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  #111  
Old 04-23-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Brent reprisents one side of this but I don't find any fault in his argument.
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  #112  
Old 04-23-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by Brent Swain View Post
Most of these twin keelers would be a nightmare to cruise in. An anchor rode wrapped around the trailing edges of these keels would be impossible to unfoul, especially on a lee shore in the night. I originaly had that problem until I made some modifications. With such a tiny attachement to the hull, getting it structurally adequate would be almost impossible. A hull speed imact on one would drive the trailing edge of it up into the hull.
An angle of 15 degrees only helps a lot at angles less than 15 degrees. A 25 degree angle gives you much more leeway reduction, and reduces the interaction between the two keels, of water being forced between them.
Any keel breaking the surface is extremely noisy; slams and pounds, and as soon as it starts to break the surface, it's drag increases drastically. Excessive beam drastically reduces ones untlimate stability. Boats capsizing and staying capsized was never a problem, until beam was drastically increased.
Brent, I don't pretend to know that much but some of those boats are designed by some of the best NA around. They utilize CFD to study the hulls keels and rudders and I am sure that they are like that because it is the way they work better.

Regarding the canting of the keel what is the point to have a 25º angle on a boat that sails with 15/17º of heel? it seems to me that the better efficiency would be obtained with a vertical foil in what regards sailing heel.

Regarding the keels having a small attachment you are probably imagining that each one is fixed to the hull. In the RM case (and in many other boats with skinny keels) they are both connected to the same steel structure inside the boat that distributes the efforts by the hull.

Regarding beamy hulls reducing ultimate stability I don't know what you mean. That depends basically to how lower is the CG and normally those boats have considerable drafts a god B/D and bulbed keels. That gives them a good AVS and a very good reserve stability at 90º.

Yes, when inverted they have a big inverted stability but again how easily they return to its feet depends on the CG. It is convenient not to forgot that to the big inverted stability corresponds a much bigger positive stability, much bigger than in a narrow boat. In the end, besides a good AVS and a good reserve stability what is important is the proportion between the positive and inverted stability (a much bigger positive stability).

Regarding that thing about the anchor being wrapped about the keel, those boats are popular in France, there are hundreds of them and I never heard complaints about that neither anybody referring that as a problem with that kind of keels.

Regards

Paulo
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  #113  
Old 04-24-2013
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The process continues....

The drawings which have been posted so far represent a morphing of Bill Atkin's original design into something more modern, and yet by intention would look and feel a lot like the boat that Wolf currently knows and loves.

This round of revisions adjusted the hull lines. The water line was stretched and a little more flare was added to the bow. This allowed the entry to be made a little finer. The run was made a little flatter and a small amount of beam was added at the rail and waterline, especially aft. The canoe body was made shallower and in general the overall displacement was reduced, while interior volume and surplus carrying capacity should be a little bit bigger due to the larger waterline plane.



As per the earlier post, the sail plan has been reworked as well. The proposed plan should take better advantage of the more efficient keel and rudder, should reduce the need for sail changes and reefing, should reduce the size of the sail inventory, and should be a better rig to use offshore.



Collectively these changes should produce a boat which is easier to handle, sails better in all wind ranges, is more seaworthy, is dryer and smoother in a short chop, and which is considerably faster and more weatherly than the original. It should track as well as the original, and sail more upright. The overall motion should be similar. The limit of positive stability and the area under the stability curve should be much better as well.

Speaking a general way, in my opinion all the best design projects have a specific client in mind. In this case, up to now, the changes to the original design attempt to improve the original design while remaining within what I believe to be the acceptable limits imposed by my perception of Wolf's tastes and preferences. And while we might be able to drag Wolf a little further, I am not sure that we need to at this point.

Both Bob Perry and I believe that there is still a lot that could be done to make this an even better boat than one that we have shown in this post. So we would like to go on and continue to develop the design further. But I also see this as a chance to show how designing for a different specific client changes the design brief and conseqently the design.

As a loose working model for that different client, we thought that it might make sense to use an imaginary client who is a limited production, quality distance cruising boat builder, (think of a company like Pacific Seacraft) who is updating their line. You might think of this kind of like a hypothetical updated replacement for something like the PC 34. The basis of the design brief as we continue this process is to produce a better sailing small distance cruiser for company whose clients tend to be from a conservative market.

So that is what will drive the next steps....

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

Jeff.. any reason not to carve away the 'deadwood/skeg' ahead of the rudder up to the waterline - ie follow the canoe body? Seems it might improve turning/tacking manoeuvrability a bit...what's left should still be a good base for gudgeons/pintles and result in a more effective rudder.
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff, in my limited knowledge of these things, I can understrand how a sea anchor contributes staility by adding drag, but overall the "drag" on a hull? Slows it down but I've never heard to that as adding stability of any kind. Directional stability comes from many things, drag is just a parasite that may be along for the ride AFAIK.

