Centre of Lateral Resistance

[Avocet]

I want to find the Centre of Lateral Resistance of my boat. I was thinking about lashing the tiller ahead and pushing the boat away from the pontoon (assuming no tide and wind) with a stick more or less amidships. Am I right in thinking that if I find a point where I can push the boat away and it stays paralell to the pontoon, the CLR will lie somewhere in the same vertical plane as the stick?

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[Mirelle]

Yes, subject to a few minor variables (eg the water will flow more easily round the bow than round the rudder, though in fact this will be a more accurate guide to what really happens!)

In fact, it is better to pull than to push. The RN actually did attach a warp and pull ships sideways, in the days of the sailing Navy, to find the CLR.

If you have a sheer plan (profile) drawing of the underbody, there is another way. Copy it onto thick paper, and cut round the edges of the underbody and along the waterline. Balance the cut out on a knife, first slanted one way, then another, and at the point where the two positions of the knife blade intersect you have the CLR.

You might enjoy reading "Cruising Yachts: Design and Performance" by Dr T. Harrison Butler. It is in print.

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[Jacket]

Thats exactly how we find the centre of lateral resistance of landyachts, so I can't see any reason wht it wouldn't work with a yacht. But push it slowly, else you'll have dynamic effects, which will be much larger, coming into play. And of course you can only do it on a calm day, else windage will have a major effect.

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[jet_morgan2000]

The CLR should also be 'roughly' under the CoP of the mainsail. That should give you an 'approx' starting point.

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[qsiv]

The piece of paper method will give an approximation, but is only accurate for a yacht that has no forward motion. As most yachts are designed to move forward through the water the keel (at least) will hopefuly have some sort of hydrodynamic lift and the CLR is likely to be different from that found by paper and knife.

Harrison Butler was a life long exponent of his own metacentric shelf theory, which frequently gave beautifully balanced yachts - but there isnt a modern yacht designed with even a nod in his direction. At the very least most modern yachts become hugely imbalanced (in metacentre terms) as the heel - usually they pitch bow down as the wide stern sections (that give such nice bedrooms and sitting rooms on modern boats) lift the stern and the slim bows cant match this without adopting a bow down attitude.

For a more modern view try <A target="_blank" HREF=http://www.amazon.com/exec/obidos/tg/detail/-/0396077390/ref=pd_sim_books_1/002-6800987-3366416?v=glance&s=books&n=507846>Marchaj's</A> refernce work. His reference book on <A target="_blank" HREF=http://www.amazon.com/exec/obidos/tg/detail/-/0877422273/qid=1063523034/sr=8-2/ref=sr_8_2/002-6800987-3366416?v=glance&s=books&n=507846>seaworthines</A> is enough to to convince of the innate danger of many modern designs.


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[Aramas]

The problem with doing it the way you propose is that the result will only be useful if you do the same kind of experiment with the rig, and even then you'll still have to experiment more to find the appropriate lead. The various rules-of-thumb for calculating the lead of the centre of sail area over the centre of lateral resistance are based on geometric centres, not centres of lift. The centres of lift are extremely dynamic, and change with variations in heel, pitch, sail trim, leeway, wave action, windspeed, wind direction etc..
Of course, it might work out perfectly, but then you'd have about the same odds of getting it right if you just did it by eye - and the odds would actually be quite good.

And anyone that designs even a flat out racer that trims down at the bow when heeled should have their rubber ducky confiscated. NA's now have access to software that makes such foolish errors inexcusable - it only takes a couple of minutes to run the hull through a hydrostatics program and see how it trims at various angles of heel. It's a no brainer.

In my somewhat less than humble opinion, HB's theory is an artifact of the paper design era. So shoot me

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[Avocet]

Thanks all,

I ought to mention that the boat I intend to do this with is only an 8ft rowing tender that I'm toying with the idea of making a small centreboard and a simple single sail rig (maybe something like an Optimist) for. I was just wondering where I might put the mast and centreboard to start with. Our boat (proper) is actually a traditional long keeler with a full length rudder hung on the trailing edge of the keel and supported all the way down. It only has a beam of 7'6" so I don't think it changes its CLR much when heeled - in any case it feels beautifully balanced (just a touch of weather helm) even when sailed on its ear so I'll be leaving it well alone!

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[richardandtracy]

I'm told [ie I can't really confirm through experience] the bow wave has an effect on CLR. Just to make things even more difficult.

Regards

Richard


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[qsiv]

Absolutely - and the CE of the sailplan will ALWAYS be aft of the static point (as determined by the paper method), unless you are using rigid wings, in which case it will USUALLY be aft.....

in the contect of an 8' tender however I believe that such empirical methods will work just fine - if a liitle scope is left for adjusting mast rake (to fine tune the CE) then I'm sure a balanced solution will be found.

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[qsiv]

I'd really contest the bow down thing - it is all but impossible to avoid this with modern, beamy sterns, as the this photo shows - these are well designed boats, by repsected naval architects who know (much better than me) what they are up to..

I dont mean that the bow becomes increasingly immersed, simply that the fore and aft centreline of the boat necessarily adopts a bow down angle as it heels - this equally guarantees that the angle of attack of the keel will change, and as such the lift will move from it's static point.

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[Jacket]

In which case, I'd look at designs for similar sized and style boats to see how much they have the centre of effort leading the centre of lateral resistance, as the "correct" amount varies for different styles of boat and types of rig.

If you want, PM me with the type of rig you're thinking off, and I'll have a look at the plans I've got- I've got hundreds of them kicking around at home.

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[spark]

Measure the profile of the boat.
Draw it to scale on a nice big bit of paper.
Draw the rig as you think it should be.
Work out the geometric centre of effort (CE) of the rig.
Draw a vertical line from CE to below the waterline.
Draw a parallel line approx. 10% of the waterline length aft of the CE line.
The centre of lateral resistance (CLR) of the hull should be on that line.
Overlay a sheet of tracing paper and draw a dagger board of suitable dimensions with its geometric centre on the CLR line.
Trace the underwater profile of the existing hull.
Transfer this total underwater shape (i.e including dagger board) onto a bit of card, cut it out and do the knife balancing trick described by a previous poster.
If the balance point doesn't correspond with the geometric centre of the dagger board redraw the dagger board in a slightly different position and repeat.
Whe you build the rig make sure that:
a) the mast rake can be altered and the foot of the sail is cut a few degrees above the horizontal (so that if you have to rake the mast aft to balance the boat the clew doesn't droop)
or
b) the mast can be moved fwd or aft (this is prefeable to a).

Don't worry about the dynamic CLR - in a boat that size the crew can trim it to suit conditions.

If you have any doubts or queries about the above try reading "Yacht and Small Craft Design" by Gordon Trower (ISBN 1 85223 709 0) - it got me going many years ago and has all the basics clearly explained.

Have fun.




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[MainlySteam]

QSIV I have to strongly agree with you about the bow down thing.

The design of all boats is a series of compromises and it would appear to me that often those without the experience do not understand that. Bow down is an acceptable tradeoff for performance in both race and cruising boats as long as the compromises in achieving that are acceptable (reduced forward load carrying ability in a cruising yacht, for example).

John

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