Cat hull spacing VS motion comfort?

Discussion in 'Multihulls' started by DennisRB, Jul 12, 2016.

  1. DennisRB
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    DennisRB Senior Member

    If the hulls of a cat are closer together compared to LWL, what effect does this have on the motion comfort of the crew? Or another way of putting it, if you design a regular 12m cat with a 7m beam but stretch it to 15m with empty sailing length leaving the beam the same, what trend might be apparent in the motion comfort? What if the said 12 x 7 meter cat was changed to 12 x 5m?

    Seems to me a common complaint about cat motion is the "corkscrewing action". It seems reasonable that this kind of motion gets worse as the design gets closer to "square".

    Has anyone got any info on this?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    If the beam is 'effectively reduced what occurs?

    Well, this affects the motions because the beam is an input into the natural periods of motion, for the vessel, as well as the metacentric height wish is also proportional to the natural periods of motions.

    Thus a reduce shall see a slight increase in the natural periods of motions, everything else remaining the same..since if you take it to the logical conclusion, if the two hulls touch and 'join', one is left with a 'monohull'. The amount of benefit depends upon the original hull form and its characteristics.

    Corkscrewing is merely one hull up fwd, say port side, meets a wave crest before the stbd hull, in a quartering sea, thus the port side hull is lifted. As the wave travels along the length of the hull the port side bow starts to go down the crest the other side while the stbd hull is now being lifted. The same occurs aft too as the wave passing by the aft end. Hence the corkscrewing. The effect is more pronounced the wider the hull spacing.

    Simple solution change course!
     
  3. DennisRB
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    DennisRB Senior Member

    Thanks again Ad Hoc. So the corkscrew motion will indeed be reduced, but the roll period will increase? I assume from here on its difficult to quantify what is more comfortable on the crew. I expect in a quartering sea the narrower spacing may be more comfortable (less corkscrewing) but in a beam sea it will be less comfortable (more roll). However I do note that roll does no seem to be much of an issue on the relatively wide type of cat I am referring to, so perhaps the reduced corkscrewing motion of narrower spacing may be more comfortable overall?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Yes and yes.
    It all depends upon the seakeeping of the original hull form to begin with.

    Yes.
    This is also what we did on a previous contract. We used narrow hulls and narrow spacing, which almost mimicked a monohull. The principle reason for doing such was to increase its statical stability over that of a monohull. But a typical cat, with its widely spaced hulls, was too stiff, in motions. Thus reduced the spacing considerably, and the desired effect was achieved.
     
  5. redreuben
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    redreuben redreuben

    Wharrams are marginally narrower than mainstream but they also flex somewhat, dare I suggest a solution ?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously, the discussion is different depending on whether it is sail or power.
     
  7. DennisRB
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    DennisRB Senior Member

    Great Ad Hoc, so my thinking was fairly correct.

    Redreuben, if the solution requires sailing a waste of good plywood with 30% overhangs held together by hemp I would rather just get seasick. :p Harsh? Maybe but then again we all don't like the same boats. But I expect you are suggesting adding some flex to a more modern design, perhaps feasible in an open bridgedeck cat?
     
  8. DennisRB
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    DennisRB Senior Member

    Where is the obvious difference? I was talking about a light displacement, minimalist* motorsailer cat, well really just a sailing cat but aren't they all motorsailers anyway?

    *Still a full bridgedeck with good accommodations, just a lot less than a charter version of the same LOA as described in the "cruising cat design" thread. Something like the Oram I posted.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously the need for sufficient stability to accomodate sail forces, has designers erring on the side of wider. Also you don't travel as fast going to windward to any degree, as you potentially can under power alone, so the motions are different, and you have the damping effect of sail, etc etc.
     
  10. DennisRB
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    DennisRB Senior Member

    Oh yes. Obviously then. In my mind the "narrow" version was not going to be all that narrow, and it would be rigged no higher than an average cat of the same beam, it would just be longer. So it would not be in much more danger of capsizing. Seems some designers like to go wider than others. Kurt Hughes seems to go very wide compared to many other designers. Seems like beam increases weight dramatically. Which also increases righting moment but results in a slower boat.

    Just trying to quantify good and bad attributes to the boat I have been fantasizing about. Looks like the 14.9m x 7m cat rigged, powered and luxuriated like an average 12m would gain in motion comfort not only from the empty sailing length at the bow and stern, but also from a more comfortable motion due to less corkscrewing. Wins all around for motion comfort compared to a regular 12x7m cat.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    To have the same margin of safety, you would have to down-size the sail rig, and speed potential under sail will be lost, if closing up the hulls. And don't forget in some conditions, having hulls optimally spaced to avoid the worst of inter-hull wave interference, is a consideration. But I agree excess width does make for snappy motions, though people can have different reactions to that.
     
  12. DennisRB
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    DennisRB Senior Member

    No it wont be slower. The imaginary boat is not a large boat made narrower. Its a smaller boat made longer. Which means the same rig will be slightly less likely to capsize the boat. The boat would be made longer without anything but empty sailing length. This does weigh something, but not much if you don't fill it with anything.

    However, the displacement to length ratio will be much better so it will have less resistance resulting in more speed not less. Well in very light conditions the extra WSA will make it slower under sail, but fire the engine up and it will be operating over the Froude number at which frictional losses outweigh the residuary resistance.

    Speculation to be confirmed with michlet, but I have learned trends from other users. The imaginary boat is light enough with narrow hulls that hull interference wont be an issue. 15m x 7m is not a particularly narrow boat anyway. Lengthening the hulls will both reduce the length to beam ratio and increase the length to displacement ratio which reduces wave making considerably. That will more than offset any possible extra interference drag. This sort of speculation is easy to work out with a program like michlet. I really should try harder to figure it out shouldn't I?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Come the revolution, all catamarans will be like your "improved" version, in the meantime everyone will labour under the constraints of practicality and physics. These are highly evolved craft, no drastic deviation from common practice is likely to be worth it, on balance.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    You may find this of use.
     

  15. DennisRB
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    DennisRB Senior Member

    Your post is unnecessarily antagonistic.

    Where did I say the cat I would like to suit my own SOR would be improved over what is commercially available? Where did I even say it would be drastically different?

    Well I never said that. Clearly the market does not want a boat like this. Which is fine as I never said I was trying to be a commercial success. I never even said I wanted to be a success of any kind.

    I just want to have fun doing a design concept for something that I would actually like to own, and possibly even be able to build.

    Boats much like this already exist and are not a delusion as you suggest.

    [​IMG]

    Here is a pic of your highly evolved (for the market place) craft.

    [​IMG]

    The 2 boats pictured share the same LOA, the lagoon weighs almost 5 times as much and is also about 1.5M wider.

    I just don't want or desire that kind of boat. I would be more than happy with the accommodations of a lagoon 380 but see no reason to not install them into a longer boat in a similar way how how Bob Oram done it. I actually like sailing. I actually want to sail and marinas are not in my itinerary. Sailing highly evolved barge like craft is no fun for me.

    This has nothing to do with making improvement for commercial reasons. Just meeting my personal SOR.
     
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