Pros and cons of a 2 rudder configuration :
2 rudders or a single rudder on centre-board boats ?
On most centreboard boats with shallow draft, particularly above a certain size of some 50 feet long, the 2 spade
rudder configuration has been the favored solution. When tacking against the wind once the boat is heeling, the leeward
rudder is ideally positionned, which yields "finger-tip" steering. Much pleasant for the helmsman, as well as for
preserving the autopilot or reducing its electrical consumption.
Moreover the larger immerged fin surface provided by the 2 rudders, diminishes the need to use additionnal boards (e.g. daggerboards) aft for downwind route stability in heavy seas.
On the negative side, a bow-thruster is required in most cases to manoeuver the boat in tight and busy marinas. No big deal though on larger boats, since most of them would be equipped anyway. Another less obvious drawback, but important to expedition sailboats, is the vulnerability of the rudders in case of a crash with an "OFNI" such as timber wood, immerged containers, ice, etc.
Notably, while on the leeward side objects should hit the hull first and be deflected away likely, the windward or upwind rudder is to remain close to the ocean surface with no protection from the hull. The rudder shafts are very thick solid alloy and the aft compartment or lazarette where the rudder shafts are located, would include a water tight bulkhead ... however a damaged rudder could jeopardise the trip, certainly divert the boat to the nearest harbour and yard to repair, or worse stop the journey or exploration.
This 2 rudder configuration has been used by well known architects and builders for aluminium offshore sailing centre-board boats, including Garcia (Salt), Alliage, Alubat Ovni (as from the Ovni 455), Allures, etc
On the other end a single rudder configuration yields much better protection to Ofnis, the rudder being positioned on the same axis and aft the centreboard plus a bustle and/or skeg in front of the engine propeller. However given the shallow draft, a retractable rudder is required, to get a fin deep enough in the water where it can do the most good. But retractable means a less efficient fin profile, and particularly the addition of a mechanism to take the rudder up or down, often the Achilles'heel of this configuration.
While a kick-up rudder is a really safe alternative in case of Ofnis when going downwind with the centerboard up, or exploring shallow and poorly charted waters, several owners have reported corrosion issues with the hydraulic cylinders used to unfold the rudder.
By opposition to the cylinders used for the centreboard well above the water line, the rudder cylinder is immerged in salty water most of the time.
On the smaller and older Ovni boats, the mechanism only involves ropes. This has been the case on some larger "high latitude" exploration aluminium sailing boats such as SEAl or Pelagic chartering in Antarctica, Alaska or Greenland...
See for instance SEAL rudder mechanism image below (boat built some 10 years ago) :
Another single rudder alternative is the one used by Stéphane Peyron on his Xplorer 44 feet Allures boat (Design Berret-Racoupeau, Saillard) : http://www.stephanepeyron.com/#/oneLastWinter/3/fr
Like most folks with previous sailing and ownership experience in aluminum center-boarders, i would prefer to adopt the 2 spade rudder configuration, but as we plan to visit some remote and unchartered areas, i would also want a simple and safe solution.
Just want to have the best of both worlds, right !
But how ?
In other words can we, or has it already been done, envision a simple, safe and efficient steering configuration ?
A solution with 2 kick-up rudders implies a position far aft on the boat stern transom or scoop, see this example :
Grand Intégral (Design Peter Gallinelli) :http://www.sailworks.net/gdint/
certainly Ok for exploration and mooring, but less ideal in busy marinas or when docking in rough conditions, where the rudders could be damaged by the wharf or another boat ...
The most sophisticated solution i have ever seen and must say with a quite successful integration of the swing rudders into the boat transom is on "Glory of the Sea" - Periple 50 by Jacques Peignon designed by Jean Berret (Berret-Racoupeau)
See this video (in french) : http://www.dailymotion.com/video/x9nhky_glory-of-the-sea-le-voilier-des-gla_sport
This boat has gone far south in Antarctica (c.a. 70 degrees south) and even remained over there during a whole winter (not our plan !).
A solution which has also been used on this boat project with 2 retractable spade rudders
Other solutions could include 2 compensated rudders behind a skeg (e.g. see several Joubert designs and J.F. Andre Patago 50), but what about "finger-tip" steering enjoyed with the spade rudders ?
In conclusion, which configuration is best ?
Well, a good answer : "depends upon the boat's program" and certainly a matter of (good) compromise as with many boat issues !