CB Lateral Resistance

[Pernicious Atavist]

Okay...I should know this, but.....WHERE is the lateral resistance in a centerboard or leeboards expressed on the boat? And, would it change longitudinally with varying positions? Methinks it would be where the board contacts the boat--at the hinge pin. But, methought other things that have been wrong, so.... :rolleyes:

[Steve Lansdowne]

Seems to me (just guessing) that it would change with the position of the board. On the Melonseed the daggerboard is not straight but rather curved back toward the bow below the hull, thus looking somewhat like a large inverted comma. I understand that this configuration allows for a longer cockpit than would be the case were the board shaped straight and designed to drop straight down from abaft the designed daggerboard slot. In this situation, at least, the position of the board beneath the hull affects performance in a way that suggests that it is where the "pivot point" exists in the water, rather than where it connects to the hull, that makes the difference.

[Aramas]

'Expressed'? Like breast milk?

Do you mean 'where are the lateral forces transmitted to the hull structure'? If so, generally the centreboard transmits forces in the form of a couple between where it enters the hull and, depending on board layout and geometry, a point higher in the centreboard case - usually the pin or the top of the board.

[Ian McColgin]

Physics ususally uses a representational point that may or may not have much to do with a point of contact. For example, the mass of a billyards ball is assumed to be at the center of the ball. One must aim accounting for the surface of the ball, but that is just to establish the force vector on the center, or to intruduce another effect such as spin to improve a bank shot, which still comes down to a center vector.

So too for a boat, the center of lateral resistance is for the whole boat. The static center is easily found. Just view the centerline section of the boat and find the center of area for the underwater part. It will usually be somewhere on the centerboard but not absolutely always and it's certainly not the same as the centerboard's individual center of lateral resistance.

The center of lateral resistance can be affected by the hull's shape but not enough for the proverbial government work . . .

Especially considering that the static center is a nice thing to know, a good start, but not the important point when the boat is heeled and moving. On most boats, the center will move forward as the boat moves due to the hydrodynamic lift of the board and perhaps of the hull. It's hard to calculate and varies with speed and heel anyway. That's one reason traditional NA's gave the sail area a bit of "lead." Depending on the scale of common drawing, some NA's settled into a lead of about one thumbprint.

[Pernicious Atavist]

so...."close" will work...this boat--an "oystering skiff," also sports a skeg, which isn't accounted for in clr, and in low winds i plan on a jib, both of which chnge the attitude depending on whether they're used, as does sailing her heeled over abit going upwind...geez.........

[Ian McColgin]

A couple of thoughts as you play. Water is denser than air. Smaller changes underwater
produce bigger effects than bigger changes in the rig. That's why a lot of traditional sloops sail well on main alone and why a lot of new style mast head big genny sloops actually sail pretty well with the main furled.

So, play with board angle. On a pivoting board, the center of resistance moves forward, inducing weather helm, the further down the board goes but that effect is small in board angles munder 25 degrees or so.

Weight matters hugely. Go forward to induce some weather helm and aft to take it our.

Sail trim matters. You're talking a big horsepower when reaching sail. Pointing too high will move the sail's CE aft and you'll weathercock into irons. Keep her well powered five or even six points off the wind and tack with vigor. Most people overtrim and then fight back with excess weather helm.

G'luck

[Pernicious Atavist]

thanks, ian! :D

[Buddy Sharpton]

The effect of the board is where the center of area of the board happens to be. If the board is staright down- thats where it is. If the board is partially swung up, the center of that board and the resistance swings aft. Where the board is pinned has nothing to do with how the load is balanced on the hull. There will be a longer arm, more torque on the trunk as it swings aft, but the boat is built tio handle that without coming apart. The whole point of a centerboard besides reducing draft, is to assist balancing the boat on any point of sail or heel and thus ease the weather helm and any brajking force from an overused rudder. Two things a fixed keel boat can't do.

