Distance Keel to Rudder & Effects

Rcsailing.net Technology Discussions
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In the past, rudders were either mounted on the trailing edge of a fixed keel … or they were mounted as far to the stern as possible (considering linkage, rudder logs, tiller space/placement or steering wheel quadrant size.

In many cases, rudders are transom hung which obviously provides the greatest distance from the keel. Some transom hung rudders are seen with fairing at the keel depth, which is designed to provide a bit of added advantage for the water flow along the keel and back and around the rudder. Tra<s><font color=“red”>m</font id=“red”></s>nsom hung rudders are often viewed as having “unpleasing” flow with turbulent water rolling out from under the keel of the boat and upwards - often climbing up the sides of the rudder, much like a surface piercing foil.

Meanwhile, hull hung rudders found under the hull of the boat are intended to reduce the turbulence at the top of the blade, as water streams back along thehull and the rudder surfaces.

Transom hung rudders have an advantage that they can be easily raised (or tilted) for shallow water and beaching. It is interesting to note, that many high speed planing dinghies and small multihulls (skiffs, racing class dinghies, etc.) all continue to use the transom hung rudder. Whether class rules have made it so, whether they have or haven’t been challenged, or whether the added few feet further aft of the keel/center/dagger board provide an added advantage is a question I am pondering.

Are there factors and formulas that are used for locating rudders from the keel? Does a rudder that is a greater distance from the keel promote a more stable direction and straighter hull “tracking”? or - Does a rudder closer to the keel offer better and faster tacking at the disadvantage of very sensitive steering?

I have witnessed on the <font color=“red”>big</font id=“red”> cat, the distance between boards and rudder seems to increase the area of lateral resistance and slow rudder response. I also have been sailing extremely fast downwind, only to find myself with overly sensitive steering (so called “sportscar” steering where it was very easy to oversteer). Was this oversteering a result of hull speed through the water - the distance between rudder and keel, or simply my imagination?

This brings us to our little boats and optimum rudder locations. Would dual rudders (bow/stern) also result in oversteer - or is there enough distance that the hull between both rudders would act to hold directional stability?

Are there any lessons to be learned from both transom hung and under-hull rudders, single or dual, close or maximum distance from keel or boards - or is surface drag a more important issue which prevents experimentation or testing of distances between keel and rudder or numbers of rudders?

The most current issue of Model Yachting (US 1 Meter section) indicates some rule questions regarding transom mounted rudders versus “Len<font color=“red”>g</font id=“red”>th Over Al<font color=“red”>l</font id=“red”>” definitions, and I am curious if any will push the envelope and try a one meter with a transom hung rudder - and see what performance gains there might be - if any.

Thoughts anyone?

<font size=“1”><font color=“red”>edits</font id=“red”></font id=“size1”>

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As you increase the “arm” you increase the “moment” so as you increase the distance between the fulcrum and the point of application of force, you increase the amount of turning moment generated… assuming equal force application, you achieve a greater amount of turning moment with the same amount of force applied.

I think boats such as a Lazer or Force 5 use a transom mount for the most basic reasons of all… they are the easiest to manufacture, install, replace, and use. No holes in the boat and easy access.

I believe that a balanced rudder is the most efficient of all the options available. It gives an honest feel, and provides efficient execution of it’s duty without an undue amount of force required to operate it. They are more fragile, being exposed on all sides to any type of damage… which makes them more of a problem for big boats that models, since the latter can take considerably more damage than their full sized counterparts.

I can see no advantage to a model in the 1M class range using a transom mounted rudder from the point of view of reducing drag, and if anything would probably increase it. I don’t see where there is a problem finding servos to operate the balanced rudder that all models in this class and it’s immediate neighbors already use, so there is nothing to be gained from reducing the force needed.

