Stability/Seakeeping of Deep V catamarans

Discussion in 'Stability' started by waterbear, May 3, 2019.

  1. waterbear
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    waterbear Junior Member

    Those are just hull form examples. If both boats are the same LOA and mass, and the dory has a higher CP, then the dory will have a smaller section amidships and much less draft.
     
  2. waterbear
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    waterbear Junior Member

    He was making a joke, claiming the rocking motion was a design feature. As someone else said, pitching gets old fast.
     
  3. waterbear
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    waterbear Junior Member

    First I've heard of Poe's law, thanks for that.

    I'm a definitely a sucker for the marketing, although maybe not Rumars' version! Somewhere buried in my mind is a little voice telling me I need one of Mr W's boats, and when I get one I will be automatically transported to french Polynesia where I'll spend my days lounging on deck drinking pina coladas surrounded by nekkid ladies fanning me with palm fronds.

    In reality I'd be paying exorbitant prices for some raft plans (omg, have you seen the prices?) to fund Mr W's polyamorous lifestyle. Not unlike a cult, no?

    [​IMG]
     
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  4. Dejay
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    Dejay Senior Newbie

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  5. Dejay
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    Dejay Senior Newbie

    Weird, now there are three threads discussing similar things about seakeeping and pitching of V hulls. Latest comment explains the pitching response to waves very well:

    piching control https://www.boatdesign.net/threads/piching-control.62036/page-2#post-853719

    Waterbear, if you need better answers you probably need to specify your question better and separate out things like seakeeping in regards to ultimate failure like pitchpoling, seasickness inducing motions, interior room and windage, spray and wetness of the ride making you tired and things like draft. Once you do that I think you will have your answers.

    Personally I want to go with a round and slim hull with mostly straight sides and higher bridgedeck clearance, since I want to go slow with very low power requirements but smooth and gentle motions.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It's probably the close symmetry between the ends that causes the most problems, not the vee-shape.
     
  7. waterbear
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    waterbear Junior Member

    I've read elsewhere the flat bottom contributes quite a bit to the damping. Logically plunging a box shape into water will take more energy than a V shape.

    Not saying it's more important than the asymmetry, I certainly wouldn't know.
     
  8. waterbear
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    waterbear Junior Member

    I'm guessing no one really knows the answer because there isn't any good data. I honestly think you'd need another 1979 fastnet race filled with catamarans to get a good handle on it.

    I also don't see anyone using Deep V hulls in new designs, including Wharram. His newer boats like Mana and Amatasi are essentially three chined, not deep V. Richard woods remarked in his forum "You are right that a Veed hull, canoe stern boat pitches more than transom sterned boats. One reason I haven't drawn any for over 30 years."

    Anyway, It sounds like you're designing and building a catamaran? Is it electric? what size?
     
  9. Dejay
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    Dejay Senior Newbie

    I think you can theoretically get answers to a lot of it. At least comparatively to other designs, if you have enough data on them. Things like metacentric height and calculating what moment a wave of a certain height and length would impart on a boat with different hulls and what accelerations you could expect. But then it becomes a lot of work to learn and do all this. That's what you pay a naval architect for I guess :) And the answer won't be very precise or universal.
    Or build a couple of models and test them in a swimming pool to get a feel. Or ask owners of prospective boats for a sail with them. Or use CFD to get a real good picture, but then you're talking real money.

    And yes :) I have the dream of a solar powered electric cruising multihull as a permanent liveaboard / island hopping off grid home. Currently thinking something like a ILAN type power trimaran in the 15m / 50' range, there seems to be a sweet spot there in regards to solar power gain and low total resistance if the boat is light enough. Something like >60 nautical miles at 8 knots in the summer in the Mediterranean, in the winter like 4-5 knots. "I want to believe" and I am eventually going to make a thread about it! But I still feel I should read and learn more and do more homework and make a better sketch and some more calculations like for e.g. weight. It all takes time and I get easily distracted haha.
     
  10. waterbear
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    waterbear Junior Member

    That sounds awfully ambitious.

    I've actually known three people who built cruising catamarans here in Alameda:

    1. The individual I mentioned earlier. To recap, he spent 10 years designing and building a 60 foot catamaran, never finished, then he got cancer and died.

    2. Similar story. Guy spent 10 years designing and building a 42 foot catamaran, never finished, had an aortic dissection and died.

    3. Guy built a Wharram Pahi 42 in 2 years, then sailed it to central america and spent roughly 8 years cruising on and off.

    In cases 1 and 2 the the builders designed their own boats and got bogged down in the details. They spent countless hours reinventing the wheel, figuring out how to do things and second guessing their decisions. Guy #3 just bought the plans and followed the directions. He was fast and sloppy and his workmanship was ugly, but he got it done and he went cruising.

    I know this sounds rude, but I'll say it anyway: If you embark on the process of designing and building a 50 foot solar powered trimaran, you will probably never go cruising. If you want to go cruising you should think long and hard about what you need, not what you want or think you need.

    Richard Woods says he no longer designs boats over 40ft because they take way too much time to build, they're too much boat for a couple to handle, and it's more space than most people need. He recommends sticking to the 30-40ft range for offshore cruising. His own live aboard is a Skoota 28 power catamaran which he takes offshore to the Bahamas.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Sad stories there. Might cause a few people to rethink the fact we don't live forever, and you have to have reasonable health to enjoy it all.
     
  12. Dejay
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    Dejay Senior Newbie

    Well it's in a "pipe dream" stage currently and I hope to find a designer that is interested and can maybe offset costs by selling plans. Or adapt an existing design. But there seem to be no plans for solar powered boats available. I haven't even found many power trimaran yachts.

    So at the moment I'm just sketching ideas and learning more about the principles. To know what could be possible and what I need from a design and what compromises could be made. I do confess that boat design is pretty intriguing :)
     
  13. waterbear
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    waterbear Junior Member

    One problem with solar boats is there's just not that much energy falling on the boat from the sun. Say you have a 50 foot long catamaran, 30 feet wide (15M x 9M) and there is an elevated canopy with a large array of solar panels that covers the entire boat. A 15M x 9M canopy is 135 sq. meters of solar panels. If you're cruising near Athens Greece, you will have a total of 7kWh/sq. meter/day falling on the canopy in summer, or 945kWh for the whole array. Now the panel might only be 17% efficient, so we get 160kWh from our giant array of panels for the day. If we run the motor for 10 hours, that's 21.4 horsepower for 10 hours.

    Now in the winter the solar irradiance in Athens is only 1/5th of that, or 1.5kWh per sq. meter, which gets you ~4hp/hr for 10 hours under average conditions. Athens is sunny 350 days per year, so performance will be even worse with clouds.

    All up a system like this could easily cost $50k USD with a modest LiFePo4 battery pack. Considering the average power output is roughly equivalent to portable 9.9hp outboard motor, it's a hard sell. When you consider most boats spend very little time moving it's a really hard sell.
     
  14. waterbear
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    waterbear Junior Member

    Agreed.

    Even if they finished it might not have ended well. Quite often it seems the boat or the lifestyle doesn't live up to expectations. In their case it would be very difficult to sell an amateur designed boat.

    There's a guy in the UK who built a very nice Tiki 38 called Gleda, which was to be his permanent cruising home. He spent 5000 hours over 7 years building the boat, and when it was done he traveled 2500 miles from the UK to Spain over two years. Then he abruptly sold it, with no explanation. I'm guessing when he embarked on the project he didn't really know what he was signing up for, and the reality didn't match the dream. It's got to be heartbreaking to find that out after you've invested so much of your life.
     

  15. Dejay
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    Dejay Senior Newbie

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