I was contacted to do structural calculations for the Windrider Rave V in early December and started in earnest in early Jan. The earliest work consisted of drawing a free body diagram and finding all the global forces acting on the boat. All forces were represented as point loads for this stage of the calculations. With all the forces mapped out I set about trying to create and solve a system of 16 equations in the front view and a system of 6 equations in the profile view. To do this I used a free mathematics software called SymPy which stands for Symbolic Python. After much effort refining the relationships in the system of 16 equations and changing my assumptions I failed to get any sensible results and was forced to abandon that effort. Fortunately Larry’s suggestion of reducing all the forces on each V foil into a point load at an estimated center of effort yielded a system of 5 equations for which realistic results were easily found. The findings from this were that to maintain foiling at low foiling speeds with lower winds both windward and leeward foils need to produce up forces. To maintain foiling at higher speeds and higher wind the windward foil will need to produce a down force or the boat will heel excessively if not capsize.
In the mean time the system of 6 equations for the forces acting on the boat as seen in the profile view was working well and giving useful results. We quickly found that the initial configuration of the Rave V was at risk of taking a nose dive when traveling at max speed because the driving force of the sails wanted to flip the boat over the forward foils. To remedy this I checked to see how far back the driver would need to move to keep 70lbs acting upward on the rear foil while the boat was in static equilibrium. Turns out the passenger needed to move back 3’ 4” to raise the force acting on the rear foil from 2 lbs to 70 lbs. Clayton informed me that the hull on our boat only had room to move the driver back 4” so then I checked the balance of the boat for when the forward foils were further forward. and found the boat was much more responsive to foil placement then driver placement. We settled on moving the driver back 4”, the foils forward 4.6”, and setting the min weight on the rear foil to 35 lbs.
We continue working to find a foil arraignment that will require an up force acting on all the foils at all times. This will ensure that the boat is always stable and help reduce the need for actively controlling the foils angle of attack.
One thing I noticed when doing these calculations was that any foiling boat with a rear lifting foil/rudder would have very high loads on the rear foil during takeoff when it is generating the least lift or no load on it when going at high speeds. In the latter condition the boat would be about to flip over onto the bow. This is due to the fact that as sail generates a force moving the boat forward it also generates a very large moment that wants to make the boat pitch over the forward foils. Moving the forward foils further forward to keep the boat balanced at high speeds only increases the loads on the rear foil at takeoff speeds and in low wind conditions. The high vertical loads on the rudder can, require larger foils to achieve takeoff and will probably require some foil trim adjustment to decrease the lift generated at high speeds.
Many mass production foiling boats seem to compensate by carrying small sails which reduce the max moment they will experience at the cost of reducing speed and increasing the wind speed required to start foiling. The other option is to put the lifting foil/rudder assembly in the front and move the main lifting foils and the mast back. This way the boat will have the lowest load on the front foils at the onset of planing and higher loads as speed increases. The nice thing about this is that as speed increases the downforce and lift on the forward foil should increase simultaneously. This may allow for the boat to carry larger sails and may eliminate the need for a rear foil trim adjustment.
The time frame for designing and building the Rave V did not allow for a thorough investigation of this option but future foiling boats designers might want to consider a bow rudder/foil.
