A yacht’s hull design will determine:
a. if it will have a soft comfortable entry in head seas
b. if it will have a gentle easy motion in a sea way
c. if it will be stable with a gentle easy motion while at anchor
d. if it will have sufficient speed to out run bad weather
e. if it will have a “dry” ride with proper water deflection
f. if it will be fuel efficient at cruise speeds
Each of these factors were the primary goals in William Crealock’s designs for Offshore Yachts. To successfully achieve these goals Mr. Crealock chose a semi displacement design that combines the best features of planing hulls and displacement hulls.
Planing hulls typically have a low to moderate amount of "V" or deadrise at the bow decreasing to a nearly flat bottom at the transom and are hard chined, meaning the hull sides meet the bottom near the water line at a sharp angle with no radius. A planing hull runs on top of the water rather than through it so is capable of high speeds depending on sea conditions.
Due to their relatively flat bottom shape they are subject to pounding or slapping in choppy and rough seas. Planing hulls are typically the least comfortable underway and at anchor of the three basic hull designs. The main advantage of a planing hull is the higher speeds it is capable of achieving when sea conditions permit.
Displacement hulls typically have deep drafts and rounded hull forms which require stabilizers and/or paravanes to decrease their roll and increase comfort. Weight is not critical so displacement designs are characteristically heavy and are not capable of planing. Top speed on a displacement hull is approximately 10 knots for 45’ to 65’ boats. They are limited to their “hull speed” which means that regardless of horsepower the yacht can never go above its designed hull speed based on its water line length.
A displacement hull’s efficient cruising speed is normally 6 to 8 knots and can achieve that speed with relatively little horsepower. The main advantages of the displacement hull are its fuel efficiency at low cruise speeds and its smooth ride accomplished by is deep hull and weight. These design characteristics are best suited for long offshore passages at low speeds.
Semi-displacement hull designs have finely “Veed” forward sections or deadrise which gradually decreases to a moderate “Vee” at the transom, and a hard, or sometimes slightly radiused, chine. A semi-displacement hull combines the planing capability of the planing hull and the soft ride and low speed cruising speed efficiency of the displacement hull.
The Semi-displacement hull design is arguably the best of both worlds offering speed when needed and a comfortable motion in a sea way without the roll of a displacement design. Typically most semi-displacement vessels do not cross oceans but have the range and capability to do so.
William Crealock's semi-displacement designs for Offshore Yachts incorporate a deeply "Veed" forward section with the optimum degree of the quarter buttock line to assure what has become Offshore's maxim, "the softest ride on the water". An Offshore's deeply "Veed" bow combined with the ideal weight and strength of her construction to ABS standards, the location of fixed weights and liquid weights low in the hull and at the center of buoyancy produces the most comfortable ride possible - NO SLAPPING, NO POUNDING, AND A GENTLE EASY MOTION UNDERWAY AND AT ANCHOR.