Bottom Hull Shape for a Tri, Cat or Proa?

Discussion in 'Boat Design' started by bjdbowman, Nov 23, 2018.

  1. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    All is relative. No, of course you can't pack the comforts of a 40' cat into this concept. But it is still possible to go hiking in the mountains for 2 weeks, without "resupply" carrying all in a 70 pound backpack. Only most people just can't imagine living such a spartan lifestyle. Adding 150+ liter water tank. 2kw genset at ca 20 pounds. 0,5 liters an hour at half load. 100 liter tank+ petrol, 180 pounds. 2kW motor and 3 sqm solar + 2kwh li ion packs add another 100 pounds. If there is wind, sailing is the way. Without wind, such a craft Will probably make close to 6 kts on a 1kw motor. Or 4.5 on 0,5 kw. Meening 3 sqm solar would be sufficient for propulsion even without any batteries. Unless dark or cloudy. That's about 1200 pounds for 2 non obese adults including all above mentioned. Of course that is a very incomplete list. But still, an indication on how light it is possible to travel camping style. I belive a Corsair 24 would have no trouble what so ever with such a payload. Or a Naviator 700 (mini stiletto style cat).
  2. Ilan Voyager
    Joined: May 2004
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    Location: Cancun Mexico

    Ilan Voyager Senior Member

    Hiking with a 70 pounds backpack, that must be a hard core pleasure, the BSDM of hiking, the all macho black leather, chains and whips. Sure that when you put on the ground the 70 pounds back pack, you must feel a great sense of relief, maybe happiness...Sorry I'm not a trained sherpa with peculiar genetics. I preferred to hike with packing goats. They do not talk, are good companions and take around 30 pounds each.
    I would like to know where and for what price you get a 20 pounds, 9kg, reliable 4 strokes 2kWh genset. Not a hitech gadget but a strong hard working genset.

    Oh man...
    What is the interest of having such a complicated power system, so heavy and so cumbersome on a small boat? Accumulating the inconveniences of gas engines, plus electric motors and batteries, plus solar panels. All that for a holiday boat in Florida, making day coastal trips in nice weather. I would like to see how do you install all that on a 24 feet sailing multi, specially the 3 m2 solar panel a little piece of around 8 feet by 6.
    No battery direct solar...Unhappily you'll need urgently the engine when it's dark under a menacing storm cloud, or at night while searching the lights of the entry of the harbor in the middle of all these too bright city lights...
    A 5 HP Honda outboard, 60 pounds, 2000 bucks brand new seems more convenient. These small Hondas can run on the fumes on the gas tank, just sip a few drops a gas, and are as noisy as a purring kitten. That will do the job without complication nut with a 5 years warranty.
    Sure Florida coast is so remote and desert, inhabited by dangerous cannibals that you'll need for survival 150 liters of water...
  3. Niclas Vestman
    Joined: Aug 2017
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

    Well, sure some People might have slightly unorthodox ideas. But as long as they are polite and friendly, I see no harm. But I do mind passive agressive, sarcastic behaviour. There are several "-isms" for making people conform to an ideal line of thinking and doing things. None of them are any good, but most of them have proven disasterous.
  4. Ilan Voyager
    Joined: May 2004
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    Location: Cancun Mexico

    Ilan Voyager Senior Member

    Niclas, it's not "-ism", it's simple common sense. That weren't unorthodox but mistakes or impractical assertions.
    I've been known in my field for being rather unorthodox having a few times clever ideas but also sometimes bad ideas. The best is to laugh of the bad ideas. The absurd shows that something is wrong. Do not worry I have done my share and gave some nice occasions of laughs, I've survived and learnt to be more cautious.

    The Occam's razor of ideas is practicality. That must be feasible, as simple as possible, reasonably reliable, economically interesting and most important give some accountable advantage. Otherwise it's dreaming about vacuous ideas, that goes nowhere.
    There are plenty of complicated bad ideas that try to look clever and hitech with impossible claims, You Tube is full of that.

    I did not insult, treated of bad names, or ridiculed the person himself. I have just showed the mistakes and explained why. I did not talked like authority. I reckon I pushed a bit the fun about the backpack.
    More seriously. A 70 pounds backpack and 30 km walking under the sun was an extremely harsh punishment used in the French Foreign Legion. The soldier could die from exhaustion and dehydration, and that happened. That gives you a preview...
    Ad Hoc likes this.
  5. upchurchmr
    Joined: Feb 2011
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    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    I'll take Ilan's comment as reasonable, practical, and respectful.

    You want a nasty shortsighted comment just get me wound up.

    There is absolutely nothing wrong with knowledge reasonably provided. We need a lot more of that. In a lot of walks of life.
    If forms a basis for a realistic conversation which can provide lots of people with useful information.
    And still room for alternative ideas if actual facts are provided.
  6. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Unfortunately that is where personal subjective interpretation comes into a simple engineering reply.

    If a client asks can i fit XX and YY and ZZ into a small 10 foot barrel of a boat and go around the Oceans for months on end using just solar power making my own water carrying AA and BB too etc etc - as that is my dream, and the naval arch says no. I can guarantee you said person won't be too happy. Their bubble has been burst, their dreams shattered. Of course they will view the reply as possibly aggressive, rude disrespectful etc. Those are all subjective emotions - not facts or engineering. Why ask a subject matter expert a question to an answer they seek in the first place - if one's mind is already made up?!

