Comparison Hulls
A comparison of Power Catamaran Hull Types
(Reprinted from the Spring 2000 Issue of Power Multihulls Magazine By Prof.Eng. Bob van Niekerk)
The registered trademark BOBKATTM covers a range of power catamarans from 2.5m to 33m that have similar looking hull shapes, but with detailed changes for different sizes and speeds. The shape took 23 years to develop after which it underwent a series of refinement for the next 10 years. It is still not perfect, but when compromises were forced on the design, safety and comfort always took precedence over the outright speed.
The almost symmetrical bows prevent wandering when quartering seas and provide plenty of buoyancy when needed. It is almost unknown to take green water over the bows even in the Cape of Storms' worst seas. Stepping the bows in the front section of the tunnel allows asymmetrical sponsons for the rest of the way to give that all important banking to the inside, which is considered a prerequisite for safe sea keeping - and keeping your passengers on board.
The convex shape incorporates the equally important non-trip below the wide chines to further improve safety in beam seas and quartering swells in a large following sea. The rounded section does not slam and gives a comfortable ride in rough water, even when jumping the large swell at high speed. Spray rails are used at the bows to keep spray down and to provide a dry ride, even with a Southeaster off the bows.
The patented tunnel chines running fore and aft behind the bow-step are there to mix this solid water with air to form a two phase medium that is compressible and therefore reduces the tunnel slamming experienced by other tunnel shapes. The tunnel is also the highest of the hull shapes listed and when the foils are fitted on the Type 'J' BOBKATTM the effective tunnel height allows high-speed travel in severe sea states. The 22m patrol boat, for instance, can take 3m high head seas at 26 knots without discomfort.
At other headings there is almost no limit to speed accept in following seas when the boat starts flying off the crest and loses propulsion while airborne. The tunnel chines lift up in a flattened S - shape near the stern as does the tunnel roof, to provide an increased tunnel area for waves to enter when travelling at speeds below 20 knots in following seas. This not only lifts the stern but prevents that sudden forward lurch you get with other transoms. For the sports fisherman this feature also allows for backing down at speed when fighting a large fish without any danger of swamping.
The overall aspect ratio of 3:1 with a sponson ratio of 10:1 reduces wave making resistance, especially on the foil, while giving excellent lateral stability. The progressive increase of sponsons width with draft gives a load ability which is further enhanced in the foil-assisted Type 'J' BOBKATTM shape. The metacentric height of most catamarans results in a restricted roll action of small amplitude that people soon accept and prefer to the motion of monohulls, where alarming roll angles can develop in beam seas and rough waters. The Type 'J' BOBKATTM shape is fast, economical, and most seaworthy with a built-in characteristic of looking after its occupants.
