Modular Cruising Catamaran

Discussion in 'Multihulls' started by ImaginaryNumber, Jul 19, 2009.

  1. apex1

    apex1 Guest

    May I remind you peers, that this:

    was the question!

    The only possible answer (apart from opinions and telltales) is YES, performance could be improved!
    NO, buoyancy is not decreased (if......see above).

    So, what are we discussing here? The fivehundredseventytwo steps of a design spiral?

    Thank you Mr. Ostlind for taking care of my character..........

    But I feel nicer when people like you just ignore me! Your premature attacks do´nt really contribute to topic, they just show you have´nt understood how to discuss different points of view. And I do´nt feel responsible to provide any info about other members of the forum, nor do i vouch for any.

  2. Ad Hoc
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    Ad Hoc Naval Architect


    Nice to see someone else can understand and follow the topic/thrust of the thread, ie Richard's comments on narrow hulls. Everything else is well....unrelated.
  3. Richard Woods
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    Richard Woods Woods Designs

    Mainly to Ad Hoc

    I think you can agree that yacht designers design yachts - period (as Americans like to say). And that anyone can call themselves a yacht designer. Whereas a naval architect may be a stability expert or a propulsion specialist and not necessarily design vessels at all.

    Almost certainly a naval architect will have had some formal training, but a yacht designer may not have done so.

    Which leaves those of us who have yacht design qualifications in a dilemma. Do we call ourselves "naval architects", like Ed Dubois does, or yacht designers like "Bill Dixon" does? Both these highly regarded people did the same Yacht and Boat Design course as I did.

    I suppose you could say a naval architect is akin to a scientist, whereas a yacht designer is like an engineer.

    It is a very poor scientist who has a theory that he is not prepared to test in practice. And an even worse one who doesn't modify his theory when practice disagrees with theory.

    I use my boats as floating experiments. So my Gwahir and Wizard each had two rigs, my Striders a total of five different rigs, I have had three rigs on my Merlin in 4 years. The basic hulls don't change, so any speed differences are the result of changes in the rig.

    And although I could have just checked gps speeds when sailing the boat in isolation, I didn't. Instead I would race it against other similar multihulls (up to 50 other multihulls at times). So I could compare, say, a Strider's performance in certain winds against a Gwahir while using, say, a F24 as a benchmark.

    When I say a Merlin is "much quicker" I don't mean by 1 or 2 knots, I mean by 0.1 knot. For those who race know that differences that small are major changes in speed.

    A few years ago I raced an F31 against several sisterships. In the first race we kept the furled screecher up to windward. In the second race we took it down. We were "much" faster in the second race. Meaning that over a five mile beat, say 40 minutes, we were 100m nearer the competition.

    That is what racers call "much" faster. No test tank or computer theory can predict that sort of speed difference.

    I know that if it is a windy day I am more likely to win if I sail a Merlin than if I sail a Strider. Conversely, I also know, because I have done it, that a Strider can beat a fleet of Firebirds boat for boat in light winds, but be easily beaten by all of them in a blow.

    Sorry this is way way off topic, and I won't comment again like this, but I want you to understand that I don't just make glib statements but rather back up considered opinions with proven full size data.

    Richard Woods of Woods Designs
  4. Ad Hoc
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    Ad Hoc Naval Architect

    "..but I want you to understand that I don't just make glib statements.."

    I'm not suggesting you did, plenty of hot air by others on this thread have done enough of that. I was just, as Richard/Apex was also doing, investigating the statement about narrow hulls.

    As for yacht designers...well, this is all very subjective.

    You're correct that yacht designers can be called that simply because that is their "field" (it is ostensibly a branch of naval architecture). But is is also true that anyone can call themselves a naval architect (certainly in the UK), without any formal qualifications. (I've just had a very interesting debate on this very subject with T.Blakley at RINA).

    My best mate who is a yacht designer, whom you may probably know and maybe very well too, says i design straight lines (Im a NA), whereas i counter that he designs plastic bath tubs (He's a YD)..we've been ribbing ecah other about this for over 20 years...