[Pernicious Atavist]

:D

[Dan Hobson]

Buddy is right on the money. Buy the man a pint when you see him. I was thinking that if you wanted to play around with a rough model that would prove this, try building an airplane with wings that pivot. Nothing fancy. Just a couple of wing like shapes that pivot on one point and stick for a fuselage. When you swing the wings to different positions, (symetrically) look where the balance points are as you try to balance the little model on two pencil points.

Oh, and someone please correct me if I'm wrong.

At 90 degrees the model should be made to balance say roughly at the center line of the wings. and the ballance point is going to be any where along that line. But as the wings sweep back, the balance point becomes more of an actual point. It not only moves back as the wings sweep. It moves in toward the center line of the model fuselage. Translating this to the CP of a centerboard of a boat the CP would start forward and low and move back with the sweep and up as well.

Gravity is only similar to hydrodynamic forces. and only in one plane.

Does that mean it becomes a weaker force? Less arm?

As well the actual cross sectional shape will vary the center of pressure with the angle of attack to the stream. With some foils the center of pressure will move forward (or back) at increasing angles of attack. You would have to check out a book on air foil sections to see how that all works. But the shape passing through the fluid has a natural center. The shape passing through the fluid at different angles of incidence will influence the position of the center of pressure moving it forward or aft.

[PVanderwaart]

Water is denser than air. Smaller changes underwater produce bigger effects than bigger changes in the rig.True. The range of lead (fore/aft distance from CE of the sails to CLR of the hull) is about 0-15%, much less than furling the genoa. The leeboards on my 15' catboat were hung about 5" too far aft by accident. The boat sailed, but with a tendency to lee helm. It was much better when the error was corrected.

[Denman James]

Dan Hobson's response triggered a memory of an exercise in finding the center of an odd shaped object.

Making a cardboard cutout of the underwater profile of the hull would allow you to locate the approximate center of lateral resistance (not including the effect of hull cross sectional shape) by balancing that shape on the edge of a ruler. You could then make profiles with the different centerboard locations and angles to see what effect the variations had.

Originally posted by Dan Hobson:
Buddy is right on the money. Buy the man a pint when you see him. I was thinking that if you wanted to play around with a rough model that would prove this, try building an airplane with wings that pivot. Nothing fancy. Just a couple of wing like shapes that pivot on one point and stick for a fuselage. When you swing the wings to different positions, (symetrically) look where the balance points are as you try to balance the little model on two pencil points.

Oh, and someone please correct me if I'm wrong.

At 90 degrees the model should be made to balance say roughly at the center line of the wings. and the ballance point is going to be any where along that line. But as the wings sweep back, the balance point becomes more of an actual point. It not only moves back as the wings sweep. It moves in toward the center line of the model fuselage. Translating this to the CP of a centerboard of a boat the CP would start forward and low and move back with the sweep and up as well.

Gravity is only similar to hydrodynamic forces. and only in one plane.

Does that mean it becomes a weaker force? Less arm?

As well the actual cross sectional shape will vary the center of pressure with the angle of attack to the stream. With some foils the center of pressure will move forward (or back) at increasing angles of attack. You would have to check out a book on air foil sections to see how that all works. But the shape passing through the fluid has a natural center. The shape passing through the fluid at different angles of incidence will influence the position of the center of pressure moving it forward or aft.

[Pernicious Atavist]

mystery ship....geez, reckon i have to post a decent pic...when i get it! :rolleyes: i do owe y'all that much!

[bainbridgeisland]

The approximate center of lift for almost any centerboard is 1/3 the breadth of the centerboard aft of the leading edge and 1/3 of the centerboards depth below the hull.

This works because: Almost all foil shapes have the center of lift about 1/3 aft the leading edge. Tip losses cause the center of lift to move up from the geometric center.

By the way, most hulls and rudders contribute a lot to reducing leeway. This is especially true for boats with fine waterlines forward. For extreme cases (about 10 degrees half angle) the centerboard can be placed further aft than normal and reduced in size. This is because the fine bow is effective lateral plane. An extreme example of this is a Hobie 16. They use only the fine hull and rudder for lateral plane.

