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

    Bridgedeck Designs, for Power verses Sailing Cats

    Marie brought up 2 interesting points, power vs sail vessels, and the 'double arch shape' that Tennant experiemented 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 35-45 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, and at sea, the good bridgedeck designs of the power cat are most likely the ones to be incorporated on the motorsailer
     
  2. Spiv
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    Spiv Ancient Mariner

    Brian,
    Speed is a major consideration.
    Alik also agrees that hard knuckles would not be much good to a sailing cat and also create a lot of undesired sloshing at anchor.

    Also, the times that I cursed the most the designer of my old 42' sailing cat with 600mm bridgedeck was when motoring through steep 1 to 1.5m waves.
    Once I had a 5h demolition ride at no more than 4kn into a 25kn wind, any faster and I would have had structural damage.

    If you look again at my drawing of the Mantacat at speed, the design of the 'double arch' relies on the speed to create turbulence/ foam on which the powercat rides. If you slow down, then you start pounding, till you really slow to a few knots and start climbing the waves rather than cutting through.
    Never been on a sailing cat with 'double arch', but my guess is that it would not work the same. Other benefits might apply: extra stiffness, some pounding reduction etc.

    IMO for a sailing cat you must make it long, light and with high bridgedeck, this way you climb over big waves and slide through little ones without bashing.
    Anything diverging from that will be a compromise.
     
  3. masalai
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    masalai masalai

    Look at mine, on the trip up to South Percy beaut... BUT

    Know the performance traits and travel to their advantage for comfort (fastest travel option...
    a) - Small messy bay chop (westerlies on the East coast) where fetch is 1 to 3 miles, travel close inshore, enjoy the view and pleasant ride....
    b) - When the south Easter brings a good swell go offshore to ride the surf and get a nice push (my best was 19.5 knots), or against the wind/sea play the tidal currents or (coming back wait for calmer weather and stay close inshore)...
    c) - If you cannot avoid the small 1 to 1.4M slop tack off a bit so that the wave first touches one bow and directly in the same position the stern of the other hull, - coming or going, the same applies and adjust your speed to best comfort in ride...

    I find that the 45degree chamfers leading to the bridgedeck lift the boat nicely and, with the high clearance no slamming... The latest variation has the chamfer extending up to the deck at the bows for nicer lift and less chines to tape and fair ... and slightly smaller trampoline area... The hull section in the water is symmetrical, has a strengthened bottom so can be beached... I have mini-keels to protect the sail-drive legs and still allow grounding in shallows with no hassles draft is about 1.3M
     

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  4. boatn00b
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    boatn00b New Member

    So...

    Symmetric hulls will pile up/accelerate water in the tunnel

    Asymmetrics leave the flow/water level in the tunnel basically as it is?

    So banana shaped hulls (idiotic in any other sense obviously) would suck the water level down (save cross flow across hull bottoms)? Configure bows like drooping wing leading edges maybe (not serious really...)? Turn bows inward when slamming is more of a problem than efficiency...
     
  5. masalai
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    masalai masalai

    Errrr, Not Quite...

    Older (original) Hobie cats, for example, had lots of "banana" so they could turn quickly in the surf, lots of fun to sail at the time...

    Slamming, (where the waves beat against the underside of the bridge-deck), is reduced by the 45 degree chamfer on the inside - above normal water-line / wetted-areas to add buoyancy as the wave-form passes ... AND ... by increased clearance under the bridge-deck... 700 to 850mm clearance on my boat ... AS WELL AS ... extending bow and stern beyond the bridge-deck to increase buoyancy and reduce weight at the ends...

    Some boats have a lower bridge-deck clearance and rely on 'spray' and air-pressure mix to reduce slamming, but the consequence can be a very stiff ride (like a "pile-driver", if you are so medically afflicted :D )

    High speed (150mph) boats like "maritimo"... http://www.boatdesign.net/gallery/showphoto.php/photo/15580/ppuser/22903 have straight inside faces and turn at very high G forces...

    I have seen others that seem to have a fuller hull form on the inside (what advantage that is I do not know)...

    The shape of the wetted area determines the "efficient cruise speed" as the wave generation is the most significant factor in limiting maximum speed and efficiency when in "displacement" mode... Again I am NOT an NA or Engineer and am not using strict Engineering terms... but you should get the general idea...

    Wetted area, (rounded as opposed to hard chines has minimal adverse impact for everyday cruising), and may come into play when seeking that last 1% performance - but I reckon that crew capabilities far outweigh that... In multihulls, the greatest performance bugbear is WEIGHT...
     
  6. boatn00b
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    boatn00b New Member

    I was thinking convex/concave/straight (parallel) tunnel sides (at the waterline) as seen from above/below. Purely theoretically, of course...
     
  7. masalai
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    masalai masalai

    No real advantage at "displacement" speeds, - A big pain in the arse to ensure PERFECT parallel alignment, both horizontally and vertically and no twist of the hulls in general, - and the connection to the bridge-deck will be less than optimum being at a right angle, exposing a very high stress factor at the join...

    With the 45 degree chamfer panel there are the advantages of :-
    - facilitating the placement of access steps down to the hulls,
    - in the placement of the bulkheads there will be better load spreading,
    - the 45 degree panel can also be designed to spread loads and stresses,
    - when the bulkheads are positioned, strengthen the join with a couple of runs of glass tape on the inside of the hull... and several other considerations to spread the loads and diagonally stiffen the hulls...
     
  8. Pericles
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    Pericles Senior Member

  9. yipster
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    yipster designer

    been looking at your boat mas, nice and about done eh? that hitchhiker wil come along as well some day
    keep aiming for those tropical coconut islands! do like that chamfer and no slaming eh?
    spoke with a guy that sailed cats acros the atlantic and like to hear you sailors talk
     
  10. masalai
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    masalai masalai

    Hi Yipster,
    Long time no see,
    Boats usually seem to me to be 'tailored' to meet 'local' conditions, and I find that in Australian cat designs, which seem to have greater bridge deck clearance and a bit more beam overall (similar to South African designs, as the Southern Oceans are a hard task-master, and will severely punish most designs that venture too long in that region...
     
  11. Boston

    Boston Previous Member

    been following this one for a while and I might have mentioned it before but how about a design that allows for a lot of venting between a central cabin and the sponsons. Kinda like this one

    [​IMG]

    the underbelly of the central cabin area is still pretty flat in this rendering ( not my rendering or design by the way ) but if it were more v shaped it might handle pounding pretty well.

    just a thought
    cheers
    B
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I have had power cats without the wavebreaker/nacelle and I always thought tunnel slap was worst when quartering than head-on to waves.
     
  13. rapscallion
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    rapscallion Senior Member

    So, how much slamming does the G32 experience? Or a Jarcat? Perhaps bridgedeck slamming is the explanation for jan's latest catamaran design "project x" as it was called before it was named strings.
     
  14. rapscallion
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    rapscallion Senior Member

    1 person likes this.

  15. Pericles
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    Pericles Senior Member

    On another page, in 1941, Wells Coates sketched a yawl, to be rigged wirh what he called "Wingsails".

    http://www.wellscoates.org/laura.htm

    His catamaran had a fully battened mainsail set on a very curved mast. Could that have been his Wingsail? He certainly was ahead of his time.
     
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