# easy way to calculate CLR?

Discussion in 'Sailboats' started by minno, Mar 2, 2016.

1. Joined: Aug 2014
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### minnoJunior Member

Hi All

Just wondering if I can find the center of lateral resistance by putting my hull in the water and pushing on the side?

minno

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### TANSLSenior Member

I believe that in this way you can not get reliable data. Why not take you a drawing of the elevation of your ship and calculates the center of the area ?. Even though the drawing is not accurate, the results will be more reliable than your procedure.

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### PARYacht Designer/Builder

You can get fairly accurate by pulling her sideways, certainly enough to get within a inch or two, which is good enough for most things.

Tie a line to where you think the CLP will be (take a guess about 55% of the LWL aft of the bow) and slowly pull it perpendicular. If the bow leads the stern, move the line aft a touch, if the stern leads the bow, move it forward. Eventually you'll pull and both ends will move at about the same rate. This is good enough to workout sail plan lead, on all but a high end racer and was employed for hundreds of years, before computer modeling came along.

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### minnoJunior Member

Thanks guys

I got to thinking and I'm wondering how the rudder will change the CLR, maybe I should tack a board onto the stern for the test to simulate a rudder?

It'll be a while till I get that far, but I'd better start thinking about where I'm going to put my amas, should the CLR of the amas be even with the CLR of the hull?

I suppose I should start a build thread so I can keep the info all in one place.

minno

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### PARYacht Designer/Builder

It might be more helpful to just tell us what you're trying to do.

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### minnoJunior Member

that's easy Par, I'm trying to figure out the CLR

although at this point I'm seriously considering multiple Dboard slots and mast partners

minno

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### PARYacht Designer/Builder

Why do you need the CLP location, as it's not a particularly handy thing to know, unless you trying to "do something", so what are you trying to do.

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### minnoJunior Member

ahhh, I see

I'm trying to figure out where to place partners and dagger board trunk on a 16' sculling trimaran I'm working on, sorry, it never occurred to me that CLR might be used for anything else.

minno

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### TANSLSenior Member

Although how to determine the CLP has nothing to do with "what do you need it for", that point is decisive in other studies, for example, calculating the heeling due to wind.

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### rwatsonSenior Member

Do what the old timers did.

A cardboard silhouette of the underwater hull section, and balance it on a straightedge.

Worked for thousands of boats over hundreds of years.

11. Joined: Nov 2003
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### PARYacht Designer/Builder

You can also use the push pin method, which NASA used on every single fin, employed on every single new rocket design through the Apollo program. Of course the math was done, but all these fins received the cardboard cutout test on the wall.

It seems he's attempting to design a new rig and appendages or maybe just making major modifications, TANSL.

12. Joined: Aug 2014
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### minnoJunior Member

@ R Watson

I'm not sure what you mean, it sounds like you're talking about finding the centre of balance fore and aft, not sure at all what that has to do with CLR

@Par

No idea how the "Push pin" method works or what it determines, sounds interesting though, do you have a link to a more thorough explanation?

I am rigging a sail strictly as backup power, this is a sculling boat. added a pic ao you can see where I'm at at the moment.

I am also adding large ammas for use in getting in and out of the water when I'm free diving and for pulling crab pots, etc.

Other than a cut out for the self bailing transom which will be well above water line the outside of the hull will be exactly as the designer drew it.

On the inside I'll be adding bulkheads to support the pylons, cockpit sole with supports, hatches fore and aft, and partners.

I've decided against a dagger board and will use exterior chines on the ammas, which should mean less weight and less turbulence/friction on the main hull when sculling.

For my tank test I'll:

-Place a 140 pound ballast in the cockpit and move it to determine the centre of balance both fore and aft and port and starboard.

-put ~260 pounds of water/ballast in the cockpit, level it fore and aft and mark the waterline.

-push/pull laterally while keeping the hull level to approximate the CLR and mark it.

-do the same with the boat heeled at ~22.5 degrees (about as far as it should ever heel with the ammas on it) and mark it.

I think I'll do much the same with my dingy just out of curiosity.

gotta go, tendinitis is acting up

minno

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### latestarterSenior Member

As the cardboard is uniform in density, the centre of balance, which is the centre of gravity of the underwater silhouette is also the centroid of its area so would give an estimate for the CLR. It will only approximate to the CLR because, unlike the uniform density of the cardboard, the lateral pressures on a moving hull are not the same over its length and depth.

There is the same problem for finding the exact centre of pressure of a sail plan as the wind pressure varies.

The "Push pin" method is also know as the "Plumb line method" see link https://en.wikipedia.org/wiki/Centroid#Plumb_line_method

I can not see your picture.
http://www.boatdesign.net/forums/faq.php?faq=vb_read_and_post#faq_vb_attachment_explain

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### minnoJunior Member

Thanks latecomer, I'll have to try that on my next boat (also have to make sure the boss doesn't see that, wouldn't want her to know I'm already considering yet another boat. )

Did the tank test this morning, turned out that Par was all of 1/4 of an inch off with his 55% aft of the bow suggestion .

it's a bit deeper in the water than I like with ~450 lbs of ballast, a bit more than the estimated maximum displacement of ~400 lbs, normal displacement should be ~300lbs.

not sure why the picture didn't show, seems to be working this time though.

minno

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### BobBillSenior Member

Hi Par, simple question...the .55 of LWL approximation for CLR also work for cats and/outriggers? Thanks. Just wondered, after I read.

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