Replacing Phoenix's Rudder Part 1: Identifying the Problem

At some point in Phoenix's history, a previous owner replaced her rudder and installed a large, flat, barn door rudder. Our assumption from the beginning of her restoration was that this was done to combat weather helm. Why else would you put a barn door rudder on her?

For the uninitiated, weather helm is a term that describes a boat that sails with its tiller or wheel slightly angled to the windward side of the boat. A few degrees of angle -- 3 or 4 degrees -- is considered ideal: the rudder steers the boat and provides lift (like the flaps on an airplane wing), the helm feels light and is easy to steer, and if you let go, the boat will round up into the wind and the sails will luff (i.e. not sail away in the event of a man overboard). With too much rudder angle, the helm feels heavy, is difficult if not exhausting to steer, and the boat loses efficiency. Rather than provide lift through the water, the rudder drags, eventually stalls and acts more like a brake than a wing.

In design terms, weather helm is a result of an imbalance between the boat's center of effort (CE) -- geometric center of the sail plan -- and center of lateral resistance (CLR) -- geometric center of the underbody. Essentially, the CLR is the imaginary pivot point for the boat and the CE is the driving force location acting on the sail plan. If the CE is too far aft (on top of or behind the CLR) the boat will have weather helm.

Convinced we needed to combat weather helm and move our CE forward, we made nearly every textbook modification there is:

• We already had the bowsprit, but added a new, high aspect tri-radial genoa replacing the old bagged out genoa
• Reduced the rake in the masts, standing both main and mizzen masts up to only 1 degree of rake
• Purchased a new mainsail that could be easily flattened
• Installed a longer (94") traveler for the main sheet
• Reefed (reduced) sail often as the winds picked up

With all of these improvements, Phoenix sailed remarkably well for a boat her size in light air, easily moving 50-60% of wind speeds. However once the breeze picked up above 14 knots with full main and genoa, the helm would get increasingly heavy and difficult to steer. The heavy helm was felt even upwind with the genoa alone.

After sailing Phoenix with the new rudder angle indicator we installed with our Pypilot autopilot, we were very surprised to see that we only had 2-4 degrees of rudder when the helm felt heavy.  All this time we thought we were dealing with weather helm when clearly we were dealing with something completely different!

We decided it was time to look below the waterline and take a hard look at Phoenix's underbody, and specifically at the rudder. We began researching rudder design, spoke with several naval architects, did our usual homework, and determined that what we were experiencing was actually excessive rudder resistance or feedback, turbulence and drag caused by our oversized barn door rudder.

There are very few Christina or Andromeda ketches out there for comparison, but we knew our rudder was VERY different from the original design and from the only other production-built Christina we know of.

Andromeda 48 Rudder Design

Christina 49 Hull #2 Rudder

Phoenix's Old Barn Door Rudder

Unlike the Andromeda design, both Phoenix and her production-built sister ship have bent Monel rudder stocks fit into reinforced Monel rudder shoes. This allows for a larger propeller, and enables us to remove the propeller and shaft without dropping the rudder. That's really where the similarities end.

Our rudder was much deeper and longer (fore and aft) than either known counterpart. The top portion of our rudder was cut off at the waterline rather than running parallel to the hull. A smaller stainless "skeg" was attached to the fiberglass skeg, presumably to protect the deeper addition and to attach sacrificial anodes, but it was wasn't faired in. Not to mention, whomever made Phoenix's barn door rudder seems to have gone out of their way to add more drag by creating an exoskeleton of straps, plates and bolts to attach the flat plywood board.

The extra depth wasn't necessarily a bad thing, as it gave the rudder some clear water below the skeg. However, a flat board rudder was really the least efficient rudder design possible. While easy to manufacture, they stall early -- typically after about 5 degrees! The flat board caused water to eddy under the bottom of the rudder rather than being redirected around the trailing edge, which reduced the steering efficiency. Instead of allowing the water to flow naturally across the rudder and provide lift like an airplane wing or sail, the flat board created turbulence and drag with every turn of the wheel. Changing the design of the rudder to make it more like a proper foil would be much more efficient and allow for streamlined water flow across the rudder.

NACA foils are the most popular shapes used in rudder and keel design. Developed by the National Advisory Committee for Aeronautics, the shape of a NACA foil is described using a series of digits following the acronym "NACA." Many boat designers use a NACA 0012 for rudder design -- a symmetrical foil shape with a 12% thickness to chord length ratio (i.e. it is 12% as thick as it is long). The NACA 0012 is best for boat speeds up to about 6 knots; faster speeds (6-8 knots) can warrant smaller percentages, such as the NACA 0010 (10% as thick as it is long).

The additional length of our rudder blade (fore and aft) was also part of the problem. While adding extra area to a rudder can make it more efficient, there is a fine line between adding efficiency and adding too much resistance -- or feedback. Making a rudder too long puts undo back pressure on the trailing edge, and also contributed to the heaviness we were feeling on the helm as the breeze picked up. This excessive rudder feedback can be mistaken for weather helm since they both make steering difficult; however the root cause is rudder size and resistance rather than the CE or sail plan.

Our metal "skeg" was another issue. While it protected the deeper rudder from crab pots and other debris and allowed for a sacrificial anode, the lack of fairing disturbed the water flow and added even more turbulence in front of the rudder and drag.

Yet the biggest problem was the "Franken-rudder" exoskeleton of bolts and straps along the rudder's leading edge creating a large mount of drag. The plates, straps, bolt heads and nuts that were protruding from the surface produced too much turbulence over the rudder and reduced lift. This was yet another major factor that caused the rudder to stall early. It was as if we were sailing with a rudder full of barnacles that we could never remove! To make matters worse, anti-fouling paint didn't stick well to the exposed straps, so without continually scraping, we literally were sailing with a rudder full of barnacles!

All of our research pointed to the fact that we needed a better rudder. So, we developed a plan to build a new rudder for Phoenix and scheduled a haul out at a local marina. Our new rudder would have:

• A shorter chord length (fore and aft) than the existing rudder but still longer than the Andromeda specification, 
• A NACA 0010 airfoil design, 
• All of the rudder straps and fasteners buried inside the rudder for a smooth, efficient flow, and 
• A small  amount of area in front of the rudder post to provide some balance and "power steering" to the helm

Our plan to modify Phoenix's rudder

Just like that, a plan was hatched. Stay tuned for Part 2 -- the new rudder design, and how we built it!