If you’ve seen the videos of our deck saloon sailboats manoeuvring effortlessly in the tight space of a marina, you may wonder how we can make it look so easy. The simple answer is that it looks easy because it is easy. Years of practice are absolutely not required as we prove on every test sail; after 10 minutes of simple instruction anyone on board who is more than eight years old can manoeuvre a Sirius with the same confidence. Part of the reason our yachts are so easy to manoeuvre is rudder design, combined with the position of the propeller.
A rudder needs to work well for both sailing and motoring, but many believe a rudder can only be good for sailing or motoring, not both. Most boatbuilders nowadays just focus on producing a rudder that’s efficient for sailing, but it doesn’t have to be one or the other. You can indeed have both. We even have a solution for hard-to-manoeuvre twin-rudder yachts, which have no prop wash to steer with.
The three rudder options we offer
We offer three rudder options. The first is a single, partially balanced spade rudder with a skeg for added protection. This is the standard setup on all our fin-keel yachts and also on the twin-keel versions.
On all our lifting keel yachts we use twin rudders because when the yacht is dried out, the height from the hull to the sea bed won’t allow enough span (rudder height) to give good control under sail. Instead of one deep rudder, we use two shorter rudders, angled outwards at the bottom so that at any heel angle one rudder is always fully immersed and close to vertical for optimum performance. Twin rudders also give the security of a wider support base when the boat is dried out.
The third option is three rudders: the standard twin rudders as above plus a small, central third rudder whose sole purpose is to improve manoeuvrability under power, especially from a standstill and at low speed.
The best rudder design for motoring
Let’s first look at how our rudders are optimised for use under power. If we were making a motorboat, the rudder would have little or no area forward of the rudder stock. It would only need to be the height of the propeller, and the distance between rudder and propeller would just be a quarter to half of the propeller’s diameter. Most motorboats are steered entirely by prop wash (the flow of water from the propeller passing over the rudder). As mentioned above, our twin-rudder boats also have the option of a smaller, central third rudder. Like a powerboat rudder, it steers the boat by efficiently deflecting prop wash and also directing a laminar current onto the two rudders.
Life isn’t so simple for our single rudder designs; they have to be equally efficient under sail and power. To get maximum manoeuvring efficiency from prop wash we locate the propeller at an optimum distance from the rudder, so that the cone-shaped, moving mass of water is the same height as the rudder when it hits it. The positioning has to be precise. The rudder blade needs a large surface area for efficient performance under sail, and if it’s too close to the propeller there will be a lot of pressure on the rudder, making it heavy on the helm with lots of vibration under power. If it’s too far away from the propeller, the energy of the prop wash dissipates into the surrounding water before it hits the rudder. One reason we prefer saildrives, rather than shaft drives, is that they allow us to fit the engine in what would typically be an aft cabin (this is one reason why the aft cabin of the Sirius 40 DS is transverse), which puts the propeller in an optimal position and gives the owner an excellent mid-cabin plus a workshop. We do also fit shaft drives to some of our yachts, but they generate more noise and vibration and this moves the engine a bit further forward. You can have whichever drive you prefer.
When a yacht is motoring, the area of rudder blade forward of the post (or stock) is doing the most work. It’s under the highest pressure from prop wash and helps to deflect more water over the high-pressure side of the rudder. The area forward of the stock also helps to balance the helm. Without it, the rudder would require more effort to turn. But if there is too much surface area forward of the stock, the rudder will become unbalanced and twitchy under sail – the helm will require constant attention, which is clearly a bad thing on a cruising yacht. It’s all about striking the right balance (if you’ll excuse the pun) between a rudder that provides a good steering response and one that is not hard work to turn at high speed.
The water pressure is highest at the point where the prop wash hits the hull, and because our propeller is closer to the rudder than on most other boats, a lot of pressure is exerted on the top (or root) of the rudder. One way to avoid it experiencing too much pressure is to remove the area forward of the stock where the pressure is highest. We do this with the addition of a skeg. Under power, the skeg absorbs the water pressure and directs the flow of water over the rudder. It acts like the wing of an aircraft and the rudder is the control surface on its trailing edge – if the whole wing of an aircraft were to move in flight it would be very unstable, it’s the same principle. The skeg stabilises the flow of water, so by the time the water hits the rudder there is minimal turbulence. This translates into less movement and vibration of the rudder under power. Adding a skeg is an expensive complication in the production process, that’s why it is rarely seen in production yachts. Twin-rudder yachts don’t suffer the same way because the turbulent water from the propeller passes cleanly between the rudders, and in our triple-rudder design the third rudder is small enough and designed not to be affected.
The best rudder design for sailing
Our yachts are not just designed for party tricks in marinas, they are also designed to sail well. So any rudder design we use has to work at least as well, if not better, for sailing as it does for motoring. To achieve this the rudder blade must be deep, so when the yacht heels there is still enough of the rudder in the water to maintain control. Under sail, it’s the trailing edge of the rudder that is doing the work and is under the most pressure.
