Centerboard shape

[Noklin]

The CS 20 plans show a desired cross sectional shape for the centerboard. The plans also show that the lower end of the board is tapered from side to side. The taper prevents the bottom of the board from being shaped to the desired cross sectional shape.

Why does the bottom of the board need to be tapered from side to side? Why not shape the board to the desired cross sectional shape to the end of the board and leave it flat on the bottom? Does the flat bottom shape cause additional drag?

I realize that the taper only affects the bottom 2 or 3 inches, but I am still curious about the logic.

Would the additional thickness at the bottom not reduce the cross flow from side to side and increase the board efficiency?

[Charlie Jones]

I would think that would work just fine- IF it was a dagger board, always operating in a specific angle to the water.

However, a centerboard is often adjusted, on different points of sail. Angled back more or even almost raised sailing down wind. Additionally, the CB doesn't wind up in vertical postion when lowered. Now if the lower end was cut at an angle so that when fully lowered that angle was parallel to the waters surface, I'm sure it would work.

Would be harder to glass well though.

Now this is just my own thoughts- Graham may have technical reasons for doing it like he does. I don't redesign I just build to what the man draws

[Noklin]

Charlie, I understand what you are saying, don

[mustangermatt]

My center board is a constant cross section from top to bottom (naca 012) with the bottom surface 90 degrees to the leading edge.

I do this because my board does go completely vertical to maintain the high aspect ratio for windward performance in light air. As you angle the board the chord lengthens in relation to water flow, good for fast but less lift slow.

The only times I adjust the board aft is running, and when I reef by dropping the foresail (main to the rest of you :-) This allows me to steer on just the main (again mizzen to every one else).

This arangement seems to work very well.