The resistance of a planing hull is known to be highly dependent on trim angle. For several reasons, trim is difficult to measure to the level of accuracy normally attained with other towing tank measurements such as resistance or speed. In a recent study intended to validate CFD methods for planing hulls, 4’ and 8’ long geosim models of the Generic Prismatic Planing Hull (GPPH) were built and tested at USNA. Significant differences were found between the trim of the two models so a separate test program was conducted which focused specifically on the trim measurement of these two models in calm water. Five different trim measurement methods were used simultaneously on one model and then used again on the other model. Trim angles were compared between measurement methods and between models. Trim measurements with the same model agreed well and are the basis for an evaluation of measurement methods. The trim measured on the two different size models did not agree well even though the same instruments were used in most cases. The paper discusses reasons for the confirmed differences in calm water running trim of the two models and suggests ways to take advantage of this knowledge to make the best use of towing tank tests for planing boat performance prediction.

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