I would like to note that all of the things in your your extended quote are secondary to length being the first order variable for ride quality. So I, who know nothing, encourage you to do that first.
The items you left out in the extended quote as being "secondary" are the most important ones in the context of this proposed project. You describe length as being the first order variable, but it is completely 100% "off the table" here.
Longer is better arguments are a total waste of time when longer is not on the table. Many boats have been and continue to be built by home builders that are in this size range and smaller.
It really is not worth continuing this unless we can set "longer" aside. We have a case of the irresistible force meets the immovable object. You are hung up on longer, and I absolutely refuse to consider longer. The ONLY useful solutions in this case are those that fit within my parameters, parameters which you refuse to accept. This is about my project, not yours. Perhaps we should discuss your project instead of mine.......... It makes no sense to interject your length into a discussion involving my project that will be built within my parameters. It's about as rewarding as having sex with a lady who continuously complains about the length of your penis..... It just ain't gonna get any longer!
I do not have a "defined criteria"..... I'm not sure how one would define it. Most interior space is just space, and a sense of space is often just as important as the space itself. There is a lot more psychology to this than science. Obviously room for stowage is very important, as is room to cook in the galley without feeling cramped, and work in the opposite work space, which will be my shop area without feeling cramped. The head and dressing room areas, likewise start to feel cramped when dimensions become too narrow or short. 1M at elbow width, widening upward, is about the minimum reasonable width sitting on the head for example. 600+ mm width for the sole in the main areas of the hull cabins, widening out as the hull widens. The knuckle on Oryx is a design feature I will copy, not because it provides significantly more real usable space except some additional counter top which is important in a galley, but because it creates a significant sense of openness and space compared to just the normal hull side. I will probably raise the sole to provide some bilge stowage, as this is where weight needs to be. The heaviest stuff needs to be as low as possible, and as central as possible. In the case of the KD 860, that means between BH3 and BH5 beneath the sole on both sides. Fuel & water, anchors and chain (when on passage), heavy tools.
Do you have a defined criteria for interior space?
When I did the math (costs), it seemed to me that a hull that added to my interior volume was too expensive for me. It was much cheaper to increase the bridge deck and make the hulls finer, longer, lighter, and farther apart.. I could still use these now skinny hulls for storage.
| In lieu of increasing length to achieve this space, which is entirely off the table for me, if not for you, the alternative would seem to be moving the center of buoyancy forward.
I think this is true for the cats you are looking at, because they have too much aft buoyancy. I don't think that it is true from a blank sheet 'properties of boats' point of view.
I disagree with you about these cats having too much buoyancy aft. That's really a value judgment, a designer seeking to work within the same design constraints I'm looking at, and achieve the best all around result. You can't have everything in a 30' cat, so as a designer you don't just say "A 30 cat is garbage, I'll build a 50' cat and have everything", you ask "How can do the best within these parameters". The commercial builders have all but abandoned catamarans in this size range, largely because there is minimal profit in them, and buyers want floating condos.
Increasing bridge deck encourages maximizing use of that area, a larger cabin, and more facilities, etc on the bridge deck. A cooking island, refrigerator, running water, more furniture, etc. More weight higher up on the boat. The opposite of a Wharram, where everything is in the hulls, and the bridge deck is a huge open expanse of just empty space.
I won't as you implied somewhere, be grafting the cabin from one boat onto the hulls of another, though I will probably increase the saloon (BH4 to BH5)length from 1.53m to over 6M, as I expect to spend considerable time there, and it also increases the space in the galley, and shop / nav area. I'll probably increase the distance from BH5 to BH6 even more, making it reasonable to have a head to starboard, and an aft single berth to port. I'll also "chop" the front as shown on Bernd's site:
, which
combined with using the knuckle from Oryx and placing ports high,
will provide a better all around view. 
from the
bridge deck cabin compared to the standard design with almost no
visibility at all. 
. I'm considering the possibility of a single free standing mast in the hull instead of on the bridge deck. I hadn't even considered this just a week ago, but a few weeks ago I hadn't considered sandwich construction, and not long before that I hadn't considered building at all. The mast in this case would be stepped just aft of BH3, more or less in it's original location, but the boat would be increased in length from 8.6 to 9M.
The big problem with lengthening the boat is that every dimension changes, and every bulkhead changes, and or moves. The displacement increases, and the distribution of that displacement needs to remain the same in relative terms. Everything is flat sheet, so the shape has to accommodate flat sheet, hence every bulkhead changing. I'm already playing with this on scale models using construction paper..... linerboard actually, about the weight of shoe box material. It is proving interesting, as there curvature about only one axis, but this combines with twist and tilt. Beautiful compound curves are actually easier to design than smooth shapes from flat sheet. It looks far simpler than it is, but I already knew that ;-(.
H.W.
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