[Rhodes22-list] how straight does a rudder need to be?

[Ronald Lipton]

Mary Lou,

The rudder provides lift that, when the boat is moving without 
accelerating,
in combination with the lift provided by the hull and keel, balances 
the heeling force.
Turning the rudder changes the component of the lift due to the rudder 
and
causes a mismatch between the heeling force and the total lift, causing 
the boat
to turn.  If the rudder is off center the lift will probably not be 
balanced at 0 degree
tiller angle because of the asymmetry of the disturbed water from the 
keel.
This effect is hard to calculate. The only information I could find 
shows
about a 10% effect on the lift due to the disturbed water from the 
keel, but it is
under very different circumstances. Given that you are having problems 
with
helm and that there is an offset gudgeon I would be tempted to go ahead 
and
center the lower gudgeon,

Ron
(home sick today - so I have some time)

On Jan 3, 2005, at 11:27 AM, Bob Weber wrote:

> Ok here is my take from the "Dummie" that guy keeps writing all the 
> books for.  A rudder on an angle would not necessarily turn the boat.  
> There are plenty of duel rudder boats where both rudders are on an 
> angle.  Now if there is twist in the foil (rudder or CB) that would 
> would cause the boat to change directions.  Us simpletons use the 
> advice of Sherlock Holmes, "Eliminate all that is false and the truth 
> will be revealed".  I still go for tank testing.  Bob
>
>> From: Mary Lou Troy <mltroy at verizon.net>
>> Reply-To: The Rhodes 22 mail list <rhodes22-list at rhodes22.org>
>> To: The Rhodes 22 mail list <rhodes22-list at rhodes22.org>
>> Subject: Re: [Rhodes22-list] how straight does a rudder need to be?
>> Date: Mon, 03 Jan 2005 08:25:06 -0500
>>
>> Roger, thanks for your analysis, it does help though not in the way I 
>> hoped.
>>
>> You are saying that if the bottom gudgeon is off-center to port, we 
>> will make things worse if we fix it. That was my second thought - 
>> after thinking that maybe we'd found the problem. Because of all the 
>> angles involved (tiller position, the fact that the rudder in use is 
>> tucked somewhat forward, the effects of a slight heel to starboard on 
>> port tack, misalignment with the keel, etc.) I've been breaking my 
>> brain trying to figure out if it is more complicated than that. Guess 
>> it was indeed wishful thinking.
>>
>> The bottom gudgeon is indeed 5/16" off center to port which makes the 
>> bottom tip of the rudder 1" to the left of vertical with the rudder 
>> almost all the way down and the rudder head centered.
>>
>> Thanks again.
>>
>> Mary Lou
>>
>>
>>
>>
>>
>> At 07:41 AM 1/3/2005 -0500, you wrote:
>>> Hi Mary Lou,
>>>
>>> You didn't say which direction your rudder blade is tilted.  So, I 
>>> assumed
>>> the lower tip of the rudder blade was tilted towards the starboard 
>>> side as
>>> viewed from behind the boat.  Then, I analyzed the forces & resulting
>>> torques that would be generated as a result of that rudder blade 
>>> tilt and
>>> compared them to the forces & resulting torques generated when the 
>>> rudder
>>> blade is not tilted.
>>>
>>> Viewing the entire hull from above, the tilted rudder blade 
>>> generates a
>>> small net clockwise torque relative to the untilted rudder blade.  
>>> This will
>>> cause the bow of the boat to have a tendency to turn towards 
>>> starboard.   On
>>> starboard tack, this would be weather helm.  On port tack, this 
>>> would be lee
>>> helm.  The boat will turn more easily to starboard than to port.
>>>
>>> If the rudder blade is tilted in the other direction; then, all the 
>>> above
>>> comments would be reversed.
>>>
>>> So, if the rudder blade is tilted to starboard as described above; 
>>> then,
>>> "fixing" the misalignment in the mounting gudgeons will reduce your 
>>> problem.
>>> If the rudder blade is tilted to port; then, realigning the mounting
>>> gudgeons will probably make the problem worse.
>>>
>>> Hope this helps, Mary Lou.
>>>
>>> Good luck!
>>>
>>> Roger Pihlaja
>>> S/V Dynamic Equilibrium
>>>
>>> ----- Original Message -----
>>> From: "Mary Lou Troy" <mltroy at verizon.net>
>>> To: <rhodes22-list at rhodes22.org>
