Center nacelles and wave pounding in cats.

Discussion in 'Multihulls' started by xarax, Sep 1, 2007.

  1. brian eiland
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    brian eiland Senior Member

    Does that take into account where the bridgedeck begins along the length of the vessel?
     
  2. Alik
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    Alik Senior Member

    This is minimal clearance at midships section. Front 25% of wet deck should be either raised or cut out.

    Moreover, one should look at structural considerations. Say, GL HSC rules apply much higher slamming loads on bridgedeck structure if local vertical clearance is below 0.05*L.
     
  3. brian eiland
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    brian eiland Senior Member

    Here is a more favorable shape I found in my files...even while it extends almost to the bow.

    I forgot what this belongs to :confused:
     

    Attached Files:

  4. mariedukat
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    mariedukat Junior Member

    Wngdeck

    Hi Brian, keeping in mind that DOMINO is a power cat, not a sailing cat, our wing deck is highly anti-slamming. The Hight above water is one component, the knuckle at the base of each hull allows for added lift, the double-barreled round wing deck, widening aft, allows for energy dispersion. In 12,000 miles and all sorts of seas, we never slammed. Check the design on Malcolm Tennant website or on our blog, http://dominocatamaran.blogspot.com

    http://dominocatamaran.blogspot.com
     
  5. Pericles
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    Pericles Senior Member

    In 2007, I mentioned the centre nacelle of an Incat ferry as a source of design inspiration for a sailing catamaran, with the added bonus of being deep enough to mount the Omer wing sail rig designed by Ilan Gonen. http://www.omerwingsail.com/

    In addition, an diesel generator and a single dagger board could be mounted inside the nacelle.

    Looking at another favourite of mine, although there seems to be adequate clearance, I wonder if the Gunboat 66 slams at 30 mph or if anyone cares? :p :p


    http://www.gunboat.com/66-gallery.php#
     

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  6. mariedukat
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    mariedukat Junior Member

    Domino

    Yes, Pericles, the Gunboat is an ultimate design. Here is DOMINO (white) compared to PELICANO (blue). Their profiles are slightly different, Pelicano being much wider and longer.
     

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  7. Spiv
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    Spiv Ancient Mariner

    Sailing Cats

    Hi Brian,
    good idea to try and consolidate this great knowledge we all share into one thread.
    I'll copy and paste what I can find first and as time allows I will write some new.

    This from a 2008 post, I still consider this true:

    Charles Kanter in his great book "Cruising Catamaran Communique`" explains the various factors affecting slamming: fine bows fat sterns, finesse coefficient, over 'square' design etc; but he gives a general rule of thumb: bridgedeck clearance should be at least 1 inch for every foot of hull centerlines distance.

    Therefore, for a 'square' 42' cat the beam max should be 21', the centerlines distance would be approximately 19' and the clearance should be at least 19". That equates to 8%.

    I found that to be not enough, my last 12.5m (41') cat had centerlines beam of 6.5m (21') and a bridgedeck clearance of 620mm (24"). She would slam badly, tossing things off the table, we even had damage to the saloon seats..
    So, my view (shared by a local cat builder on his 47th cat) is that the same size cat should have a minimum 900mm (35') or 14%.
    I was looking on Kurt Hughes web site and most of his 13 to 15m cats have 920mm clearance.

    If your cat ends being 35', I would guess a BMax of 18' and CL beam of about 15'; using 'my' min 14% you would need 2.1'.

    That is bridgedeck clearance, forebeam should be higher as it is going to hit waves much earlier and before the boat starts to rise."
     
  8. Spiv
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    Spiv Ancient Mariner

    Power Cats

    This also from a 2008 post:
    ,
    I had several power cats of different size (23 to 40') and driven others on test runs to try and make up my mind on what to buy. All cats are not the same.
    I know this is off the tread subject, but I hope Bob doesn't mind...