    A Naval Architect (a fully qualified & trained one I'm speaking about) can indeed be a fully fledged 'designer' (of boats/ships etc) or, as i know some who worked at VT, just designed ER brackets all day!!! Again, conversely a yacht designer may not have any formal training and simply "ohh, this is nice shape or idea", but does not know how to quantitatively "design it" to a professional standard and ensure its safety. (They often sub contract this out). So it works both ways.

    Whatever we "design" requires training and education. But more importantly, recognising the implications, professionally, what we are doing and being able to support claims of the design, in one form or another and to make sure whatever is 'floating' is safe.

    Again, slight digression too...sorry!
  5. catsketcher
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    catsketcher Senior Member

    Ah but what is performance!

    Gday guys

    I should have made it clearer earlier about performance. I think this is a very nebulous concept and this is why I don't like super narrow hulls. A cruising boat must perform in may situations. It must tack, resist leeway, manouvre, resist pitching, carry variable loads, stay dry on deck, have people walk around without falling off etc etc. My high performance cat will not have one number - high speed - designating high performance.

    My concept is a bit off key with "high performance cars" and the like where numbers such as top speed and horsepower designate performance. I probably should have a new phrase such as highly evolved or the like. This would at least try to convey that there are a myriad of competing issues involved in the design spiral.

    A racer has only one object - to win. Performance can be easily determined by these easy and objective criteria. A high performance cruiser - whoa - that could be anything - it could be a really easy steerer, super comfortable in a seaway or a whole combination of facets.

    The problem is that there are few numbers to help the buyer out here. Tacking ability, crew comfort, dryness, stiffness, noisiness, are more important to me that an extra 0.2 of a knot yet they have no numbers to show that my design is any better or worse so they fall out of the lexicon. We hear the same thing with economics - economists measure what they can and we try to show that this makes us happier or not. It is sometimes very hard to get numbers on the most important factors in a situation.

    So I think we will agree to stand by our original points and I will muse on the inadequacies of models in describing the way our boats handle out at sea.


  6. masalai
    Joined: Oct 2007
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    masalai masalai

    Build site costs are the least expense - have a look around, there must be a block of land where you could rent for a year, and on-sell the shed to the block owner or work out some arrangement with the owner?

    I am renting an open ended shed, about 55ft x 32ft or something near that, with a very carefully levelled concrete floor, for AU$3800 for the year, including electricity, and the use of a "trailer" to deliver the boat about 3000 metres to the water... I drive daily 20km to the shed (not on Saturday or Sunday except for a strictly social visit - - no work on weekends as one needs a break)... See my thread on the build in the link below... the cat is 21ft x about 40ft...
  7. apex1

    apex1 Guest

    Thank you Mas for coming back to topic. After throwing all the necessary compliments around, could we discuss the possibilities of building (and connecting) the bridge deck accommodation in segments? Any thoughts?
  8. marshmat
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    marshmat Senior Member

    Segmented bridge deck: Yes, it ought to be feasible.

    An example: Chris White's "Explorer 44" trimaran. Each individual component (three hulls, plus fore and aft crossbeams) can be built independently and easily trucked to a final assembly site. At the final assembly site, the beams are bonded to the hulls and she's ready to launch.

    Another example: Any of James Wharram's larger cats. These do not have a full bridgedeck, though- they have lashed crossbeams and, in some boats, cuddy-like deck pods that are structurally independent.

    I can't think of any full-bridgedeck cat, though, that is demountable for piece-wise assembly. That's not to say it couldn't be done; I've done a lot of sketches myself trying to figure out how it might work. If you're OK with a "stack" of small bridgedeck segments, they become much easier to fit in a low-windage profile, much easier to fit on a truck, and I think less structurally complex as well. Perhaps a forward segment containing berths, a midships segment with dinette and nav/watch station, and a cockpit segment (its location in the "stack" depending on whether you want the cockpit ahead of or behind the deck house). At 8' thick each, three such segments, permanently joined together at the final assembly site, could form a bridgedeck comprising perhaps two-thirds of the boat's overall length.

