Catamaran hull question

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

  1. subzero
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    subzero Junior Member

    What is the purpose of the fin between the upper two hulls on a catamaran? I have it circled in pic
     

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  2. jehardiman
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    jehardiman Senior Member

    Reduces wave slamming load and stiffens the cross deck plating.
     
  3. subzero
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    subzero Junior Member

    Ok I was just wondering as I see many different designs and they are all a bit different.
     
  4. subzero
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    subzero Junior Member

    another question regarding non sail, motor powered inboard cats, why do some cats have keels and others very little ? what would be the advantages and disadvantages of both of these
     

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  5. jehardiman
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    jehardiman Senior Member

    Both the keel and the skeg (small "keel") are to protect the propellers and rudders. Cat's generally don't need them for directional stability, though they can improve maneuvering.

    Edit to add: It also makes a place to put the stern tube and bearing without having to have a strut.
     
  6. subzero
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    subzero Junior Member

    thanks again, I realize that this forum is more directed towards sailing vessels however I am intrigued by a few 45'x 22' commercial fishing catamarans I have seen. Would powered cats of that size generally have better roll resistance broadside, and less pounding head on in rough seas than the typical 45'x22' mono haul lobster boat in north atlantic in winter? Would cat tend to throw more water up and onto the vessel in freezing spray conditions?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless my eyes deceive me, the picture of the catamaran with the "wavebreaker", also shows a decent "fillet" between the vertical inner hull sides, and the "wet deck', also a device to reduce slamming on that flat horiozontal surface.
     
  8. jehardiman
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    jehardiman Senior Member

    Actually, there are a lot of people on the forums that handle commercial design, so dropping this in the Boat Design forum is the proper place.
    As to the question of a power cat or a mono, that all depends. Cats generally have higher initial stability, but poorer habitability motions, especially in a cross sea. Head on, it could be a wash, as to ease the motions the lower waterplane of the cat will rise less. Realistically it will depend on the hull shape, CG, sea spectra and the wave slope, remembering that it is wave slope, not size that endangers small vessels. A full up analysis of requirements, loads, and seaway would be needed to decide the choice of one over the other because there is no "one best answer", everything is a compromise.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    High differential loading goes against cats, if it means when heavily laden the tunnel is too close to the water, then you will have slamming issues.
     
  10. subzero
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    subzero Junior Member

    how do they react to being broad side stationery in seas?
     
  11. jehardiman
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    jehardiman Senior Member

    Depends on the wave period and height (which drives wave slope) to the specific geometry and CG of the cat. A cats response in roll and heave are very peaky, so if you know your operational area, you can de-tune the response. However there are two flip sides to that; first depending on wave size, wave slope, and freeboard you could be more likely to have a boarding sea incident because the cat won't lift to the wave; second, if you do ever get into a seaway near the natural period of either heave or roll, it will become exciting unless dampers of some kind are fitted, but those just increase the response in other wave periods.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    If it is important to have a boat that won't heel much under changing athwartship loads, the cat is the obvious choice. Even little trailer power cats can have a half dozen people fishing on one side, without much change in attitude.
     
  13. subzero
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    subzero Junior Member

    would a strictly power cat between 40-55 Gt with good draft and ample freeboard have totally different rough sea handling and sea keeping abilities than a strictly sailing cat?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    There are all sorts of designs of power cats, and if there is one thing they all have, it is athwartship stiffness, beyond that much varies. The sailing cat is obviously designed differently, and is not a direct comparison, but of course both kinds can be thoroughly seaworthy. A sailing cat is likely to be less stiff longitudinally.
     

  15. Waterwitch
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    Waterwitch Junior Member


    Seems like with twin screws and ample horsepower, of course a power cat will travel differently than a sailboat. A Canadian lobsterboat what is called a Novi boat has a very flared bow which I would think would deflect spray more than the narrow bows of a a cat as you asked.
     
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