If there was anything that I would see as a challenge to the existing setup, it would be the use of a dual system employing a canard on the bow section, but those have been ruled out of most classes. I have flown canard equiped aircraft, and they do offer greater flight control movement with less effort. Balance is much more critical. (The Wright brothers had this going for them)

When you seperate the rudder from the leading edge (keel) you create a 2nd drag coeficient but you increase potential stability (or instability if it is not properly aligned) Just as when you are knocked off balance you spread your feet to give greater stabilit

As with virtually anything else… you have to give up something to get something. The more sharp the steering, the greater the impact on speed. The more streamlined rudder next to the keel is less efficient at generating the turning moment, but does not suffer the same drag penalty.

A transom mounted blade rudder would tend to “Skid” the boat around where a balanced rudder (which could still be transom mounted and undercut the stern section) would be more efficient than a blade type. You would also need to consider the depth of the rudder as a transom mount would tend to be more shallow due to it’s higher mounting point compared with a rudder mounted on the keelson… this could be as much as twice the depth. Now you are needing to increase the verical dimension of the transom mounted rudder to compete. Add to that the effect of the boat when heeled and the proximity of the rudder to the more turbulent water at the surface compared to water below the keel of the boat… it all keeps coming up that a transom mount has nothing to beneficial to offer a model.

You can always hedge your bet and have your rudder hung behind a skeg, and you can still balance it that way as well. (see Photo of 6 meter set up this way.) I don’t recommend it… just offer it to the mix of possible solutions.

Download Attachment: [ 6METR1A.JPG](http://www.rcsailing.net/forum1/data/larry Ludwig/2004822232626_6METR1A.JPG)
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Hi Dick

There is a concept called “rudder volume”, where the general effectiveness of a rudder is calculated as “rudder area” times “distance from centre of hull buoyancy”. AFAIK, there is no formula for rudder volume or effectiveness, only past experience. The concept helps, though, because it says that, for a given effectiveness, if you can hang your rudder further away from the COB, you can reduce the rudder area. Now that is useful – lower area, lower drag. Hence the trend to hang the rudder off the transom if the LOA is restricted in some way, and hope that the reduction in rudder area more than offsets the increase in turbulence.

Lester Gilbert
http://www.iomclass.org/
http://www.onemetre.net/

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Mmm, dinghies use transom hung rudders because its easier to make on a smaller boat, & it would be most likely in a lot of the rules of dinghies. but once you get into the bigger keelboats, the rudder blade of a transom mounted rudder would be heaps bigger, making it way harder to put into the rudder box. I know when you get to about 23 ft, the rudder blade on the faster boats are big, i have put them in, they are pretty heavy as well.

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.
Life’s A BITCH, LIVE HER TO THE FULL!!!

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My point of view on this is that the drag through turbulence that is created by having a under slung rudder against a transom hung rudder is only something to be concerned about if we can sail the boat to perfection. No man, no matter how good, can sail a boat to perfection.

Tank testing and boat performance does show that the control created by having the rudder under the hull is better than transom hung rudder’s. The correct distance depends on many different things.

Size of keel
Type of boat
Just to name two.
With model boat’s the idea for having the rudder under the boat is to keep maximum waterline length.

Peter

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Hey dick . . .

Ill respond to this one, as I havent replied to much of anything recently. That Question in the US one meter class actually came from myself. If anyone would like to read the “official question” went to the tech comm, it can still be viewed at:
http://www.ourwalden.net/rcsailing/techcomm.pdf

The reason I wanted to go this route was because I believed that I found a potential rudder design that would be advantagous in the the US one meter class. . . and several others. I decided that I could most easily test the concept in a stern mounted fashion, so I went to the rules to find out if I would have to cut 2 inches off of my boat to accomplish this legally. Upon reading the rules, I determined that I might not have to, and thus, the whole hub-ub with the tech comm.

The ruling of the tech comm was that I had indeed found a hole in the rule, and that I could use it, however, the hole would most likely be closed by next year. I personally support closing this hole BECAUSE it could allow for boats of infinite length, or practically up to almost 2 meters. I DONT like the thought of that.