    I do think that at 35 feet, as the original poster suggests, it will be very difficult to come up with a full bridgedeck design that has sufficient bridgedeck clearance (a lot of designers seem to suggest about 3' minimum clearance for an ocean-going cruiser, while charter barges sometimes have as little as 1') while maintaining standing headroom. If you do achieve both, the likely result will be a very tall, high-windage profile, the cabin top being 10' or so above the water. It can be done, but I've never seen it done gracefully in anything 35' or less. How essential is it that we have an enormous area of standing headroom, especially in spaces (dinette, nav/watch) where we'd normally be sitting down?
  9. masalai
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    masalai masalai

    ImaginaryNumber, It is not clear what your intention is as regards build in 12 ft wide components? is this a ONCE ONLY option required to transport cat to the launch site - - or - - is it to be a regular collapsible re-transport event... To my way of thinking, the latter option would be a right pain in the arse... in the former option, see my previous post build nearer to the launch site, as I am doing... 12 months build and anticipated launch cost around AU$300 ~ 350K... I hope:D:D:D:D
  10. apex1

    apex1 Guest

    As far as I understood, the problem was to build at home and complete at shore.
    If that is going to be a collapsable construct I would´nt like to be involved.
    But lets make some sort of "book of requirements".
    How much space is required at bridge deck level?
    Can that living space be cleverly arranged to be divided into three sections?
    What about the mimic to merry hulls and bridge? To join the segments should´nt be a problem.
    When we have a rough idea how big this deck structure will come out, we can (endless) discuss the hull size and shape.

  11. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Richard Woods:
    No need to apologize for being doubtful about my ideas. The purpose of my posting on this forum is to learn from those with far more experience than mine. At some point I will have to hire a designer/architect/engineer to convert an idea into buildable blueprints. Right now I gratefully relying on this forum’s expertise to see whether the concept is even worth pursuing.
    English translation:
    Here is a German website for a houseboat/catamaran which has nearly a full-width bridgedeck similar to what I am proposing. If you don’t read German (like me), Google provides a fair text translation. This is NOT the catamaran that I envision, but it does show the possibilities of having a single-level, full-width bridgedeck. To get from the FlatCat to my modular cat I would increase the clearance of the deck, replace the hulls with something sea-worthy, and make the superstructure shorter and more aerodynamic. The other big design difference is breaking up the cabin into three sections, with the two connecting joints running transversely, not longitudinally.

    At the risk of hijacking my own thread, what are the pros and cons of an A-Frame type mast?

    Richard Apex1:
    I understand that the total buoyancy can be the same as for a shorter and fatter hull, but it is the distribution of the buoyancy that I am wondering about. It seems to me that in certain wave patterns a long skinny hull might “dig in” in a wave trough, and that it might also “high center” at the wave crest, causing deck slap. A shorter and fatter hull might be less apt to have these type of problems, but then pitch-poling could be an issue. So I’m trying to get a feel what relative length to breadth ratio a catamaran should have to counter the higher bridgedeck that I am proposing?

    I see that both Richard Woods and Catsketcher are agreeing that 10:1 – 12:1 is a good proportion for a cruising cat. Thanks for the input.

    Richard Apex1:
    Thank you, though I have also learned from some of the off-topic comments too!

    Ad Hoc, if the specifications aren’t too ambiguous, could you suggest ways to solve the cabin segment to cabin segment connection problem, and also the bridgedeck to hull connection? Let’s say that the catamaran is 20’ wide, so each segment is 20’ wide and 8’ long. The three segments then have a combined length of 24’. Ideally (to me) the assembly process could be reversed. That would imply that everything was bolted or lashed (or ??) together. This is not a firm requirement, but it might be attractive to owners (me) when needing to haul-out when they don’t have ready access to travel lifts wide enough for a 20’-wide vessel. Lifting lugs might be incorporated into each segment that would allow it to be lifted on and off the hulls with a crane.

    Matt Marsh:
    Yes, the windage has been a concern from the start. I don’t care about grace, but I do care about stability and controllability, and I’d like to keep the boat as small as is prudently possible. Standing room throughout the whole bridgedeck isn’t required. But since I plan to cruise for extended times in cooler climates I want enough standing room to help fight off claustrophobia when going outside isn’t inviting.

    Thanks again for all the thoughtful suggestions,
  12. rayaldridge
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    rayaldridge Senior Member

    I'm not any kind of pro, let alone a real one, but he's not known to me as anything but a guy who for some reason won't post under his own name. If Richard Woods isn't a "real pro" I don't know who is, and Richard doesn't seem to know who he is either. I have little patience with this sort of pointless coquetry, but maybe I'm just a grump. In any case, and this may be unfair, I can't bring myself to take seriously the statements of any person who isn't willing to stand by them under his own name and reputation. After all, this is not politics, and no one here is going to jail for saying the wrong thing.

