Modular Cruising Catamaran

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

  1. ImaginaryNumber
    Joined: May 2009
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    ImaginaryNumber Imaginary Member

    I share your reluctance to design a "collapsible" boat. ;) Maybe the word "demountable" would be less evocative. Designing this catamaran to be demountable is lower in my priority list, and I'd be happy to have this group's suggestions pertaining just to a permanent attachment mechanism. But I'm not clear why the idea is anathema. Many catamarans are held together just with deck beams, including some rather large cats. So it seemed to me that these three proposed segments could be thought of, and designed as, very large, strong, and rigid deck beams.


    A most helpful suggestion.

    Can you suggest an on-line template that I could use for the SOR? That might save us all the frustration of my generating unnecessary or unhelpful specs.

    Allow me to dispel any misconceptions: this isn't intended to be a BOR-type of project (at least for me)! :)

    But I am grateful for those of you who are helping minimize my NIRs. :D

    John
     
  2. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Start by identifying where you desire to cruise, for how long each "voyage", independent operation or regular calls to a marina for sleeps/shower restaurants etc., Is the region prone to sudden storms, are there lots of "safe" creeks etc where you may find shelter...

    How many will sail with you? (are you running a hotel, or is this your "getaway" for peace & quiet with your partner, where you both will explore the region (gunk-holing, I have heard it called)...

    Can you do without 'toys', can you adopt to a simple lifestyle... the technical stuff comes later...
     
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  3. apex1

    apex1 Guest

    There is no difference, just forget it!

    Of course, nothing else was my intention!

    Unfortunately not, every design is unique.

    Well that was a misconception! Yours will be a BOR not SOR! Ships have a SOR of some several hundred to several thousand pages. Yachts are fine with BOR of some dozen.

    What I wanted to express was, do some legwork and draw a rough arrangement of your accommodation. There are hundreds of drawings and pictures on the net in exactly the size that might fit your needs.
    When that is done, we can discuss the pro´s and con´s of your design and how it might be refined to provide one or the other advantage in one or the other situation.
    But do´nt expect a drawing (sketches) from one of the pro´s here! When you show some serious effort done by yourself, you may be impressed how much additional work others are willing to give for free. But first your "legwork" please.

    Regards
    Richard
     
  4. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Well, for starters, let me address the point made by several folks about The Requirements (whatever acronym you choose):

    You need some!

    And they need to be clear! And realistic!

    And you need to understand your priorities- which requirements are firm, and which would be the first to go if it turns out you can't, in the end, afford the moon?

    (And, if I may borrow some advice AdHoc gave me, write it all down! You may know, in your head, exactly what you need down to the finest detail. But no collaborators, designers, consultants or builders will have a clue what you're trying to do unless you write it down and talk it over. "No written SOR" only works if you are the designer, the engineer, the builder, the captain, and the only crewman. If anyone else is involved, the requirements have to be written out and discussed!)

    So grab your notebook (or blackberry, or openoffice, or whatever) and start writing. There's no set format for a statement of requirements. You just write down how you want to use the boat, what features and characteristics you need, how you want it to perform, etc. Sometimes it's a paragraph, sometimes it's the size of a quantum physics textbook.

    On to the boat: Yes, a big cat can be made to fit on a truck: http://cat2fold.com/
    How do you do it? Brilliant engineers, plenty of cash, and plenty of compromises.

    In any case, you can't disassemble a large multi-part cat in the water; no matter how you design it, the individual components still need to be hoisted, supported and stabilized somehow while it's being put together. Cat2Fold has a lot of complex engineering built in to stabilize everything as it expands and contracts. Most cats will need a crane of some kind to help.

    If you really have to build inland, it's certainly possible to design for building in pieces that'll fit on trucks and stick it all together at the marina. But I wouldn't count on doing this any more than once in the boat's lifetime. Masalai's shop near the water is a dream to many of us....
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    I think everything regarding "requirements" and "a way forward" has been said :)

    Excpt that so many people come onto this website with "I have an idea, what about.."...great we all have ideas. I would like to press a button and be teleported to my holiday destination, not waiting around at airports, sitting in a metal tube next to screaming kids etc. BUT, and it is a real BIG BUT, just having an idea and discussing the "idea" will get you nowhere and nowhere real fast.

