Use a keel as a rudder? - Page 2 - RC Groups
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May 14, 2017, 02:36 AM
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Every action has an equal and opposite reaction, the rudder needs to react against the force of the fin. Nothing to react against, nothing happens.
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May 14, 2017, 08:07 AM
Resistance is futile
circuitfxr's Avatar
Thanks for all the great feedback. There is a lot more to consider than I originally thought. Just shows that there is a good reason why things are done the way they are.

I am curious though about why the rudder design on an EC-12 type hull would work as well as it does given the information presently shared here.

The rudder on the EC-12 is similar to an extension of the keel. The shark fin style keel appears to have the back 1/3rd of the keel fin cutoff and reattached with a hinge. This would be a 1/3rd spinning keel design, that appears to work fine.

My guess would be that the fixed portion of the keel gives the stability to prevent sideways drift while the turning part (more towards aft) would be able to give some directional resistance? I was originally figuring that if the entire keel was able to turn, it would give a better rudder authority to perform better in high wind conditions.

I had not considered the "crab walk" sideways situation or the possibility that the hull would turn topside and the keel remain stationary. The hull certainly has a lot less resistance "on" the water than the keel will have "under" the water.

Thanks for the physics lesson everyone.
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May 15, 2017, 11:34 AM
Big Boats Rule!
boater_dave's Avatar
Google "canting keel boats" and you will see lots of options dealing with loss of fin projected area as the main ballast keel cants out of the way. By canting, the righting moment goes up, but the keel area drops. The loss of area dictates that additional appendages be used, but that increases overall drag. It's all a balancing act. Add into the mix any real boat class rules prohibiting such technologies, or what race/conditions the boat was designed for. Some long distance ocean racers are maximized for reaching, others for windward/leeward course racing.
With all that in mind, could a boat be designed where the only steering element is the tail section of the main ballast torpedo? Who knows? Is the loss of drag (of the traditional rudder) worth the loss of steering authority? Having a small blade mounted way back on the transom has tremendous authority in pushing the transom around.
If you ponder the evolution (in general) of real sailboat design it has really been the inclusion of modern materials with much better properties that allows this kind of technology. You could never build a fin and bulb style keel if all you had to build with was wood.

May 15, 2017, 12:09 PM
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DLord's Avatar


On a Canting Ballast Twin Foil boat the keel fin doesn't provide any of the lateral resistance at all. All lateral resistance is provided by the twin rudders.
One wheel turns the rudders in opposite directions for steerage , another wheel or lever moves both rudders the same direction(collective) to eliminate leeway:

May 15, 2017, 08:32 PM
Registered User
Originally Posted by DLord
On a Canting Ballast Twin Foil boat the keel fin doesn't provide any of the lateral resistance at all
I like that you think like a material scientist and go to ideas that would be awesome, but not possible with modern materials and then do them anyways.

However I am struggling with "none at all for lateral resistance. Are we talking Volvo 70 "it's not even in the water" or lateral resistance is peak at 90* to the wind and this will hold 30* type magic?

May 15, 2017, 08:57 PM
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KoneWone's Avatar

Nzl 20

Been done ...

Build log for 120 cm version of the boat here
Last edited by KoneWone; May 15, 2017 at 09:31 PM.
May 15, 2017, 11:18 PM
Resistance is futile
circuitfxr's Avatar
Thanks Kone wone. That is a very interesting setup on that new Zealand boat. Very similar to what I was imagining. I had not considered the double keel attached forward and aft of the bulb. Seems some of my crazy ideas aren't so crazy after all.
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May 16, 2017, 03:31 AM
Registered User
Having two or more devices dangling below the hull is no different to having a fin and a rudder. If steering is accomplished by having a fixed fin with a moving panel on the leading or trailing edge, that is effectively a fin and rudder as well, so a single steering fin is likely to remain something for the future.
May 16, 2017, 08:46 AM
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DLord's Avatar
Originally Posted by pwallace
I like that you think like a material scientist and go to ideas that would be awesome, but not possible with modern materials and then do them anyways.

However I am struggling with "none at all for lateral resistance. Are we talking Volvo 70 "it's not even in the water" or lateral resistance is peak at 90* to the wind and this will hold 30* type magic?

Not sure I understand your comments or question. On a CBTF boat the keel fin is just a strut to hold the ballast and is not designed to prevent leeway.
The twin foils act as both the keel(minus ballast) and rudder on a "normal" boat but have additional capabilities, for instance, the ability to totally eliminate the effect of leeway on the hull-which is not possible with a normal fixed keel fin.
May 17, 2017, 03:20 PM
Registered User
I designed and built a keel that had a trim tab that was the aft 40 percent of the area. This was done in the late 1970s, when carbon fiber was pretty much unheard of. The leading 60 percent was thin aerospace aluminum, half the camber of the average keel thickness popular then. The aft section was a thin stainless "rudder". Over the whole aluminum leading edge I glued on plastic sheets, and taped the plastic trailing edges so that they could shift a bit when the trim tab was engaged. The trim tab was in a central position for downwind sailing, being half as thick as most of the current keels was noticeably faster off wind. The trim tab was positioned going upwind such that it flattened on the lee side and produced a foil on the windward side, thus making it behave like an airplane wing with more lift than the average boat could manage. I introduced this keel in the M class worlds in Ottawa. The windward leg of the first race was set up so that it took 3 tacks minimum to get to the first (windward) mark. All the Ms started, heading off on the first tack. I started last, locked in my trim tab and made that first leg in one tack and headed down wind. I passed the reach mark and was half way down the leg to the 3rd off wind mark when the next boat rounded the first mark.

I was disqualified because the officials claimed that I had a moveable keel. That rule was installed in the vane days when heats where divided into separate windward and down wind legs (no way to control vane boats on a triangle course). It was popular for a while to move the rig and the keel aft on the running leg, hence a movable keel rule. I did not move my keel, I added a trim tab which had no rule prohibiting it. (In fact trim tabs were used in the A class boats for a while. The "guy" advent in the vane supplanted the trim tabs which was the original way of keeping a boat from sailing on the wrong tack).

As it turned out, if I hadn't smoothed the foil shape with the plastic sheets (which had hidden the trim tab) my trim tab would have passed the rule. I offered to remove the plastic sheets after the race, but it was to late. I think that my invention, if allowed, would have made all the world's M boats obsolete. It does show that established classes (development ones) need to protect the class from really imaginative developments. I was 18 when I introduced my innovation, some 40 years ago.
May 18, 2017, 02:57 AM
Registered User
I was also at the ottowa worlds :-)

in the UK my father had a trim tab on a design in the mid 80,s. boat called deception.

it was interesting when being to leeward of a boat and you applied the trim tab, the boat just sailed higher on the wind
May 19, 2017, 03:07 AM
Registered User
Interesting that fins with moving components have been tried, but telling that they seem to have fallen by the wayside compared to the conventional fin with a single rudder at the stern. Possibly the improvements brought about by the unconventional designs were restricted to too small a range of conditions to be useful?
May 19, 2017, 03:36 AM
Registered User
on the Marblehead that we had it worked very well. now rules say the fin has to be non moveable to it was banned :-(
May 19, 2017, 09:50 AM
Registered User
Dick L.'s Avatar
Personal observation: I love (??) development classes that aren't!

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