The concept that the stern mounted rudder was meant to test has been put on the side burner. I do believe that it has significant speed advantages, however will have to be implimented through the hull. I will most likely start sailing competitvely with the concept next year. Had I used the concept this year, it would have had a 1 year shelf life and then all would know what I am up to. Executing this rudder through the hull will take a little more time, but hopefully be worth while.

How much of a speed gain. . . not yet sure, not tons, but <font color=“red”>most likely (edit, added)</font id=“red”>enough to make a difference across a day of racing.

Hope this clears some things up . . . but at the same time, confuses things a little more!

-Todd

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Thanks to Todd and everyone else.

The Model Yachting post along with Todd’s further detail was interesting. What prompted the original question was that on many of the newer dinghies and catamarans, I have noticed the board locations seem to be moving rearward and closer to the rudder. At the same time, as noted in my post, I was “educated” on the oversteer problem with my big cat, and working toward an r/c multihull with improved tacking as a goal, began to wonder about distance between board/keel and rudder.

I have often theorized that a bow rudder instead of a rear rudder might be more effective and efficient in steering a multihull through a tack, since it was turning and “pulling” the bows/hulls through the turn, as opposed to a rear rudder that pushed the hull one way through the water creating additional drag as the stern tried to move sideways - and at the same time “pushing” the bows in the opposite direction. (perhaps the difference between trying to “push” a rope or “pull” it?)

I understand the issues of underwater damage that a bow rudder could encounter - but in our little boats where there certainly isn’t any cost differential between bow versus stern rudder location, thought it might be worthwhle to try as an experiment- given that most sailing venues are free of whales, floating shipping containers, etc.

If one were to move the rudder to the bow, would you keep the same distance as if a conventional rear location, or could/would it be feasible to bring the rudder back closer to the front of the keel or boards?

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Hey Dick,

Interesting thought.

I think that the concept of “rudder volume” that Lester describes would say that you want to move the rudder as far AWAY from the keel/boards as possible.

Here are some things to think about:

  1. The keel or boards are the main lifting bodies. The rudder should generally have little or no lift when sailing a straight course. With the rudder in front of the keel, the water flow over the keel will be somewhat perturbed by the wake of the rudder. This will make the keel a bit more prone to stall and/or ventilation.

  2. A forward rudder is similar to a canard type arrangement on an airplane. While canards have been used on some planes, they are generally less prefered than a rear elevator. The reason for this is stability. If you mount the elevator on the front of the plane (as a canard) you mode the center of aerodynamics ahead of the center of mass. This results in an unstable configuration - sort of like trying to throw a dart backwards. It wants to flip over. Think of your your boat sailing along. suppose the bow started to turn to port. With a forward rudder, that turn would be an angle of attach that would result in lift to port. Since the rudder is on the front, that would cause the bow to turn even more to port. Now consider the conventional rear rudder. boat turns to port resulting in rudder lift pointed to port. But since the rudder is on the back of the boat, that lift vector will cause the boat to head to starboard - thus resoring the original heading. So a transom rudder is inherently stable.

  3. There is a lot of turbulence near the bow especially in large waves. If the boat is pounding through large waves with the bow regularly coming out of the water, it might not be wise to have the rudder up there.

  4. The bow of the boat is generally a narrower V shape than the transom which is generally a wider U shape. Thus the transom forms a better endplate for the top of the rudder and prevents ventilation.

It might be worth trying a bow rudder, but I would be careful as some or all of these effects may negate any advantage you might gain…

  • Will

Will Gorgen

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The problem with canards is that they are more difficult to control accurately. There are few examples of aircraft that fly well with only manual controls of a canard (and Rutan has them all) any other aircraft using canards require a computer and fly-by-wire controls to accurately keep the machine under positive control.

The more you turn, the more you turn…