How much buoyancy is needed for catamaran hulls?

Discussion in 'Boat Design' started by dustman, Aug 14, 2023.

  1. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Senior Member

    How much buoyancy do I really need? There will be no accommodations or storage in the hulls so will have much less volume than most. My thought is about 4 times the total max displacement divided between the hulls. I'd like to minimize the volume for weight and cost savings, but maintain a safe margin.

    What are the factors?

    -Heeling from sail forces.

    -Sailing at speed into/over/through waves.

    -Vertical accelerations.

    Hull form, fineness, resistance to plunging related to hull shape. (Hull shape will be an approximation of semi-circular except towards the ends, high length/displacement.)

    What other considerations/insights would you add?

    The only real examples I have for comparison are small racing catamarans. I will have much lower sail area/displacement than these, will probably never see more than 10kts in moderate conditions.
     
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    "will probably never see more than 10kts in moderate conditions + I will have much lower sail area" is not equal to "Sailing at speed into/over/through waves."

    You need to clarify the SOR.

    I would suggest that the MINIMUM displacement for EACH hull, is (Total Crew Weight + Boat Weight) X 2.5

    Actual shape sounds non-critical, the way you have described it. Hardly worth detailed analysis.
     
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  3. dustman
    Joined: Jun 2019
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    dustman Senior Member

    Not sure exactly what you are getting at. Moderate conditions mean to me 15-20mph wind, 7-8' seas, my estimated boat speed would be around 10mph with full sail in over 15mph wind(yes, I know, that depends on a lot of things). That seems too fast to me for this boat. I plan to reef in anything over 15mph sustained wind.

    SOR is getting me from Biscayne Bay, FL to and around the Bahamas from early March to mid June, without dying.

    Shallow draft, 30" or less, breakaway keels and rudders, beaching ability.

    Minimum distance in a 12hr period of 80mi in 10mph wind from 45 degrees to 180 degrees, 6.6mph average.

    Reliable internet access for weather updates and maybe some work.

    30+mile range/day at 5mph on electric propulsion(800w solar, 5kw batteries), min 7mph max speed.

    Minimal accommodations.

    Reliable solar still(min 1/2gal day, 10ft2 collector).

    Rainwater collection(min average at .5 in rainfall(June) per 30 day period of 15gal, requiring a minimum of 50ft2 collection area, approx 90ft2 planned)

    15gal freshwater storage.

    60 days dry food storage.

    Small dinghy.

    Thanks for that input. Are you willing to share with me how you arrived at this number? What factors that come to mind? What comparisons you are making?

    I figure that long slender hulls will be more susceptible to submergence, thus may require a bit more reserve buoyancy. On the other hand it seems that they may be less susceptible to being held under, though I have no real evidence to back that up. They will be shaped such that they won't "scoop" water like some others might, and any solid decking will be well back of the bows. A vee hull would seem to tend to submerge further than a round hull, when falling off a wave crest for instance. I notice that wharrams plunge pretty good, but are saved by a large amount of buoyancy.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I think that you are making your life complicated and that the "problem" has a very simple solution: you must have exactly the same total buoyancy, counting both hulls, as the weight of the boat in the maximum loaded condition.
    With that and the shapes of the hulls you will calculate the draft. If what you need now is to calculate the depth: calculate the summer freeboard and add it to the draft. (I must advise that the minimum summer freeboard is also not easy to calculate)
     
    Last edited: Aug 14, 2023
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  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    @TANSL

    only for sake of conversation; would you not design for typical loaded condition and then accept some immersion for maximum loading?

    I am asking; not telling.
     
  6. TANSL
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    TANSL Senior Member

    It is necessary to study all possible load conditions and select the worst of all (which does not have to be the maximum load). These calculations are also greatly influenced by the flooding point or, in smaller ships, the condition in which, with the passage to a band, the deck is introduced under water. So many factors are involved, for example, free surfaces in the tanks, that the maximum load is not always the most compromised condition. Excessive trim, in a condition with little load and many free surfaces, can cause flooding through a point, or a hatch, which in another condition would be totally out of danger. Many, if not most, of these "special" loading conditions are solved by increasing the freeboard. In what amount? : What is necessary so that the water does not enter through the deck.
     
