Practical Application of Interceptors on Small Non-Planing and Slow Planing Powerboats

A surplus U.S. Coast Guard 26 foot Motor Surf Boat (MSB) was purchased and outfitted for use as a pleasure craft. This boat had been previously repowered by the Coast Guard and is capable of considerably higher speeds than the original design. The hull form has curved buttocks aft and a rounded transom which caused excessive trim at these higher speeds. The owner investigated a number of possible solutions to correct this excessive trim and selected installation of a fixed interceptor. This solution was believed to be simpler than installation of trim tabs, especially considering the rounded shape of the transom. However, there were two areas of uncertainty related to use of this devise on the MSB: (1) would an interceptor be effective on this hull form at the pre-planing and planning speeds; and (2) what size interceptor would be required to correct the trim problem? The following were the design considerations (1) the total project cost must be less than $3,500 The design must be able to allow for the installation of different interceptors without hauling the boat out of the water (3) the interceptor must be able to be removed from the boat leaving no trace of it’s existence if it did not improve the boats performance. To determine the length of the interceptor the transom was marked with masking tape at 4 inch intervals off the center line of the transom to aid in visual observation of water flow and set of pictures were taken of the stern wake as well as the water flow off the transom. It was believed that a minimum depth of an inch would be need to interrupt the water flow. Expectations (1) the interceptor would have no effect on the boat at run angle under 1500 RPMS and would only present a small fuel penalty (2) the interceptor would be effective above 1500 RPMS and make the boat more mannerly. A relatively small interceptor was found to be very effective reducing trim, improving fuel consumption and increasing maximum speed. This paper presents design details and trials results quantifying the performance improvements.