Rudder Efficiency

[seandwyer]

Hey,

The blade of my rudder is held in place with a hinge joint (bolt and wing nut) as I believe most are. It's great because it yields easily when an obstruction comes along, but when I sail, I have noticed that as speed increases, the rudder begins to raise from the drag as it runs through the water. My question is - what is the most efficient angle for a rudder - straight down, or something else depending on the moment? I know that depending on conditions, the center board should sometimes be partially raised to increase performance or efficiency - is the rudder the same? Seems like I have also read where guys have rigged a bungee sort of affair on the rudder to keep it fully extended, but still capable of kicking up if something hits it, or when beaching. I think this is what I'd like to do as well, but first I want to better understand the efficiency issues. If anyone can explain this - or if anyone has an easy to follow design to bungee the rudder in the down position, I'd be grateful.

Thanks Guys!

[navahoIII]

I second that! Good question.

[MrPlywood]

LOL - there's nothing like having to lean over the stern to push the lower part down after a beach launch, or forgetting to do so and hitting the prop with the horizontal rudder. Priceless...

[ctenidae]

I use the bungie method- works well for me.

I would imagine that straight down is the most efficient, as it would direct the greatest quantity of water.

[Peterw11]

My rudder pivot is baulky and the blade is kind of stuck at a 45 degree angle. The wing nut is rusted on, so there it stays until I get around to replacing it. It's been that way since I first got the boat last spring.

It reacts very swiftly to movement, in that I don't notice any lack of steerage or efficiency. I move the rudder, the boat responds, immediately.

Plus the angle at which it's jammed makes for a gentler impact when it hits anything underwater.

That said, I would think that the overall efficiency remains pretty much the same as long as the same amount of rudder remains under water.
The bearing surface area is the same, whether straight vertical, or at an angle.

Or am I wrong about that?

[jdubes]

Interesting question. I'm always changing things on my DSI to see how it responds. This is one of the reasons why i think this is such a great boat. I've learned so much related to the dynamics of sailing doing this. I live on a pond that gets very strong winds in the summer. And i go out in anything 20's-30's, sign me up. Often when I'm going downwind with a spinnaker, i pick up the rudder and my keel blade because my position to the mark allows that. This boat cruises like that, i can get this thing to throw up a wake. Granted in this setup, the boat is slightly unstable.

To be quite honest, I'm glad the water friction picks it up going down wind. Up wind it's a totally different story.

[ctenidae]

That said, I would think that the overall efficiency remains pretty much the same as long as the same amount of rudder remains under water.
The bearing surface area is the same, whether straight vertical, or at an angle.

Or am I wrong about that?

I'm no hydrodynamicist, but I don't think it works that way. If you figure that the effectiveness of the rudder is measured by the amount of water it's able to deflect, then it's most effective with the greatest leading edge (ie, straight down).

Given that the further along a plane the water moves the more drag and turbulance it creates, you would want as narrow a blade as possible to get the amount of deflection you needed. (As evidence, think about how much work the rudder becomes to handle when it's floated up and you're dragging its whole length in the water)

So, combining the greatest leading edge with the narrowest blade, you'd want the rudder straight down. I'm sure there's some equation for determining optimal length and width, etc etc, but I don't know it.

[ctenidae]

Interesting- reading this article: http://www.fastcomposites.ca/services/tools/design.php

It seems that having the rudder float up might not be a bad thing, but it needs to go oposite to what it wants. When moving fast, you want a longer thinner rudder (straight down). When going slower, you want a wider shallower rudder (kicked up).

[MrPlywood]

I'm guessing that a vertical rudder is easier to steer because the total surface area (the moment) is closer to the fulcrum. Right?

Great link, BTW. Thx.

[persephone]

Just a blind guess on my part, but I'm going to guess that a rudder is at it's maximum efficiency straight up and down at all times. The only reason for my guess is that I have never seen a racing boat such as an Americas Cup boat with a swept back, or pivoting rudder. At least not in the modern era.
One would think that if there were some advantage to this it would be done at the high end of the competition ladder (or that it would be prohibited by some rules).
Also, the DS rules specify that a soft (breakable) wood pin is allowed to keep the rudder down. Perhaps implying that some might consider permanently fixing the blade in the down position for some advantage.

[Peterw11]

I'm sure there's something to the efficiency of a vertical rudder as opposed to horizontal, but given the fairly slow max speeds a DS can attain, it may be more perception than reality.

That said, on the rare occasions that I have had mine at close to max hull speed (what, 7kts or so?) I notice a distinct difference in the amount of pressure required to move the tiller. As speed increases, the pressure required virtually doubles.

I'm sure that's a function of blade orientation, so there maybe something to the theory, after all, but not being a racer, isn't that big a deal to me.

[K.C. Walker]

I think of a rudder as having a split personality. It can be thought of as a brake or a wing, though it is both.

As a foil it acts by "lifting" the transom in the direction that has the most lift. So as you change the angle of incidence of the rudder you change its lifting characteristics and it pulls the transom to steer in that manner. It's very tricky to keep attached flow to the foil and it's best with a perfect wing shape, a high aspect ratio, and with a low angle of incidence. It's critically important to keep the rudder nearly vertical to make all this work. And, this is by far the most efficient method of steering the boat with a rudder.

Steering using it as a brake is somewhat akin to sticking a paddle flat faced into the oncoming water to steer the boat with drag (I'm sure you've had this experience with a canoe). It works but it is not fast or efficient. If you're rudder is not vertical it becomes less efficient as a foil and becomes more of a brake. The more it is raised from vertical the more it becomes a brake which is part of why the helm becomes much heavier. Again, think how hard it is to hold the paddle flat faced in a canoe when moving fast.

The foils, both the centerboard and the rudder, are big sources of drag. So keeping the rudder in the best condition and in the best position (vertical) does make a large difference in speed and efficiency. This is true in any wind conditions but particularly noticeable in very light air and when it's really blowing.

Of course, steering the boat as much as possible without using the rudder is even more efficient.

KC

[Peterw11]

"Of course, steering the boat as much as possible without using the rudder is even more efficient."

There are a couple of videos on Youtube that show how to steeer without a rudder. Pretty interesting.

[GreenLake]

@Peterw11: Can you give a link or the search expression to get to these videos?

[GreenLake]

Aspect ratio relates pretty directly to the achievable ratio of lift to drag for a foil. That's why sails are tall and narrow.

If you don't need the maximal lift (because your boat is well trimmed?) then it might be possible that some other configuration eliminates some of the drag. It would also eliminate lift, of course, but if you don't need the maximal lift that may not be an issue.

The whole thing is perhaps comparable to partially lifting a centerboard. That results in a slanted leading edge as well as an increase in chord length and decrease in total exposed area. In addition, the center of lateral resistance from the centerboard moves backwards.

We know from experience that for some points of sailing this is advantageous over a fully lowered centerboard. For example, when not going upwind or not carrying full sail.

You'd expect similar effects for the rudder (except that the exposed are is not reduced, because of the location of the hinge). The point where the rudder generates lift would move back (and because the DS is not all that long, that increase in lever arm should have some effect).

For those who like to try things out, and have access to another DS, it would be a neat experiment to try to sail two boats on parallel courses and experiment with different rudder positions first on one of the boats then on the other and compare results.

I wouldn't rule out that there are some speeds/points of sail where there's a small advantage to a partially raised rudder, even though I agree with K.C. and others that high speeds up wind is the scenario where I'd expect that the standard configuration wins.