daggerboard shape: suggestions?

[daiquiri]

Originally Posted by Tim B

@Daiquiri: "sharp square tips are better than rounded ones" - please read the section in Hoerner referenced above, as I don't think you're right here. The tip shape in the Y-Z plane (assuming X is fore/aft) if square will cause secondary vortices from the edges which add drag. in the XZ plane (ie. in planform) square tips will hurt performance when sailing with a pitch angle (particularly bow-down), as you get sharp corners and "rearward-facing suction areas".

I have to disagree, Tim.

I have that book, please take a look at the page 7-6. It shows graphically the differences between various tip shapes. On the drag polar (figure 10) you can see that the sharp tip, the one you have shown in your pic, is advantageous at near-stall angles of attack, where a boat's keel will rarely happen to work. Below that, it is just the same as a square, sharply cut-off tip.

In the figure 11 of the same page you'll see the increase/decrease in the apparent aspect ratio, with various tip shapes. It is indicated as Delta A1 column. Again, you see that, everything else being equal, a rectangular shape gives the highest increase in apparent AR. The worst performers are the rounded-tips. It is also explicited in the text on the same page.

If you're referring to Hoerner-type tips (Figure 8.a at the page 7-5), then they are good for airplanes but not so useful for boats, because they work better for cambered sections. Besides that, they are more complex (and hence time and energy consuming, and hence costly) to make.

Cheers!

[Tim B]

Hoerner describes the comparison between what looks to me like a tip with a tip-cap revolved around the chordline, and a square tip. I have no doubt that in that case Hoerner's assertions are reasonable. However, the parabolic tip is a different matter (as described and shown in the previous post). It should give better performance than the square tip, due to the removal of separation around the tip edge. It would be interesting to do a representative study on this at some point, but I am currently too busy!

How much effect you would actually see I am not sure, it's certainly worth having, as making a tip-cap is not that hard, but whether you would notice much difference on this type of boat is not clear.

Tim B.