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May 13, 2017, 01:01 PM
Resistance is futile
circuitfxr's Avatar
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Use a keel as a rudder?


I have been giving some thought to the current keel / rudder setup being used on racing yachts. The thought occurred to me that a keel and rudder both have an aerodynamic shape . Typically the job of the keel is to provided "counter balance weight" to the wind force applied to the sails. The job of the rudder is to provide a directional steering force to the hull. The question I have is:

Why can't both functions be accomplished by having a steering keel? I accept the fact that the central placement of the keel does not lend itself to providing a maximum steering force. However, if the keel were to slant toward the stern, and the "bulbous" weight were to be repositioned to the forward half of the slant, would it not allow a "steerable keel" to do the function of both keel and rudder?

I am not physics expert, so I don't know if what I am proposing is even technically feasible without causing structural damage. My thoughts are such to effectively reduce drag and weight by eliminating the need for a separate rudder.

This is just a crazy idea I have. I would invite some feedback from people more knowledgeable in this subject before I begin any prototyping. Has this idea already been considered? What difficulties or challenges are involved? I have seen rudders attached the rear of a keel, but this is not what I am thinking about.
I want to use the keel itself as a rudder AND keel. Any thoughts would be appreciated.

Another crazy Alienater idea!
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May 13, 2017, 01:24 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
I have seen pic's from another member here of a sail boat mono-hull that had 2 smaller keels oppose each other and at outward angle at about the bilge line amidships that I believe were movable to some degree of sailing control but not sure that steering is (their) main function. I would foresee the additional stress of forces would rip the boat in half, JMHO Of course I'm no engineer of such dynamics either ,!! Tim aka Cap'n Hax
May 13, 2017, 01:46 PM
Registered User
I would think that it would not work real well. The rudder also act a bit like tail feathers on an arrow to give some directional stability.
May 13, 2017, 02:16 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
Another thought on this is that with the keel/rudder amid ship, I would also venture to say the boat with its forward momentum would slide sideways on her forward course rather than TURN in which ever direction the keel pointed. Here's an example,: The boat on a north course on port tack, you want her to head more west,( turn to port ) turning the normal rudder aft (point right starboard) moves the boat to a NW heading. If you did it with keel seems she would still be moving North just at a parallel but displaced over line and still on port tack rather than tack to starboard. in other words I don't think she would tack back n forth from port to starboard but merely continue forward on a rather "snakey" line crossing her own original line. Hope this "thought" makes sense. Tim aka Cap'n Hax P.S. In the event she would TURN it would probably be a MUCH exaggerated radius. Me thinks Lucy,!!!
Last edited by haxawsnavy; May 13, 2017 at 02:30 PM. Reason: added P.S.
May 13, 2017, 02:32 PM
Registered User
Its known that you can sail a boat without a rudder by the sails. Trim the jib and ease the main to turn down. ease the jib trim main to turn up. You can keep a course for the most part, but maneuvering is quite slow (of course the beer might have had something to do with that when we did it on a Lighting) I'm with Marc on this. Plus what is the path of least resistance? Would the boat turn on the keel or the keel turn on the boat?
May 13, 2017, 02:41 PM
Resistance is futile
circuitfxr's Avatar
Would the boat turn on the keel or the keel turn on the boat?

Now that is a possibility I did not forsee....hmmmmm
Latest blog entry: Finally got a SOLING 1 meter
May 13, 2017, 02:51 PM
Registered User
glad to help...

Quote:
Originally Posted by circuitfxr
Would the boat turn on the keel or the keel turn on the boat?

Now that is a possibility I did not forsee....hmmmmm
May 13, 2017, 02:53 PM
Registered User
There is CLR and CP to consider. The keel does more than act as a place to hang ballast
If you swept the keel back far enough to act as a rudder you would end up with brutal lee helm.
The reason you can steer a sailboat with the sails if it loses a rudder is you are moving the CP forwards and aft around the CLR by sail trim.
May 13, 2017, 03:03 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
Quote:
Originally Posted by huzway
Its known that you can sail a boat without a rudder by the sails. Trim the jib and ease the main to turn down. ease the jib trim main to turn up. You can keep a course for the most part, but maneuvering is quite slow (of course the beer might have had something to do with that when we did it on a Lighting) I'm with Marc on this. Plus what is the path of least resistance? Would the boat turn on the keel or the keel turn on the boat?
+1 Agreed see this vid,....
Sailing: Steering a sailboat with the main sail ALONE! (3 min 25 sec)
Tim aka Cap'n Hax
May 13, 2017, 03:57 PM
Registered User
Many big boats and some small have this feature with a flap on the keel for trim. If you look at Aussie II for example you will see the big wheel and a smaller inside wheel for rudder and trim.

This is just speculation, but I think the reason the trim is chosen to be small is because the mechanism of action is drag. The small boats with keel trims are able to point higher and I am not sure if it's because they are using it as an "airfoil" to increase the righting moment or some other trickery but it does work. It's just not really enough to force the boat to tack. If you look at the history of pond boats the rudders were small and often tucked behind an appendage for reduction in drag because they were just trims. With RC you can see the rudders became quite large and sometimes stick out the back of the transom.

In general the fastest skippers are also using the rudder the least for all the reasons above.
May 13, 2017, 04:16 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
Quote:
Originally Posted by pwallace
In general the fastest skippers are also using the rudder the least for all the reasons above.
Yep, turn the paddle in a canoe and it slows or causes drag/brake that side which causes a turn much like land tracked vehicles,!! A "Keel Rudder" IS a BIGGER paddle than a "normal" sized rudder,!! However some human pedal powered 2 person boats have the rudder in the bow . HEE-HE-hee-he,!!! Tim aka Cap'n Hax
Last edited by haxawsnavy; May 13, 2017 at 04:21 PM. Reason: added text
May 13, 2017, 06:20 PM
The wind is free, go sailing!
Scratchy101's Avatar
Quote:
Originally Posted by circuitfxr
Typically the job of the keel is to provided "counter balance weight" to the wind force applied to the sails. The job of the rudder is to provide a directional steering force to the hull.
You've omitted another very important feature of the keel, and that is to resist the sideways motion (leeway) of the boat through the water.
When turning the keel you will lose this force and the boat will slip sideways through the water when going to windward.
May 13, 2017, 08:36 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
Quote:
Originally Posted by haxawsnavy
Another thought on this is that with the keel/rudder amid ship, I would also venture to say the boat with its forward momentum would slide sideways on her forward course rather than TURN in which ever direction the keel pointed. Me thinks ,!!!
That's what I was trying to convey, Thx for the clarification Scratchy,!! Tim aka Cap'n Hax
Last edited by haxawsnavy; May 13, 2017 at 08:37 PM. Reason: spelling
May 13, 2017, 08:44 PM
Po' boys does w/ Po'boys ways
haxawsnavy's Avatar
***deleted***
May 13, 2017, 10:00 PM
Registered User
Think of two wheels on a bike. The forward wheel is a rudder. How do you turn a unicycle?


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