Rave V - Designer Notes
Larry Knauer, Lead Designer of the RAVE V has provided these detail comments regarding the design which is moving into production. If you care to comment, please do so and Larry can respond.
The design object for ability to point into the wind will be a desired off wind angle of 35 degrees and a total tack angle of less than 90 degrees to be comparable to most modern mono-hull sail boats and about 5 to 10 degrees better than most modern multi-hull boats. The design desire is to be able to foil at these closer beat angles as well. This can only be accomplished by aligning the sail forces to the water side forces to be similar to a mono-hull boat design. To do this the center of pressure for the sail must be outboard and leeward to the center of water pressure for the hulls and foils. The design solution to do this is to have an A frame mast arrangement with there being two primary masts connected at the base near the forward AKA and on the AMA and the top of the Masts joined together in an A frame shape. This design significantly reduce the bending loads on the mast and moves the center of sail force outboard pat the center of pressure of the foil water side forces. This allows for a significantly lower weight for the mast and rigging. Because the center of force is moved outboard and the force acting on the sail has a lifting component the sail force area can be much larger. It is estimated that each sail can be 1.25 time larger and have the same heeling forces on the foils and AMAs. This larger sail area and the moving to the outboard position will help the boat to foil at much lower speeds in addition to the benefits of better pointing angles. The exact location of the mast attachment to the AMA will depend on optimizing the sail forces to the foiling forces. There will also need to be an assessment of the rake angle forward or aft to optimize the sail forces.
It has been well known since the 1930s that an elliptical wing plan form is the highest lift o drag ratio and the half ellipse is next best. The use of the Spitfire trailing edge half ellipse combined with the semicircle “winglet” joining the tops of the masts which eliminates the “Wing” sail tip vortices or the need to have the top of the sail wash out with twist is considerably more efficient. The losses at the tip of a wing or top of a sail represent about 30% of the induced drag caused by the lift of the wing or sail. The combination of the half ellipse and the “winglet” results in a substantial increase in sail area efficiency. This should result in a 5 to 10% net sail area efficiency. So the 320 square foot sail on the Rave V will have the effective area of 352 square feet on a conventional sail plan form.
The masts are to be airfoil shaped design and the sail will be raised and lowered similar to any other sailboat. A rotating bearing for each mast at the base will be used and it is desired to allow the top of the masts to twist slightly to assist in optimum sail to apparent wind angles. The top of the mast will also have a blind pin connection similar to putting winglets or wing extensions on a modern sailplane (glider) to simplify rigging and a hinge point at the top of the semicircle mast winglets as well for storing with the mast up and AMAs folded inboard for movement on the trailer in dry storage bot yards. This twin mast twin Main sail design eliminates the need for a Jib and results in needing only two main stays for raising and lowering the masts as well as stabilizing the masts when up in place. These forward and aft mainstays will be adjustable to allow for changing the rake angle of the main sails.
A Simplified version that eliminates the need for a foresail is being considered and the option to sail under the twin mainsails simultaneously is being evaluated. This approach will eliminate the need to furl the Main sails and eliminates the furling Jib. The mast moves slightly forward to maintain the common center of pressure as the baseline design with the four sails. This approach would be simpler to sail and reduce costs. The trade on sail area and force distribution will determine if this simpler approach can foil at as low a speed as the baseline design.”