Stefan's New 80' Power Cat

Discussion in 'Boat Design' started by brian eiland, Aug 21, 2011.

  1. brian eiland
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    brian eiland Senior Member

  2. Doug Lord
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    Doug Lord Flight Ready

    Stefans boat

    ==================
    I love it-beautiful in many ways. I'd do the interior differently but not the exterior. Thanks, Brian....
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    Gilles Ollier made similar cats in Carbon Fiber at Multiplast shipyard a few years ago. Symmetric long slim hulls for better efficiency. The recipe is well known and works. Like any design solution, it has its limits, but for a cruising boat is a very good one. The longest, lightest and slimiest boats for a given task.

    The plumb bows do not cause any problem and are enough buoyant if well designed. The gravity center has to be carefully placed and the volumes (and so the righting moments) with a good distribution along the hulls. In fact these "cutting waves" bows are designed to pass through the water. A flare on a such hull could be potentially dangerous. Some fast destroyers has been designed in the same way.
     
  4. masalai
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    masalai masalai

    You will note that it is an Oram design..... :D :D :D
     
  5. brian eiland
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    brian eiland Senior Member

    Slim Bow Problems

    As expressed by several others on this subject thread:
    http://www.multihulls4us.com/forums/showthread.php?t=3559

    ....I also questioned those very slim bows, and their 'diving' into the relatively small swell in these videos. What's going to happen in a good size swell?

    Certainly the bows are slim enough that they are not going to create wave forms in the bridgedeck area between the hulls that will slam that surface. But the slimness is going allow the bows to dive down, letting that bridgedeck area come very close to the swell. I believe the bridgedeck is properly located a good distance aft of the bows, and high enough off the water as might be reasonable for this design.

    But if I were to modify one thing it would be to NOT use those plumb bows. I would add bows that flared fwd and outward as they reach deck level. This would give an extra added buoyany to the bows as they dove deeper.

    I sometimes don't understand this 'follow the pack' mentality with plumb bows, or even more recent axe/reverse bows?? While these type bows might be great on near shore racing designs, I don't think that much of them for offshore cruising designs. Then all of a sudden someone determines these bows don't offer enough buoyancy, so they come back and add bulbous bows to them ...another NOT so great idea for a variable speed vessel, and particularly a multihull one????
     
  6. masalai
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    masalai masalai

    With you there brian,
    The most recent version of my boat has one chine less on the inside of the hulls - basically the 45 degree chamfer from just above the waterline is carried all the way forward to the bows... My boat is dry now but that just adds that little bit extra "insurance" by way of reserve buoyancy... I guess the changes would be somewhat like the second image 'DSCN4279a.jpg'

    The plumb bows are OK as they maximise waterline length for a given LOA - - and berthing, as well as lots of other fees seem to hit a surcharge at 40ft...
     

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  7. Rick Tyler
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    Rick Tyler Defenstrator in chief

    What's with the trend of putting circular ladders with no real handrails on big yachts? Imagine trying to go up, or even worse down, that ladder in a storm. I'm sure the interior designer thought it looked cool, but I think it just looks dangerous.

    I'm not a naval architect, nor do I play one on TV, but those plumb bows don't look to me like they have enough buoyancy for large waves. At least the boat doesn't have a wing or roll bar.
     
  8. Corley
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    Corley epoxy coated

    I'm not a fan of big open spaces on cat bridgedecks in general, I'm sure they look cool in marketing brochures but cats do move around a lot when at sea particularly when in a beam sea and a good selection of easy hand holds are important for safety. Its an impressive boat though and Bob Oram would not scrimp on safety or seaworthiness in his designs.
     
  9. Ilan Voyager
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    Ilan Voyager Senior Member

    Thanks for the information, I'm going to see the portfolio of this NA.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Must be more money in hairdressing than is popularly imagined.
     