    What cats have in common is that they (in general) go faster and are more comfortable than monos of same length and power as they (should) ride on a cushion of foam. All my cats did more than 25kn, the Manta 33' did 36kn and it was by far the best.

    The idea of a wavebreaker has been used by different design, but the manta carried all the way to the stern and the half bridgedecks were round.
    Also, it had asymmetric chines so it would climb on the plane smoothly and react to waves smoothly.

    If you are considering doing some mods to your cat, add the wavebreaker, keep it off the waterline and carry it all the way aft. I am sure it will make a big difference, especially at speeds over 20kn.

    [​IMG]

    [​IMG]
     
  9. Alik
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    Alik Senior Member

    I don't see much advantage of having such developed and long wavebreaker specially on planing cats. Yes, they do soften the slamming, but if the wavebreaker itself is lower than normal surface of tunnel then it does not work.

    We have done some tunnel photos/video on number of cats. Off course we were unable to test in extreme slamming conditions otherwise we would loose the camera.
     
  10. Spiv
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    Spiv Ancient Mariner

    Sailing V Power

    My first post refers to sailing cats, my last to power cats doing 15 to 30+kn.

    I thought for a long time about a wave breaker for sailing cats, but I am not sure if it would work at slower speeds.

    On sailing cats, I think it is safer to stick with high bridgedeck clearance and round chines.
    As Richard Woods suggests, the addition of a 'knuckle' above waterline, is all I would recommend.

    This section is of a lightweight 17m cat Richard Wood designed for me.
     

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  11. Alik
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    Alik Senior Member

    Fully concur.

    On powercats, we use 'wave breaker' shape at bow mostly as placement of anchor and natural stiffener. We do not use wave breaker at full length of wet deck as it actually reduces either vertical clearance or headroom.

    Smooth knuckle works though sharp one would create unpleasant noise (due to wave sloshing) especially at rest.
     
  12. Boston

    Boston Previous Member

    dont any of these designs have pressure relief, seems to me that one reason slamming is so sever is that the upward thrust of the water is completely resisted by the intransigent nature of the deck. If there were a space between the hulls and the bridge pod that allowed "breathing" ( pressure from the upwardly moving fluid to escape the hull structure ) then the intensity of the slamming action might be relieved somewhat. Say a typical hull configuration suspended between the hulls of a cat with some form of rigid mesh between, wouldn't elevate slamming completely but might make it bearable in less than ideal conditions ;-)
     
  13. Frosty

    Frosty Previous Member

    That would make a bit of a mess of my lounge carpet Bos.
     
  14. Boston

    Boston Previous Member

    well they could roll up in ugly conditions, ;-) Thing is why does there have to be a large solid flat surface rather than a nice streamlined one with space for the deflected water to bleed off between the hulls? Could be something rigid but porous that might hold that carpet just fine in pleasant conditions. Maybe even with a canopy for some shade. Whole things stows away for rough conditions.
     

  15. brian eiland
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    brian eiland Senior Member

    Bridgedeck Designs, for Power verses Sailing Cats

    Marie brought up 2 interesting points, power vs sail vessels, and the 'double arch shape' that Tennant experimented with quite a bit.


    The height, shape and span, both transverse and longitudinal, of the wing deck, will vary from design to design, and from designer to designer. When looking at the wingdeck structures of power catamarans it is necessary to recognize that they may be operating in conditions that will never affect a sailing catamaran.

    Most sailing catamarans will never be going any closer to the wind and waves, under sail, than 40-45 degrees. This to some extent can reduce the slamming effect of the waves. But keep in mind that a sailing rig itself can impart a downward force to the bows on many points of sail.

    The power cat on the other hand may very well be 'punching' straight into a seaway and this should be kept in mind. Likely this is the more difficult case to design for.

    In the case for a motor-sailer which is a great interest of mine, and particularly a vessel that can perform reasonably well in both modes, at sea, the good bridgedeck designs of the power cat are most likely the ones to be incorporated on the motorsailer
     
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