New rudders

Several reasons compelled us to rethink and replace our rudders this year.  Performance-wise, Reach seemed to have significant weather-helm when sailing upwind (~12˚rudder angle), which causes a lot of drag.  One of the first things we did after hauling was drop the rudders.  There were some physical features that we though we could improve on.  

1.  We noticed that our rudders had gotten deformed in the intense sun during a long haulout when building the new sterns  two years ago in Guatemala.  

2.  The shape of the old rudders is a trapezoid (chord length 22” at top & 14.5” at bottom), not an optimum planform for water-flow.   

3.  The gap at the top of the rudders is inefficient, allowing water to pour upwards instead of flowing aft.

4.  The leading edge of the rudder ahead of the shaft is only 3” (~14% of chord length) shifting the balance point aft and making for stiffer steering.

Mark was all ready to design and build a new set of rudders using an ellipse shape and making mold of his own.  Then we learned from Scott that a Florida company Foss Foam who makes rudders commercially for major boat builders and had made a few Manta rudders.  When contacted, they did not have any Manta rudder molds (nor did anyone else formerly associated with Manta), but offered to do a custom job with us. 

Bob and his son were great to work with on this project.  Mark went to visit their shop and they dug through a yard full of molds until finding a few suitable options.  One seemed perfect, but the mold hole was too small and wouldn’t fit our 2” shaft.  Another was chosen that is from a Concer 47 rudder.  It was 4" longer than we could place on Reach and still be above our keels, so Foss simply filled in and shaped the lower portion of the mold for pouring ours.  They use poured polyurethane 20 lb weight for all of their rudders into a mat-biax fiberglass outer layer.

Their shop handled all aspects of rudders and we decided to have them rebuild the stainless steel metal cage of our rudder.  

The original 2” shafts were solid stainless and still in good shape, but the welded plate was mild steel and started rusting soon after Mark opened up the rudder.  

Foss built onto our shafts new cage structures with a lower post-extrusion as they do for their commercial rudders, that look like this one.

The resultant rudders look great and have redesigned properties.  We hope to gain a bit of lift from this new foil to assist in weather-helm and overall performance.  We’ll report back when under sail, of course. 

The new foil shape with thickness & chord dimensions, plus calculated flow field at 10° angle of attack:

1.  The new rudders are higher aspect (height:width) than our original ones (1.6 vs 1.18).  

We’ll keep them painted white, as recommended in general by Foss Foam,  to avoid thermal damage from the sun.

2.  The shape of the new rudders is now a NACA* foil of 0020 ~ “00” indicating no camber (i.e. symmetrical shape) and “20” indicating a 20% chord thickness to length ratio.   

Our original rudder was a NACA 0018.  

Old vs New rudders EAR

The new thicker foil section was gained by keeping the same thickness, but decreasing the chord length.  This minimizes additional drag and should give the rudder more lift at higher angles in theory.

  • Data Points = 1˚ rudder angle
  • Green = New rudder
  • Red = Original rudder
  • Cl = Coefficient of lift
  • Cd = Coefficient of drag

New rudders:  42% more lift at 5˚, 44% more lift at 10˚ and 53% more lift at 15˚ rudder angle with only slightly more drag.

3.  The gap in the top of the rudders is filled in with this mold shape and should prevent spill-over of water flow (rudder not all the way up in picture).  

The new rudder height is 30” from shaft entrance to bottom of ellipse, increased from the 26” original size but still 2” above the bottom of the keels.

4.  The leading edge of the rudder ahead of the shaft is now 4” (20% of chord length), which should make for more balanced rudders.

*National Advisory Committee for Aeronautics (NACA)

© M&M 2019