trap 64 feature Centerboards by Gerry Mawson

CENTERBOARDS
by Gerry Mawson

CLASS RULES OF THE INTERNATIONAL FLYING DUTCHMAN CLASS MARCH 1, 1975. CLASS RULE 50 and 52. CENTREBOARD.
50. The underhull part of the centerboard in its lowest position shall conform to the equivalent part of the fullsize Mylar plan with a tolerance of plus/minus 6 mm on bottom and trailing edges (excluding keelbands) and plus or minus 12 mm on the curves between the forward and lower edges and aft and lower edges. A stop shall be fitted on the centerboard to prevent it from being lowered farther than 1060 mm under the hull. The mark of Rule 19-a-b shall be directly next to it. The use and position of a centerboard bolt, notch or hole are optional. The construction and material of the centerboard are optional. instruction: The board shall be laid with the leading edge on the leading edge of the drawing on the Mylar plan. The tolerances can be easily controlled at the trailing edge and on the foot. The weight of the complete centerboard shall not exceed 14 kg.
52. Thickness of the underhull part of the centerboard shall not exceed 23 mm.

The FD centerboard is a vital piece of equipment! Not only does it attempt to eliminate leeway it also provides a convienient foothold when capsized, not to mention an efficient grounding device in water of less than three feet depth! Joking apart, a good board makes the whole boat feel better, it runs smoother and seems to make less leeway. Bad ones are simply horrible! This article attempts to define good and bad characteristics and describes several approaches in making a board. NOTE -if you have a year and $300.00 or $400.00 read no further, MADER boats of Germany produce a fiber/foam board that in my opinoion cannot be bettered by home made products. Buy one and avoid all the effort described below!

LIFT/DRAG AND ALL THAT What makes a board good? First we need to understand its function. The FD board is a hydrofoil giving lateral lift to help prevent leeway. Comparison to an aircraft wing is not really valid. The board is operating in an incompressible medium (water) and has a very high loading. The nearest type aircraft wings are seen on suppersonic fighters! Since the rules fix profile and depth no major advantage can be gained here. Section is however, free -but(!) FD class rules limit the board thickness to 23 mm. For many years this was 20 mm. and some older boats will not take a 23 mm board. This rule forces a very thin centerboard on us. So thin in fact that lift from sectional shape is a pretty marginal consideration.

SECTION -This is a GOOD topic for discussion at the bar, further there exist books of theory full of glorious mathematics that I do not understand! Several points about board section seem clear however:
1. The section should be an even streamline shape with no hollows on flats.
2. The leading edge should not be sharp.
This is contrary to jet plane practice. Jet planes fly fast, when they do go slow they use mechanical contrivances to reshape their wings for the different airflow patterns. Since trim tabs, slot flaps and other "whatyoumacallits" are not allowed on an FD the section has to be a compromise that can provide good lift at near stalling speed, and also low resistance at high speed. Sharp leading edges suffer at low speeds from a tendency to have flow separation from the lift surface of the foil. Not quite the same thing as a stall but no good to us anyway. At high speed this is not a problem.
3. The thickest part of the section should be between 30% and 50% aft of the leading edge.
4. The trailing edge need not be wafer thin. A theory exists that it can be as much as 3/16 Of an inch wide and square edged. Certainly 1/8 inch thick trailing edge is commonly used and works fine.
5. Perfect symmatry is essential, if your board is uneven from side to side a vibration (bad vibes) will occur. This breaks down the even flow of water over the surfaces and causes more drag and less lift. So much for shape and theory, your problem is how to make the wonder board from material you get without a Watergate type break in at NASA. The basic engineering problem is that you have a highly loaded, very thin cantilever foil. Worse, this is mounted in such a way that a very high concentration of stress occurs right where the board exits from the hull. Poorly engineered boards break at that point!

HARDWOOD BOARDS
The most common approach to board construction is to use hardwood. Mohogany or teak are fine and lighter grades Of these timbers work just as well. Since the FD board is wide and thin it is not good to use a single wide plank. This will warp even if you could manage to buy one. Instead it is usual to construct the board from 2 inch to 3 inch wide strips edge glued together (see the diagram below).

