POWERBOAT DESIGNS
	Chris Craft Cobra 21 Redesign Eric Sponberg shakes hands with client Jack Rouse on the beautiful job Jack did on building his reproduction Chris Craft Cobra 21 with Eric's new deep-V bottom design.  Jack is a Michigan State Spartan grad, but he didn't mind too much shaking hands with a Michigan Wolverine.  During the construction, Jack also purchased an original Chris Craft Cobra 21 from its original owner, Bill Nicholson, next photo.    That's Jack and Bill together at the Lake Dora Boat Show in Tavares, FL (formerly the Mt. Dora Boat Show) in late March, 2008.  Jack's reproduction Cobra Double Take is on the left, and the original Cobra 21, Intoxication is on the right.  Bill bought Intoxication new out of the 1955 Chicago Boat Show--it was the boat show boat.  The Chris Craft Cobra represented Chris Craft's first venture into fiberglass construction--the tail fin is made of fiberglass.  Only a little more than 100 Cobras were built, about 60 or so of the 21s, and even fewer of the 18s, and they sold for only one model year, 1955.  The Cobras are now one of the most sought-after classic boats because of their iconic design.  Bill used Intoxication up until 1970 when he put it in storage, and Jack bought it last year.  Note the difference in bottom shapes. Jack purchased the original Cobra 21 plans, which included construction details and lines, from the Mariner's Museum in Newport News, VA.  He gave copies of the plans to me to pull offsets which I then used to create a 3D surface model of the hull and deck, without the tail fin.   Then I modified this 3D model with the new deep-V bottom.  This bottom design is very similar to the bottom that I have on the CC 20 reproduction speedboat which has excellent handling characteristics.  This new bottom on the Cobra had to fit the slightly different length and beam dimensions of the Cobra hull. After creating the lines plan above, I took the station cuts through the model at each frame location in the original construction and drew full-size patterns of each frame.  These patterns included the thickness of the hull and deck planking as Jack was going to build in his new boat.  I plotted patterns both on Mylar, below, as well as in a format that could be NC routed by a third party.  In the end, Jack used the Mylar patterns. As shown above, each Mylar rectangle measures 36" x 48", and they have nesting marks so that they can be assembled accurately.  Nesting the stations saves on the amount and cost of Mylar.  In an NC routing pattern, however, the stations can be fully separated and transferred by AutoCad.dwg or generic .dxf files to any company that provides routing services.  I have had success with other formats as well making patterns for other boat designs and companies. The deck on Double Take is 1/4" marine plywood overlaid with 1/4" mahogany planking.  The bottom is three layers of 1/4" marine plywood.  It is strip-planked forward where the bottom curvature is greatest, and this is married into the full plywood planking aft.  On the topsides, the first two layers are 1/4" marine ply with a top layer of 1/4" mahogany planking. Jack bought two fiberglass tail fins, one from a supplier in New Hampshire, and the other as a spare from a supplier in Oklahoma.  The cockpit details between Double Take and Intoxication are virtually identical. The photos below show the differences in bottom shape more closely:   Double Take is on the left, and Intoxication is on the right.  Note the convex sections in Double Take's bow versus Intoxication's concave sections.   Here are close-ups of the respective sterns showing the difference in bottom shape.  Bill Nicholson told me that he added the anti-cavitation plate above the rudder on the transom on Intoxication after he gave his wife her first ride in the boat in 1956.  It seems that they were running in kind of choppy waves, and in the middle of a turn, the rudder ventilated and he lost control of the boat--and the Mrs. was pitched overboard!  Fortunately, they remained married until her passing a few years ago.  The anti-cavitation plate cured the problem and they never worried about that again.  Below is a comparison of the two hulls from the side. Obviously, with the deep-V hull, Double Take has a deeper draft than the original design.  Installed power is a 6.0 liter Crusader engine, 375 HP at 5200 RPM.  The propeller is 14" x 18" DxP, and the engine sits backward driving through a V-drive gear at 1.29/1 reduction. In all, this has been a marvelous project.  Jack has only limited time running Double Take up till now, but has been tweaking things to improve speed, performance, and handling.  I have not had a chance myself yet to ride in either boat, but that day may come in the not-too-distant future.  It will be interesting to judge the differences in handling between the original hull and the reproduction with the new bottom. There is some noise on the horizon about putting this design in production as a do-it-yourself kit, so watch this space for more news, or contact me by phone or email for an update.
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