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

[Rik Sandberg]

Mary Lou,

I just went through something similar with our M-15. Not exactly the 
same, but kinda. Our rudder blade was a homemade thing and was not 
shaped symetrically. The boat would turn much better one way than the 
other. Also, we would have much more helm angle on one tack than the 
other when trying to maintain a straight course. I got a new rudder 
blade for the boat and the difference was astounding.

I don't know what all Roger analysed and lord knows I'm in no position 
to argue physics with him, but I think that your rudder being off center 
to port might just cause the boat to want to turn to starboard. This 
will happen because the rudder is not centered behind the keel and so 
the turbulence created by the keel is not flowing across the rudder 
evenly, ie, there is a faster flow (less turbulence) across and thus 
more lift on the port side of the rudder than the starboard side. We all 
know I'm no physicist, but that seems like a very real possibility to 
me. And, it follows with the findings I mentioned on our M-15.

IF it was me, I'd center that gudgeon and expect that would help.

Rik

Mary Lou Troy wrote:

> 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
>> >
>> >
>>
>>
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>
>
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