[Dan Hobson]

Denman James had a good suggestion. But gravity is still only an approximation of the hydrodynmacs. Cardboard is also a good idea. I still feel that a symetrical model with the hull form represented as a line is going to give a better representation of how the CP moves on the board. The hull form will have it's impact. But center boarders have flatter bottoms in general. And the point is to find the CP and understand it's movement.

The center of the exposed geometry of the board is the place to start. It's the place where the board would balance on a point if it were just a flat piece of plywood. If you built your model to disregard the hull all together, Just two identical center board shaped peices of plywood that pivoted on a bolt, they would always ballance on a line through the centers of geometry no matter what angle you set them at.

But then you have to leave simple gravity behind and start thinking about the fluid dynamics. The shape of the foil section will influence the position of the CP. Lee pressure from the rig will generate a few degrees of difference between the direction of the boat through the water and the true direction of the board, so it will have an angle of attack to consider, but only by a very few degrees. The only time the angle of attack will be great enough to move the CP hydrodynamically is in a turn. And that's a brief occurance.

Tip effects are also pretty dependent on angle of attack. They are generated around a foil when higher pressure on one side escapes around the end of the foil. Tip Vortex Drag. Again these forces are at their worst in a turn and can steal momentum.

The actual true CP is of course a weighted average of the wetted area of the boat. The hull shape counts for a lot. With a shallow draft or skimming dish type boat the center board and spade rudder are big factors. If the two were equal in size the CP would fall directly between them and move about half the distance of any change in the position of the center board alone.

[bainbridgeisland]

Ahh... guys, the true center of lift is virtually never the center of area. The 'CLP' used by boat and yacht designers is not really the center of lift (or center of lateral resistance). It is just the center of area in elevation view we have agreed to use as a reference point.

By the way, 'the center of effort' of a sail is also not the true center of lift. Again, it is just a reference point we have agreed to use.

This is why 'lead' is important. It is also why lead varies with hull type and rig. The lead is the fudge factor we use because we do not normally calculate the true center of lift for either the underbody or the sail plan.

[Dan Hobson]

That's what I was thinking about bainbridge. It's not typicaly calculated. That's what makes it such a poser and makes it such an interesting muse for me. So what if you wanted to actually try?

By the way, what is Lead? Is it melted and poured in to a useful shape? Is it a dimensionless constant that is calculated from a particular set of points or other dimensionless constants?

I am not an expert, but I've read a few books. I'm hoping to fill in some of the deeper holes in what I know by picking the salt encrusted brains of others.

[Tom M.]

"Lead" is a term. It refers to the center of effort of the sails being farther forward than the center of lateral resistance (hull, rudder and centerboard combined)

[Pernicious Atavist]

so...IF i were to TRY to determine the clr by considering hull--hard-chined, leeboards, and skeg...(ouch :confused: ) how would i do it? the simpleton method would be good, i think....
then again, maybe that's what gets me into so much trouble...i think too much :rolleyes:

[bainbridgeisland]

The center of lift for a centerboard, the entire underbody or the sail plan can be calculated to a reasonable accuracy for steady state conditions.

Trouble is that the calculations are very complex and time consuming if you desire good fidelity. You could literally spend years calculating the lateral resistance of a hull, rudder and centerboard combination. This is partially because the calculations would need to be run for each combination of wind and sea conditions. Oh, and most Degreed naval Architects are not proficient with the calculations. It is a specialty field within Naval Architecture.

In the case of a dinghy, it would be much less time consuming to build 3 or 4 of them with different centerboard positions and then see how well they sail.

Having said the foregoing, there are simplified methods to estimate or bracket performance of a centerboard. Centerboards and rudders are easier than hulls or rigs. Honestly though, unless your centerboard is an unusual shape, the rule of thumb I gave you will be within a few inches of the center of lift.

If you are interested to know more about how these things work you might look up a copy of 'Sailing Theory and Practice' by Tony Marchaj. Tony does a good job of introducing how sailboats work without any advanced math. The book is easy enough to follow that a dedicated high-school student can follow along.

'Lead' is pronounced leeed. As pointed out, it is the distance that the center of effort of the sail plan (CE) is forward of the center of lateral plane of the underbody (CLP). It is expressed in percentage of load waterline length.