>>> Sent: Sunday, January 02, 2005 8:45 PM
>>> Subject: [Rhodes22-list] how straight does a rudder need to be?
>>>
>>>
>>> > Happy New Year!
>>> >
>>> > Can you all stand to think about our starboard turning boat again? 
>>> (There
>>> > is a quick recap below if you don't remember the issue)
>>> >
>>> > We didn't get a chance to try any of the in-water tracking tests
>>> previously
>>> > suggested but we've done a fair amount of measuring of the boat on 
>>> the
>>> > trailer. We can't detect any asymmetries in the hull but did come 
>>> up with
>>> a
>>> > slight asymmetry in the mounting of the gudgeons for the rudder. 
>>> Our
>>> > question is: is the variance enough and in the right direction to 
>>> cause
>>> the
>>> > boat to consistently want to turn to starboard? We don't want to 
>>> "fix" it
>>> > and have the boat turn more strongly to starboard.
>>> >
>>> > The situation is that the upper gudgeon appears to be centered and 
>>> the
>>> > lower gudgeon is 5/16" off-center. This causes a slight slant to 
>>> the
>>> > rudder. The lower tip of the rudder is about 1" off of center. Is 
>>> this
>>> > enough to cause the observed turning to starboard?
>>> >
>>> > Thanks again
>>> > Mary Lou
>>> > 1991 R22  Fretless
>>> > Ft. Washington, PA /  Swan Creek, MD
>>> >
>>> >
>>> > Here's the summary of the posts from earlier this fall:
>>> >
>>> > We've had lee helm issues with Fretless since we got her. The 
>>> learning
>>> > curve learning to sail her and the fluky wind and current 
>>> conditions on
>>> the
>>> > Chesapeake have slowed us down diagnosing the problem.
>>> >
>>> > She has always sailed better on starboard than on port. When we 
>>> set her up
>>> > this spring we lengthened the forestay and took up the backstays. 
>>> In a
>>> nice
>>> > breeze with a full main and genoa this gave us a small amount of 
>>> weather
>>> > helm on starboard and less lee helm on port. It also made us 
>>> really pay
>>> > attention to the difference between her behavior on port and 
>>> starboard.
>>> >
>>> > Roger mentioned looking at how she sits on her lines and indeed 
>>> she does
>>> > have a slight list to starboard sitting in the slip (in spite of 
>>> the
>>> > outboard, 7 gallons of gas and the battery all stored to port. 
>>> Bill Effros
>>> > said he thought that if the boat was too heavy to port it would 
>>> turn to
>>> > starboard. These two thoughts seem to contradict each other but 
>>> we'll work
>>> > a bit more on her trim.
>>> >
>>> > Bill & Slim mentioned heel as a contributor to weather helm but I 
>>> think we
>>> > heel about the same on port and starboard. Those fluky winds make 
>>> it hard
>>> > to tell.
>>> >
>>> > Jim Connolly mentioned a similar port/starboard problem but then 
>>> mentioned
>>> > that he thought he had a spreader problem that might be 
>>> contributing. As
>>> > near as I can tell, our mast is straight and centered. We tune the 
>>> rig
>>> with
>>> > a Loos gauge and have measured with a halyard to the chainplates. 
>>> We'll
>>> > take a look at the spreaders though.
>>> >
>>> > Lastly, Steve and Slim mentioned that maybe something was out of 
>>> whack
>>> with
>>> > the keel, the centerboard or the rudder. We've taken a look at 
>>> those
>>> > possibilities. There's nothing immediately visible with the keel 
>>> or the
>>> > rudder. The centerboard drops cleanly and does not bind or thump 
>>> about
>>> > significantly but we've never taken a really good look at it. 
>>> Fretless has
>>> > an unusual repair (?) to her centerboard trunk (there is a flat 
>>> plate
>>> > glassed over the forward part of the trunk - between the trunk and 
>>> the
>>> > centerboard cap) that prevents us from removing the centerboard in 
>>> the
>>> > usual fashion). Raising and lowering the centerboard has no effect 
>>> on her
>>> > tendency to want to turn to the right.
>>> >
>>> >
>>> > __________________________________________________
>>> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>> >
>>> >
>>>
>>>
>>> __________________________________________________
>>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>
>>
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>
>
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