Metal Multihull Cruiser - Seeking Input

Discussion in 'Multihulls' started by redou, Dec 6, 2007.

  1. redou
    Joined: Dec 2007
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    Location: Louisiana

    redou Junior Member

    Hello. I'm trying to design a metal (aluminum) 36 foot twin hulled multihull cruiser and would appreciate some help.
    Safety of metal construction.
    Can be sailed and docked singlehanded.
    Outperform monohulls under most circumstances.
    Safety of twin hulls.
    Deck space.
    Simple rig to handle.
    Low maintenance.
    Can carry a cruising load.
    Weighs 16,000 lbs loaded down.
    Shallow draft - more access

    Would the members mind sharing their thoughts on the attached simple drawings?
    Please excuse drawing quality.

    36 foot multihull, one large hull, one small hull - aluminum construction - one mast or two - meant to be a seakindly heavy multihull cruiser.

    1) hulls are both 36 ft LOA
    2) one hull is beam 7 ft. - the other hull is beam 3 ft.
    3) total beam is 18 feet
    4) large hull is 12,000 lbs loaded total weight of loaded boat is 16,000 lbs.
    5) small hull is 4000 lbs loaded - has enough buoyancy to float entire vessel should large hull be holed
    6) large hull has v bottom 45 degrees rise both sides
    7) small hull has v bottom 45 degrees OR elliptical bottom to minimize wetted area
    8) Draft large hull is 3 ft. - Draft small hull is 1. 7 ft.
    9) engine in large hull.
    10) rudders on large hull, both hulls or 2 different sized rudders?
    11) Boards to be lowered or pivoted from deck to aid in upwind and minimize leeway.
    12) mast(s) located 1/4 to 1/3 distance between hull centerlines - closer to big hull further from little hull

    My interest is in sailing characteristics of such a vessel. Would anyone care to comment on the following questions?
    a) Hard to turn or manouvre in limited space? Compared to a 36 ft. monohull heavy cruiser?
    b) upwind ability in various sea conditions? with boards down? Suggestions on the shape and location of boards?
    c) ability of vessel to 'heave to' or ride out a storm? modifications to make this possible?
    d) suitability of one or two junk (jonque) sails possibly with curved or hinged battens if masts not on centerline?
    e) speed of boat - 34 ft waterline - compared to monohull cruiser
    f) could one person dock and undock such a craft?

    The inspiration is the Ndrua, although smaller hull is not shorter. I think shorter small hull will slow boat down with shortened waterline. Boat is not shunted like the Ndrua, but tacked like a catamaran. My logic is one larger hull to carry people, equipment and stores. Smaller hull provides righting moment for sails, deck space and buoyancy in case larger hull is holed. I calculated wetted surface of both hulls combined comparable to heavy monohull cruiser.
    I am not sure about location of masts relative to different sized hulls but
    think 1/4 distance from large hull centerline to small hull centerline is justified
    by smaller hull carrying 1/4 weight. Any and all comments would be most appreciated as I do not know much about sailboats. Thank you for your time.

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  2. Alan M.
    Joined: Jan 2006
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    Alan M. Senior Member

    When you have the smaller, lighter hull to windward isn't your righting moment going to be significantly reduced compared to when you are on the other tack?
  3. redou
    Joined: Dec 2007
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    redou Junior Member

    Hello Alan M.

    Good question. I did some diagrams and calculations before I answered. It appears that if the mast is three feet off the centerline, right on the side of the larger hull, the righting forces (moments) balance out. In other words, the boat should have the same righting moment on both tacks. I figured the mast as one
    moment arm, the waterline center hull to center hull as another
    moment arm. Buoyancy going down and weight going up.
    It appears the smaller hull will immerse 3.5 inches at 20 kts wind on the opposing beam, 6 inches at 30 kts and 11 inches at 40 kts. These calculations assume still water. I'm sure the ocean will be less merciful.
    Thanks for your interest. Regards, Redou
  4. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Your earlier posting on the same topic has your answers.

    I have yet to see an efficient catamaran or proa design fitted with unnecessary ballast. Why MORE than DOUBLE its weight by using aluminium?

    I do not like the idea of your seeking information and not being able to accept some cold, hard facts from people a LOT more polite than I. Don't design and build. I doubt anyone here would like to endorse your design or competence, based on presented information. The likelihood of death by misadventure is highly likely as your design stands.
  5. nero
    Joined: Aug 2003
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    Location: Marseille, France / Illinois, US

    nero Senior Member

    you are in cypress country! This is the wood that I have strip planked my 14.5 meter catamaran hulls from. They are 18 mm thick with 2 layers of 10 oz uni glass going across the grain and 1 layer of 10 oz 0/90 biaxel glass going with the hull. They are strong and solid.

    Strip planking is ant work. It doesn't take a lot of brain power, instead it envolves repetition. It also gives a very fair hull after grinding the drips off.