    You only have to read the contributions on this thread (and others very similar) by those who have no concept and cannot grasp this very simple fact.

    Write it down and make it real, to paraphrase above. Once it is written down and sketched up, you'll be amazed what does or does not become clear very quickly. And half the questions you can asnwer yourself!

    PS..nice to see the children dishing out negative points again...please don't stop. It is nice to know how immature and childish users on the website are, rather than having an enquirying and open mind, still its childs play, pressing a button, same on a computer program which one hasn't clue what it does :)
     
    Last edited: Jul 26, 2009
  6. rayaldridge
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    rayaldridge Senior Member

    No no no. You don't get to move the goalposts, just because you've scented danger. Ad Hoc stated baldly that "No true. The length displacement ratio is significantly better for long thin hulls compared to short fat ones and hence the wavemaking resistance is lower. This more than offsets the increase in frictional resistance at low speeds too."

    I ran the numbers on two hulls of identical concept, and they indicate that Ad Hoc has no idea what he's talking about.


    The two hulls I ran started with a 20 foot dory-like hull that I've been working with for my last two designs. The hull displaces 12.072 cubic ft., has wetted surface of 39.4 sq. ft, and a waterline beam to length ratio of 10.4 to 1. I stretched and narrowed this hull to 25 feet. keeping displacement constant. The skinnier hull displaces 12.054 cubic feet, has wetted surface of 45 sq. ft, and a length/beam ratio of 17.9 to 1.

    You don't even have to run both hulls through the KAPER module to see that Ad Hoc is completely wrong. I believe we picked 4 knots as the low end of the low speed spectrum. At 4 knots, skin friction on the 20 foot hull amounts to 5.374 lbs. Resistance due to wavemaking is .666 pounds. In other words, at this speed skin friction is almost 9 times as important as wavemaking resistance.

    But wait, Let's look at the skinny hull to see if, as Ad Hoc asserts, the wavemaking resistance declines sufficiently to offset the greater skin friction brought in by the greater wetted surface of the long skinny hull. The 25 foot hull at 4 knots has 5.933 lbs. of resistance from skin friction, more, as you would expect. Does the decline in wavemaking resistance make up for it? Well it declines by half, to .312 lbs.

    Unfortunately, at 4 knots, total resistance for the long skinny hull is 6.245. For the shorter, fatter hull, at 4 knots it's 6.040. It would appear that Richard Woods is probably correct in his assessment of the light-air performance of his own boats. Isn't that surprising?

    The range for this exercise in number crunching was chosen to reflect the size of the hulls involved-- in light air, boats of this size may be happy to do 4 knots. For speeds under 4 knots, the disparity grows-- the shorter fatter hulls will be increasingly better at ghosting. For speeds over 4 knots the long skinny hull soon catches up. For a bigger boat, these relative values would pertain to substantially higher speeds.
     
  7. Guest625101138

    Guest625101138 Previous Member

    Ray
    You are correct. There is an optimum length for the lowest drag hull for a given speed. There is a point where going longer will increase drag for exactly the reason you point out.

    If you go back to post #22 I gave the basic dimensions for the lowest drag cat hulls with ship displacement of 800kg and speed of 4kts. It was not a wet finger in the air exercise. I analysed 17,284 hulls to arrive at that specific hull. Took almost 5 minutes.

    Rick W
     
  8. marshmat
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    marshmat Senior Member

    Valid points, Rick and Ray.

    There are several caveats that must be noted, though:
    - KAPER is meant for canoes and kayaks; its estimates are not particularly accurate for any hulls that deviate from normal double-ender canoe/kayak shapes
    - No resistance code can predict the drag on a hull to four significant digits. Most of the common methods will have the uncertainty estimate applied to the second significant digit.
    - A set of estimates based on a numerical model of one particular hull at one speed does not necessarily reflect the properties of a different hull shape
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    RayD

    Ok, so to take this further, where did those numbers come from
    1) Published data on said hulls
    2) Tank testing you ahve or just did
    3) Computer program?
     
