Presentation on theme: "Performance Evaluation of Sailboat Rudders"— Presentation transcript:
1 Performance Evaluation of Sailboat Rudders MIDN 1/C HuebnerEN49510DEC01
2 BackgroundGoals-Compare a new spade rudder to the existing rudder and variations of both.Based on testing propose and design a new rudder for the new Navy 44s.
3 Design BackgroundCreate a scale model of the existing rudder for use with existing modelsFrom this I calculated how much lift would be generated at an angle of attack of 4 degreesUsing that lift as a target I designed a new rudder with a higher aspect ratio, which allowed me to use less area
4 Design Goals Maximize lift for a given area Maximize aspect ratio Drag is a result of area, so minimizing area also minimizes viscous drag, but increases induced drag.the tradeoff for less viscous drag is worth it.More lift generated on the rudder translates into more sideforceThe keel has to provide less of the total sideforce which means less overall dragMaximize aspect ratioHand in hand with maximizing liftLimited by depth of rudder
5 Design ConcernsRunning aground and damaging or immobilizing the rudderThis is the limiting factor for depth of rudder70-80 percent of draft3° from keelSkeg vs. SpadeVentilationKeep the rudder away from the surfaceThe effective aspect ratio is doubled if the rudder butts against a flat plateMore lift is generatedMid-Proofing
10 Testing in the 120’ TankResistance+ Yaw+ SideforceVelocity
11 Testing in the 120’ Tank Running Downwind at 9.5 Knots Sailing Upwind at 6.5 KnotsNote the rudder ventilation
12 Results From Model Testing What defines the better rudder?Upwind tests – Most sideforce for least rudder angle and dragUpright Tests – Most yaw moment for least rudder angle and dragTactical diameter tests – Smallest radius and most speed carried
13 Upwind Testing For all conditions the existing rudder provides more 0.522 lbs predicted from VPPFor all conditions the existing rudder provides moreSideforce for the same drag.
14 Sideforce ProblemsThe required sideforce was not obtained within the rudder angles tested.This was due to an optimistic prediction of yaw angle from the VPP (1.8o)Extrapolation of rudder angle data provided values of 5 degrees for the elliptical spade and 10 degrees for the existing rudder.This gave model drag values of and pounds respectively, correlating to speeds of 6.2 and 6.0 knots. A 0.2 knot speed increase.Additional runs at 3o of yaw showed a linear relationship between yaw and sideforce, realistic rudder angles and a consistent difference in relative drag values.
15 Upright Testing Here the spade rudders show more yaw moment for the same rudder angles.
16 Upright TestingAgain, the spade rudders perform better than the existingbut the elliptical planform also looks better due to its moreefficient shape.
17 Tactical Diameter Testing Free release Testing in the 380’ tankFull scale testing with existing Navy 44
19 Tactical Diameter Testing Result Summary:The elliptical spadeLessens the turning radiusEven with a longer waterline
20 Conclusions Upwind Tests Upright Tests Tactical Diameter Tests Elliptical planform provides more sideforce for less rudder angle, which means less drag. Skegs were more efficient than spades at low angles of attack.Upright TestsElliptical planform provides more yaw moment for a given rudder angle, which means faster response.Tactical Diameter TestsElliptical planform turns tighter due to larger yaw moment.Spade rudders carry more speed through the turn, a benefit for collision avoidance and also for tacking nad rounding marks in racing conditions.Improved turning radius is a primary concern as the new 44 will have a longer waterline.