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#1
07-23-2003, 10:36 PM
 Guest Guest Posts: n/a
center of lateral resistance

Would some one explain what 'center of lateral resistance' is? Thanks.
#2
07-24-2003, 07:02 AM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2917 Posts: 1,981 Location: St. Augustine, FL, USA
The center of lateral resistance (CLR) is actually a misnomer. It is also called the center of the lateral plane (CLP), which technically is a more correct name. That is, it is the geometric center of the underwater profile of the hull. It is used in comparison with the center of effort (CE), also a misnomer, which is the geometric center of the sailplan. In comparing these two area centers, the fore/aft distance between the two is called the "lead" (rhymes with "reed"). In order to have proper balance and just the right amount of weather helm, the CE should "lead" the CLP by a certain amount, which is dependent on the shapes of the hull, the appendages, and the sailplan. The vertical distance between the CE and CLP is the heeling arm, and is indicative of how much the boat will heel when underway (longer arm = more heel at any given wind speed).

Naval architects and yacht designers use the CE and CLP for general design purposes because they are convenient and easy to calculate. In reality, the aerodynamic forces do not really go through the CE, and the hydrodynamic forces do not really go through the CLP, at least not constantly, although they may do momentarily. It is actually quite impossible to calculate the precise centers of the actions of the aero/hydrodynamic forces with any degree of accuracy because they are constantly changing as the boat moves through the waves. Therefore, using the geometric centers is a useful tool that gets the job accomplished and can be used for comparison between differing boat designs.

Eric Sponberg
__________________
Eric W. Sponberg
Naval Architect, retired
www.ericwsponberg.com
#3
07-26-2005, 07:13 PM
 wind Junior Member Join Date: Apr 2005 Rep: 10 Posts: 22 Location: Argentina (Bs.As.)
Question

Eric, can explain to me how I can calculate the CLR (the hydrodynamic)?

The method, please, thanks
#4
07-26-2005, 08:15 PM
 Skippy Senior Member Join Date: Nov 2004 Rep: 12 Posts: 568 Location: cornfields
Just draw a side-view picture of the submerged portion of your boat, and find the center of it. That's all you have to do.

It seems to me you can define the CE and CLR as the actual forces, as long as you remember that they don't correspond exactly to the CLP and the middle of the sails. And also that they move around somewhat. I can see equating the force centers with the geometric centers as being a misnomer, and of course the geometric quantities are much easier to deal with. But I wonder if it's possible to approximate the force centers more accurately using known dynamical properties of the hull and rig. They're known to vary in certain ways with things like boatspeed, heel angle, etc., so why not include that information and use them directly if you have enough resources and require better results?
#5
07-26-2005, 11:38 PM
 wind Junior Member Join Date: Apr 2005 Rep: 10 Posts: 22 Location: Argentina (Bs.As.)
Thaks

Ok I understand, I do it, but i want to know how can I obtain the really CLR the hydrodynamic, one more question.

In a boat with canting keel, when I go upwind I have the canting with a canting angle 40 degrees, ok and the canard down, I calculate the CLR, then i have o angle of canting and i sail with the main open, and i donīt have the canard, my question is where I put the canard in my design to balance the boat, thanks for all, and sorry for my english.

Becuase when I move the cannard up - down the CLR moves, i think, when you have the canard up you go down wind and you can despise it, thanks for all again, bye.
#6
07-27-2005, 12:17 AM
 Skippy Senior Member Join Date: Nov 2004 Rep: 12 Posts: 568 Location: cornfields
Calculating the real CLR is very difficult. You need lots of software, lots of math. The easiest way to calculate the canter is just draw two pictures, one with the bulb up, canard down; the other with bulb down, canard up. And of course, when you're running you don't want as much weather helm, so you can factor that in.
#7
07-27-2005, 08:39 AM
 wind Junior Member Join Date: Apr 2005 Rep: 10 Posts: 22 Location: Argentina (Bs.As.)
Some friends said to me that we put the CE ahead of CLR, because the real CE is more to stern, how can I see how much is ahead the CE.

In yacht design, of Larsson, in the YD-40 he put it aprox i think, 0.06 X DWL (lead), But in a boat with high speed, the lead is important??, if my boat came into Gliding, what must I consider, thanks
#8
07-27-2005, 09:28 AM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2917 Posts: 1,981 Location: St. Augustine, FL, USA
Wind,

There are two methods to calculate the center of lateral plane: In Autocad, and with scissors, paper, pencil, and a triangle scale (what could be referred to as the kindergarten method)

AutoCad: Assuming your profile is in AutoCad, a 2-D drawing, using a polyline, trace around the entire underwater profile area. With the REGION command, declare the polyline a REGION (Type REGION on the command line, and select the tracing polyline as the region). Set the Origin of the drawing at some convenient point on the drawing, such as the front end of the waterline (TOOLS-NEW UCS-ORIGIN). Then on the command line, use the MASSPROP command to determine the mass properties of the region (MASSPROP, select the region you just created) and a window will pop up with the mass properties of the region. Included in those properties will be the centroid coordinates of the center of area (or mass) from the origin. Using those coordinates, you can put a point or a set of cross hairs at the centroid location.

