How Not To Design A Cruising Boat Keel

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By Rich

I was able to hunt down and identify the make and model of the French production boat I “raced” earlier in the week, which I observed making significant leeway relative to my Sabre while sailing upwind in a stiff breeze.

With the model identified (a Beneteau Oceanis 473), I took a ride through Jabin’s Yacht Yard to see if a similar model could be viewed out of the water and if its keel design could provide any clues as to why it performed so poorly going to weather relative to my 1989 Sabre. I found not one but two 473s, and It didn’t take long to identify the problem: a very poor keel design. Looking around the yard one can only conclude that most of the production cruising boat manufacturers seem to have thrown basic keel design principles out the window. As a primer, take a look at this white paper on keel design. The paper deals with a variety of aspects of foil shape that will result in good performance (which, like a sail or an airplane wing, means the best lift / drag ratio possible). One critical concept will come through if you read through the entire paper: the only part of a keel that really produces lift is the airplane-wing-like flat section whose shape is not distorted by either a bulb (on the bottom) or a fillet (at the hull). Specifically, the author points out that “the addition of [a bulb] for a fixed draft [reduces] the available span of the keel.” What is meant by the “available span” is the portion of the foil that actually produces lift. The paper also specifically calculates a span / draft figure for different designs to formalize this notion.

Now with that critical concept in mind, let’s contrast the my Sabre’s keel / centerboard with the Beneteau 473’s shoal keel design. First, the Sabre:

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Note that as viewed from the side, both the Sabre’s lead keel and the centerboard present wing-like flat plane surfaces to the water, with the keel portion being a low aspect ratio NACA section foil. The entire span of both appendages produce lift – and there is plenty of surface area between the two in relation to the overall boat size. Here is the view from directly aft:

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The keel portion is pretty thick (not ideal) but the centerboard helps to compensate by being thin with a long span and short cord length – pretty much the ideal foil shape for a keel. If you have any doubts, head over to the Volvo Ocean Race site and look at the drawings of the Volvo Ocean Race 65. With absolutely no limitations on foil design, these deep (long span), short cord length foils are exactly what naval architects prefer for optimal performance. Now let’s contrast this design with the Oceanis 473. First, let’s start with a side view:

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The first thing you’ll notice is the the keel’s overall surface area is quite small relative to the size of the boat. But the EFFECTIVE size of the keel does not include the bulb or the fillet at the top. Let’s look dead on from the front to see how little “real” lifting surface remains with this design (I photographed two examples of 473, which is why one bottom is painted green and one red):

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Holy cow! It’s only a couple of feet in the middle of the keel between the fillet and the bulb that constitutes actual lifting surface. And note how short the cord length in that section is relative to the length of the bulb:

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Suddenly it’s little wonder why this boat’s keel is so ineffective. The part that is hard for me to understand is why they chose to design a keel this way when there are plenty of superior shoal keel configurations – well beyond the keel centerboard. Here is that old Hunter I found in Rock Hall. Note the large surface area of its shoal keel and nice flat-plane-wing-like shape undistorted by a fillet or bulb:

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Here’s another counter example: a newer, Jim Taylor designed Sabre with a shoal keel rather than a centerboard. Again, notice from the side how much surface area Taylor provided with the design:

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Now take two views of the keel to note that Taylor used almost all of the draft as effective span and substituted in a flat duck-bill set of wings for a bulb:

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These are much better keel designs, but if you walk through your local boat yard you’ll see that most of the recent production boats have keels just like the Beneteau 473’s. Why don’t they make boats with large, efficient foils anymore? Your guess is as good as mine, but my first guess would be that there is something about these keels that makes them cheaper to engineer and manufacture, and that customers aren’t prioritizing sailing characteristics in their purchase criteria.

So once again this old fart is shaking his head and concluding that for whatever reason, they “just don’t make ’em like they used to.”

3 thoughts on “How Not To Design A Cruising Boat Keel

  1. Maybe the designers are exploiting current yacht owner ignorance. It’s simply possible that owners have just not noticed the leeway of their boats yet.

  2. First, I’m still trying to come to terms with the fact that you cited a Hunter as a “good” example – the last thing I expected *you* to write. Second, in truth this doesn’t surprise me at all. I bet if you look at *racing* boats you will in fact find their keels designed optimally for racing (see the Volvo site). People that buy Beneteau and similar production yachts are for the most part not racing them – at least not seriously. What they do want is two things: shoal drafts so they can explore more areas a deep keel wouldn’t allow for and “roll dampening” to make the ride more *comfortable*. The only way you’re going to dampen roll with a short lever-arm (keel) is to stick a massive amount of weight at the end of the arm and hence distort the foil. Comfort (and as proven by Hunter interior volume/amenities) trumps performance in this market segment every time.

  3. Wing keels might be nice in deeper waters but they also work a lot like an anchor when they run aground. Have several in our marina that end up stuck when the waters low and they are a pain to pull out. So with the choice of a bulb or wing keel the bulb was my preference when looking for a new boat.

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