Shaping the board is simply hard labor. Power planers and sanders are good only for the rough shaping work. The board must be finished by hand. Starting with the blank cut to profile and planed to thickness, proceed as follows:
1. Mark the centerline all the way around the leading and trailing edge.
2. Mark the leading edge about 1/4 inch each side of the center line. This mark is the line you will plane down to before rounding off the leading edge with sandpaper.
3. Mark the sides with the point which will represent the thickest part of the board. Do not plane off this point at all.
4. Sharpen you plane.
5. Go to it, do not plane from the middle outwards, start near the edges and work into the middle later.
6. Final shaping is best done with coarse sandpaper. Do not hold this in your bare hand. Take a piece of wood and fix carpet or rubber to one side, wrap the sand paper over this and work away.
7. Do not shape the part of the board that is always inside the trunk. This is best left with parallel sides.
With a hardwood board little more is needed except paint or varnish. A refinement is to stick plastic cheeks on the top to give a really close fit in the trunk. I recomend that you paint boards a light color. Dark ones soak up heat when exposed to the sun and tend to split or warp from the internal stresses created. This approach will provide you with a durable centerboard which is ussually quite stiff. The main disadvantage is weight. This can be as much as 15 to 20 pounds.

SOFTWOOD BOARDS fig 2

It is of course possible to build a board as described above out of softwood such as ceder or spruce. They usually break. Also they are always very flexible but this does not seem to slow the boat any. Weight works out well, about 5 -8 pounds. Really if you do make a light board like this you need to beef it up. This can be done two ways. First you simply introduce hardwood stringers into the light board, (see the sketch). This works fine but leaves the board somewhat frail and prone to damage from hard knocks. Robustness can be easily gained at the expence of weight by skinning the board with glass fibre cloth and resin. This is not in itself difficult except that getting a good even finish is a lot of work. My own experience is that the expensive Epoxy resins available today (such as from Goregeon Bros.) have no advantage over regular polyester resins.

ALUMINUM BOARDS
Enter the arms race -build a real metal board! This is a) difficult, b) seems to provide no advantage. The basic method is to epoxy glue thin (.020") alloy sheet over a light wood core. The effect if done well is a very stiff board of moderate weight.
Longevity is always a problem with these boards. The very high stress concentration at the metal/wood bond eventually causes delamination leading to buckling and failure. Since no one has proved that a metal board has any better speed potential than other types I recommend that you stay clear of this area.

HIGH TECHNOLOGY MATERIALS
By this we mean those amazing materials developed in this space age, such as Kevlar (DuPont trade name for AROMATIC POLYAMIDE) and carbon fibres. Both these materials are man made fibres having a low density coupled with very high tensile strengths. Carbon fibres also have a very low stretch factor (modulus of elasticity) enabling very stiff structures to be engineered from them. Neither material can be used alone, they are usually laminated into composite structures to increase strength and regidity. Thus your super-board is likely to be a wood core with a kevlar or carbon fibre reinforcement on the outer surfaces.

First carbon fibre. This material is currently losing popularity, it is darn expensive and when made up into an epoxy laminete it suffers from two serious defects. a) The resultant structure although very strong and stiff is brittle. If overloaded it will snap without warning. b) The material has very poor resistance to abrasion. Thus it needs skinning-over to protect the carbon fibres. This is the problem that led Rolls Royce to abandon the use of this material for the gas turbine compressor blades on the Lion jet plane. In short although you can make the lightest, strongest and stiffest board possible using carbon fibre, the end result is not satisfactory being too chancy and prone to sudden breakage.

Kevlar is a better bet by far. It usually comes in woven cloth form and is amazingly tough stuff. Regular shears simply wont cut it, you need carbide edge shears! You basically use kevlar cloth much like fibre glass. A layer is laid up on each side of a light wood or even foam core. You should use epoxy resin with kevlar, regular polyester does not have enough bond strength. A point to watch if you do tackle a kevlar board is not to let the cloth overlap the edges of the board. If you do so it is virtually impossible to finish off the edge. Sand paper simply erodes the resin leaving the much harder fibre sticking out like a floor brush!

A kevlar reinforced board built on say a cedar wood core is possibly the lighest and best you can make. Since kevlar has only poor resistance to compression loads it is necessary to beef up the core at the point of exit from the hull. A hardwood lamination at this point solves that problem.

SUMMARY
1. You wont go far wrong with a really good centerboard made of some moderately dense hardwood.
2. Nearly all highly stressed composite structures have very high internal stress concentrations. Thus they delaminate and eventually break under use.
3. Kevlar is probably the best reinforcing currently available (At a price.)
4. Building a really superior board is an amazing amount of labor. However, the end result is worthwhile.

Good sailing and may your bent centerboard never break -ours did!