|
WHAT IS SEMI-PLANING?
The semi-planing hull operates in a gray area of performance, between pure displacement speeds and true planing. Generally, displacement speeds are considered those below a speed/length ratio of 1.5. The semi-planing region extends from 1.5 to about 2.5. Above this is the pure planing region for normally proportioned monohulls.
In the planing region, a boat can take advantage of the forces working on the hull to lift the boat, reducing immersion and wetted surface. This consequently reduces resistance, both in wavemaking and skin friction. The semi-planing boat does not take advantage of these forces and continues to push through the water in the same manner as a displacement vessel.
This condition is akin to a planing boat running constantly at the hump speed - or the point of maximum resistance! Hull shape plays a major role, of course: a finer bow and stern will mitigate the resistance problems, but at the same time these can be the source of performance and handling concerns.
THE SEMI-PLANING HULL
The typical semi-planing hull is a development of the pure displacement hull - with sharp and deep forward sections fairing into full rounded aft sections. The major difference between displacement and semi-displacement hulls is in the buttocks.
On a displacement hull, the buttocks will run up aft while the waterlines are coming in toward the centerline. If the desire is for more speed, both the waterlines and the buttocks must be straightened to reduce the tendency to squat. A semi-planing hull is essentially a displacement hull with straighter buttocks ending at a broad transom that has some immersion.
CHINES
Usually a semi-planing hull has no distinct chine. This is a carryover from the displacement hull, where the task was to have the water flow easily around the hull for minimum resistance. At higher speeds, we want to separate the water from the hull cleanly both at the transom and at the sides.
Rounded sections will not allow this: water will cling to the hull and usually find its way on deck! The deep-V has multiple spray strips and a chine to clean water from the hull, at once reducing wetted surface and making the boat very dry.
STABILITY
Most semi-planing hulls have little deadrise. So, they do not have the natural tendency of a hull with good deadrise to right itself after a roll. At planing speeds or near planing speeds, a hull with some deadrise and a distinct chine takes advantage of the dynamic planing forces to increase its stability. Rounded bottoms and flat hulls cannot.
The modern Hunt deep-V is designed to have its chine immersed aft at rest to give a good static and low-speed stability. Early deep-V’s had their chines completely above water and thus had narrower water planes; they tended to roll. This is not true of current Hunt designs.
Another disconcerting aspect of round-bilged, low-deadrise boats is their tendency to roll outward in a turn. The deep-V hull banks inward.
SEAKEEPING
Sharp, deep forward sections, common to semi-planing hulls, can be troublesome and even dangerous. In a following sea, the forefoot will dig in and cause the boat to tend to broach. The shallow aft sections and small keel are not enough to balance out the deep lateral plane of the forward sections. In fact, the inability of the rounded aft sections to resist rolling can do nothing to counter a turn into a broach. In contrast, the shallow forebody of a deep-V will not trip or root. The evenly distributed lateral plane urges the hull toward a straight course; the stabilizing v-shape fights rolling in all seas.
The sharp bow sections of the semi-planing hull do not pound, but neither do those of a deep-V. Additionally, the deep-V’s considerable volume above the waterline prevents plunging, limiting the times that solid water is shipped. The deep-V is perhaps best known for its ability to run fast upwind in rough water without slamming.
SPEED
By definitions, the speed of a semi-displacement hull is limited unless huge amounts of horsepower are applied, simply because you are always pushing the boat through the water. The deep-V holds world records for speed and endurance in rough water. The Blue Riband, for example, emblematic for trans-Atlantic crossings, is held by a deep-V motorboat, not the giant displacement ocean liners for whom the contest was originally unleashed.
At Hunt, we usually optimize our deep-V designs for ordinary speeds: 18-30 knots. While we occasionally design boats and vessels for higher speeds, their characteristics are less ideal for normal activities, whether in family trips or pilot operations. Speeds on the lower edge of planing seem to be where most people prefer to operate.
CONCLUSION
Semi-planing hulls represent a point on the evolutionary scale of hull design that has been surpassed by the modern high-deadrise or deep-V hull. For work offshore, in all sea states, in exposed waters, and at speed/length ratios of 2.0 or greater, the deep-V will exhibit superior performance. The U.S. Coast Guard has specified the deep-V hull for its latest self-righting motor lifeboats; the U.S. Navy has studied frigate-sized deep-V’s and found that a smaller deep-V can offer equal seakeeping, speed, comfort, and payload at less cost than larger, conventional round-bilge or swath hulls.
|
