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Re: Bob Perry's take on Wolfenzee's dream boat
Here is the second one on choice of Keel Type...
Choice of Keels:
Paulo has asked an interesting series of suggestions/questions about keel types and keel options. I will respectfully suggest that some of these suggestions are not consistent with the goal of producing a boat which maintains the character and virtue of Wolf’s current boat, but would be valid points if we were starting from scratch without a ‘client’ and so were free to move the design any direction we chose.
Again, not wanting to speak for Bob, I personally see our design brief making it something of an imperative that the design stay relatively close to the design of Wolf’s boat in terms of not greatly altering the beam, cross sectional properties, or profile. I could be wrong on that, but that is how I have been operating.
In other words, we do not have a client who is interested in a wide beam design no matter what their virtues may be. And items like twin keels and rudders are design concepts which work well with the current of wider hull forms, but which are probably not a good design approach for a comparatively narrow beam, moderate to light form stability design of the type being developed.
To explain why I say that, I need to go off topic for a moment and talk about the dynamics of the newer style broad beam boats. I began looking into them a few years ago due to some comments that Paulo had been making. I have watched a lot of video of them underway and at speed. At a recent Chesapeake Bay Sailing Yacht Symposium, I had the chance to discuss them with folk from the Farr office and sail makers, and hear a lecture on the design of the new Volvo boats. Its wildly interesting conceptually, but a very different world than this project.
To begin with the dynamics of these newer boats is all about being able to carry a huge amount of sail relative to their displacement. To do so, they have enormous form stability, wildly so! They get around many of the liabilities of large form stability by being designed to be sailed in two-distinct modes, either heeled perhaps 10-15 degrees or sailed almost dead flat. The heeled mode is used on most points of sail and results in semi-displacement speeds. The dead flat mode is mostly used in deep reaching and running which are clear cut planing conditions.
When heeled in the 10-15 degrees the boat in the water has a comparatively narrow, low wetted surface and symmetrical water plane and immersed canoe body. It functions in effect as a comparatively beamy multihull leeward hull. The axis of the immersed hull shifts to leeward off of the boat’s centerline so that this narrow hull form is angled to windward and adding lift.
In that configuration, the placement of the twin rudders and something like twin keels are located on the centerline of the immersed hull and are slightly rotated towards that axis, reducing drag in that mode, but increasing drag when sailing flat. This small additional drag when flat is offset by the big sail plans and speed advantages when planing.
The normal price of carrying twin rudders and twin keels is in part mitigated by the wide beam of the boat allowing these foils to be lifted partially clear of the water reducing their wetted surface.
And that brings us back to why these do not make sense in our case. For twin keels and twin rudders to work anywhere near as effectively as they do on these newer designs, there needs to be a few things present, and these come with big beam.
To begin with I disagree with Paulo’s example that a keel canted perpendicular to the heel angle generates twice as much lift force per unit of area as a keel that is heeled at 10-15 degrees. If you look at the math the difference in area due to heel cant is only about 30% at a 15degree heel angle. There is a small decrease in lift which also comes from the water moving horizontally across the foil, rather than slipping at an angle toward the tip, but even with that in mind, it is unlikely that the canted vertical twin keel will in fact increase unit effectiveness to as much as 50% increase, which means that you are still paying the price for the drag of the windward keel. So unless you can pull the windward keel close to fully out of the water as they do with dagger boards on the bigger open class boats you still have a lot more drag relative to lift on the twin keel design.
As these high form stability boat heel, they quickly lift the windward side of the boat out of the water, and that allows the immersion of the ‘lazy’ foil to be decreased and so their drag to be decreased. But on a boat like we are designing for Wolf, as the boat heels, the weather side of the boat rotates but does not appreciably lift vertically out of the water at the point where the windward keel or rudder is located and so do not experience a decrease in drag.
Furthermore, to work efficiently twin keels and twin rudders need to be separated from each other so that the compression wave from each do not interfere with the flow on the other keel. (There was a lot of discussion at SNAME about new research on this just being done in Europe.) On Wolf’s boat we do not have the beam to allow the keels to be pushed far enough apart to create these separation of flows.
There is more to this as well. The stability generated by these broad beam boats comes many from shifting the center of buoyancy very far to leeward at small heel angles. If you watch these boats as they heel, like any boat with a lot of form stability, the center of gravity lifts vertically as the boat heels and being above the instantaneous axis of rotation, moves slightly to leeward as well. Because of this, the height of the vertical center gravity on these wide beam boats is less critical than on a low form stability boat, which generates its stability by being dependant on both the center of buoyancy moving to leeward and on the center of gravity moving to windward.
The relatively short keels of a twin keel boat work acceptably well on these beamy boats, but their shorter draft and therefore their short spans would not provide an adequate center of gravity shift contribution to stability on a boat like the one that we are doing for Wolf.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay