Fast Displacement Hull Form

We have developed a new hull form, the patented “Fast Displacement Hull Form” or FDHF. The development of this concept was triggered by the desire to develop hull forms which are efficient over the entire speed range and not only in a restricted speed interval around the maximum speed. The FDHF is applicable to all vessels and out-performs conventional displacement and semi-displacement type hull forms at all speeds up to full “planing” speeds (in excess of a Froude number of 1.0). At semi-displacement speeds, the resistance values displayed by the FDHF are 15 to 20% better than those of well-designed hard chine hull forms. Comparison of model test results for the FDHF with those of numerous hard chine models show that only at fully planing speeds will a very well-designed hard chine hull be more efficient than the FDHF.

Other benefits of the FDHF, especially when compared to hard chine yachts, are improved sea keeping behavior and maneuverability, as well as a lower sensitivity to an increase in displacement. In November 2009, we signed a contract with Heesen Yachts to design their new 65 meter flagship. This is the first yacht featuring the fully developed application of the FDHF concept. Since then we have received many more commissions to develop new yacht concepts based on the FDHF, ranging from smaller yachts around 20 meters up to 75 meter in length.

Benefits for both the Yard and the Owner

The benefits of a low resistance, high performance hull form are interesting to both the builder and the owner. For the builder, the main advantage is the lower installed power for a given speed. This translates directly into lower building costs. For a semi-custom series featuring the same hull, the benefits are even more significant as there is no longer the need to develop different hull forms for different speed ranges. Using the FDHF, both the displacement as well as the semi-displacement speed ranges are served equally economically by the same hull form, a single platform, and therefore a single engineering package.

The advantages for the owner are less operational costs because of less fuel consumption, more space in the hull, because of the requirement for less bunker capacity, as well as less noise and vibrations because of the lesser installed propulsion power. In addition, the yacht will be equally comfortable at all speeds as the traditional resistance hump, of which especially hard chine yachts suffer in the transitional speed regime from displacement to semi-displacement speeds, is dissolved.

Tank Tests at the Wolfson Unit

The Wolfson Unit has conducted towing tank tests on a large number of round bilge/semi displacement motor yacht forms and hard chine forms since 1968. A database has been compiled by them containing the resistance and powering data for the majority of the tested motor yacht hull forms which calculates a comparative performance coefficient for each resistance data point relating the speed, length, displacement and effective power. This then allows direct comparison with other tested hull forms at discrete Froude numbers.

As Dickon Buckland from the Wolfson Unit explained recently: “For example, if hull efficiency at the cruising speed is the main design driver then the designer should be looking for a hull form that is placed high up in the rankings at that particular Froude number. The rankings are expressed as a percentage at each Volumetric Froude number increment, 0% being the least efficient and 100% being the most efficient.”

When commenting on the results of the tests with our FDHF hull form Dickon Buckland writes “When compared to other round bilge hull forms at speeds in excess of 15 knots (cruising speed) and up to 44 knots (almost planing speed) the tested motor yacht’s ranking is typically around 100 – 140% indicating that the design is currently ranked as the most efficient in the Wolfson Unit’s database. When compared to hard chine forms in the data base the ranking increases to around 100 – 200%.”

References