A Different Perspective on a Keel

Yeah, adding some steel ballast is one thing, but this is a sailboat he's talking about and wholly different. Steel isn't a very good ballast material, I don't care what you put on it, corrosion will cause issues. Then there's the hydro issues.

 

PAR,

As I originally noted, I offered the query as a discussion question. I am not married to an I-beam and thank you for your feedback. I currently have and am sold on a full keel or 3/4 keel for extended offshore cruising and have a different opinion fin keels. I do think that fin keels are great for inland and coastal waters. Again, thank you for sharing.

 

Hi Par,
You comments on the "hydro issues" was why I originally posted the question. The steel issue is another situation. There are many sailboats built throughout the world in steel. The hulls and keel are steel and they require attention to the treatment and painting to reduce corrosion.

I will be building in wood-epoxy because I like the characteristics of wood and a polymer shield between the wood and water. Most likely I will pour a lead keel (as part of my full keel) but I enjoy exploring different options. There are sailboats built for different purposes--some racing or rally and others extended off shore cruising.
When I go out in rougher winter weather my full keel tracks well while the fin keels are rushing for the protection of the marina.

I do appreciate your comments and am hoping others will share their perspectives about the hydrodynamics.

 

JSL,
Thank you for the interesting post. It appears that you had a sturdy full keel rather than the wing keel. Although a lot bigger scope than what I will be building, having the sides filled in adds an interesting concept kind of like a solid rectangular steel box rather than a wide I-Beam.

 

gfusch.

Here’s a few things to consider about using a Wide “I” Beam as a keel.

Let’s look at it as a matter of gains and losses.

On the Gain side:

You will have a ballast keel which needs little fabrication,

It will have a very handy mounting flange which will require very short bolts, which can be placed wide apart, making an extremely strong mounting system which is relatively simple. The loads on the bolts will be minuscule compared to the weight of the Beam.

The Beam itself has enormous strength in resisting bending, which is not only good for itself but good for the Hull bolted on top of it too.

The wide bottom flange will not only increase the effective draft when the boat heels, but will do so even when the boat is level, by acting as an end plate.

And finally, its flanges will help it dampen both rolling and pitching. Dampening pitching, as PAR has pointed out, may slow the boat down, but it will also contribute greatly to comfort.

On the loss side.

The width of this “I” Beam is as great as its depth.

There are several disadvantages to this:

One, a lot of extra whetted area (WA). This WA area will create a lot of frictional drag on your boat, making it slow in light air.

Let’s calculate this WA.

Let’s assume the “I” Beam is 19 inches tall, 19 inches wide, and weighs 250 lbs per foot of Length.

Now let’s assume we will use a length of 190 inches, or 15.83 ft. So now we have the weight nailed down at around 3900 lbs, more or less, depending on how we trim the ends.

The WA for the top flange will be about 19 inches, minus say 1.5 inches for the center web, multiplied by 15.83 ft, divided by 12. This comes to (19-1.5)*15.83/12, or 23 sf.
We will count this only once, because the top flange will be bolted to the Hull.

Next, we’ll count the center web, assuming the top and bottom flanges are, say 1.0 inch thick and get something like this: (19-2)*15.83/12*2 (because there are two sides to it exposed to the passing water) and get 44.8 sf.

Now all of this area is god because it is almost all directly keeping the boat from sliding sideways.

Now for the Bottom flange.

On the inside it will have about the same area as the inside of the top flange had, about 23 sf.

But it also has a bottom surface.

On this we have to count its entire width of 19 inches, giving us: 19 * 15.83/12, or about 25 sf.

Now let’s total up the WA

23 + 44 + 23 + 25, or 115 sf total.

This is a lot of WA for what will probably be a 32 to 35 ft sailboat. And this is just for the keel.

And maybe only about half the bottom flange area, or about 14 sf will actually be helping the boat to windward. The other half would be just adding drag.

I suggest using a more conventionally proportioned “I” Beam, which is deeper than it is wide. This will probably give you most of the advantages of the flanges with less drag.

I further suggest you make the keel no longer than 10 times its Depth, and make the Hull, including the overhangs, about twice as long as the “I” Beam, if not a little more.

This would allow constructing a relatively simple to build round or “V” bottom Hull on top of it, which would avoid most of the pitfalls of a “built down” hull, which PAR aptly pointed out, as well as the construction complexity.

Most likely, this “I” Beam keel will be bolted to a Deadwood, which will extend 1.0 ft or more up to the Hull, at both ends. This deadwood would have to be faired at each end, which will mean a lot of wood cutting. All the more reason for using a narrower “I” Beam. This deadwood will also add both WA and Lateral Area.

The resulting deadwood/keel structure will be long enough to allow the boat to stand on its keel, but short enough to allow reasonable maneuverability.

 

Sharpii2,
Thank you for the WA figures. This adds an engineering context to the discussion and promotes thought. I was looking for perspectives on whether the bottom flange of a wide I-Beam would improve performance as a wing or cause other issues. You spelled it out well as the additional drag may hurt more than the bottom flange would help and made a better argument for a taller web with narrower flanges on an I-beam. Again, this was just one of those random thoughts that I thought would make a great discussion which it did. PAR provided some excellent points as well. Thank you for sharing.

 

As a wing, you'd have very limited results, all offset by vortice generation, that wouldn't be there on a vertical foil. If the I beam was considerably deeper, you get some modest gains, but not enough to offset the additional drag and turbulence generation.

The lower flange could be foil shaped, though the web would benefit more from this treatment. The problem in terms of hydro, is it's extremely low aspect and the mentioned drag. Simply put you will get a barn door effect, from the flat plate lateral area, offering leeward skid resistance, but without reasonable streamlining and considerably more depth, next to nothing on pointing ability. To get benefits like a wing, you'll need shapes that will work well across the Froude range the boat will operate in. You can expect some improvements, simply because of the end plate effect, but these will be discounted because of the dramatic increase in drag creation.