Discussion in 'Boat Design' started by vicbauwens, Sep 8, 2013.

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vicbauwensJunior Member

Hi all,

So, now that I have worked my way through, and dare say, understood PYD, I turn to the expert audience with a question.

Using the free software Delftship, I couldnt resist the appeal of modelling a number of current designs and crosscheck these hulls with the rules of thumb given in PYD.

In general I find the results the be in accordance, except for one issue; When applying the method for finding CLR as given in PYD (0,45 x height of 25% chordline of the finkeel, extended to the waterline) it seems that current designs would have less lead than what PYD "prescribes", and sometimes no lead at all.

Apart from the twin rudder situation, which understandably would place CLR further aft since its working in an unspoilt stream, I seem to be missing something here.

I most welcome your comments and pointers, please don't shy away from the maths, I can stomach some number crunching.

many thanks, vic

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DCockeySenior Member

One of many previous discussions of this topic.

http://www.boatdesign.net/forums/sailboats/center-effort-vs-center-lateral-plane-36687-2.html

Some of my contributions to that discussion:

"CoE" as the geometric center of the sail area is generally not the location the net aerodynamic forces act through.

"CLR" as the geometric center of the some or all of the underwater profile is generally not the location the net aerodynamic forces act through.

But the lead methods when used with knowledge from similar vessels are a reasonable empirical method.

Moving the mast aft increases weather helm. Moving the keel/centerboard forward increases weather helm. The question is where to locate the them relative to each other.

What is commonly refered to as "CoE" is NOT where the aerodynamic forces act while sailing. Likewise for the "CLR". So trying to figure where CoE should be relative to CLR on a first principles physics basis doesn't work. Lots of boats have the CoE ahead of the CLR, and have a weather helm. However putting the CoE in a similar relation to the CLR as other similar boats known to be successful is a good pragmatic, emperical way to locate the sails. Fabio Fossati in "Aero-Hydrodynamics and the Performance of Sailing Yachts" in discussing the use of such methods has said "they [lead method] are only valid for producing a boat design that we can consider balanced with respect to a series of previously analysed projects; otherwise they are excessively disconnected from the fluid dynamics governing the problem, and from this point of view there is room for in-depth research, ..."

The actual balance of forces and moments is very difficult to understand if considered only from a 2-D, side view perspective. Trying to do so is usually mis-leading at best.

Consider a boat with a cat/uni rig running downwind with the boom at almost 90 degrees. The actual "CoE" of the sail will be to leeward and ahead of the actual CLR, and the boat will have a strong weather helm.

If you want to understand the actual physics there are a number of good books available, including:
"Principles of Yacht Design", Larsson & Eliasson
"Aero-Hydrodynamics and the Performance of Sailing Yachts", Fossati
"Sailing Theory and Practice", Marchaj
"Sail Performance", Marchaj (cursory about balance, lots on aerodynamic forces including their effective location and direction)​

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vicbauwensJunior Member

Dear David,

Thank you for taking the effort of replying.

Allow me to say that with a Msc in civil engineering under my belt, I understand that the concepts of CE and CLR are merely simplified models that should not be regarded as anything but rough guides.
Also the fact that I posted this thread is because I couldn't find any pointers in the other threads on CE/CLR to what I was actually asking.

Let me try to rephrase more clearly:

CLR is conventionally placed in the geometric centre of the underwater body and then the rules of thumb seems to match more or less with what I see in real life.
PYD however goes through quite some trouble explaining why the geometric CLR should not be used for design purposes because the geometric approach places CLR too far aft.
Their reasoning seems well founded and I am willing to go along with it. However, when I apply the "PYD-rule" for finding the CLR and apply the guidelines on lead they suggest, virtually none of the yachts I crosscheck seem to fit the bill...

Regardless of the model used, if you stay within the constraints of the model, it should work. In my findings it doesn't, since few designs seem conform to this rule .

My point being:

1) Am I missing something?
2) is there a certain evolution in reducing lead of CE over CLR?
2) is PYD a bit loose in the model it suggests, in spite of their seemingly thorough argumentation?

thanks, vic

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DCockeySenior Member

I don't think your question revolves around "math".

All of the CE - CLR - Lead models are fundamentally empirical and approximate. As you noted: Regardless of the model used, if you stay within the constraints of the model, it should work. Those constraints can include hull shape and proportions; keel and rudder type, size and configuration; and sail shape and proportions. Another factor may be assumptions about how much load the rudder should carry and resulting force on the steering gear.

My guess is the method described in PYD for fin keel boats worked well for the types of boats it was based on. What vintage and types of boats was the PYD model based on? Unfortunately the authors don't provide that information. You indicate that it does not appear to work for the boats you have investigated. Possibly they are different than the ones the PYD model was based on.

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vicbauwensJunior Member

Thanks David,

Frustrating, but insightful. I learn that naval architecture needs tank testing or CFD in order to escape the educated guess that is called 'based on experience'. For info, I don't mean that in a disrespectful way, it is just very annoying to face a wall in the learning curve.

I guess Open Foam will be the next focal point in my quest then. Cheers for open source software.

All the best and thansks again for the effort.

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TeddyDiverGollywobbler

To answer to your second question, '2) is there a certain evolution in reducing lead of CE over CLR?':
Yes, sail boats have more righting moment (wider stiffer hulls) and are planned to sail less heeled than in the past.

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vicbauwensJunior Member

Hi TeddyDiver,

But of course, why didn't that cross my mind earlier.

I think that concludes my query for now.

Thank you very much!
grtz vic

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FirstLightJunior Member

Same Boat

Hey Vic,

Been in the same boat trying to come up with decent first shot at mast location.

I did the est I couple from a theoretical standpoint and feel like I have a good starting point.

One angle I took was to import photos from 5 different designs of line plans in a similar category of the boat I am designing into my hydro software and calculate the COE and CLR. From there I was able to determine that the average of the lead was COE 2% forward of the CLR with range of 5% in either direction. That lined up with my theoretical calculation within 2% so I feel pretty good about it. Just the same I'm providing structure in the bilge to reposition base base 6 inches in either direction. Seems like a small investment in weight and infrastructure for flexibility to move rig a bit.

Cheers..

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vicbauwensJunior Member

Hi FirstLight,

The influence of stiffness, as indicated by TeddyDiver, indeed seems to be a major driving factor. Check this, which I found to be very revealing: chevaliertaglang.blogspot.com.au/2012/04/volvo-ocean-race-yacht-drawings-design_17.htm

I agree with you that incorporating a bit of freedom to move the mast around is a very practical solution around the problem. The bowsprit "trick" is basically up the same alley, although neither are imho very elegant from an engineering point of view, which basically dictates the removal of all guesswork from the table.

My learnings in the field of yacht design are nothing but an intellectual hobby to me and my prime interest is mostly to see how deep I can push my understanding into this (large) field. It's a nice escape if you will from my professional engineering background which is of less romantic nature...

So no practical shortcuts around the problem for me, but nevertheless thanks for sharing your thoughts!

grtz vic

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