Elements of Fast InterClub Foils - Jesse Falsone

The subject of centerboard angle came up one Sunday between races. Some people did not believe me when I told them that raking the board forward is fast. Well, it is. But CB angle is only one factor among many in good centerboard design. The following information addresses the elements of fast InterClub foils.

Centerboard Design

The best CB and rudder for the IC are ones which have maximum thickness (7/8") and use a standard NACA airfoil cross-section. The maximum thickness provides the greatest lift/drag ratio when coupled with the NACA section. True, a thicker foil causes more drag, but it is a very small increase and the extra lift you get greatly enhances upwind performance. The standard NACA foils section ("00" series) is the most forgiving and a decent performer. It tolerates imperfections better than any other section and won't stall as easily.

Planform refers to the centerboard's outline. A casual observer may not notice, but planform area varies significantly between centerboards, and this can cause wide performance margins. Generally speaking, an IC is a slow boat requiring the largest possible centerboard. The rules state that the tolerance on the CB planform measurement is +0.0" to -0.5". What this means is that the CB can not measure any less than 0.5" from its maximum planform dimensions as specified by the original S&S drawings, and can't be any larger than the maximum dimensions. This equates to roughly a 5% potential difference in planform area (Max CB area is @394 in^2). I have measured centerboards where the planform difference is at least this much. Typically, custom foils are at or very near the maximum dimension.

Centerboard Construction

The best IC sections will have a wood or foam core with an epoxy or S-glass lay-up. These construction methods are much lighter and stiffer than the all glass foils supplied by Vanguard. Waterat Sailing Equipment offers a Sitka spruce core with an epoxy laminate. The Cb averages 6 lb and the rudder 4.75 lb. The surface is painted with a durable white finish. Also available at a cost premium is an S-glass laminate over a foam core which should be slightly lighter and stiffer. Lindsay Custom Yachts probably offers similar construction methods. My refinished Lindsay blades are about 50% lighter than Vanguard blades.

See the IC Links for manufacturers of custom foils.

Fit in CB Trunk

The IC CB trunk is wider than the head of the centerboard. In order to get a snug fit and prevent transverse (lateral) movement, shims on the head of the Cb are needed. A snug fit in the trunk allows for better speed upwind and more powerful roll tacks and gybes. Vanguard can supply these ($10/shim). If I were having custom foils built, I would ask the builder to do it after taking caliper measurements of the trunk opening. When I refinished my board, I used a pre-laminated fiberglass mat material that had a rough side (mat texture) and a smooth side. I just layered it until I got the right thickness. I estimate that I used about 3-4 layers on each side to build up the surface (about 6-8 mm). I was careful not to lay the mat where it would be below the trunk. The downside to my method was that by layering the entire area of the CB that sits in the trunk, I effectively increased the frictional resistance of the board rubbing inside the case. As a result, my board is more difficult to move up and down. Some grease on the head of the board has helped.

Neil Fowler's boat (#755 - modeled after Jack Slatery's), has a forward CB slot that is form-fit to his centerboard. When his CB is down all the way (5" tip rake), the leading edge of the CB mates perfectly with the front of the slot giving the best possible hydrodynamic flow. Neil suggested that this is easily accomplished by waxing up the leading edge of the board (so the epoxy won't stick) and using West System with microballoons to fill the gaps. Make sure the CB rake is correct! Banks recomends 4", but you may need slightly more if you're pin location is aft of the maximum (77" - see CB Pin Location below).

Centerboard Pin Location

The centerboard pin location is another important speed and pointing determinant. Generally, you want the pin location to be as far forward in the trunk as possible or as far as the rules will allow. The maximum allowable pin centerline location is 77" forward of point T2 or about 79" measured diagonally from point T1. A major difference between old and new fiberglass IC's is the leading edge of the centerboard trunk. Older boats have a vertical trunk leading edge which does not permit the CB pin to be placed as far forward as desired. The newer boats were built with a trunk with a leading edge that slants forward allowing the centerboard to be further forward in the trunk when it is raised. I was able to get a maximum pin location of 76.25" from point T2 on IC #235 and still be able to retract the centerboard completely.

Centerboard Rake Angle

Raking the centerboard forward effectively accomplishes the same thing as moving the pin hole forward - it helps induce some weather helm. This helps you steer better upwind (you make less corrective action) and aids in pointing. The amount of rake needed is widely disputed. Banks suggests 4" (6 degrees) and Neil Fowler has said that he uses 5" (7.5 degrees), so somewhere in this region should be fast if you're pin location is correct. The detriment to raking is that the centerboard can stall more easily, especially in chop when you hit a wave and slow suddenly. Neil's board is always raked forward while sailing upwind.

There is a wealth of information on the web about foil design. Try brousing the 505 North American Web site or search the web under "centerboard design" or the like.