Why twin rudders have become popular
There is a trend for yachts to carry their maximum beam a long way aft to enlarge the aft cabins. Sirius yachts don’t need the aft cabins to be the master cabins because the accommodation is on two levels, and we prefer to keep our cockpits low down for greater comfort in rough seas. For single-rudder boats, the problem with a broad, overly buoyant stern (apart from excessive heeling and being vulnerable when docking) is that as the boat heels, the root (top) of the rudder is lifted out of the water. If the top 30cm (1ft) of a 1.3m (4ft 3in) rudder blade comes out of the water, more than a quarter of the blade’s effectiveness is lost. The top quarter of the blade becomes useless and the rest of the blade is operating in less dense water. The angle the blade is working at also makes it less effective and when under pressure it will ventilate: a vortex of air swirls down one side of the rudder, which loses its grip in the water, causing a sudden loss of control as the boat rounds up into the wind. To stop that happening, the crew needs to play the mainsheet constantly in gusty winds, which is exhausting if you’re sailing long distances, and the yacht will need to be reefed (and unreefed) more often.
To avoid this effect, many yachts have twin rudders that are angled outboard on each side of the stern. As the yacht heels, the leeward rudder is pushed into deeper, denser water and becomes vertical, making it more efficient. Twin rudders tend to be narrow-chord with a short span (short and skinny) and they must be set well apart to make them efficient while sailing. The problem is that there’s no prop wash over the rudders when manoeuvring in harbour, so unless the yacht is moving relatively fast through the water you have no steerage. When the yacht is stationary, she cannot be turned by applying helm and giving the engine a burst of power. Only when there is water passing over the blades at a speed faster than is prudent in confines of a marina will the yacht start to turn. One way to overcome this trait is to fit a retractable stern thruster in addition to the bow thruster, but they come at extra expense and need to be operated simultaneously with the rudder and engine and bow thruster, as well as using more power. Also, they take up space inside the hull and they’re something else for debris to catch in.
Sirius yachts don’t have fat aft quarters, they have a more balanced hull form for better all-round performance and comfort and safety at sea, so we don’t need twin rudders or stern thrusters – a single rudder works well. When a Sirius heels, the rudder blade is deep under water and remains fully immersed.
The reason we use twin rudders on some of our yachts
We do, however, use twin rudders for a different reason on our lifting-keel and twin-keel yachts, which are designed to dry out. With a draught up to 75cm (2ft 5in), a single rudder wouldn’t have a long enough span to work properly. Where we fit twin rudders, they have a relatively long chord (wide blade) and because our hulls are only moderately wide at the stern, the rudders are quite close together. A burst of power from the engine can be diverted by the rudders, creating a turning effect – not as much as with a single-rudder boat, but more than most twin-rudder yachts.
We can fit twin rudders on any of our yachts. We offer the option of twin electric drives (for propulsion and power regeneration) and we recommend twin rudders, one behind each drive, to give the very best manoeuvring ability. Not only is the thrust directed over both rudders, the two drives can be powered individually so with one drive in forward and the other in reverse she will pivot on the spot.
Construction of the rudder and skeg
As mentioned above, our single rudders have a half skeg that serves many purposes. As well as absorbing the energy of turbulent prop wash at the top of the rudder it also helps direct the water passing onto the rudder blade while sailing. It boosts the efficiency of the rudder and also protects it from hitting submerged objects. Inside the skeg is a stainless-steel structure that is through-bolted and bonded to the hull. We don’t hang the rudder off the skeg, though; three bearings would create a rudder that is harder to turn when heeling and wear out the bearings faster. Instead, we use two self-aligning bearings. A well-designed, strong rudder design doesn’t need a skeg for support, only for protection.
A rudder to dry out on
We use an alloy stock in our rudder blades that is optimised for seawater and strength. The blade is built in two parts, laminated onto the stock on the inside, and filled with closed-cell foam then laminated along the leading edge. On our yachts that are designed to dry out, the rudder has a wide foil on the tip to give a larger surface area for the rudder to rest on. When our twin-keel yachts dry out on concrete or any other hard surface, the rudder will be about 80mm (3in) off the ground and she will sit happily on her keels – we wanted to see how stable, so we had 12 guys jumping up and down on the stern and they were unable to get her to move.
On softer surfaces, like sand and gravel, the keels sink in by about 2-5cm (1-2in). Our boats’ longitudinal centre of gravity is slightly aft of amidships, so the stern tends to sink in a bit further. The wide foil, or foot, at the tip of the rudder spreads the load and prevents the rudder blade from sinking in. The rudder has a Delrin sheave to transfer the weight onto the hull and it is designed to provide a stable tripod stand for the hull. The rudder is deliberately a little shorter than the keels for added safety – when you run aground while feeling your way into your perfect spot to dry out, the keels will touch the bottom before the rudder does. On our lifting keel models, the twin rudders support the stern of the boat so she will dry out upright.