When are two rudders necessary?

Hi everyone,

I am currently designing a 7 meter sloop day sailor and the idea is to include modern design features such as a hard chine at the transom and a slightly reversed bow (very similar to the new VO65).

Since the boat is quite flat aft and relatively wider in comparison to the other stations of the canoe body, I was asking myself if it's necessary to include two rudders?

I would appreciate if someone could give me some guidelines in order to realize when it is necessary to use two rudders instead of a single one and where should these be located for adequate boat handling.

Thank you very much in advance.

 

You will need to do a careful analysis about whether a single rudder will become ineffective when the boat is heeled more than a few degrees. That depends on the righting moments rig size and height, metacentric heights, along with quite a few other variables.

 

FDESP80, Welcome to the Forum. The reason for putting twin rudders on any boat is so that as the boat heels, the active rudder is pointing more or less straight down into the water, while the inactive rudder is retracted and out of the water. Such rudders are usually canted at about 12° to 15°, and wide enough apart so that when at those heel angles, the active rudder is more or less in the center of the boat's wake and pointing straight down. Boats with wider sterns such as you suggest benefit from twin rudders the most because very wide boats tend to lift their sterns out of the water as they heel. Many think this is a good thing to have (wide sterns) but in fact it is good really only for downwind sailing. Wide sterns make a boat quite a poor sailer when sailing at heel on the wind.

That said, if you have twin rudders, the ideal place to put them is on the transom so that the windward rudder can be retracted and does not drag in the water at all. The boat uses only one rudder at a time. When you go on the other tack, you deploy the retracted rudder as it now becomes the leeward rudder, and the other, now windward rudder, is retracted. When sailing downwind, and the boat is not heeled very much, you can use both rudders, but keep them both partially retracted because you do not need the full area of both rudders (presuming that you have designed them correctly) on that point of sail.

I hope that helps.

Eric

 

Eric
Can you explain the science behind that, do you benefit from the rudders force acting in the plane that you want it?
or the active rudder only has half the drag assuming the other one is out of the water?
Elan have done a cruiser racer with non transom hung dual rudders, I always wondered?
cheers

 

I recently helped build a boat similar in hull shape to VO60's and the one you are considering. A single deep rudder foil was used. Light air makes for poorest performance because of high wetted surface. Even heeling does not often make them equal to a hull designed for light air.

On a boat this size, I don't see that any benefit of dual rudders is worth the extra design effort, building and particularly extra nuisance in handling or drag if you don't lift one. Very few designers resort to dual rudders for small boats.

 

Powerabout, The lift force on the rudder is always going to be perpendicular to the plane of the rudder (and perpendicular to the flow), and the drag is always going to be parallel to the flow (90° to lift) by definition. The amount of lift and drag are directly proportional to the areas of the rudders (bigger rudders have more lift, smaller rudders less lift), and the lift and drag coefficients are going to be a function of the planform shape, aspect ratio, and section shape. All rudder forces are directly proportional to the square of the boat speed.

Let's say an example boat has twin rudders mounted on the transom, the leeward rudder is deployed, and the windward rudder is retracted. As the boat heels over sailing on the wind, usually 10° to 20° in most sailing situations, the lift on the working rudder is going to be pretty close to parallel with the plane of the water surface--it might be slightly up or down, but the majority of the force will be parallel to the water surface. This is what you want--you want to be able to turn into or away from the wind, parallel to the water surface. If there is too much downward component from the rudder, it will try to pull the stern down, creating more hull drag and slowing the boat down. If there is too much upward component from the rudder, it will try to lift the stern, which drives the bow down and away from the wind. The boat's leeway increases--it's harder to point to windward.

Now let's say that we have inboard rudders on our example boat, installed forward of the transom, such that you cannot retract the rudders. That is, both rudders are working all the time. When sailing on the wind and the boat is heeled, the rudder that points more straight downward will be the rudder that does most of the turning. But if both rudders are linked together (they usually are) then the other rudder is going to be doing those awful things of raising or lowering the stern, either creating extra hull drag (pulling stern down) or increasing leeway (pushing stern up). So it does not make sense, really to have twin rudders that cannot be retracted. Therefore, such rudders should be mounted on the transom.

However, if Elan has equipped their boats with twin rudders, I would suspect that their angles to the hull are not nearly as high as 12° to 15° as I stated above. Maybe they are only a few degrees angled off vertical. That minimizes their adverse effects when heeled. Then, since you have two rudders working all the time, you don't necessarily need full-size rudders as on a single rudder installation. That is, the two rudders are smaller than one big rudder, and at least probably of equivalent total area. If they are really clever, then they'll have one rudder (windward) free-stream (not turn) while the leeward rudder is the one that turns. Then you are left with only form drag and wetted area drag from the inactive rudder. I don't know the rationale behind Elan's twin rudder design--I suspect part of it may be due to marketing hype.

At any rate, I hope that helps.

Eric

 

Many thanks to Eric and Co. for your explanations! This forum is indeed very helpful.

Regarding the design of the rudder: Are there any guidelines to figure out what the optimum surface area of the rudder should be for your design? What dictates the span and maximum chord?

In order to make things simple for a design of these dimensions, I would like to include one rudder only. The hull has been designed with some overhang at the stern and hence the idea is to instal the rudder as a foil that fits through a box (like a centreboard box of a dinghy) located at the aft perpendicular. Maybe this way, there is enough rudder span immersed at small and even large heel angles?

If so, what dictates the size of the rudder?

Cheers!

