rudder shape

Here's a question for the lot of you:

What rudder shape will make a ski feel most stable in general upwind/downwind conditions?

I'm trying to reduce the boat roll induced by a particular prototype rudder shape which was made by a friend. This rudder is fairly vertical, 8" long (20 cm), a NACA shape, and has the post positioned about 1/3 of the way back. When I demo'd the rudder, it brought up a problem that I don't know how to answer: using the rudder makes the ski lean/tip almost violently towards the direction that you are turning, and this is unsettling at high speeds on a downwind. So, the question at hand is how do I change this design to make it roll the ski less? Is the roll due to the great rudder depth? the rearward rudder post placement? The more vertically oriented the rudder is? Or is it a combination of all of these?

My gut tells me that the rudder induced boat roll is mainly a function of the amount of rudder surface area infront of vs behind the rudder post, but that's just a guess.

Erik Borgnes

The surface area is certainly a large factor in this problem. But ultimately I think it is the lift that is created from the changed angle of the rudder.

Assuming the rudder is a normal cross-section which will look like an areoplane wing (thicker at the front and tapering at the tail), then the rudder is effectively a wing that creates lift.

When the rudder is straight, the lift on both sides is even and offsets - so no tipping of the ski.

When the rudder is turned, you increase the angle of attack on the 'wing', so the water is high pressure on one side and low pressure on the other. This creates lift on the low pressure side of the rudder. So, when turning left, the right side of the rudder generates lift and will tip the ski into the turn.

To reduce this, you can reduce surface area and/or try to reduce the lift that is built into the shape of the rudder. However, you may also encounter problems with cavitation where air is sucked down the rudder and making it useless until water flow is resumed.

Hope this helps.

Is the rudder relatively thick or thin compared to others? I found lift increases with thickness,creating a sharper turn. A thin sheet metal rudder was not so responsive in rough water, but then it was stern hung.
You may be right about shaft position.
A piece of softwood is fast and easy to shape for a prototype. If it works well it can be a plug for a mold. If it doesn't, try another.
Jim

Jim, yes the rudder is fairly thick compared to others.

Now, what happens when you take that same rudder (which rolls the ski quite a bit during turns) and angle it back to 30 or 45 degrees? I believe that it rolls the ski less during turns and I suspect that this is because the lifting effect of the turning rudder is now directed towards both depressing the bow and rolling the ski - albeit less for the latter.

Hi Nell,

I'll add my 2 cents for what it's worth - I'm sure there are some gurus out there with more knowledge than me ... there is a lot going on with such a simple thing as a rudder!

A thicker foil shape should produce greater lift at lower speeds than a thinner foil. It will also produce greater drag at higher speeds. So that is probably the first thing to think about with your rudder shape.

Having a thicker rudder at the tip will also add to the leverage effect of a long rudder (via greater lift at the bottom of the rudder).

There is also a relationship between the chord (ie thickness - if I recall the terminology correctly) and the length from leading edge to trailing edge. I don't have this info but I'm pretty sure there are some rules of thumb re this ratio and it's implications.

When you angle the rudder you are effectively lengthening the distance from leading edge to trailing edge for the same chord measurement. This (I think) should reduce the lift characteristics slightly. You will also have a rudder that is not as deep as before and therefore acts as a shorter leaver to tip the ski with.

The rudder should still turn through the virtical plane in both the upright position or at an angle. Since it is the flow of water over the rudder that matters, it will still follow the line of the bottom of the hull and should not create any lift in the tail of the ski or corresponding nose dip. This assumes you have not found a way to tip the rudder on it's side slightly by turning the rudder.

My gut feeling is that you want the thinnest, shortest rudder you can get away with (lower surface area and therefore lower drag) but still turns the ski adequately without cavitating. A flat plate would do for paddling in a straight line but it will create more drag when turning than an aerofoil shape. It might also promote cavitattion when turning at higher speeds (like surfing down a wave)and therefore become useless to direct the ski.

Hope this helps.

