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  #31  
Old 04-19-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

The premise was that you could build a more efficien( better performance) keel with iron than you could with lead.

To retain the same VCG of the keel the iron keel would have to have a much bigger bulb and a lot more frontal area I also think the fin portion of the keel would have to be bigger so even more wetted surface and frontal area is added.

It does not make any sense and I don't give a rip who did the study.

Think of it this way. If an iron keel could provide "better performance? why does every serious racuing boat in the world use lead for ballast. If spent uranium not not illegal for racing keels they would use that like they did back in the earkly '70'. It's denser than lead.

Open you eyes, not another book, the answer is easy to see in any boatyard. The more dense material gives the designer more freedom of design. Bulbs are slow. They cut into the clear span on the fin and reduce apparent aspect ratio. They add wetted surface and frontal area. Bulbs work because the allow for a very low VCG. This is far easier to achive with the more dense material.
SVAuspicious and lancelot9898 like this.
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  #32  
Old 04-19-2013
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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.

Wolf:
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.

Shallow Rudder:
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.

Keel position:
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.....

Jeff
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Last edited by Jeff_H; 04-19-2013 at 11:13 AM.
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Re: Bob Perry's take on Wolfenzee's dream boat

So the change to a bulb was simply Jeff's q&d change to lower the VCG while not adding draft. Given the options for a longer and deeper or thicker foil though, was this done with specific calculations to lower VCG? Or is this just a guestimate that it would be the way to go?

And of course once the underbody starts changing shape, do these lines reflect similar changes to keep the balance and performance of the boat? Or does that get refined later in the process?

Bob, just how big a lottery ticket are we talking about here? Powerball? Or lotto?
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  #34  
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff:

I appreciate your explanation of why you put the fin forward but as I told the people who worked for me, "I don't need a long explanations of why you did something wrong." Your keel is too far forward to insure proper helm balance. Simple as that.

You need more sweep to the leading edge due to the low aspect ratio of the fin.
I'm not convuinced we even need a bulb on this fin at this point. With minimal draft a bulb will cutt into our fin span and that's not good. But if you feel we need a bulb I'm not adamantly against it either. Your call on this.

On the rudder:
I think we can retain the transom angle that Wolf has now but we will add a tapered "spine" down the center on the transom that will allow us to mount an outboard rudder on the spine with a more vertical stock. Maybe get 6 degrees or more out of the rudder rake. Do you follow me here Jeff? I can sketch it if you like. In other words the gudgeons will not be mounted on the face of the transom but mounted on the aft face of this 4" wide "spine". I'd like to get some of the rake out of the rudder. Give him half a skeg just to keep him quiet. Do not put the prop in an aperture. We want this boat to have manners in reverse.

Also, I don't think we need to bow to every wish of Wolf. I anticipate we may have to drag him kicking and screaming into some of the 21st century.

Lets massage the sections so we can get the D/L down to around 275. You ( Jeff) will probabl have to reduce deadrise amidships and go for more of a U shaped bow section. I'd maintain deadrise at the transom if only for looks.

Having fun yet?
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Re: Bob Perry's take on Wolfenzee's dream boat

Bob,

I completely understand what you are suggesting. Your suggestions are not all that dissimilar to some of the ideas that I had been considering but put aside our of wanting to stay as close to Wolf's orginal design as possible.

I too had been concerned about the bulb reducing the span of the keel foil. I can certainly flatten the bulb should we chose to retain it. In that way, we retain the endplate effect without losing as much span.

I see the first design as perhaps the tweaked version, and now we are moving closer to the 'Caraciture' version......I like this part.

And yes, believe it or not this is fun.
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff:
On draft:
Wolf sails in my backyard. He can live with 9' draft. I say we go 5'10" to start and se what that looks like. It will mean a shorter fin but that can be good so long as we keep the fin long enough to sit the boat on.

If he objects to 5'10" draft we can go to 5'6" draft.

If he objects to 5'6" draft we can duct tape him to a chair and make him listen to the orchestral works of John Adams until he cries Uncle!

At 5'6" draft we should be able to do a bulbless fin given that we will have the planform required to get the lead low on a deep sump. Like the Valiants.

Maybe we should keep in mind an adjustment to the L/B. He's pretty narrow now with a full bow. Maybe we should think about an L/B around 3.85. Do you know what his current L/B is? It looks to me like it's around 5.0 now. He'll piss and moan over it but in the end he'll have a much better boat that has some stability. If he objects we'll give him more of the John Adams music.
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Re: Bob Perry's take on Wolfenzee's dream boat

Balance is very important to me and my boat as it is is nicely balanced. That said the center of lateral resistance is in the ideal place, if you put a smaller rudder on it and move the keel forward you would increase weather helm. I was not concerned with protecting the rudder from groundings but from hitting debris in the water (logs,etc). Someone suggested a skeg rudder which extended below skeg and was balanced, I would make the section below the skeg "breakaway" so if struck it not take the rest of the rudder with it (this would also allow the opportunity to experiment with different shapes). I never said draft was a limiting factor in redesign because I realize fin keels are by their nature deeper.
I don't mind being called "Wolf" ( I actually enjoy it), I have also been called Zee (this one made it into the real world), you may call me David, but it is a common name.

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Old 04-19-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Adams doesn't even like his own music:



I would agree with whatever they say Wolf.
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Re: Bob Perry's take on Wolfenzee's dream boat

Smackers:
Very funny! Well done.
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Re: Bob Perry's take on Wolfenzee's dream boat

Wolfer:
Good on the draft. I think for now 5' 9" would be a good place to start. I'll put my John Adams cd's away now.

I was also thinking a partial skeg but I would not do a breakaway section, although I have done rudders like this. But we are not going to get into that degree of detail on this preliminary study.

What do think of adding the beam I was discussing?
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