  11. Ilan Voyager
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    Ilan Voyager Senior Member

    It's not follow the pack, at least for those who re-initiated the plumb bows (that was used extensively at the end of the XIXth century). If I remember well Gilles Ollier went to the plumb bows with the Jet Service catamarans, I think the 2nd of the name, at the beginning of the 80ties (almost 30 years ago now!) after a very serious study.
    I mean not by divine inspiration but after a serious study at the Bassin D'Essais des Carenes at Paris. That gave very interesting data, specially about the motion of the hull through the waves and the way of cutting the wave.
    The result in scale one validated the study: the Jet Services catamarans were the fastest oceanic sailboats of these time.

    At about the same time the French Navy did similar studies for slim fast destroyers and patrol boats. The longitudinal distribution of the weights and volumes and the calculation of the polar inertia are primordial. The immersion of the front part of the boat is progressive and follows a non linear curve carefully calculated. Computer are mandatory as thousands of calculations are needed (fastidious but not very hard...)
    The gains on mean speed are consequent as the boat does not waste energy in vertical motions. Incidentally it's far more comfortable for the people inside the boat and for a sail boat the sails do not spill out with the motions of the mast.
    INCA made a similar research and arrived to a more complex solution for his ferry cats. But the goal is similar: suppression of the vertical motions and keeping a steady forward motion.

    If you watch well the videos the cat is using very little reserve of buoyancy; it has plenty to use for a stronger sea. The bows are designed to go through the water to the decks have to be as small as possible to clear out. Flares and larger decks would have and adverse effect; if being immersed in solid water the drag would stop the boat, and possibly cause a longitudinal capsize (called spoon effect in the French yachting circus). Also the stresses could overcome the strength of the bows.

    On a pro fishing boat for example you need a wide working platform everywhere whatever the weather (or almost). So flares and water deflecting bows are mandatory (with a rather heavy displacement and big scantlings) to soften the movements of the boat and withstand the stresses induced by the waves breaking against the boat. It has a price: heavy, slow but it's for fishing...and about the sea motions, fishing boats are not designed for comfort. That moves hard and rolls sometimes brutally but fishermen are tough pros with strong stomachs.

    On a very slim hulled multi the purpose is very different. Sweet movements, vertical and lateral accelerations kept to the minimum. And maximal ratio speed/efficiency. No struggle against the sea, no big bow waves, no big splashes of the boat crushing the waves. No need to go on the bows in bad weather, you stay dry inside the boat. Even better you do not need to stay in a zone of bad weather (but the cat must be able to cope with that).

    With the oceanic racing tris the design has gone further as it's very efficient. The proof is on the records done in circumnavigations in any kind of sea and weather. I think that 20 knots of mean speed around the world for a man alone on a trimaran is the best proof that slim hulls with cutting wave bows works on any kind of sea.
    First a guy alone is able to skip a big tri; the boat is smooth, stable and sure for not exhausting the skipper in a few days while being fast, not only in some surges but during days and days. Sail multis are notoriously harder to design than motor cats. Brian it seems that you have not understood the philosophy of such designs. Think about gliders exploiting the winds, or seagull sleeping on the waves. No struggle with the sea, no big engines, nor high bows, just a japanese sword cutting the 6 inches bamboo. The maximal use of the minimal power. Very zen indeed.

    There are some good videos on YouTube about racing multis in hard sea. Have a look: the analysis of the motions of the boats shows how work this kind of hulls. It's directly "translatable" to a similar sized motor boat going at the same speeds. Those lucky enough (I'm in this pack...) to try such boats become addicts. I do remember a 21 meters motor trimaran going comfortably at 20 knots in sea state 5 with just one engine of 250 HP...0.264 gallon of fuel per nautical mile at 20 knots...max speed 28 knots. This tri crosses from Cabo Verde to England, so for me it's high sea...

    Evidently a too heavy catamaran do not get the same results with very slim hulls. The problem of these design is lightness so the accommodations are normally rather small for a given length. A light boat cannot have heavy accommodations...
     

  12. FMS
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    FMS Senior Member

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