 

The rudders on Elan are pretty angled, base on the available pictures I would say close to 15° (http://www.basicsailor.com/wp-content/uploads/2013/04/Elan-350-e1364839685143.jpg and http://i667.photobucket.com/albums/vv40/ceeceebee_photos/elan21.jpg).
In order to establish the benefits of their configuration, a further consideration has to be added to what Eric has written (great info as always, btw).

As the boat heels, the effective rudder area (the one which produces the side force) varies with the cosine(heel) law. On a single-rudder boat the variation is negative (less lateral area with heel). On a twin-rudder boat with rudders angled outwards by 15°, the lateral area increases with heel. Just as Eric said in his post.

But, as the boat heels, the yaw moment from sails generally increases (see here: http://www.boatdesign.net/forums/boat-design/weather-helm-heel-question-50598.html#post691028). The rudders task is to balance the increased sail-induced yaw moment. However, the single-blade rudder has to do it with less lateral area available. Therefore, the tiller angle of the single-blade rudder has to increase in order to compensate for this missing area. The twin-blade rudder angled outwards, having more lateral area at heel, needs a smaller tiller angle increase to achieve the same effect.

Now, the induced drag of the rudder is proportional to the tiller angle squared, everything else being equal. So the twin-blade rudder like that of Elans will have a double frictional drag (in the worst case of a completely-submerged windward blade), but will also have a much smaller induced drag than its single-blade counterpart. Whether it is beneficial or not, will depend on the global boat-resistance analysis, referred to the design goals. At the end, the detailed analysis might prove it beneficial even without the windward-blade kick-up capability.

The above explanation considers the same geometrical blade area between the single and twin rudders, which is not strictly true but gives an idea of parameters which have to be considered. The blade areas of a twin rudder are usually smaller than the blade area of the single rudder, because in a heeled condition the twin rudder blade operates in a better condition for creating the side force.

Finally, a consideration has to be given to the blade aeration too. A single-blade, centrally mounted rudder will be more exposed to the effects of the blade aeration, but also to the adverse effects of the water aeration in the proximity of the sea surface. It can make a difference between broaching or not broaching in some conditions, in particular when the sea gets rough.

Cheers

 

It also appears that the Elan uses much smaller rudders than what would be expected from a single rudder installation. Which means that at even moderate amounts of heel most if not all of the windward rudder is out of the water.

 

This is a subject that will demand a lot more space and time to explain than is available here. You should consult a yacht design manual (or manuals) to read up on the subject if you have not already. For example, Skene's Elements of Yacht Design by Francis S. Kinney (8th ed.) says that the rudder on a sailboat should be between 8% and 10% of the total profile area. Principles of Yacht Design by Larsson and Eliasson says that the rudder area should be about 1% to 2% of the sail area, with 1.4% being the overall average for modern cruiser/racers. Another must-have book is C.A. Marchaj's (pronounced MAR-Ki) book, "Aero-Hydrodynamics of Sailing."

Obviously, you don't want the rudder to be deeper than the keel, and it should be a few inches (or equivalent mm) shorter. You have to figure in aspect ratio, and generally, aspect ratios are generally not more than 2.0, although on some really hot shot racers you may see very tall, narrow rudders with aspect ratios of 3 to 4. The higher the aspect ratio, the greater the lift at small angles of attack, but the greater tendency for the rudder to stall at lower angles of attack. Finally, you need a suitable section shape, and since your rudder probably won't have a rudder stock, it being in a cassette, you don't have to worry about fitting a stock inside the rudder blade. Therefore, you can use a narrower rudder section. Narrower rudder sections stall more easily than fatter sections. I tend to use rudder sections that are between 10% and 15% foil thickness.

Further reading and study will allow you to home in on the appropriate rudder dimensions.

Eric

 

Thank you Eric! This should get me started!

 

Speed squared!

1)Compare multihull rudder sizes to mono.
2)What's required during maneuving versus upwind.

Two rudders are vastly better than one.

Increase the angle on them if the transom is not wide enough to get the windward one clear on heel, good idea anyway, help increase aspect ratio.

 

you only need small rudders if you are going fast and the boat is light - multi

 

Even multihulls maneuver slowly but now you've got two.

 

There are a few other ways to have dual rudders, & still have them be retractable, aside from going with the transom hung option. Particularly as transom hung rudders lose some efficiency when compared to those which are underneath of the hull.
1. Would be to go with rudders in kick up cassettes. These are seen both on multihulls, & high end racing mono-hulls.
2. Is to go with rudders in a drum, as discussed here. http://www.boatdesign.net/forums/boat-design/rudder-drum-25041.html
3. Fixed (non-retractable or kick up) dual rudders, mounted in the standard fashion akin to single rudders.

For #'s 1, & 2, as well as with transom hung rudders. If you want to have a chance at system redundancy, & or to be able to swap out rudder blades easily, the cassettes/drums need to be built a lot stronger than the blades themselves. So that if the rudder strikes something severely, it's only the blade which is damaged, not the entire system.

All of the options have their pros & cons. But hopefully you have enough info to get started on making your decision.

Ah, one other item as yet not mentioned. Such rudder systems have become a popular item on mono's in the last decade or so, is because they're on round the world racers, & or serious, deep ocean racing boats. Where rudder casualties have become a lot more common. And with the way most of the twin rudder systems are designed. One can carry a spare blade onboard, and also can haul the damaged blade up on deck, or down below in order to patch it up.
Some boats even going so far as to have the same foil sections for their dagger boards as they do for their rudders, for redundancy's sake.