Twin-rudder raceboats have been with us since the mid-1980s. In the last 10 years or so, they’ve also become increasingly popular aboard cruising boats, including those available for charter. It is therefore a good idea to be aware of their idiosyncrasies.
In some circles, twin-rudder boats have a reputation for being difficult to handle in tight spaces. This is a fair assessment, insofar as approaching an awkward berth in tricky conditions the same way you would with a single rudder could become unexpectedly exciting. That doesn’t mean, however, that maneuvering with twin rudders is any more or less difficult than with a single one—merely different.
So, what are the differences? First and foremost, it’s important to remember that whenever the engine of a single-rudder boat is in forward, the propeller pushes a good deal of water past and over the rudder directly astern. This in turn makes the boat immediately responsive to the helm when, say, pivoting around in a narrow channel or approaching (or leaving) a tight berth in a gusty headwind.
Aboard a twin-rudder vessel, on the other hand, the prop wash passes between the two rudders, depriving you of this kind of instant maneuverability. One solution, of course, is to carry more boatspeed, thereby increasing the amount of water passing over your rudders. However, making a habit of doing so in a tight spot risks a maneuver eventually going spectacularly wrong in some fashion.
On the plus side, twin-rudder boats tend to be much easier to handle in reverse, having less “prop walk” and tending to gain steerageway at relatively low speeds thanks in part to their increased blade area. As is the case with a single-rudder boat, the bow will naturally want to turn downwind in any kind of breeze, making it easiest to maintain control when the wind is coming from directly aft, with a little more speed required to keep the bow from falling off when the wind is coming in over the bow or from abeam. The maximum wind speed at which it is possible to control the bow in reverse will, of course, vary from boat to boat. But in my experience, the limit is usually around 10-14 knots.
Back ‘er up!
Those who learned to sail aboard older designs, especially full-keel and fin-and-skeg yachts (as opposed to those with deep, semi-balanced spades), tend to avoid docking stern first. This should come as no surprise given the way these boats tend to have smaller rudders, larger turning radiuses and loads of “prop walk” when you engage the engine in reverse. (See sidebar) They also tend to be heavier, which means having to use a good deal more power to get them moving.
Aboard twin-rudder boats, on the other hand, the situation is the complete opposite, given their maneuverability and improved steerage backing up. Not only that, but using a conventional single-rudder, bow-on approach in a stiff crosswind you run the risk getting into some real trouble, due to the aforementioned lack of prop-wash effect, which makes it that much harder to keep the bow from blowing off to leeward in a crosswind.
Pulling alongside in reverse, on the other hand, allows you to maintain a measure of control. As an added benefit, the fact that the freeboard aboard most boats is lower aft also makes it that much easier getting that first dock line across, as well.
Unfortunately, when pinned onto a dock by the wind, the lack of prop-wash effect means springing the stern out aboard a twin-rudder boat may not be effective—a situation that is further exacerbated by the fact that the broad transoms found aboard many twin-rudder boats means attempting to spring the bow out will often not work very well either.
Because this is arguably the most difficult situation you’re likely to face when handling a boat with twin rudders, it is, therefore, worth considering whether the wind direction and strength are going to pose a problem when the time comes to leave.
If it looks like you may be in for some trouble you might want to try moving elsewhere. Alternatively, in the event you do find yourself pinned, you may want to try using warps secured to another boat, another dock or a tender, to help pull you clear.
As a side note, it is for this same reason it sometimes makes sense to fit a bow thruster to a slightly smaller twin-rudder boat than would be necessary for a boat with a single blade. Of course, the final decision whether or not you do so depends on the type of moorings generally found in your area and your intended cruising grounds.
Ultimately, there’s no magic or mystery to handling a twin-rudder boat; these are all fairly obvious points to any experienced sailor. Nonetheless, these sorts of boats do require a slightly different way of thinking, a way of thinking that may not be immediately obvious in the heat of the moment when faced with a suddenly precarious docking situation. It, therefore, pays to think and plan ahead.
Prop walk: 101
Most sailboat propellers are “righthanded,” which means when viewed from astern they rotate clockwise. “Lefthanded” propellers, when viewed from astern, rotate counter-clockwise. A righthanded propeller (which rotates counter-clockwise in reverse) will also cause the stern to slide, or “walk,” to port when you first start backing up. This can either be a hindrance or a big help, depending on the situation. Imagine, for example, you are coming into a dock bow-first, port-side to. Putting the engine in reverse will not only slow you down, but pull your stern in that much closer. Coming into a dock to starboard: not so much. Another time you can put prop walk to use is when pivoting a boat around in a tight circle, alternating between forward and reverse as you do so.
The physics behind prop walk can be a bit tough to wrap your head around. The main thing to understand is that when a propeller and shaft are mounted at a downward angle—as many are—the distance travelled by the blades on the upstroke is greater than on the downstroke, creating an uneven amount of thrust. This, coupled with the lateral deflection of water off the hull, pushes the stern sideways in addition to the direction of travel.
Prop walk also happens in forward, but can be compensated for even at a dead stop by the propeller wash flowing over the rudder—assuming a single rudder on centerline. In reverse, once the boat begins moving the rudder can be used to compensate as well. The problem is getting going or slowing to a stop, when the boat is barely moving and there is little, if any water if flowing over the rudder. Note, because the shaft on a saildrive leg is mounted on the horizontal, prop walk is far less a problem.
A quick and easy way to figure out if your boat is right or lefthanded if its already in the water (and you can’t read the designation on the prop itself) is to simply put the engine in reverse when it’s tied up in a slip or at the dock. If you see prop wash coming out from under the starboard side (which would cause the boat to walk to port) you have a righthanded prop. With a lefthanded prop, throwing the engine into reverse should result in prop wash flowing to port.