Centerboard area

Hi,

Im new on this forum I hope could find good counsel. I race at a local home made skiff class 24 footer. It is made from marine plywood laminate with a thin layer of fiberglass very lightweight construction. The design its homemade too, draw with a stick over the sand, applying a eye percent rule.

The design its very based on the 18 footer aussie skiff and the entire rig its from melges 24. The only rule its the 24 feets loa. Tha ballast its none exept for the five crew member on trapeze.

For now, my question is how much centerboard and/or rudder area should I need to be effective. Its there a formula? right now Im usin 6 feet deep (below the hull) by 18" tapared to 16" wide, and the rudder has 4'6" feets deep by 14" tapered to 10" wide. Remember there's no rule so I can use whatever i want.

The picture shows a 20 footer.

 

I would say, make the board as deep as you can stand it, and then start cutting down on the width.

It's going to be a tradeoff between reduced wetted area for less drag, vs having too narrow a "groove" and it being hard to avoid stalling when coming out of a tack. To some extent the right answer is going to depend on your sailing style and abilities.

You might even consider different boards for different wind or sea conditions. Save the boards that seem to be too narrow or too wide and see if they work better in light winds or heavy.

 

You could work out the optimum centreboard area pretty accurately if you know your speed and the righting moment (weight of the guys in the trapeze). With that data you could calculate the required lift. Then you figure out at which lift coefficient your board is most effective and after that you can calculate the area.

The problem is that the board will be optimized for only the assumed set of numbers. Different speed and different guys in the trapeeze will require a different board area. Also the optimum lift coefficient is quite close to stalling. That is a problem when the boat accelerates since you have the full righting moment and require maximum lift but the speed is not there yet.

I once designed a tiny centreboard for an experienced but lightweight 505 crew especially for heavy conditions. With that board they could keep up with the heavy guys and because they use gate starts and didn't go for a swim too often stalling was not a problem.

 

Thanks Karsten I allready hear that with more speed less centerboard area, less speed more centerboard area. Im considering "thanks tspeer" make two more centerboards. But still work with the eye percent rule. I would like to have the numbers to be more accurate. Did you know a web page or a book were I could find the formula.

THANKS

 

First you have to work out the righting moment. Multiply the weight of your crew (in Kilogram) by the distance (in metre) between the boat centre line and the centre of gravity of the person (somewhere at the hip). You get the righting moment in Kilogram * Metre for each person. Add up the righting moment of each person to calculate the total righting moment of the crew. Multiply this righting moment by 9.81 to turn the kilograms into Newtons.

Now you have to work out where your sail force and lift from the centreboard acts on the boat. For the sail force 1/3 of the mast height above deck is probably a good guess. The lift from the centreboard acts at about half the length of the centreboard. Now calculate the vertical distance between the centreboard lift and the sail force.

If you divide your righting moment by this distance you get the required lift on the centreboard in Newton assuming that the boat doesn't heel. In reality the boat will heel but the rudder will also produce some lift. So we just ignore this.

The formula to calculate the required centreboard area is:

A = (L * 2) / (Rho * V^2 * cl)

A = Area in square metres
L = Lift in Newton (as calculated above)
Rho = Density of the water in kilogram per cubicmetre (just use 1000kg/m^3)
V = boat speed in metre per second (2kn = 1 m/s roughly)
cl = lift coefficient

The optimum cl depends on your profile. For laminar profiles it's often quite low to stay within the low drag bucket. For normal profiles 0.3 is probably a good guess. The centreboard starts to stall at a cl of about 1.0 to 1.2. If you manipulate the formula you could work out at which speed your board is going to stall while producing maximum lift.

The boat speed is squared and therefore it has a big influence on the required board area. If you have a hand held GPS take it sailing to come up with a good number.

Good luck

 

Bethwaite's book "High Performance Sailing" discusses this and the are of the rudder surface vs c.board surface is related. If I remember correctly he talks of a rough 30% ratio. But at different points of sail a different amount is ideal. This is why you see in some development classes that allow it ruddders whose foil is vertically adjustable.
The way I understand it is that based on the board you have there is so much rudder you need to have efective steering with minimal drag, beyond that your drag will slow you down more than the less efficient steering force.
Balance and holding a line is an issue, steering/turning the boat is not much of an issue.

 

I think you'll find the limiting factor on reducing centreboard area will be the extent to which the board stalls at low speeds and coming out of a tack. Anything that will handle that situation will work fine at high speed as regards side force. There's probably an interesting balance between having a low drag high speed section, and having to have more area to prevent stalling, and having a smaller low speed section with more drag at speed per unit area. I have nothing to offer on where that balance might lie. On a lot of modern boats the rudder is set up to take some of the side load from the sails, which ought to mean you can have less board area nearer the bow. However I've sailed a boat set up like that where the daggerboard stalled out well before the rudder did. That situation is a cow to handle, and I would say best avoided.