NAMBA Propwash October 2017.pub

As you can see from the drawings, the width increases from the A model to the D model. The wider D model has the lowest calm water drag. This is due to the better “aspect ratio” (width to length) of the wetted area. This is similar to the effect on a wing. Steps added to model C substantially reduced the calm water drag, due to the reduced wetted area with a better aspect ratio. owever, tests in waves show an advantage for the narrower A hull. A longer narrow hull produces lower vertical accelerations in waves. The steps in the C hulls didn’t affect the performance in waves compared to the unstepped C hull. A much lower trim angle helps as well. Running the hull with only a 1 to 2 degree trim angle significantly lowers the impact acceleration. The picture below shows the wetted area both by solid water and spray on a planing hull. Strakes are often used to reduce this wetted area. They give flatter planing surfaces that help lift the bow out. They also provide an edge to help with cornering. Some hulls have 4 strakes, but others use two. High Speed Boat Design - Part 3 Planing By Lohring Miller NAMBA Safety Chairman In Part 2 we discussed the ways air flow can be used to reduce water drag. This time we’ll explore ways to reduce water drag on planning surfaces. The standard design features of planning monohulls are: 1. Complete avoidance of convex surfaces (except for the bow area which is out of the water at planing speeds) to avoid the development of bottom suction pressures. 2. Sharp edge chines at the intersection of the bottom and sides to insure complete separation of the transverse flow component from the bottom. 3. A wide transom with a sharp trailing edge to insure complete separation of the longitudinal flow from the bottom- thus insuring that the entire transom is ventilated to the atmosphere. 4. Straight horizontal buttock lines at the aft end. 5. Vee-bottom transverse sections with the deadrise increasing towards the bow. The deadrise is required to reduce the wave impact loads in a seaway and to provide lateral wetted surface required for course-keeping stability and maneuvering. In contrast to other hulls, the performance of a planing hull is very dependent upon the longitudinal location of the center-of- gravity that controls the trim angle of the craft when planing. The trim angle in turn has a major effect upon the resistance/ weight ratio. Typically, the resistance of a planing hull is a minimum at trim angles between 3-4 degrees and increases for both higher and lower values of trim. If the center of gravity for a given hull cannot be varied, then transom flaps or transom interceptors (see interceptors below) can be used to change the trim. Traditional planing hulls plane on their transoms. Steps can be used to reduce drag. Below are views of a modern deep vee design series. All the bottoms have the same 22.5 degree deadrise (the angle between the bottom and a horizontal plane). Up to 30 degree deadrise has been used in offshore boats. PROPWASH 14 October 2017 Planing Hull Wetted Area