1. Performance Evaluation of Sailboat Rudders
MIDN 1/C Huebner
EN495
10DEC01

2. Background
Goals-
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 Background
Create a scale model of the existing rudder for use with existing models
From this I calculated how much lift would be generated at an angle of attack of 4 degrees
Using 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 sideforce
The keel has to provide less of the total sideforce which means less overall drag
Maximize aspect ratio
Hand in hand with maximizing lift
Limited by depth of rudder

5. Design Concerns
Running aground and damaging or immobilizing the rudder
This is the limiting factor for depth of rudder
70-80 percent of draft
3° from keel
Skeg vs. Spade
Ventilation
Keep the rudder away from the surface
The effective aspect ratio is doubled if the rudder butts against a flat plate
More lift is generated
Mid-Proofing

6. Rudders Tested
Existing Hull
Midn 44 Hull

7. Planform Comparison Old Hull Midn 44 Hull
N. B. The elliptical rudder on the Midn 44 Hull is 3.5” deeper than the existing rudder on the old hull.

8. Testing Setup
Tow Post
Yaw Arm

9. Testing Setup
Rudder Adjustment

10. Testing in the 120’ Tank
Resistance
+ Yaw
+ Sideforce
Velocity

11. Testing in the 120’ Tank Running Downwind at 9.5 Knots
Sailing Upwind at 6.5 Knots
Note the rudder ventilation

12. Results From Model Testing
What defines the better rudder?
Upwind tests – Most sideforce for least rudder angle and drag
Upright Tests – Most yaw moment for least rudder angle and drag
Tactical diameter tests – Smallest radius and most speed carried

13. Upwind Testing For all conditions the existing rudder provides more
0.522 lbs predicted from VPP
For all conditions the existing rudder provides more
Sideforce for the same drag.

14. Sideforce Problems
The 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 Testing
Again, the spade rudders perform better than the existing
but the elliptical planform also looks better due to its more
efficient shape.

17. Tactical Diameter Testing
Free release Testing in the 380’ tank
Full scale testing with existing Navy 44

18. Tactical Diameter Testing
Existing Rudder vs. Elliptical Spade
Existing Rudder
Elliptical Spade

19. Tactical Diameter Testing
Result Summary:
The elliptical spade
Lessens the turning radius
Even 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 Tests
Elliptical planform provides more yaw moment for a given rudder angle, which means faster response.
Tactical Diameter Tests
Elliptical 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.

21. Questions?