Very interesting array of twin keelers, very different from what they used to be. Kinda scary to see rudders being used as supports though.
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff, that boat is starting to look really sweet. Nice work.
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Re: Bob Perry's take on Wolfenzee's dream boat

The changes to the ballast & keel on this thread reminded me of a question I have never found an answer to.

When putting a bulb tip on a keel it seems to be standard practice for the bulb to flare out & down and the bottom to be flat - presenting something of a triangle in section view.

It intuitively seems to me that it would be more effective to reverse that so the keel tip/bulb interface was flat to create a definite, sharply defined end-plate and maximize the foil depth of the keel - presenting an inverted triangle in section view.

The difference in righting moment would be extremely small but it strikes me the flow characteristics would be quite a bit better that way

?????? Anybody???
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  #118  
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Re: Bob Perry's take on Wolfenzee's dream boat

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Originally Posted by hellosailor View Post
I can understrand how a sea anchor contributes stability by adding drag, but overall the "drag" on a hull? Slows it down but I've never heard to that as adding stability of any kind. Directional stability comes from many things, drag is just a parasite that may be along for the ride AFAIK.
I am not 100% sure what context was said or that is being asked within. Drag does not add to stability per se, but reducing drag can help with heel angle and weather helm in a gust. When a boat gets hit with a gust the force from that gust has to get disbursed somehow. Some of it goes into producing drive which in turn accelerates the boat, some of it produces drag trying to slow the boat down. some of it goes into heeling, some of it goes into leeway, and so on. In theory, the more force of the gust that you can use for acceleration, the less that should be available to do the less desirable things. So if there is less drag, and the boat can accelerate, it will heel less and develop less leeway, when a gust hits. This is very apparent on my boat when she has a dirty bottom.

Directional stability is generated by a collection of forces generated by the hull, the keel, and rudder passing through the water. It has traditionally been thought that a long keel in and of itself generates a lot of directional stability, but much of a long keel operates in turbulent water, and so does not have the kind of steady flows that would automatically mean that any boat with a long keel will be directionally stable.

But beyond that on a boat the size of the one in question, good balance between the rig, hull and foils will be a larger determinant of how likely the boat is to hold a course relative to the wind.

And frankly for distance sailing, a boat with a light helm which can be easily steered by a windvane is probably a better choice than a boat which will track solely on the length of its keel.

To answer Ron's (Faster) question, we kept the skeg on part because Wolf had been very concerned with tracking. I had hoped that the skeg would help with directional stability at higher boat speeds. I am not sure whether we will retain the skeg as we move into the next phase of this.

I have to admit that I drafted the bulb, and have not done that with a whole lot of scientific care. At this point its a place marker and more of a symbollic illustration.

But to answer your question to the best of my knowledge the fillet at the top of the bulb is there to reduce wetted surface and mimimize the turbulance that would occur as the water passes downward and aft over the bulb.

A bulb acts in a very different condition than a keel root especially on slower boats. This area is exposed to water sliding down the foil and colliding with water being turned aft from the free stream over the top of the bulb. Its my understanding that a square edge at that point does little to help with that condition but does add drag.

Jeff
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  #119  
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Re: Bob Perry's take on Wolfenzee's dream boat

JonB:
A bulb is never going to work well as an endplate. It just doesn't have the aspect ratio to do the job. You "mushroom" out the bottom of the hull to lower the VCG ,as you said, and to also maximize the span of the fin. If you had a flat top to the bulb I think you would create a huge tip vortex with those sharp corners.

I've ben very busy these last two weeks with my PSC 60'er project so Jeff and I have been communicating but my contribitions have been pretty abbreviated.

I want to see beam addded. I want to see a good deal of deadrise taken out of the transom and amidships. I'd like to see less of the cods head/mackeral tail look to the plan view. These changes will give us an entirely different boat.

But Jeff has been working hard not to obliterate the original character of the design. We'd like Wolf to recognize the boat when we are done mucking around with it.
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Re: Bob Perry's take on Wolfenzee's dream boat

Jeff- From #90, the context is :
"Considering, the suggestions to consider retaining either a Cutter or Ketch rig, The while both rigs have virtues in some applications, both would be inconsistent with the hull and keel as it is evolving. It is important to go back to the decision loop thought process. Both cutters and ketches are at their best in boats with lots of drag for their stability."

I read that as "drag for their stability" but I think you are meaning the rigs are better off for the stability they create in boats that have lots of drag or inertia and as such are slow to accelerate?
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