Re: Bob Perry's take on Wolfenzee's dream boat
Good morning all,
There has been a whole lot going on while I was offline.....Let me see if I can comment on some of this.
Please let me know if you mind if I use your real first name. I feel a little silly and disrespectful using this abreviated version of your nom-d'-net knowing that its not your first name, but I do not want to use your first name if that makes you uncomfortable.
Why I chose to draw a bulb keel:
When I draw a set of lines, I begin to get a sense for how a boat moves through the water and how its shape changes a various heel and pitch angles. While this is not a scientific analysis, what struck me is that this design develops comparatively little increased stability until it has heeled perhaps 12-15 or so degrees, at which point a large portion of the rather full topsizes are entering the water and the boat starts quickly building form stability.
And while I have not sailed the boat, this would seem to be born out in part by Wolf's comment that the boat likes to heel and seems to come into its own around 20 degrees of heel. While this is mere speculation, I am guessing that at 20 degrees of heel what Wolf is feeling is this rapid increase in stability, and so at that heel angle the boat feels very reassuringly stable, like its on rails.
But looking at the lines, my sense is that leading up to that heel angle, wetted surface increases and keel and rig efficiency decreases. If the boat were instrumented, I would suspect that the best VMG in the form of slightly better speed through the water, and less leeway would occur at comparatively small heel angles, probably well less than 10 degrees.
With that in mind, my sense was that one way to increase performance would be to increase sail carrying ability (and/or the power of the sail plan) at flatter heel angles. To do this I considered two options, a deeper fin keel or the keel shown with the bulb. I drew the keel with the bulb because in some discussions Wolf had made comments which suggested that he was happy with the draft of his boat.
I chose the the bulb keel as a way of keeping the same draft, and hull form, while increasing stability and also creating an endplate, with the hope of making the fin which is comparatively shallow with a long chord and foot behave more efficiently.
I agree with both Bob's and Wolf's comments that a deeper rudder would be more efficient, and if extended below the skeg, and the pivots rotated so that there was some counterbalancing, the helm would get lighter and more surgical. I chose not to show the rudder in that form because Wolf has expressed concerns about the damaging the rudder in groundings. I will concede that I probably went a little too conservative, and that the throw back to a loosely rendered ersatz 'Constellation' rudder probably was not the best choice.
I purposely pushed the center of lateral plane more forward than it had been. I acknowldge that it may be too far forward but without a sail plan I am not completely convinced at this point. It certainly can move aft as the design develops.
But to explain why it shown where it is, I had several thoughts on this. First of all, Wolf has talked about the importance of tracking to him. I had wanted to try to spread the center of resistance of the keel and the center of resistance of the skeg, as far apart as possible to increase tracking ability. Of course this is limited by the realities of maintaining the longtudinal center of buoyancy in the same location as the longtudinal center of gravity, and balancing the rig.
Since I was eyeballing this, I make no claim that I did not get carried away. But I am not fully sure of that. Again as a rough measure, cutting away the deadwood aft, would slightly reduce buoyancy aft, and so as buoyancy in the way of the fin, moved forward, I wanted to move the center of gravity ballast slightly forward as well. Again,since I did not do any calculations, this is only a swag, but my sense is that the CofG of new ballast is roughly 8-10" forward of the old ballast.
To take advantage of the greater stability, and lower drag, I had intended to design a more modern sail plan. My sense was that this would be a taller fractional rig, with a shorter boom, and with a minimally overlapping headsail that would extend a greater fraction further up the mast than Atkins had shown on his sail plan. The keel position anticipated my sense the new rig conguration would push the CE a couple feet forward of where it had been.
I can't swear that keel position is even close to right.
Paulos interesting keel study:
Here is my take...Everything that I have ever seen suggests that a deep draft fin keel without a bulb, but perhaps with small winglets is the most efficient keel form in terms of lift to drag. But because the current crop of 'fast boats', have enormous amounts of wetted surface when sitting flat, and make big gains in speed either heeled at small angles to balance the water line, reduce the wetted surface and get the spare rudder out of the water, or in breezy reaching conditions, sail flat in order to plane, the design brief therefore seems to be all about being able to carry as much sail as possible. To carry as much sail as possible, the designs push to have as much stability as they possibly can.
And it seems like they are going through extraordinary measures to get that stability. So much so that any weight that is not in the bulb, is seen as the enemy. That even includes such seeming counter intuitive measures as making the keel foils as light as possible (carbon fiber, or hollow stainless steel blades) so that more lead can go in the bulb.
My take on the paper that Paolo posted is two fold, in the paper they state:
"The four keels should have the same righting moment in order to be comparable. This means that the static moment of the yacht which counteracts side forces from the sails should be the same. In the redesign of the T-keel, this righting moment has to be kept constant. For an easy comparison between the keel designs the draft will be kept constant as well, and the fin size and shape will be identical between the bulb keels."
Designing these keels all with the same righting moment makes sense for research purposes, but it would not be a logical contraint for a yacht designer designing a real yacht.
The paper is predominantly comparing the hydrodyamic issues independent of the impact of equal weight keels and stability with the goal of comparing the relative VPP performance gains of improved hydrodymics. Ultimately this would help inform the trade-off in improved hydrodyamics vs. improved stability. The strange constraint of 'equal weight keels' and equal stability is what is pushing the bulb keels to be iron vs lead.
In reality, for the chosen hull forms, the race boat designers probably have it right, using a light weight, high strength foils, and a high density small surface area bulb.
Anyway, back to the conversation at hand, or in my case, coffee break over, back to work.....
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 04-19-2013 at 11:13 AM.