Performance Engineering
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Performance Engineering |
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The Cheetah 30 was conceived as a trailerable sport boat whose design objectives where 1) high performance, 2) easy water access, and 3) a price that didn't equate speed with expensive.
When clients ask me for a fast boat, I usually ask them with how fast a boat can you handle. Anyone who has ever wanted to sail an International 14, or Australian 18, but just weren't sure if they where up to the task, knows exactly what I mean. The performance objectives for the Cheetah 30 were to provide a safe and controllable high performance boat. This is not a boat exclusive to young hot shots. But it is not for the faint of heart either. You don't have to be a gymnast to plane offwind in steady speeds in the high teens. But you do have to enjoy sailing at these speeds. The Cheetah 30 is designed to provide exhilarating rides while feeling safe and in control. The Cheetah's large sail area will easily provide excellent performance in light air and exhilarating rides offwind in heavy air. The appendages are deep and high aspect ratio for close-winded performance, minimum drag and easy control.
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The hull was result of years of experience in designing planing sailboats. My experience with International 14's, Ultimate 30's, International Canoe's, and the Vanguard 15 combined with tank testing and computational fluid dynamics have resulted in the Cheetah 30 hullform. The chined hull sports a plumb bow for maximum waterline and sections optimized for planing. The entry provides a dry ride and easy motion. Midship sections are tuned to maximize planing. The figure above illustrates the maximum planing pressure region located under the mast. A full stern section provides additional stability necessary for this powerful rig.
The sailplan is very generous but easily handled. With 100% jib and large fully battened main, the Cheetah 30 will tack and accelerate quickly. The high aspect ratio rig affords a large 1100 sq. ft. masthead asymmetrical spinnaker. Easily jibed, this large kite provides the power for exciting offwind rides with the ease and control of an asymmetric. A rollerfurled masthead genoa/genniker is tacked to the bowsprit sleeve for very light wind conditions or heavy air reaching. This sail and all its roller furling gear is easily removed when not in use.
Offsetting this large sailplan is a 1000 lb. ballasted bulb keel drawing 7 feet. The combination of deep high aspect ratio keel and tall high aspect ration rig gives the boat excellent upwind performance, good ultimate stability, and light displacement with a moderate ballast/displacement ratio. The deep draft keel also provides excellent roll damping in gusty conditions. Like all parts of the boat, the carbon fiber keel was engineered using the best technology available. The bulb was set back to not catch weeds but maintains good lifting characteristics through optimized flow analysis.
The rig is nearly identical to the rig I designed for Chattanooga Chew III, an Ultimate 30. It has no backstay or runners only highly swept spreaders. With an asymmetric spinnaker it is important to be able to jibe quickly keeping your speed up. Downwind legs are also very tactical and require quick crew work. Without runners there is no crew work in a crowded area of the boat, and jammed runners in a jibe are no longer a problem. The lack of a backstay affords a high roach main and less weight aloft. An added benefit is that rig tension can be adjusted from a single control line in the center of the cockpit. The control line can be adjusted by the mainsheet trimmer without the need for a winch. This control line allows up to 10 inches of mast bend control underway. Another advantage of this rig is the added simplicity in rigging and derigging. Raising the mast only requires pinning the shrouds and headstay. All shroud tuning settings can be left as they were the last time out, only requiring the headstay to be tightened.
The spinnaker pole length ahead of the bow is 9 ft. and near minimum for an asymmetric kite on a boat this size. Given the centerline location it is the longest pole that could be provided. This length allows the kite to jibe easily ahead of the forestay and keeps it from backwinding the main. By comparison it my opinion that most asymmetric sport boats have too short a spinnaker pole. As a point of reference an International 14 has a spinnaker pole length of 9 feet and Chattanooga Chew III had a pole length of 23 feet forward of the bow.
A center cockpit console is the location for the main sheet, vang, pole launch and retrieval, and all are easily handled by the main sheet trimmer. The main sheet is also double-ended fore and aft of the traveler which allows either the skipper or mainsheet trimmer to control it in different situations.
The Cheetah 30 was designed for ease of production and low cost right from the beginning. Hull construction is vacuum bagged E-glass and PVC core with vinylester and polyester resin. This cost effective laminate in a high performance boat is made possible by engineering the internal structure using a philosophy of load isolation and structural optimization. The one piece internal structural is unusual in its design.
It isolates most of the major loads on the boat to the liner structure not the hull or deck. In essence, the liner takes the loading of the mast, rigging, spinnaker pole and keel; and simultaneously provides structure to support separate hull and deck loads. Secondly, the stucture is designed to be integrated. Consider the transverse bulkhead in the figure above. This bulkhead bears the shroud loads, the keel loads, and also acts as a frame for the hull. The net result is a hull that is light weight, and very strong, yet inexpensive to build. There is very little fiberglass in this boat that is not contributing structurally.
The liner also provides conduits for running rigging and other controls that end up exiting at the center cockpit console. The pole launch line for instance, passes through the forward bulkhead and proceeds to passed through the liner, and up to the cockpit console. The liner design also provides for plenty of sitting headroom near centerline under the coach roof in a fore and aft orientation. Storage is provided under each side for porta-potty, anchor, and other heavy items. This keeps weight central and low. The liner incorporates two quarterberths. The v-berths are pipe berths that are lightweight and custom fit to the liner and hull.
The rudder and keel are constructed of unidirectional carbon fiber and epoxy. The keel ballast is internal cast lead in the bulb. As an internal bulb, all keels will have the same accurate external fairness. Secondly, having the bulb encased in a carbon shell will ensure that grounding will not alter the alignment and therefore performance of the keel.
The Cheetah 30 has a number of innovative features which allow for easy access to and from the water.
First the Cheetah 30 is the only boat that can go from a 7 ft. sailing draft to a 14 in. trailering draft. This is accomplished by having the keel hoist mechanism built into the keel trunk. The keel hoist system has two positions; one that raised the keel to deck level and one that raises it to trailering level. At deck level the keel draws 4'-6" which will allow it to fit in" even the most difficult slips. At 14 inch draft the Cheetah 30 is easily ramp launched. At 2300 lbs., the Cheetah 30 can be pulled easily by recreational vehicles, vans, or light trucks.