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  #91  
Old 04-21-2013
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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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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff:
I am fine with the rig you drew. It looks good to me.

You hit on a good point. The lazy cutter sailer will drop his jib and sail full main and stays'l to avoid having to reef the main and that's not so good for balance. And dropping just the staysail with a yankee type jib is disastrous. I have tried that and with that big hole between the main and the jib it is not an efficient rig.
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Old 04-21-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
Wolf:
Ok by "camber" you mean "mast bend". Nothing wrong with mast bend so long as your mainsail luff curve has been built to match it. If not you can pull the camber out out of the mainsail and get a very flat luff.

More terminology:
By " headstay" I mean the stay that goes to the head of the mast.
I use "forestay" to describe the stay inside the headstay.
Are we on the same page now?

I like that arrangement at it gets the center of pressure forward on your smaller jibs and staysails. This helps helm pressure,
In describing my boat William Atkin referred to the "headstay" (this was also the stay the working jib or headsail was flown on)as the stay from the stemhead to 2/3 the way up the mast and the stay that went to the top of the mast as the "topmast stay".
The original design was a "knockabout" also referred to as a "stem headed sloop" (a sloop without a bowsprit) but if I fly a jib topsail or flying jib it becomes a "stem headed cutter". Knockabout is one of those old terms for things that still exist but is nolonger used (technically most sloops are knock abouts, but most people have never heard the term).
I am going to have Hasse sails measure me for sails so I can get an accurate measurement of my camber and rake for when I have a new main made down the road.

Last edited by wolfenzee; 04-21-2013 at 03:53 PM.
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Re: Bob Perry's take on Wolfenzee's dream boat

Wolf,

Thank you for the explanation of how you are using the terms. What is interesting is the way that you quote using Atkins using the terms. On a gaff rigged boat, with a fidded topmast, the headstay would be the stay that went to the top of mast, and would be roughly in the position of your forestay (as Bob and I are using the term). A topmast stay would be the removable stay which went to the top of a fidded topmast. His use of the terms Headstay and Topmast stay suggest that he also had a gaff rig in mind.

The term 'knockabout' was not a rig per se. The term referred to a boat without a bowsprit no matter what its rig. In other words, there were knockabout sloops and ketches and schooners.

In that era, adding a second jib would not make your boat a cutter. By definition, depending on the specific era, minimally the position of the mast aft of the stem made a rig a cutter. Under the older definitions, a sloop could have multiple headsails. In fact in Atkins day there could not be a 'knockabout cutter' because the very definition of a cutter included a 'housing bowsprit' (a bowsprit which could be retracted). As the definitions of sloop and cutter evolved, a cutter could include designs which have no bowsprit, but have the mast further aft, in which case they were called, "Jib headed cutters".

In the case of your boat, at the time your boat was designed, both rigs would have been called knockabout sloops. Of course in modern usage, on the upper sail plan would be called a sloop today.....

Jeff
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Last edited by Jeff_H; 04-21-2013 at 04:21 PM.
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Old 04-21-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by Jeff_H View Post
Wolf,

Thank you for the explanation of how you are using the terms. What is interesting is the way that you quote using Atkins using the terms. On a gaff rigged boat, with a fidded topmast, the headstay would be the stay that went to the top of mast, and would be roughly in the position of your forestay (as Bob and I are using the term). A topmast stay would be the removable stay which went to the top of a fidded topmast. His use of the terms Headstay and Topmast stay suggest that he also had a gaff rig in mind.
Jeff, I just wanted to duck in here and confirm that AIUI your last sentence is correct.

Many prominent yacht designers (and owners), up to pre-war at least, had gaff rig terms in mind.. it helped avoid confusion both when (a) converting existing yachts from gaff rig to bermudan and (b) when offering either rig for a new design to suit the owner's preference. In books I have, for a bermudan rig, the part of the mast above the attachment point of the runners was considered the "topmast".
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Old 04-21-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by Jeff_H View Post
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.
.....
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.
...

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.

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.
There is some confusion here: twin keels are used in many types of boats, narrow and beamy, light and heavy and the use of a twin keel has nothing to do with the use of twin rudders. Marc Lombard in most of his designs with twin keels uses a single rudder (and two rudders for a single deep keel).

Regarding the advantages, two articles very interesting mainly this one:

http://www.wrightonyachts.com/wp-con...ftwinkeels.pdf

even if I find he exaggerates in what regards pure performance. Twin keels are about a better motion comfort and a better performance with a shallow draft, specially upwind and they are not used only or even particularly suited for very beamy boats.

Bray Yacht Design and Research Ltd. - The Advantages of Twin Keels

Some very different boats using twin keels, some better designed than others but adapted to all kind of cruising boats. Most of them are not beamy boats. and very few share the concept of the RM boats (Lombard) in what regards type of hull.


































Quote:
Originally Posted by Jeff_H View Post
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.
...
You are confusing me with someone else. I never said such thing.

What I said about drag regards direct comparisons of the same boat equipped with the two keels, having the twin keel a swallow draft. The single keel has less drag and the boat is marginally faster. The difference is so small and the advantages regarding cruising are so many that RM while building a boat with a mono Keel if a client if wishes so, recommends twin keels and in fact most of their the boats are built in twin keel configuration.

Regards

Paulo
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Re: Bob Perry's take on Wolfenzee's dream boat

Wow Paulo:
I know what I am going to be dreaming about tonight.

Not sure what twin keels have to do with Wolf's boat. We have very deep water here in the PNW. Maybe this should be moved to a "thin water" thread where it is relevant.

Headstay:
The stay that goes to the head of the mast. Just because someone said it was different back in the day does not make it right. The head of the mast= the headstay. As rigs evolved I think the terminilogy needed to evolve along with it. If it did not evolve then you end up with silly terms that are not accurately descriptive of the rig element. Are they wrong? No, I don't think so but they are confusing. For me anyway.

Knockabout:
Anyone who has studied yacht design knows what a Knockabout is. But we don't use that term anymore. Pity, but in a world of masthead and frac sloops with headstays, yes,,,headstays going to the stem it hardly seems necessary.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
Wow Paulo:
I know what I am going to be dreaming about tonight.

Not sure what twin keels have to do with Wolf's boat. We have very deep water here in the PNW. Maybe this should be moved to a "thin water" thread where it is relevant.

...
My suggestion of a Twin Keel regarded this option proposed by Jeff:



That does not seem a deep draft keel to me

Regards

Paulo
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Re: Bob Perry's take on Wolfenzee's dream boat

I'm sorry Paolo, it was Brent Swain who said that a when a double keel is vertical it develops twice the lateral resistance.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by Jeff_H View Post
I'm sorry Paolo, it was Brent Swain who said that a when a double keel is vertical it develops twice the lateral resistance.
Yes, I know. Not intentional, not a problem

Regards

Paulo
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