I'm sure the designers at Fenn, Epic and Red7 have all considered this in their designs. I'd be interested to see if I'm on the mark with any of these comments!!

Nell, I'm not sure, but I think there is something called aspect ratio. High aspect is vertical and provides more lift relative to a raked position.
The trailing edge should taper to a knife edge and if "fish form" the leading edge should be rounded. All the surfaces should be convex. Thickness should probably not be more than needed to get a shaft fixed
That said, there are rudders with a symmetrical cross sction that do work reasonably well.
Mathematical models are wonderful, I suppose, but never worked for me- 'nuther challenge. Manual implementation of an idea does work well if the idea is based on some experience. That is why I say to try the shape(s) using pine, cedar. basswood etc. It doesn't take long. When you have something that works just make a mold from it for your composite version.
Jim

Hi Eric,

Lots of 'it depends'

What is the planform shape of this new blade ?

Can you provide width and thickness ?

Dumb question but have to ask ... Is it a foiled blade ?

How much does it vary from the stock rudder ?

Is it 'vertical' right now or raked back a bit ?


aloha,
pog

Interesting topic.

I just used three different rudders on my Fenn Mako Elite. The first is the standard fin shape that angles backwards.

It's reasonably effective - except when you're going downwind at speed it gets more and more difficult to turn. My explanation for that is that most of the rudder area is behind the pin - the faster you go, the more force there is on the trailing edge. In other words, the rudder is unbalanced.

Then I tried his new rudder which is has a more vertical leading edge and has a greater area. It felt to me that it had the same issue at speed as the old rudder, but it also felt to me as though it had more drag ie when you turn sharply the boat stops. I didn't like the feel of it.

The third rudder was built for me by Dale Lippstreu - it's a similar shape to the Epic elliptical rudder except that it's slightly smaller. It also has a NACA airfoil profile.

It is extremely effective and for me it makes the boat much easier to maneuver going downwind in that you need hardly any force on the pedals to turn it. It does make the boat a little more twitchy - but I found that I got used to the increased sensitivity within minutes.

Robin, can we see a photo of the rudder Dale made, Side on and section if possible?
Thanks, Jim

Pog, don't have the rudder in front of me to measure, but it looks fairly standard, foil shape, a bit thicker than usual for most of its length, and raked back about 10 degrees or so - the typical "surfing rudder", which brings up another point. .

Because there is so much lift generated from a long vertical rudder, maybe the best rudders for downwinds would be raked back maybe 30 degrees or so, so that they create less boat roll at high speeds? Maybe the vertical "surfing" rudder design actually is a misnomer? Why is there the persistent idea that a surfing rudder is a vertical rudder? (as an aside, decades ago when I windsurfed, the standard vertical centerboard would create way too much lift at high speeds, seek the water surface, and buck me off. The high wind centerboard was raked back considerably.)

Mike, you brought up an interesting idea that a rudder that tapered in thickness towards the bottom tip would effectively maintain surface area but would have reduced lift down low, so it should roll the ski less. So, maybe more vertical rudders should be thinner in general if boat roll is an issue, though the tradeoff would be more drag/cavitation . . . Or, maybe an effective design would have a NACA foil shape in the upper half, close to the hull, and a flatter shape in the lower half?

Eric,

How 'bout the planform shape ?

Trapezoidal or elliptical ?


No absolutes but if you rake back 'X' foil shape you will get a little less drag but have trade offs of course.

Think about those days when you sailed catamarans. Remember how horrible the helm felt if the blades were not locked down OR did not have the correct rake ? At speed it took all your strength to even hold course, let alone sail effectlively.

If yo can get the width that might shed some light too. Too much is just too much of a good thing sometimes.

aloha,
pog

Nell,

The windsurfing comments are very true. I understand your point exactly as I've used various boards over the years.

The 'bucking' effect is absolutely from the lift generated from the deep centreboard. The lift is a factor of the sideways pressure you are applying to the board and is reduced as you rake the board back (smaller lever and lower lift foil at that angle).