    You are also close to Houston (Epon Resins are made there). And being in The Southland, you have longer seasons to work the resin with.

    Just a thought for you.
  6. JCD
    Joined: Jul 2006
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    Location: Coney

    JCD Follow the Bubbles!


    IMHO, and it should be taken with a grain of salt since I am the baby designer compared to the others here but, she is going to be heavy. Really heavy. Something like 444#'s per foot LOA. That is a lot of beef for a multi that size and width. Assuming a 36 foot LWL, you have a displacement to length ratio of 153. The sail area would have to be remarkably huge if you intend to make any kind of way.

    You should rethink the material or the length unless you intend to drop a couple of F16 engines in the bilge or plan to use her as a small ice-breaker.:D

  7. redou
    Joined: Dec 2007
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    redou Junior Member


    Hello. Something's amiss here. I proposed a boat that, FULLY LOADED for cruising weighs 16,000 pounds with a 34 foot waterline. Aluminum, one small and one large hull, one diesel engine, no bridge deck, no custom cabins, no vanity for the ladies, 800 square foot sail area. You describe it as an icebreaker.
    No offense taken. It's only a concept at this point although I have been practicing welding.
    If you get a moment, check out Richard Woods website. He is a British multihull designer and sells plans for a boat he describes as a "performance Multihull Cruiser." His "Transit" design, if I transcribed the numbers correctly, is 35.81 feet WL, 21.97 beam, 11,020 pounds Empty, 15,004 lbs "displacement to waterline", and has 819 ft squared sail area. Apologies to Mr. Wood for using his numbers.
    I wonder if Mr Woods is aware he designed an "icebreaker?"
    I'm not comparing myself to Mr. Woods. He is a well known yacht designer and I'm just a retired pipefitter. I'm only comparing some weight and square feet. I don't think you can build an 8000 pound fast multihull, add 50% to it's weight, load it with 4000 pounds of people, fuel, water and gear and expect it to act frisky. (Maybe if you have access to a lot of money and carbon fiber?) I think you can build an 11 or 12 thousand pound multihull out of affordable materials, load 4000 lbs on it, and expect it to act reasonable. But what do I know? Regards. Redou
  8. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

  9. JCD
    Joined: Jul 2006
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    JCD Follow the Bubbles!

    Hello Redou,

    My delusional vision of a multihull is "as light as possible". With that being did state that it would be "meant to be a seakindly heavy multihull cruiser". So my apologies are in order because you have set your design constraint and are in fact sticking to it. I apologize.

    She will be as you have said...seakindly, heavy, a multihull and a cruiser. I would be interested to see your design completed and am quite interested in perusing her stability and performance calculations. I'm not sure what your definition of "cruiser" is since your design eliminates several elements that in my opinion denotes a cruiser, but if it is your idea of cruiser, then it is valid to you.

    I didn't describe her as an icebreaker but instead made a reference to the possibility of her being considered to be used as a small ice-breaker or having powerful enough motors for her to get to hull speed since a vee hull below the waterline has the maximum area for any displacement, therefore maximum drag. Do not be offended with my suggestion it was meant as humor and not to offend. If it did, I apologize for that also.

    In the future, I will be more sensitive when responding to requests for input. My opinion is valid to me if to no one else and it should always be taken with a grain of salt as I have already suggested. As far as what you know...I don't know what it is that you know or don't know...only you know that and only you know whether what you know or don't know has produced anything worth knowing about.

    I have looked at Woods designs as I have looked at many other excellent designers. I have taken significant time to study them and their thinking in producing a design. I'm pretty confident that I have read published works by every single one of them. Some things I liked, some things I didn't. One will say heavy is good, the other will say heavy is for a monohull. However, everyone seems to agree that they can "agree to disagree" and they then follow their own thinking based on their own knowledge and experience and most often the specifics of the design. For example, a designer would consider a commission for a light fiberglass design to make the Northern passage very reluctantly and may not consider it unless it is in steel, yet, the Eskimos have been transiting the passage for millenia with canoes wrapped in animal hide. What can we say to either or?

    It used to be that a "multi" was ugly because it was high and square so they were made low, narrow and long. Now the trend is that "people are getting used to seeing high, boxy multis", soooo, lets overlook windage, nautical aesthetics and pointing ability because it sells. Not all designers agree and they still design low, sleek multis with good pointing ability that require a contortion act to take a piss while in a 50 foot waterline vessel. The contradictions continue.

    In any event...if opinions are sought, I will continue to offer them with my little clause of them being taken with a grain of salt even if shot down so long as they trigger a thought process. If not wanted, I will just follow your thread and it's progress while in the background hopeful that I can acquire something that may prove to be useful to my own thought process, or conversely, secretly laugh at futile effort. No harm no foul...neither here nor there. It's your show and my nickle.

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