  7. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    So now you're going to the Bahamas ??
    Now you're facing 7-8' seas , and only expecting 15-20 knot winds ?
    AND you are needing 60 day stores, solar and batteries.

    You keep upping the ante on every post :)

    Look, save yourself a lot of unnecessary grief, and get a set of existing properly designed plans.
    You are NOT going to create anything better than a dozen existing plans, you can get for a few hundred dollars.
     
  8. dustman
    Joined: Jun 2019
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    dustman Senior Member

    That's been my primary destination for a loooong time, you must be reading old posts.

    I'm expecting to never be on passage in conditions worse than that in that area. I've been tracking conditions there for 2 years.

    That's just 60 days dry storage, not all the food I'm going to eat(saves a lot of money). I've been planning the electric propulsion for ages. You asked for SOR, so that's what I gave you:p

    Yes, I keep adapting my design to new information and understanding, if that's what you mean.

    It may not be "better", but it will be much cheaper, suited to my specific requirements, faster, require less maintenance, and easy to handle by myself. Actually, yes, it will be better, at least for someone who doesn't mind roughing it in terms of accommodations.

    You are wasting your breath, I am going to design it and build it, that's as important to me as the journey itself. You can withhold judgement on the outcome til it's built and sailed, then tell me if it's better or not. And you can help me achieve the best outcome, or not, your choice.

    Would you be so kind as to clarify your conclusions here?
     
  9. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I can't imagine what's simpler than asking people with design experience what they would recommend as a minimum total hull volume. I already know what my draft is going to be with the chosen hull design and displacement, I just need to know how much more volume I need to add to avoid submarining my hulls.
     
  10. messabout
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    messabout Senior Member

    Dustman, you are located in Arizona? How much experience do you have with blue and green water cruising? How familiar are you with the Bahamas, the Gulf Stream, the tongue of the ocean, the flats with all those coral heads, and the colossal sharks that do their nightly cruise at Nassau marinas and elsewhere?
     
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  11. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I have limited experience, but I am WAY better prepared than most of the people who start out there. I will have been sailing a month on this boat before I cross the gulf stream. I've watched thousands of sailing videos, done hundreds of hours of research, read several books on heavy weather sailing, read many accounts of dire sailing experiences, asked many questions on all the forums, have a lifetime of experience in nature, have scoured every inch of the bahamas on google earth, have looked at the weather conditions in the bahamas almost every day for more than 2 years, have a good practical knowledge of physics, and you could hardly ask for a better person to have around in an emergency. The bahamas is a benign place to sail comparatively. As far as the construction of the boat I have been building and fixing things pretty much since I could walk, and have made a living off it for over 15 years. On top of all that, I am risk averse and am not going to put myself in stupid situations. I will have real time weather forecasts at my fingertips. I will be a LOT safer there than driving to work every day in the city. Sharks are statistically not dangerous AT ALL.
     
  12. bajansailor
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    bajansailor Marine Surveyor

    @dustman, I know you want to design your own boat, but pretty much every boat design is an evolution of a previous design, in a never ending quest to try to find the 'perfect' design to meet the SOR (but we all know that this realistically does not exist).
    I am sure that there is a design out there already that is very similar to what you have in mind - keep googling, and you are bound to find it. Buy a set of plans (or even just a study plan), and it will help you enormously - you will then have a basis design to work with which you can then tweak as much as you want.
     
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  13. dustman
    Joined: Jun 2019
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    dustman Senior Member

    I understand that the result will not be perfect. I started out wanting to build a boat to sail around the world, and quickly realized that I should build a smaller version first and work out the bugs in a safer environment.

    I bought the study plans for a wharram tiki 38 almost a year ago. There really is nothing out there quite like what I'm envisioning, a small wharram is the closest thing, and the closest thing to my philosophy of what would make a good and safe boat. I'm not exaggerating when I say I have looked at a great many boats.

    What I really need is straightforward answers to my questions, no offense intended.
     
  14. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    It depends.
     
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  15. dustman
    Joined: Jun 2019
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    Location: Tucson, AZ

    dustman Senior Member

    If there is one thing I've learned on this forum, it is that.
     
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