Kindergarten method. Print out to scale a copy of the underwater profile on a piece of paper, and glue it to a piece of cardboard such as the back of a tablet of paper. Cut out the paper and cardboard outline of the underwater profile with a pair of scissors. Bend the profile slightly with a smooth curve from one end to the other so that it is not perfectly flat. Place a triangle square on a flat table, and balance the cutout on the triangle square so that the bow extends one side, and the stern extends the other side. Balance it perfectly, and with a pencil, mark the upper edge and lower edge where the top edge of the triangle is. You should have two little pencil marks on the cutout--connect them with a straight pencil line using a straight edge (the triangle scale, for example). Now you have a line of position, just like in celestial navigation, going somewhere through the center of gravity (area) of the cutout. You need at least one and preferrably two more lines of position. Rotate the cutout about 45 degrees one way, then 45 degrees the other way, and repeat the balancing and marking process, and drawing lines between the hash marks. You should now have 3 lines of position that all cross at the same point. If you have done a poor job of balancing, you will have a little triangle drawn by your 3 lines and you'll have to do it over again until the lines all cross perfectly at a single point. This crossing point is the center of area. You can measure the coordinates to the waterline and the forward end of the waterline.

I hope that helps.

Eric
__________________
Eric W. Sponberg
Naval Architect, retired
www.ericwsponberg.com
#9
07-27-2005, 09:40 AM
 wind Junior Member Join Date: Apr 2005 Rep: 10 Posts: 22 Location: Argentina (Bs.As.)
Thanks Eric, I calculate all the centroids in rhino, but I want find the real CLR ( hydrodynamic), but I suppose I must agree with the CLR geometric, I know a lot of mathematics, but not much of hydrodynamics, I am in 3 year of naval engineering, but even do not study it, I study a lot of chemical, physics, materials, but not this, thanks Eric again. doyou design some boat with canting ballast & canard?
#10
07-27-2005, 10:04 AM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2917 Posts: 1,981 Location: St. Augustine, FL, USA
Wind,

About the only way you can calculate the real hydrodynamic centers is through model testing or perhaps with CFD, inaccurate though it may be. And even then, it will not necessarily give you discreet points of centers, only regions of possible centers. And then what are you going to do with that information??? It is not very useful, really. I doubt that you could design to any greater degree of confidence than with the static, center of area method. And that is because sailing is a very dynamic system where the centers and balance change moment by moment. The effect of the crew, the sails, the waves, and the steering are constantly changing the balance of forces. So by extending all the computer power at your disposal may not get you any further along the design path than what we already do geometrically.

I have designed boats with canting ballast (Project Amazon) and lifting keels (Bagatelle, Saint Barbara) and am working a little on a small hydrofoil design. For the sailboats I used the simple geometric methods for balance, and so far they work just fine. On the hydrofoils, we are trying to raise money to build two prototypes to test our ideas on the foil design and control system. It is cheaper to just go build a few boats and test them at full scale than to do all the computer math to study the problem. We need about \$80,000 to get going, money that will have to come from an interested owner or investor.

Eric
__________________
Eric W. Sponberg
Naval Architect, retired
www.ericwsponberg.com
#11
07-27-2005, 10:33 AM
 wind Junior Member Join Date: Apr 2005 Rep: 10 Posts: 22 Location: Argentina (Bs.As.)
Thanks Eric you are so Nice, thanks again.
#12
08-03-2005, 11:54 PM
 vinay Join Date: Aug 2005 Rep: 10 Posts: 4 Location: Victoria BC Canada
vinay
I think the method with the pencil is easy. But once done just drag the thing in water (submerged) until the it is 90 degree to the string. that is where the CLR lies, ie on the vertical line passing through the string. The altitude of the CLR on the that line is not that important. The center of boyency is more important if that concideration is needed.
If the CE (which has nothing to do with the submerged body of the hull but cener of effort of the sail, is forward of the CLS then you will have a weather helm, which is needed to move the boat forward. 2 to 5% of the LWL is usual
#13
08-04-2005, 08:29 AM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2917 Posts: 1,981 Location: St. Augustine, FL, USA
Vinay,

Everything you say is, to my mind, quite inaccurate. Although I have not tried it, dragging a cutout through the water will not work. You won't get the center on the first try, so you'll poke lots of little holes in the cutout--soon to be one big hole--and you'll never get an accurate position of the center. You won't be able to measure a 90-deg angle accurately while you drag, and even then, you need the cutout to be 90 deg in all directions. The kindergarten method that I described above is simple and takes all of about 10-15 minutes to do, even less, and it is perfectly accurate.

Also, yes you do want the CE to be ahead of the CLP (it should not be called the CLR, which means the "center of lateral resistance", a misnomer; and it is not known as a CLS--is that a typo?), but this lead is not required to move the boat forward, it is required to get the boat to balance properly with a slight touch of weather helm, and so that when you let go of the tiller, the boat will round up into the wind and stop. And 2% to 5% is not the norm, it is greater than that, usually on the order of 10-12%, plus or minus a few percent depending on various design factors. Be aware, too, that when you have lee helm, the CE is still forward of the CLP. Ted Brewer discusses lead adequately in his book on sailboat design.

Eric
__________________
Eric W. Sponberg
Naval Architect, retired
www.ericwsponberg.com
#14
08-12-2005, 02:48 PM
 steveh Junior Member Join Date: Aug 2005 Rep: 10 Posts: 4 Location: FL
Wait a minute, something's not right here. In order to have weather helm, the aerodynamic center of the sails must be aft of the hydrodynamic center of the hull. If both lee and weather helm result from the CE being forward of the CLR, what good is calculating the geometric centers?
#15
08-12-2005, 03:04 PM
 steveh Junior Member Join Date: Aug 2005 Rep: 10 Posts: 4 Location: FL
Also, there is a geometric method for finding area centers including multiple areas. This link has a pretty good description of it.
http://www.duckworksmagazine.com/05/...acing/free.cfm

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