NAMBA Propwash October 2017.pub

Brian Buaas’ rigger is a smoother version with more steps. Note the high angle on the main step that is the main supporting area at speed. The forward steps should be well clear of the water. Sport hydros aren’t quite as radical. Mark Anderson’s sponsons are representative. They still have flat bottoms but run at a lower angle of attack. The forward area has a steep antidive section and the left sponson has a small non trip area. Note that the above interceptor design also has an aft planing surface like the Dynaplane. This isn’t necessary with a more aft interceptor placement. However, this aft placement creates a larger wetted area with more drag. A further refinement of the planning surface can be accomplished by shaping. A properly contoured surface (Johnson 3 term camber) gives a much higher lift to drag ratio than a flat plane. The contour is very subtle. The dynaplane model pictured above used a CNC foam cutter to cut the shape in its step area. This is all fine for monohulls, but what about the planing areas on hydroplanes? Here the planing surface barely touches the water on its aft end. Stability isn’t an issue as in monohulls so the wetted length doesn’t need to be as long. Sponsons also can have flat bottoms. Non trip side angles were once used, but may not be needed on riggers. A higher angle of attack is often used to get a shorter wetted area. To keep the sponsons from diving a short, high angle section is often used at the bow. The Kep’s sponsons are the state of the art in rigger design. PROPWASH 16 October 2017 Interceptor Wetted Area – Aft Placement Johnson 3 Term Profile - Exaggerated KEP’s Sponsons Brian Buaas 138 mph Rigger Anderson Sport 40 - Left Side Anderson Sport 40 – Right Side Anderson Sport 40 – Right Sponson Bottom