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Topic: Higgins letter on hull form |
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alross2
TOP BOSS
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Posted on: May 1, 2011 - 1:05pm
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Here’s a letter from A.J. Higgins to Admiral Cochrane on 8/19/43 that Jerry found at NARA. I’ve retyped it as the original is a bit hard to read.
NOTES ON HULL FORM OF THE HIGGINS 78 FOOT MOTOR TORPEDO BOAT
An ideal torpedo boat must have many incompatible characteristics. Soft riding qualities do not ordinarily lead to speed and vice versa. However, an easy riding boat is desirable to keep the crew in fighting condition, so that the can accomplish their mission. After that speed is absolutely vital to guarantee a safe return of crew and fighting equipment.
/to fulfill the above specifications it is necessary to consider the hull in several sections, - the vow for instance in rough weather, the forefoot for entrance in calm, the middle body for life and stability and the stern for planing and control.
First, let us study the bow. The long rake is an extreme modification of the eureka or spoonbill bow found on all Higgins landing craft. Full at the deck line and fine at the keel, it exerts a wedging, lifting, motion on entering a choppy sea and raises this bow without shock, at the same time cleaving the sea to flatten it out for the rest of the boat. As the water flows aft, it is gradually turned and rolled under, so that maximum lift is obtained near the center of gravity. At this point the dihedral angle is fairly large, which accounts for the extreme stability of the craft under way. As we pass by the middle body of the boat we find the aerated water leaving the chine, and the solid water below rising to give the propellers a good solid bite so that maximum thrust can be exerted. All of this is accomplished by the selection of a single curve which when rotated about a point at the keel forms a double spiral tunnel effect running he g=full length of the boat. The flow of water over these tunnel surfaces produces longitudinal stability in the same way that dihedral gives lateral stability. At the stern this tunnel increases vibrations and increases propeller efficiency. This can best be understood by a study of the relation of the propellers to the sections immediately over them. The concavity makes the approach of each propeller blade to the hull more gradual than would occur in other types, and the low chine gives directional control to the thrust of water leaving the blades.
The shape of a boats transom should (in the case where performance rather than beauty is desired) be a compromise between a very full one which will lift quickly in a following sea and leads to yawing and loss of control and a very narrow one which will not affect steering but which means that the boat will be constantly pooped and that personnel and equipment will suffer. Neither extreme is desirable yet conversely one is the cure for the other. This explains the long rake of the transom, which sets like a shovel raising the stern slowly as that neither of the above disturbances occurs.
Now we come to one of the most important features of all, - maneuverability. We have demonstrated longitudinal stability which at first glance means low maneuverability. Such is not the case however, yet each 1 present in its turn without sacrifice of either. To do this we must, of course upset our longitudinal stability at the moment of entering a turn and reestablish it when straightening out on our course again. For this reason the rudders are very deep with a high aspect ratio. As the wheel is turned in either direction, say to port a high thrust with a considerable leverage resulting from deep rudders is exerted to starboard. As this is turning moment considerably below the center of turning resistance, the starboard side rises. As it rises, the upward thrust increases as the angle of attack increases. Now we have an entirely different action in our tunnel sections. The starboard tunnel at the stern resembles an aquaplane while at the bow it becomes a wedge driving the bow downward and holding on. Thus the bow pivots around the forward sections producing this most remarkable turning action.
Al Ross
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Will Day
TOP BOSS
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Posted on: May 1, 2011 - 2:20pm
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Fascinating stuff. Thanks, Al.
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Hadly
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Posted on: May 5, 2011 - 10:21pm
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I second wWill's comment.
SH
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