Clr--the Computer Verses The Balancing Pin

[viking north]

Yes and i have no doubt will be discussed as long as boats exist, however often each time it is presented in a different wording arrangement-straight forward- simple -to the point as you have posted-- Thank you I think this is the best i've ever read it explained--Geo.

[DCockey]

But that doesn't mean the "CE lead ahead of CLR" rules shouldn't be used. Usually they provide the best guidance available. Just don't become obsessive about precision when using them.

[viking north]

I'm the rank amateur here,basic rules of design are all i have to play with,and I do mean play . The biggest benifit I am gaining is the ability to ask intelligent questions(i hope)and have a better understanding when communicating with my designer and designers in general. The other benifit i gained from this thread is a better understanding of sail and boat handling as i now will not just shorten sail with too much weather helm, i can now visualize and am much more aware of the cause and effects in advance. Being more tactical in storm conditions can never be a bad thing. One of the biggest spinoffs of forums is that they are open to all levels of expertise, everybody learns, and often a basic question on a subject triggers a light bulb in the professional thats providing the answer--Don't tell me thats never happened to most. I have a greater appreciation for what the designer is trying to achieve and the tradeoffs involved especially where CL and CLR are concerned. These two devils can dull the outcome of a lot of hard work and building on their basic rules is the only polish available.Even these are fairly limited until the "hull form" becomes part of the equasion. Thus the importance of TAD's modeling to perfect the art and since i'm omitting that aspect--I pay my money and take my chances with the dancing twins.

A yacht is not defined by the vessel but by the care and love of her owner--

[RThompson]

Tad – I agree with everything you say there in (Post #12). I was not suggesting that vertical centre of lift could be found from the profile, but rather that the vertical centre of lift would (or could) be valuable. It would be of value if one was trying to numerically resolve the forces acting on the yacht - where an increased resolution and accuracy of inputs should lead to an increase in the accuracy of the output.
As you say, any resultant predictions of performance are of value to compare candidate designs rather than as an absolute indication of performance. I certainly don’t think that the precise answer given by a computer is necessarily accurate, although cfd results can shed light on the impact of subtle design variations -IF the cfd is validated. I'm no expert on it, getting quality results from cfd looks like a black art to me...
Generally a prediction is only of use if validated by some kind of real world result (historic data, tank tests, designers experience etc etc). I do take your point that finding vertical CLR from the profile as mentioned in the original question/project would not be of much use.

Viking -"rank amateur"? Actually building boats and putting to sea/sailing means you are miles ahead of so many others...

Rob

[viking north]

Thankyou Rob and thank you all for the great help on my project, I am going to be a bit of a lurker for awhile and do what I do best- that is get this thing built- I will be posting as it proceeds and would welcome any questions on the project.--Thanks again--Geo.

[Earl Boebert]

My experience with free-sailing model yachts equipped with sliding rigs (see http://www.usvmyg.org/freesail/lassvane.htm for details) indicates that that the difference in effective rig position from near-dead air to blown off the pond can be as much as 8% of the LWL. This is for fin-and-bulb designs with the CLR as far aft as structurally possible to improve tracking on the run.

Just an observation, FWIW, toy boats, completely different Reynolds Number domains, etc. etc., but I thought some of you may find it interesting.

Cheers,

Earl

[MikeJohns]

Originally Posted by DCockey

Sailboats are not two dimensional. .......... The forces generated by the sails in general don't act through the "CE", and the hydrodynamic forces don't act throught the "CLR". This topic has been well discussed in previous threads.

Yes
And with a model you should be towing it from the center of effort ( tow from atop a small robust mast) at various heel angles and then you'll quickly see the relationship between factors that affect the balance such as righting moment, fore and aft trim and underwater resistance at various froude numbers.

Then when it's all nicely balanced you'll go and take in into a seaway and all the smooth water considerations vaporise and rules of thumb suddenly start to be quite applicable

A more effective rudder can save a lot of drag since it alters the CLR massively with only a few degrees of helm. A poor rudder will become a draggy overloaded foil all too easily. One of the biggest mistakes designers make is too small or too ineffective a rudder and it's the most common first change to a design to make the rudder more effective usually by making it larger !

A rudder angle of only 3 degrees on nearly all underbody shapes with a reasonable rudder area moves the CLR by close to 10% LWL.

[DCockey]

Originally Posted by MikeJohns

Yes
And with a model you should be towing it from the center of effort ( tow from atop a small robust mast) at various heel angles and then you'll quickly see the relationship between factors that affect the balance such as righting moment, fore and aft trim and underwater resistance at various froude numbers.....

Do you mean the nominal center of effort? It is only coincidental if the line of application of the sum of the aerodynamic forces goes through the geometric center of the sail plan which is usually what's usually designated as the "center of effort"? The actual location of the line of application of the sail forces varies with sail trim and shape.

[MikeJohns]

David
Yes thanks, I understand that very well.
We do it as a model test and it's quite interesting to compare resistance curves with the upright fixed model.
The model is set up freely with a rudder and it's towed from an estimated aerodynamic center of effort. Heel is set ant different angles by offset weights and then usual resistance curves are generated. And rudder angles recorded. I've used 2m models and more recently a 1.5m model with good results.

I was really illustrating an important consideration under sail, that the driving 'line of force' from the sails moves to leeward as the boat heels, so heel angle adds a turning moment. The efficiency of the rudder arrangement to counter this moment is important.

[viking north]

Well the skeg mold is built, again i based it on the 63 series foil shape as much as possible , say 25% of the cord from the nose back, beyond that it is foil shaped in as much i had to conform to the existing keel section and keep in mind the strength factor as a bearing support for the rudder. Overall i'm happy with it and will be able to install a filler which is actually an extension of the skegs foil after section that will blend in with the leading edge of the rudder some 12in. aft. In effect a skeg aparture is created for the prop rather than an undesireable one in the rudder. I am a big believer in oversize properly shaped rudders in as much as a design will allow. Two simple basic reasons on my part-- a wide rudder say at 3 deg. creates less turbulance than a narrow one at 6 and a big rudder is the best tool to combat running seas. So it seems the old rules of thum are keeping me on track, allbeit without the input of higher associated engineering. A case in which ignorance seems to be bliss ( photos of the keel/skeg molds and install will be posted on my building thread shortly--all input welcome--good thread and input - keep her comin guys, i'm actually beginning to understand and visualize these dynamic effects--Geo.