Performance Evaluation of Sailboat Rudders

MIDN 1/C Huebner EN495 10DEC01

Background

Goals 1. Compare a new spade rudder to the existing rudder and variations of both.

2. Compare a new spade rudder to the existing rudder and variations of both.

3. Based on testing propose and design a new rudder for the new Navy 44s.

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

Design Goals

• Maximize lift for a given area • 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

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

Rudders Tested

Existing Hull Midn 44 Hull

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.

Testing Setup

Tow Post Yaw Arm

Testing Setup

Rudder Adjustment

Testing in the 120’ Tank

Resistance Velocity + Sideforce + Yaw

Testing in the 120’ Tank

Running Downwind at 9.5 Knots Sailing Upwind at 6.5 Knots Note the rudder ventilation

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

0.4

0.3

0.2

0.1

0 0.15

Upwind Testing

Old Hull Side Force v Drag Upwind at 10 deg of heel

0.9

0.8

0.7

0.6

0.522 lbs predicted from VPP 0.5

0.17

0.19

0.21

0.23

Model Resistance (lbs)

0.25

0.27

0.29

0.31

For all conditions the existing rudder provides more Sideforce for the same drag.

existing elliptical existing as spade elliptical skeg

• • • • •

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 (1.8

o ) 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 0.235 and 0.252 pounds respectively, correlating to speeds of 6.2 and 6.0 knots. A 0.2 knot speed increase.

Additional runs at 3 o of yaw showed a linear relationship between yaw and sideforce, realistic rudder angles and a consistent difference in relative drag values.

Upright Testing

Old Hull Upright Yaw Mom ent v Rudder Angle

25 10 5 20 15 existing elliptical existing as spade elliptical skeg 0 0 1 2 3 7 8 9 10 4 5

Rudder angle (deg)

6 Here the spade rudders show more yaw moment for the same rudder angles.

Upright Testing

Old Hull Downwind Yaw Moment vs Rudder Angle

20 15 10 5 30 25 45 40 35 0 0 1 2 3 7 8 9 10 4 5

Rudder Angle (deg)

6 Again, the spade rudders perform better than the existing but the elliptical planform also looks better due to its more efficient shape.

existing elliptical existing as spade elliptical skeg

Tactical Diameter Testing

• Free release Testing in the 380’ tank • Full scale testing with existing Navy 44

Tactical Diameter Testing

• Existing Rudder vs. Elliptical Spade Existing Rudder Elliptical Spade

Tactical Diameter Testing

Result Summary: The elliptical spade Lessens the turning radius Even with a longer waterline Condition Full scale Radius in Boat Lengths 1.63

Old Hull - Existing Rudder Old Hull - Elliptical Spade Old Hull - Existing as Spade Old Hull - Elliptical Skeg New Hull - Existing Rudder New Hull - Elliptical Spade New Hull - Existing as Spade New Hull - Elliptical Skeg 1.39

1.24

1.22

1.22

1.89

1.35

1.22

1.22

Conclusions

• Upwind 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.

Questions?