Boating NZ : FREE TO READ March 2014
the vertical lift is generated and the lower the trim angle, but the longer the running length. With it comes increased wetted area and, crucially, less chine immersion. See Rule Two and chine diagrams. As the boat moves through the speed range, the trim angle varies from an initially quite high 6 to10 degrees as the boat gets over the hump and on to the plane. Once on the plane, the drag is reduced and, for a given available horsepower, the speed increases until the drag and thrust are equal. As the lift from the bottom increases with speed, the amount of area required is reduced and the vessel lifts up, reducing the wetted area. This has the effect of moving the centre of pressure aft, as the running length is reduced from the forward end, which alters the trim as the balance between LCG and LCP comes into play. This alters the trim, which alters the angle of attack, which alters the lift, which alters the running length...... you can see why Savitsky’s work was so significant, and all the more impressive for being from an era of slide rules and tables. The third benefit of some vee in the bottom is to impart roll stability. As the boat rolls to one side it presents the downhill bottom panel at a flatter, reduced angle to the water and the uphill panel at a greater angle. This means that the hull bottom is generating more vertical lift on the downhill side and less on the uphill side, which has the desirable tendency to counter the roll and return the vessel upright. This effect is similar to that harnessed in aircraft design and is why wings have an element of dihedral; ie, wing tips are higher than the wing root; see Roll diagram. EQUILIBRIUM STERN DOWN ROLL OPPOSED BOW DOWN HIGH TRIM LOW TRIM When the LCG is forward of the centre of lift, the bow will drop, increasing the wetted surface area of the hull and therefore drag. When the LCG moves aft of the centre of lift, the opposite will happen; the trim angle will increase and wetted surface area will reduce. The vee of the hull opposes roll as the lower side creates a more horizontal plane in the water, which generates more lift than the raised side which is on a greater angle. A nicely balanced attitude with a moderate trim angle and running length giving just enough chine immersion aft. 52 Boating New Zealand March 2014 Feature__future concepts_March14.indd 52 19/02/2014 3:17:34 p.m.
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