Hull Shape

Cross Section

The cross-section of both hulls is trapezoid with a 30 cm wide flat bottom. The chines are well rounded. A longitudinal stringer was glued to each bottom as a protective keel shoe.


While being fore/aft symmetric, the leeward hull of Prosis is double-asymmetric windward/leeward wise. Under water and at water line, the windward side is considerably rounder compared to the leeward side. At deck level its the opposite. The reason for the asymmetric shape under water is to counter the drag of the windward hull. The asymmetric shape at deck level results in more volume leeward of the hulls centerline. At excessive heel, this leads to an increase in righting moment which hopefully will make capsizing more difficult.

The windward hull is fore/aft and wind/leeward symmetric.


Despite maintaining high volume in the ends for a high prismatic coefficient (pitch stability), the bows got a fair bit of rocker. This is due to hydrodynamic reasons as well as practical considerations like hitting floating stuff (hulls with rocker get over thing more easily, reducing stress due to impact) or beaching the boat (rocker helps to get up a beach without digging the nose into the sand. Helps to push rollers under the hulls, too).

The windward hull could have used slightly more rocker (about 5-8 cm) to ease the interactions with waves resulting in less spray.

Length to Width Ratio

The leeward hull is 40 cm wide at the water line, the smaller windward hull 35 cm. With a length of 8.6 m resp. 6.2 m, the length to width ration is 21.5 for the leeward hull and 17.7 for the windward hull.