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A true Fat Cat?

Discussion in 'General Yachting Discussion' started by Zed Marine, Oct 19, 2017.

  1. Zed Marine

    Zed Marine New Member

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    [Admins, feel free to move this thread if needed]

    Something I have noticed about most Catamarans; While many of them have wider beams than mono-hulls of equal length, rarely do I see them with a close length to width ratio. Typically the range about 2.5: to 3:1. Is there a structural reason for this?


    Length:width ratios
    H2X YACHTS: MCAT 88' 2.4:1
    Horizon Yacht's PC-58 Catamaran 2.43:1
    Axcell 65' Hybrid-Air Power Catamaran 2.9:1
    Horizon 80' Catamaran Yacht 2.97:1
    Curvelle 33-meter Catamaran 3.66~

    These two are kinda getting there:
    AfriCat 420 Catamaran 1.9:1 (power)
    Havana 72' Sailing Cat 1.8:1 (sail)

    My dream yacht is a cat. More a trawler-cat, as she's meant to cruise in full displacement at about 6-8 knots over a great distance (about 6,000nm). My ideal LOA to Beam ratio is about 1.625:1, with an length of 88' and a beam of 55'. As the thread title says, "A true Fat Cat".

    This would also break the typical mold of many cats, by having several decks. The two hulls would house the crew quarters, engine rooms, fuel, water, holding tanks, and tender/toy garages. Next deck up, two vip staterooms, 4 guest cabins. Another deck up. the master, galley (yes, i'm in North America) , formal dining, salon, and cockpit. Up yet one more deck and you have the pilot house with Portuguese bridge, and a long open air covered deck thats roof is covered in solar panels (about 50 kw worth).

    Propulsion would be from a hybrid diesel electric system. Electric motors (magnomatic PDD, highly efficient, high torque, medium rpm motor) would drive the shafts, CP props would make best use of that torque, and the solar would run things most of the time. If the owner (me) is content (yes i am) to trawl along at 6 or so knots, then the generators will see very little use.

    A large (500kWh) LiFePo4 battery pack will supply all the electrical needs of the boat. The solar will charge the batteries, and if they gets too low from extended navigation at night, the generators will kick on charge them back up. They will also come on if the boat goes over a selected speed to protect the batteries.

    Your thoughts?
  2. AMG

    AMG YF Moderator

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  3. Zed Marine

    Zed Marine New Member

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    :D Truth be told, the proportions don't look that bad...:p
  4. discokachina

    discokachina Senior Member

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    SWATH ships were developed to achieve the most stable ride in rough seas compared to monohulls, catamarans, trimarans, etc. The size of SWATH ships vary depending on their use from 25m to 60m. The length and width of a SWATH ship is dependent on how heavy and how large of a load the boat will be carrying. Pilot SWATHs that are only carrying a few pilots are at the lower end of the spectrum, around 28m, as their single purpose is to get a pilot safely on and off ships in rough sea conditions. After an owner tried two monohull yachts, one shorter and one longer that were unstable enough to cause his wife to get seasick easily. In the case of Silver Cloud she is longer than the 28m pilot ships since she has 3 decks and a large helo landing pad. She has large proportion cabins for 10 people in 5 cabins. 4 of them are located on the first deck which gives all of the guests wonderful views compared to the limited window size for the guest cabins typically located below the main deck of monohulls and catamarans. The owner's suite on the second deck is massive and has amazing 180 degree views. She carries 8 crew in 4 cabins that are on the main deck with the captain having a cabin on the third deck behind the wheelhouse. In their SWATH yacht the couple traveled the oceans of the world without his wife ever getting seasick!!! And just imagine the size of the parties you could have on the helo deck, the sky lounge, and the immense living and dining area which has two dinner tables in an very comfortable, uncluttered space. She has an elevator that services all three decks making it wheelchair accessible!! The two tubes contain all of the mechanical systems so the decks above get very little noise and vibrations. Her draft at sea is a little over 13 feet while her port depth is a little over 9 feet! The only downside I can see is the build cost. For my money I would definitely go with a SWATH yacht like Silver Cloud for all of the reasons above!!!

    Here is a quick video comparing the ride of a SWATH ship to a monohull in heavy seas. The difference in their rides is significant giving the pilots and crew a stable enough that they can arrive at their destination and return to port with little fatigue.
    Last edited: Jul 10, 2019
  5. discokachina

    discokachina Senior Member

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  6. HTMO9

    HTMO9 Senior Member

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    It has been talked about the pros and cons of the swath principle in epic width on this forum. I am a big fan of the swath principle and I like Silver Cloud very much. One reason for her length to width ratio was the max. possible beam on the A&R syncro lift (about 18 Meter) at Lemwerder. Silver Cloud is 17.85 Meter in beam and 42 Meter in length and her operational draft 4.5 Meter. But the SWATH principle is not the only source of her smooth ride in rough sea. She has 4 stabilizers on the inner side of her buoyancy bodies (Torpedos). They act as roll and pitch stabilizers and against any corksrew type movements.

    But the disadvantages of her principle should not be forgotten. As she has to maintain a constant neutral buoyancy and a pretty large draft for her principle to work, permanent trimming is necessary and her useful load is rather limited. Silver Cloud would not be able to carry a larger array of tenders and toys. Bud a perfect vessel for a proper planned circumnavigation with friends or family. When building a SWATH, there would not be very much, I would change. A SWATH will neither be a beautiful swan or a fast boat :). The journey is the goal!

    Silver Cloud is moored at Rybovich and seriously for sale at a reasonable price. Her owner has stopped his boating due to age and health reasons.
  7. Capt J

    Capt J Senior Member

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    The problems with your length to width ratio is that the weight of the superstructure will far exceed the carrying capacity of the hulls, unless you got really creative with synthetic materials.

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