Designing a ballasted daggerboard

[quilbilly]

I have a twentyseven foot sloop I built and designed. It is trailerable and henced only 8 1/2 feet wide. It has a ballasted daggerboard that originally weighed 1500lbs, last summer I added another a 300 lb bulb to stiffen it up. It is a steel fabrication filled with molten lead and coated and faired with epoxy. It is two feet wide and without the bulb it extends four feet below the hull. It is a NACA foil with a width to length of about 19%, is that a #2019? In any case I am thinking of replacing it mostly because so heavy that raising it is quite a chore. What I would like to do is replace the entire thing with a light weight fin with about an 1100 lbs bulb on the end. My calculations show that this would give the same righting moment and if I could keep the fin to a couple hundred pounds I could get rid of 500 lbs which would help a lot. I looked at the Melges 24 website and it has a 1100 lb bulb on a fin they say weighs only thirty-five pounds. My questions, I am sure I will have more, are what material do people think would be the most effective for the fin. I would like it to be a light as possible, although 35 lbs seems pretty extreme. And also is there any reason to use a thicker foil than structurally necessary. I.E. does it need to have a mimimum thickness to provide lift? Obviously the ultimately thinnest foil would be flat and the foil would provide no lift from its shape. I hope these questions make some sense and thanks for any advice Todd Miller

[tspeer]

Originally Posted by quilbilly

...And also is there any reason to use a thicker foil than structurally necessary. I.E. does it need to have a mimimum thickness to provide lift? Obviously the ultimately thinnest foil would be flat and the foil would provide no lift from its shape. ...

Actually, it's the other way around. As long as the keel isn't stalled, the thin foil will produce the same lift per degree of leeway as the thick foil. So the foil shouldn't have much more thickness than is necessary for strength and stiffness. The thinner foil will have less drag.

Given a certain physical thickness that's set by the structural requirements, a high thickness ratio, like the 19% you mentioned, means a narrower chord to the keel and less area for the same depth. If you want to go with that kind of a thickness ratio, there are more modern sections that might be more appropriate.

A section of modest thickness ratio will have a higher maximum lift than a very thin section. But extremely high thickness ratios don't necessarily continue that trend.