will be decreased by 3.1 per cent... That’ is to say, ‘that for a pair. of rons: at 10 bao each way the mean increase of resistance due to wind pressure will be 27.7 — 3.1 2 It will be seen from Table I that if there had been no wind on the mile the increase of resistance at 10 knots would have been only 3.1 per cent, a difference of 9.2 per cent. That is to say, that if this cargo vessel were run on the mile on a day when there was a 20-knot wind blowing down the mile, the mean resistance and horsepower for a pair of runs would be 9.2 per cent higher,:due to a greater wind pressure than on a calm day. As the wind speed rises so this percentage will rise. If the wind is not directly down the mile, but is in such a direction that on one run it is a little on the bow, it will have the effect of in- creasing the wind pressure resistance, due to the additional resistance of parts of the superstructure previously masked by the wind. In addition a cer- tain amount of helm will be required to counteract the tendency of the wind to turn the vessel off her course. It was shown above that with a helm angle of 10 degrees the resisance of the cargo vessel was increased by = 12.3 per cent. 19