What Is A Rudder On A SailBoat and How Does It Work ?

What Is A Rudder On A SailBoat and How Does It Work

Rudders are basically the underwater parts of your boat that controls your direction. Without them, it is really hard to maintain the steering so they are one of the most important parts of your boat. Different type of rudders with various design features gives a different performance. Do you want very high performance but a fragile rudder? Do you want a good performance however well-protected rudder? It all depends on where do you sail, how do you sail and what would you compromise. In this blog post, we will explain what is a rudder, how it works, what kind of rudder designs are available out there@

Tiller or Wheel?

Tiller or Wheel

Tiller: Smaller boats (under 30 ft) usually use a tiller to turn the rudder. Tiller is basically a wooden (sometimes aluminium or steel) stick attached to the top of the rudder rod.  The more force needed to turn it is better to make the tiller longer to increase the moment arm. They are great for racing and also for solo sailing as you don’t need to stand to control and you can also be busy with other things like tacking while holding the tiller between your legs. 

Wheel: The bigger boats usually have wheel instead of tillers. The wheel system works with additional shafts and gears to ease the movement of the rudder blade.  Wheel also gives the sense of driving as the turning direction is the same as the boat turning direction, unlike the tiller.

So what is a rudder?

So what is a rudder

The rudder designs we are using today have evolved from the steering board that was used in ancient times. The steering board was usually mounted on the right-hand side to suit right-handed sailors. In time the steering board moved on to the centre line through a stock passing through the vessel and a tiller was then attached to the stock which allows sailors to control the rudder from the main deck. Surprisingly the rudder basics are kept the same over the years. A tiller or wheel connected to a rudder stock connects to a board/blade in different shapes and profiles that create the lift under the water.

How rudder works?

How rudder works

The rudder works by deflecting water flow. When the rudder blade is straight and parallel with the flow, it has no effect on the steering. 

When the skipper turns the rudder this is what happens;

If the rudder blade is directed to another angle, the blade has to direct the water at an angle away from the boat to increase the pressure on narrowed angle side and decrease the pressure on the other side.

This pressure difference pushes the stern to the low-pressure direction. In this case, the boat will try to turn to the port side. This is basically how you steer a sailboat with the rudder. Not an easy task to master but fun nevertheless!

Rudder types on Sailbaots

Rudder types on SailbaotS

Rudder on a Full-keel sailboat

Full-keel boats or modified full keel boats usually have this type of rudder. These rudders are usually hinged to the aft edge of the keel to make a continuous surface not to create any drag after the main keel. Engine propeller is usually positioned between the keel and the rudder. 

Pros

The main advantage of these type of rudders is the rudder is well protected and strong. It is hinged at the top and bottom which helps to distribute the forces on the rudder blade through the keel. Also, Rudder or any other item cannot snag on. In case of grounding, the rudder will be always protected. This means you will be able to maintain your steering even after grounding or an accident. 

Cons

Because of design, this type of rudder requires bigger forces to move especially when speed is increased.  This is the reason why old long boats rarely have tillers as it usually requires a lot of push. 

Manoeuvrability is harder on these type of rudders as the propeller wash effect can be so little – especially with the full keel attached rudders with a propeller notch. But hey, full keel boat itself is another challenge for manoeuvring right?

Spade rudder and balanced spade rudder

Today, most fin keel boats have a spade rudder which is basically an extension straight down from the aft hull suction. The rudder rod comes down through the hull into the rudder blade itself in a most efficient and force friendly way.  It allows the entire rudder to rotate around.  

Pros

Great performance: Spade rudders are modern rudders often chosen by serial production boats by famous brands. They are usually tall and thin which gives high aspect ratios that means a lot of lift and little drag as the longer the rudder is the bigger the drag is. 

Less force: They don’t need a full keel or skeg for mounting. As the rudder blade can be moved around a balanced rod, the forces are much less compared to the other rudders. 

Cons

Fragile: As it is applicable for any optimum design, more performance means less durability. A spade rudder is a vulnerable rudder especially to debris or objects in the water which can hit the rudder and exert a force on the rudder rod which is the only structural support. Even the force of the water when the sailboat falls off a wave can have a huge impact on the rod and create high stresses. If the rod is bent, the rudder may also jam – so be aware!

Balanced spade rudder: In some designs, the rudder rod and the leading edge of the rudder blade are arranged in the most optimum way so the forces during the steering are less as possible. These balanced spade rudders usually have a little gap between the hull and the rudder blade. 

Skeg mounted rudder

Some fin keel boats also may have Skeg mounted rudders.  It is a compromised between the two rudder types above.  

In general, Fin keel boats give more performance comparing to the full keel or modified full keel sailboats. When the rudder is placed more to the aft that gets more clean water than it will have better performance however you may have durability issues as explained in the spade rudder section.

Designers over the years combined these two rudder types and came up with a solution called skeg mounted rudder.

Some designs include skeg construction right before the rudder blade.  They also call it cruisers rudder. The idea is the skeg (faux keel) which is strongly attached to the hull or part of it like the keel protects the rudder for any damage and also provides a better connection in terms of the load distribution when loaded. As you might guess you give up little performance for better durability. 

It has the same advantages and disadvantages when you compare it with the full keel rudder or spade rudder however, it is known that the forces on the helm can be on the high side.  This means it requires more force to turn the rudder blade especially in heavy weather or high speed.

Outboard rudder

The outboard rudders are mounted outside the hull on the boat’s stern (transom) with the hinges. The outboard rudders are usually provided with a tiller instead of a wheel as there is no rudder rod that can be controlled with any gears.  

Pros

Outboard rudders don’t need any hole through the hull which is great in case of damage. The rudder can be removed or serviced while the boat is still in the water. Also, the strong hinges from the top and bottom (sometimes in the middle) provide great strength like in the full keel rudders. 

Cons

Like the spade rudders, outboard rudders are vulnerable to any object that may hit. The forces on the helm will be also high at high speeds during sharp turnings. 

Can be also dangerous when docking the sailboat from the aft side as you may hit the rudder itself to the shore.

Conclusion: It's like most things in life; it's a compromise

The rudder type has more relation with the keel type of the boat than any other equipment on a sailboat. As said, if you are a day sailor and like the high performance and even participating in races, you wouldn’t go for a full keel sailboat. If you are a cruiser and making long passages and want to feel comfortable then a full keel reliable boat might be your choice. 

Whatever the rudder and skeg configuration, it is essential that the keel has a greater draft than the rudder and/or skeg. This way, in the event of a severe grounding, the keel takes the hit. We have seen contemporary rudders that actually have more draft than the boat’s keel; this is absurd! 

In the event of a grounding, and if the boat dries out, the keel and rudder must be built strong enough to support the full weight of the boat without damage and in a relatively stable manner. For this to occur, there must be a reasonable surface area on the bottom of the keel (a bulb keel does well here, as in so many other areas), with a flat or nearly flat profile far enough forward and aft of the boat’s centre of gravity to stop the boat toppling over on its ends if, for example, it is laid up against a jetty. 

However, a wide, flat keel is undesirable (some wing keels fall into this category). If the boat runs aground parallel to a steeply sloping shoreline with the tide going out, it will want to lie down on the “downhill” side. This will lay it over far more than if laid down on the “uphill” side and will exacerbate problems such as stowed gear falling around and wet-type batteries leaking. It also makes the boat vulnerable to flooding when the tide comes back in, particularly if any seas are running.

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