    As to the topic, it should be relatively easy to test the contention that a long thin hull will have less resistance under all conditions than a shorter fatter one of the same displacement. Freeship has a useful slender hull resistance module, contributed, as I understand it, by John Winters. If I get a chance this evening, I'll run a set of graphs. My expectation is that the fatter one will show less total resistance at low speeds, but I could be wrong.

    The phrase "speeds where a cat has it´s advantages" seems a bit nonsensical to me. A cruising cat has to sail in all conditions, not just those where it has an "advantage." Light air is a fact of life. If the boat doesn't sail well in light air, evil consequences will ensue, such as overuse of engines.
  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    "...I can't bring myself to take seriously the statements of any person who isn't willing to stand by them under his own name and reputation..."

    Well, that is the point that you and all those who are unable to comprehend. The post is not about me, nor who I am, nor anyone else, it is about the engineering/naval architectural facts to explore a concept.

    The posts I make can easily be supported by endless text books, reference papers etc etc. I select not to go through the long process of detailing every little bit then finding out the person does not grasp some basic engineering concepts. So, to short circuit the debate, i present an abridged version, for clarity.

    If your only way of learning or debating is by perosnally knowing who your debating with, rather than the topic at hand and the points/issued raised, your views shall always be subjective. Hence whenever you debate with a person, your views are biased towards whether you feel superior or inferior to the person, because you "know who they are", rather than can the technical claims be supported.

    If any of my techincal statements i have made are incorrect or you feel require further explanation, please state so and we can debate. But if your only form of debate is a personal debate of what size my inside leg is, or how tall I am, or can i form big words or what type of tea i like (since this is what a personal debate is about, rather than a technical one) just demonstrates a lack of cognitive critiquing and comprehension. As such, there is no point reading your comments that are biased and subjective and have no value to the topic being debated.
  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    As for your "concept". The engineering side can be done, it is not rocket since. But in order to achieve your objective, many compromises shall ahve to be made. During the life cycle of the design when presented which some of the compromises you shall be faced with, it shall slowly deviate from your original concept. In the end you may have to make so many comporises to satisfy your SOR, that you're not happy. However, you may find the some of the compromises are "I can live with that".

    The first hurdle you have is two fold, hydrodynamically and structurally.

    Addressing the structural first, may prove to be the wiser route. Since to have 3 modules, requires some serious connections and load paths to ensure the modules do not break free when out sailing.

    So, the best way, is to sketch out the 3 modulus, and work out roughly what size for each, which you ahve roughly done, BUT, then see what you can reasonably fit in each and are you happy with it.

    Once You have done that, then the task of establishing what the structural load cases are and then what structure is required to satisfy the load cases. After the first design spiral, you may find too much structure is required and compromises too much weight or space or both.

  15. apex1

    apex1 Guest

    You may save your time testing well known properties of hull forms. And number crunching without the clue of what we are talking in terms of weight distribution is´nt worth the effort.
    Every boat has to sail in (almost) all conditions, who does´nt know that? But when we talk cats, we auromatically focus on the upper end of the speed range. If not, there is little sense to build a cat with the specifications given by so far. (actually I do´nt see any, because ALL the requirements known by now, are better and easier to achieve in a mono, but thats builders choice)
    Well I have NO patience with this sort of comments! Far more than half of the forum members do´nt show their name! Who cares?

    There was´nt too much content worth to be learned.

    Thats no task, as long as you forget the idea of having the boat collapsable!
    I doubt you find one professional (to use the term again), willing to make a design of a collapsable Cat in the 40´ range. The possibility to kill his reputation is not worth the pennies on his account! Think about that again, for your own sake.
    The task are the beam / hull joints (even when permanent).

    So forget about the properties af hulls and decks and make a sketch of what you think is needed (forget to divide the structure to begin with), and lets start to make a SOR* or BOR*.

    *Statement of Requirement: common in the commercial scene (shipbuilding)
    *Book of Requirement: common in the Megayachtscene

    ***NIR: common in the yacht scene....................................No Idea about Requirement

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