Interestingly, the latest Formula sailboards have very deep, skinny, fixed vertical and (generally) eliptical shaped fins. They have a very fine entry leading edge (due to the high speeds they reach: > 30 knots) and have a very high lift to drag co-efficient. The interesting thing is that the boards they use are about 1 metre wide, so the rider has a lot more leverage to keep the board under control. The lift generated (kept under control by the weight of the rider on the edge of the board) lifts the board to keep it planing at very low wind speeds.

The other thing to note is that you can also build in some flex in a fin - either straight side-ways bend or a twist at the tip. This also changes the dynamics of the fin ... a twisting, flexing fin has very different characteristics to a super-stiff fin.

All very interesting, but a bit off the topic of surf-ski rudders! Or perhaps not ...

The comments re rudders and conditions are very interesting. There can be a large range of speeds experienced when surfing down a wave vs normal flat water paddling, and also when you are in the lull between catching waves where boat speed through the water is relatively slow.

It should be possible to develop a rudder with the right flex/twisting characteristics that makes it less twitchy at speed, but at lower speeds the water pressure on the fin would not be enough to induce the flex/twist and so it should work efficiently at low speeds and high speeds.

Sailboard sails have worked on this philosophy for many years. As wind speed increases the mast bends sideways and the tip of the sail twists to 'lay-off' the pressure at the top of the sail. This helps the rider keep control and the twist reduces tip vortices which create drag.

Something to think about.

I recently fitted a Lippstreu elliptical rudder on my Fenn Mako 6 and can confirm Rob's findings. It's made a world of difference to my downwind paddling. Where before I would broach 4 to 5 times on a typical big downwind paddle, I now don't broach AT ALL. The difference is immediately noticeable, one no longer is fighting the boat to keep it going straight and small adjustments to the pedals are enough to keep the boat pointing in the right direction. Highly recommended!

Hi Rob , Gary, Dale

Sounds like Dale should be advising Keith on rudder shapes - any chance of a photo or is it "patented" for Millers Runs ?

I would love to see pics and dimensions of these rudders.

I'm really intrigued now by the idea of a flexible rudder. I suppose that it could even have better turning characteristics if it bent with the stress, thus delaying cavitation/stalling. It might also give the ski a "less abrupt" turn and lift when surfing at high speeds almost like suspension.

How would one go about making a flexible rudder with only basic materials? Maybe just fiberglass over a thin balsa core? I suppose to do it right you would use delrin or rubber and an injection mold.

Why wouldn't a thin (maybe NACA 007) rubber rudder work? (It would also save the ski some damage if you were to hit a rock.)

Erik

Hi Eric,

Still don't know what shape your blade is but I would venture the planform shape ( for the position it is in on the hull and as long as it' not outrageously spec-ed ) is causing your problem more than anything else.

Obviously dimensions would be a factor but certain shapes are better than others @ using 'x' surface.

my .02.
aloha,
pog

Erik,

I'm not completely sure how they embed flex characteristics with formula sailboard fins. My brother has started making these fins, but the dimensions and forces are very different from ski rudders. The stiffness and flex is something to do with how many layers of carbon and the direction of the fibres.

The sailboard fins are extremely stiff and you won't get much flex even by resting it across your knee and leaning on both ends. But I've seen pictures of a board riding up on the fin with a huge bend in it - truly amazing stuff and I'd never have thought that much flex would be possible.

Ski rudders are so much smaller and far less force on them - no idea how you'd do it with standard materials.

Sorry Pat, that rudder was eliptical, about 13 mm thick at the top, carried its thickness and NACA shape down pretty close to the tip. Chord was about 11 cm. I have since chopped it up and, with the shaft, I might try something else. My guess is that you were correct in that it generated too much lift way down, and in a near vertical rudder, that caused too much violent boat roll on that ski.
Erik

the "surf" rudders im making have some flex but i have no idea how much or if they flex at all while paddling/surfing. But i can easily flex/twist the tips. Unfortunately im more a designer/craftsman than a paddler so i cant say as to how the flex feels/performs. Perhaps those who have my rudders can tune in and comment. Id post pics but im not sure how