  10. rayaldridge
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    rayaldridge Senior Member

    John Winter's KAPER module as implemented under Freeship.

    Now it's your turn to support your assertion with actual data from any of those categories. I'm afraid you'll have to be specific.

    Marshmat: your caveats are correct. The difference was 2 tenths of a pound, total resistance, so I feel fairly confident that it isn't just noise. That amounts to a 4 percent difference in drag-- significant to a racer, I suppose. At speeds under 4 knots, the difference grew. Every knowledgeable person with whom I've discussed this matter has confirmed that at low speeds, skin friction is far more important than wavemaking resistance. Which is why I found Ad Hoc's statement to be so strange. While it's true that different hulls will show different resistance characteristics, these were two hulls as alike in concept as I could make them, except for waterline length/beam ratio.

    The point is that for any hull at low speeds, skin friction will be a much more important factor than wavemaking resistance.

    As Rick pointed out, for any given speed, there will be an optimal length/beam ratio (optimal in the sense that resistance will be lowest.) Ocean greyhounds designed to be operated at high speed will have very fine hulls, since wavemaking resistance will be their main concern. A cruising boat that will have to deal with frequent light air will do better with fatter hulls, and besides, this is helpful in many other aspects of a cruising design.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    RayD,

    so, what are the caveats, or limitations of this KAPER program?
     
  12. rayaldridge
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    rayaldridge Senior Member

    No, no. No more obfuscation. It's your turn to support your assertion. After all, you made it. Can you support it?

    In case you've forgotten:

     
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  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    RayD

    I was giving you the opportunity to explore and understand naval architecture and hydrodynamics. But clearly you wish to remain ignorant in the subject and be a "I have a computer program, you can't tell me matey" type, of which proliferate on this website more than Spanish flu is currently doing.

    So, just to scratch the surface (that's all since any more would be pointless that is clear), since you clearly have no desire nor understanding of anything other than superficial, the manual says thus:

    "15.5.2 KAPER.
    The KAPER resistance method is intended for canoes and kayaks. It was originally developed by John Winters, a naval architect now specializing in designing canoes and kayaks...It is based on statistical data obtained by model tests...
    The range of valid parameters is:
    ● Prismatic coefficient 0.48-0.64
    ● Submerged transom ratio 0.0-0.04
    ● None of the other variables other than the waterline entrance angle me be zero."


    So, without even getting in to aspects which are clearly beyond you as you are not a naval architect (nor choose to broaden your mind), for those who can read and comprehend, the program has extreme limitations.

    In other words, if you have a hull outside the said parameters, the result can not be guaranteed, as they are beyond the limits.

    But if that is how you wish to educate yourself on hydrodynamics using a very limited program and one "designed" for just canoes and kayaks, ...way to go matey...enjoy your limitations :)
     
  14. Guest625101138

    Guest625101138 Previous Member

    Mat
    It does not matter what type of hull as long as you are comparing two hulls of the same type and using the same validated method of comparison. You will always find there is a finite length that will give the lowest drag for a finite speed.

    Consider two slender hulls. One 50m long and the other 100m long. Identical draft of 1m, same displacement and same block coefficient. The beam of the long one is 0.5m and the beam of the short one 1m. Both have two square chines. One will have a wetted area approaching 150sg.m while the other will have a wetted area approaching 250sq.m. Bow and stern are not bluff but designed for streamline flow - like a canoe.

    Now these two hulls are being towed through the water at 1 knot in calm water. Can you tell me which one will have the lower drag? (You are an engineer bound by the principles of physics. There is no magic Navil Architek wand that you can wave.)

    Rick W
     

  15. masalai
    Joined: Oct 2007
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    masalai masalai

    Then on those same hulls, which one will achieve the highest displacement velocity (before the power required starts to climb exponentially, the aft squats if pushed by a propeller at the stern or the hull tries to climb out and on top of the water, but at the point of any of those events commence occuring), then how much will the other hull require to achieve the same velocity?

    Just looking at things from a different perspective and genuinely curious as to the answers... Thanks
     
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