    There are no emotions in engineering. Only in the minds of those recipients requesting information.

    Many people do not like a cold hard dose of reality.

    I'd love to have a Ferrari or Aston Martin, ...but it wont persuade the salesman to let me take one home, simply because it is my dream, despite offering them just £10 for the car - as that's all I can afford.

    Everything in design is a compromise - some accept this, many do not. The facts are black and white, the personal interpretation of such - aahh..that is not. But it doesn't alter the facts.
    Ilan Voyager likes this.
  7. rob denney
    Joined: Feb 2005
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    Location: Australia

    rob denney Senior Member

    On what do you base this?

    The best shape bottom for any boat where performance and comfort are important is as long as feasible and as narrow and shallow as fits within the SOR. Unless it has to tack, it does not need to have rocker, which not only causes the boat to sink as it goes faster, but means you need a floor inside to get a flat, level surface. Putting this floor in is one of the most miserable boatbuilding jobs. Plumb ends maximise the length.

    The easiest hull shape to build is a proa as it does not have to tack, so does not need rocker and the bow and the stern are the same. Choose the design carefully and the hulls are the same, except the lee one has an extra section in the middle.
    As RWatson said, The easiest boat for a nervous wife is a harryproa. There is no changing sides, no boom anywhere near her head and no foredeck work for her to get scared that you are going to fall off. In my experience, the thing that scares novices most is flogging ropes and sails and the feeling that the boat is often out of control. The best rig to alleviate these is an unstayed mast with a wishbone or other self vanging boom and a fully battened sail. At the first sign of a drama, the sheet is released, the sail weathercocks and the boat sits quietly. Sort it out at your leisure, then go sailing again.

    The easiest boat to build is one that has no finishing and minimal contact with dusty, sticky, toxic materials. Almost certainly, this will be Intelligently Infused. INTELLIGENT INFUSION – Harryproa The only other real option is plywood. This will probably be more expensive, and the hulls will be between 1.5 and twice as heavy.
    Last edited: Dec 3, 2018
    Doug Lord, Dejay and rwatson like this.

  8. W17 designer
    Joined: Oct 2010
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    Location: Vermont, VT

    W17 designer Senior Member

    While I am reluctant to chime-in on many of these early design debates, I was pulled to this one for a couple of reasons.

    First of all, I need to say that I totally support the words and advice of IIan Voyager, Messabout & Ad Hoc. My own 70 years of designing boats of all types, totally backs up their conclusions, so yes, for anyone new at this, it’s worth taking note of their experience, without giving up on new ideas that might potentially solve old problems.

    Secondly, the originator of the thread was looking for the pros and cons of various hull shapes, say a flat bottom compared to a rounded one etc. and as I was asked to address that specific issue in 2018 by PBB, perhaps pointing the reader to the article could be helpful. See this link:

    As many will know, although I started sailing and designing small sailboats in the 50’s, I spent 40 years designing far bigger fish - ships up to 600ft. Many might then well ask, “what the heck has that got to do with light sailboats and trimarans etc??” Well, a lot more than you might think.

    Unlike most sailboats, test models are made for most large ships and as these models are typically 12-20ft long, the visual results and data collected can be directly applied to small boats, unless a lot of heeling is concerned. (My background also puts my ‘ear to the ground’ re other related ‘ship’ developments – see below). As the reader will see from the above mentioned article, there’s definitely a case to be made for the simple box section provided the hull is slim, as is the case for a trimaran. Keeping the sides vertical and the displacement as low under the water as possible, is the secret for low wave-making. This same concept led to the creation of a Norwegian trimaran ferry that subsequently convinced the US Navy to adopt a similar ‘boxy main hull’ for their new 45 knot LCS ships (Littoral Combat Ships with ‘Littoral’ meaning ‘near shore’). I invite readers to check this out (Wiki can help) and if any of you still doubt its performance, do you really think the US navy would throw $billions into ‘a significant series’, with several more repeat order on line for 2019-22, despite the prototype being 3 times over budget? Of course, this is ‘a motor boat of sorts’, so the amas (outriggers) need only be ½ length, but just look at the section of that hull and then a glance at my W17 might start to make more sense.

    In the case of the sailing trimaran W17, I have also done all I can to enhance windward performance and minimize lateral slip or ’leeway’, so as explained in my article, this will offset the potential gain from using a rounded hull at most upwind speeds, something particularly advantageous when sailing in shallow water when the W17 can still go to windward with under 2ft draft.

    So hope this ‘somewhat different take on things’ is of interest to readers.

    But yes, it’s important, before ANY new design can be successful, to establish what IS your personal SOR? (Statement of Requirements) and as wisely pointed out by ….. your noted SOR would indeed require a much larger boat, with something like my W22 being a minimum size for your needs. So. as others before me have said, either cut your needs or up your budget and boat size, as your initial plan is just not technically feasible without winning the lottery. Good luck to you.

    Please note that many more quasi-tech articles that might answer future questions, are available free at
    Doug Halsey likes this.
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