Does anyone know about Dotan's system of using wave energy to speed up a catamaran?

See

dotan article

Sounds like an expensive way to add rudder slop.!

I'm good in negative stuff so let me give everybody the sceptic view on it.

-1- How many times are the wave lengths on the watersurface about the width of the catamaran ?

This situation gives the max "bad" effect as the article claims. Of course the little eddies as depicted in the waves are proportional to the wave height and speed of travel. If my memory about this subject doesn't fail me. In my experience; wave with a wavelength of only 2.5 mtr are both small and slow therefor I feel the eddies in them are relatively small.

-2- A flexible rudder stock like that or even a flexible tiller will result in less direct steerage and possibly even in some jawing effects. Almost like steering a car with slop in the steering wheel. Remember the difference in feel between powersteering and unpowered steering. You could steer the car by your fingertips in the latter case while powersteering setups tend to have a small delay or deadzone in the steering action ?

-3- Want to loose 0.4 kg in drag on a total drag in the order of 30 to 50 kg's = 1.33 - 0.8 % => 0.6 - 0.4 % speed increase theoretically or on avarege 18 second per hours when everything is as they say.

-4- As far as I know these eddies occure near to the surface and not at depth. So it is quite possible that the upperhalve of the rudder blade experiences these while the lower halve is in "stationary" water. We may be overestimating the magnitude of the effect.

-5- With a catamaran at speed passing waves how will this system work. Will the rudder blade move continiously from side to side. Being a rotational action in stead of a translating actions this means that a large portion of the rudderblades is not perfectlt aligned with these eddies and therefor creates all kinds of turbulance. Ergo does the added drag not outweight or cancel out the positive effects.

Wouter

I am skeptical like Wouter. The concept is to build some compliance into the rudder system to accomodate differences in the "best" or optimum angle of attack for each blade relative to the other blade. As Wouter pointed out the article uses a point solution with some oversimplistic assumptions to draw a broad conclusion about potential performance improvement. The most reliable way to validate something like this is boat on boat test results with skippers switched for half of the test runs.
By logical extension, if we made the platform so one hull can move relative to the other, less energy would be lost in going through waves (theoretically at least). But it obviously dosen't work that way in the real world. Another unanticipated effect could be introduction of rudder vibration if the spring constant/mass of the more flexible rudder system is close to the vortex shedding frequency of the blades.
Speaking of compliance in a design... the windsurfing guys have done a great thing by designing the upper third of the sail to be floppy. It blows off in a gust and reduces the angle of attack. A sail like an automatic transmisson in you car. I mention this to point out that lifting surface compliance can be very effective in specific applications.

Kevin

>>the windsurfing guys have done a great thing by designing the upper third of the sail to be floppy

This is also how true wingmasts work as seen on A-cats and Taipans. And it does really work I can tell you that from experience.

Wouter

This sounds like an old "Hobie" argument about sailing around the course with one or the other rudders always out of the water! (only one at a time) It seemed that the loss of positive steering control cost much more than any "percieved" reduction in drag.
The pro's and con's of this "so called" rudder drag etc was fully investigated and resolved through full practical experimentation by C. A. Marjac before 1964, and I would suggest that any company that is trying to "sell" a product to eliminate this drag or, even more, use a system to gain speed out of some "so called" phenomina. is selling, not a successful system but a "bill of goods"
Just my opinion
Darryl

On the water surface there are many currents which affect all surfaces of the rudder blade. As an example, we mention only one current in our article.
We tested the crossbar on special equipment in the water (waves 0.5m-3m high). This device showed us the changes in the crossbar length. The length of the crossbar changed from 10mm to 30mm, with a frequency 2-5 seconds (depending on the boats direction against the wave). All changes in the size were constant, but not irregular – one second it was smooth, one second it was like a spurt. This clearly proves that in the crossbar there are forces. During all tests none of the skippers lost control or feeling of the boat. Moreover they indicated that boat goes more smoothly through the wave.

In the sailing world there is a myth that the mast has to be flexible and must work together with sails, but foils must be stiff like concrete.
Even on a car, we can see that there are flexible parts – rubber tires and amortization system.

Notwithstanding, you never lose control of the steering. If you imagine this system that is completely stiff with aluminum tires, it would crash very quickly…
This example can also explain catamaran rudder breakages, when a skipper can’t feel loads on the crossbar and tiller extension.

According to the Ackerman principle, when turning on different radiuses, tillers must have particular toe-in angle. Meaning when the turn radius is large - tillers have little toe-in angle, but on sharp turns (small radius) tillers must have a larger toe-in angle. Usually sailors choose an average angle, about 7 or 8 degrees. Our system in this case automatically allows for a more optimal angle.
Usually in order to change the direction a skipper turns the rudder about 20-40 degrees. Our amortization system can only turn for 2-3 degrees, again optimizing the Ackerman principle – adding or taking the angle.
There are no slop, dead zones, delays etc… The skipper perfectly interacts with the catamaran.

Let’s return to water currents on the rudder blade taking into consideration all resultant loads on an example of a dolphin. Dolphins are known for their amazing hydrodynamic characteristics. An ideal rudder in this case would be something flexible with a surface like dolphin skin, a surface which can process pressure differentials on all of the rudder blade area. But at the moment, it is impossible to make it.

All dolphin fins (rudders) are not absolutely stiff, but flexible.

With a sharp increase of the load on the blade a flexible rudder damps hydrodynamic oscillations, fading the pressure of the blade if you compare with aluminum rudder stock, Dotan rudder stock deals better with dynamic loads. Flexibility gives an increase in endurance of the rudder system dramatically.

The article example shows how everything would work with 10kg load. If there are no wave loads between the rudders the load is very close to 0kg. When there are large wave loads it can reach about 100kg before the rudders will break.
Anyway, even with a condition when load is 10kg you can decrease drag for 0.4kg. As Wouter’s calculations show, you can gain about 1 minute on a finish during a 3-hour race. If you take into consideration large waves that can cause loads to reach about 100kg your final result on the finish line can be many times better.

I REALLY like the look sof the kickup and locking system. How much force does it take to unlock the rudders from the working position, have there been any problem with them popping up unecesarily on catamarans? What are the prices for those things, I don't see them listed anywhere?

jfint:

Take a look at this link: http://www.dotan.com/adjustment/#3 We supply rubber stoppers of different stiffness to suit different boat sizes.
Anytime you can lock the kick up mechanism by inserting a pin: http://www.dotan.com/adjustment/#4

As to prices, again it depends on the boat size, rudder blade(carbon / kevlar / fibreglass), final weight and other things.
Best way is to send us an email. Contact Info