Catamaran hull question

Discussion in 'Boat Design' started by subzero, Jul 30, 2019.

  1. subzero
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    subzero Junior Member

    Yeah that seems to be handling the waves nicely considering force 8.
    In regards to stability, I don`t want to make another thread so I`ll ask here. A "Powered" non sailing catamaran vs mono hull both 45'x22', similar freeboard , what would be the pros and cons of the powered cat regarding stability. Yes I know there is so many variables but I`m sure someone has seen two similar vessels of each and can relate differences in general stability, angle of heel, etc...etc..
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Is sounds like you have ideas of converting a sailing cat to a powered one ?
     
  3. subzero
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    subzero Junior Member

    No strictly powered cat design
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Ad Hoc and Alik are both cat design experts hyere, but like everything else, function (and financial capacity) dictates form, and the way the boat will make its living is a big part of the input, the move to catamarans is largely because they can achieve better speeds than displacement monohulls, and in fishing, and other activities, time spent travelling to and from, is time not spent in revenue service.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Well firstly, you realise the L/B of the monohull would be stupidly low...yes?

    However, what do YOU, mean by Stability?....need to make sure we're using the correct and same definitions first.
     
  6. subzero
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    subzero Junior Member

    Stupidly low, I don`t know but that is the norm for lobster boats, some even are 40x28 but they have deep keel, good draft, sufficient freeboard and open stern with scupppers cut on along the deck. They are out in winter fishing 25- 40 miles off in North Atlantic.

    what I mean by stability is the ability to resist capsizing by returning to an upright position after being heeled over due to wind or sea conditions. Surely there must be a some downsides to a catamaran stability compared to a monohull. Large head seas short duration, having to steam broad side in trough, being broad side at rest in 2-3 metre seas. How about icing conditions? does top side weight affect the cat stability any more than a monohull?
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Well, here's the thing. You are asking about:

    So, perhaps the definition of "powered" needs to be defined. Sicne "powered" tends to imply not slow. In which case, such a low L/B hull form for a monohull is not what is selected for a high(er) speed monohull. Thus please define what you mean by "powered".

    Both shall fair pretty well.
    But the greater restoring moment will inevitably be a catamaran. It is easier to improve its stiffness - its ability to right itself - simply by moving the hulls farther apart!

    This does depending upon the wave's period length and its steepness: its length to high ratio.
    Heads, both shall exhibit similar abilities - the mono being a 'smoother ride'.
    Beam seas, the mono will roll much more, but if 'designed' correctly the roll angles experienced may not produce high horizontal accelerations. A cat will be stiffer, thus lower angle of roll, but higher horizontal accelerations, it tends to "snap" back to its equilibrium/upright - not as comfortable.

    In a generalised nutshell....
     
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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like the fishery has a restriction on boat length, and that has led to extreme increases in other dimensions, to increase displacement.
     
  9. subzero
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    subzero Junior Member

    Stiffer usually means higher gm with better stability at the expense of being quick and uncomfortable.
    restriction is 45' however there is a loophole 39' 11" means less restriction and that's why they go super wide.
     
    Last edited: Aug 4, 2019
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Yup...this is not about the pro's and con's of hull types/shapes at all.
    This is more about how to bend the bent rules, even further.

    An unsatisfactory outcome is all that will occur...
     
  11. subzero
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    subzero Junior Member

    I disagree as the cape island boat has proven itself since 1905 and has evolved as one of the safest fishing boats designed. These newer wider ones are same huge deep seaworthy hull just 5 ft shorter. I seen a catamaran hulled fishing vessel and have some questions about the design and stability.
    Can we get back on the topic , Yes this is about the pros and cons of hull shapes, and nothing to do with bending the rules. FYI Any newly built vessel has to undergo stability tests it`s the law.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The catamaran with similar dimensions will be initially "stiffer" against heel, but a mono with the kind of beam you speak off, won't have much concern about stability, you'd have to think.
     
  13. jehardiman
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    jehardiman Senior Member

    As has been pointed out, "stiffness" is a function of GM, which has nothing to do with ultimate "stability" in a capsize. This, and the ungainliness of wide vessels, is why catamarans are not the "go to" hull of choice. They excel in limited circumstances, and are a liability in others.
     
  14. fallguy
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    fallguy Senior Member

    I can think of a big downside for the cat hull vs the monohull.

    Where are you going to put the cargo in the cat?

    In the mono, this is easy, you put the cargo low. In the cat, the cargo can also go into the hulls, but I am less certain about placing heavy loads into each side of the vessel. I am a casual lover of boats, not a designer, but I would have figured I would have seen a catamaran with cargo holds if it were being done.

    Off to google I go now...

    I found a few workboat styled catamarans, but they don't have cargo holds. More like top deck storage and they look like they are generally designed for shallow draft more than deep ocean. But I am a google expert is all and I sort of like catamarans (hoping the ride is nice, too).
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Catamarans are a way of getting big boat performance, out of smaller, lighter craft. Accommodation is one aspect of "big boat" performance that is not so easy to replicate, however.
     
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