Low speed planing hull theory

Discussion in 'Boat Design' started by Doh, Jan 5, 2020.

  1. portacruise
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    portacruise Senior Member

  2. fallguy
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    fallguy Senior Member

    I never said an 8' boat must be a monohull. But I know enough about boats to realize challenges in designing an 8 foot long planing catamaran for 2 people with electric power. And I didn't say that was impossible even; despite grim realities. I did not say planing hulls because that is implied by the thread title.

    And this is why I mentioned some confusion and questioned the development of the requirements.

    Things are a bit off.
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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Absolutely zero chance of a planing catamaran here, but you could make a very slim hulled cat work at a decent speed, but it would be a hobby horse job, and there would be no significant venturing away from the mid-length permissible by the occupant. But weight control might be the problem
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  4. HJS
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    HJS Member

    It is important that the planing surface has an effective spraystrip for it to function. This applies to all planing and semi-planing boats. This is usually missing on small RIB boats that drag the tube in the water. Also, the aft of the tube is usually cone shaped. This means that the boat sucks with great drag.
    In summary, these small inflatables are normaly unsuitable for higher speeds.
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  5. philSweet
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    philSweet Senior Member

    You'll need a pair of batteries out of a Prius to do that. 400 pounds of batts minimum (and about 6500 USD). If you can get an inflatable to plane for more than 5 minutes, I'll be really impressed.
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    His battery weight calculations seem way under-estimated.
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    There was a bloke started a thread about a "top secret" fast boat project that would debut a ground-breaking battery electric set-up, that go like a bat out of hell for hours on end, and he was belly-aching that the drives available to put this magic into practice were all inadequate. I think if practical planing boats with battery/electric propulsion are coming, and they well might be, they won't debut in an 8 footer built under someone's house.
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  8. Doh
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    Doh Junior Member

    thanks for all the replies :)
    I want to make something clear. I did not really ask for any advice about feasibility or interest of the project I want to build ( I stated it clearly on my first post, or I though so X-) ).
    I only wanted some feedback about raw planing theory (which I received from some reply, and I'm VERY thanksful for that)
    For the recall. I worked a few years in renewable energy (small wind turbines) and then in electric bike as of today. Why is that important?
    Because I fully understand your skepticism and really noticed the real-life orientation of most of the replies, that's, with all the love I can give for free, what I would do in the fields I'm confident in.
    I also know that any build, should be made and though for a specific use. That's how I built my wind turbines and later my ebikes :)
    The purpose of my post was to try to check if I didn't missed some ground theory monuments, because with anything related to fluid dynamics, it sucks big time as often non-intuitive.
    For any comments on reasons, interest, legality...etc thanks you, that's really nice ( I mean it) but in this case totally irrelevant to the actual status of my project. With this I really hope not hurting anyone feelings, it is really oriented on the theory side, actually to check "standard" boundaries, restrictions and then only, find a compromised-based solution to my needs.
    What I wanted to know, (and I mostly got my answer) was :
    -What parameters helps the hull to plan earlier
    -What parameter helps the hull to reduce power at low speed planing. And where are the draw back to specific designs.
    -Some kind of order of magnitude concerning hull resistance for an inflatable dinghy in displacement, semi and planing. with related parameters
    -Maybe some kind of hull geometry favoring a smoother displacement->semi displacement->planing transition... and its draw backs.
    -All of this with in mind a reduced boat length (2,5m) and an inflatable design under less than 3,6KW
    I hope that I didn't hurt anyone feelings there. Again, I want to get as much informations as possible. I love this forum, I actually really like the "hey buddy, check your math would you?" orientation of lots of answers" because that's what I would do, even if I'm pretty good to find some workaround/compromises/adaptation as soon as I understand raw physics.
    Finally, I built micro wind turbine Mppt+C years before they were implemented in big WT like the enercon or vestas, I have some very nice electric bikes extracting benefits of brushless motors for cruising and cargo purpose and live from it today. All this has been build in my shed, because no big companies see enough interest to commercially develop such products. There is more to just "having something that works better" when it comes to project development.
    I do not want to make any kind of revolution in the electric-boat world, I actually do not want to build and sell a boat (even if I'm currently building mine for fun/learn purpose) BUT I definitively want to bring a motor on the (local) market as well as an ebike for cruisers and e-dinghy users.
    Hope this clarify my post, as stated before, I'm here to learn about theory, make my experiments and adapt my ideas, expectations to it and again, not to start any kind of "revolution" not my style, not my interest ;)
    PS: For the "2 hours planing thing" it was just that I would love to have it.. but that would means planing at 2kW with a 4-5Kw (17-20Kg) battery. as stated before, this are just over optimistic goals which will be totally down-screwed as soon as I get real world, feedback + adapt them to my project.
    Sorry for the roman, hope this doesn't goes against any kind of forum rules :)
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  9. fallguy
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    fallguy Senior Member

    The short answer is planing is unwise for the parameters you desire. And so the thread is tricky...
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  10. alan craig
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    alan craig Senior Member

    Short answer: a wide flat bottom is the most efficient planing surface.
    The closer you get to the the bottom of an airplane wing the more efficient (and more impractical) it will be. Same applies to the propeller, use a prop designed for i.c. model airplanes - and hope that you don't meet any weed. Your estimate of motor and battery weight and endurance are fine, you obviously have experience of brushless and lithium. The photo of the planing table more or less proves that it can be done. I am guessing that the 2.5m length limit is to avoid the EU Recreational Craft Directive. You might find some useful posts in the electric propulsion part of this forum. Don't forget to post a picture when it's done!
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm surmising he wants a modicum of speed, whether planing or not planing, he speaks of a semi-displacement form, well that won't be practically achievable, I would say, without having something horribly "tippy". I would say a catamaran is his best bet.
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    An interesting point was made earlier, that whilst a flat surface will hold a plane, and get on plane more easily, that wider is not necessarily better, because it sets up more wave-making resistance at a point before planing begins. The weight is obviously integral to the calculation, but it could be a case of having a hull that takes more power to reach plane than a narrower one, but will hold plane with less power than the other. Obviously that becomes important when you have marginal power available.
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  13. DogCavalry
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    DogCavalry Senior Member

    At 150 kg and 3 imperial hp, a sea sled gives 22 knots. At least, according to Crouch.

    It is worth pointing out, that a planing boat only planes on a small part of the hull. Which of course is why Savitsky define the boats in his seminal paper solely on beam, not loa. Which means you could, for example, have a square boat. Ideally a sea sled, since they, again according to Crouch, get on plane easiest in a poor power to weight situation. A square boat would be pretty limited in top speed and seaworthiness, but obviously the boat we're discussing isn't going to be offshore racing, or circumnavigating.

    Not necessarily a square boat, obviously, but that's a thing to consider down in that size range. Beam 60% of LOA, maybe?
    Last edited: Jan 7, 2020
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  14. Ilan Voyager
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    Ilan Voyager Senior Member

    This thread is going nowhere...I'm French so I was asking myself about the strange rule the initiator of the thread invoked as I know only the rules about sail boats and motor yachts CATEGORIES BATEAUX PLAISANCE http://seme.cer.free.fr/plaisance/categories-bateaux.php. There are local rules of max speed while being close to beaches, and in harbor. There are specific rules for fluvial navigation but you do not use often planing boats in rivers, at least in western Europe and there are severe limitations of speed.
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  15. sharpii2
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    sharpii2 Senior Member

    My own impression is you need a wide, flat(ish), rigid bottom of four feet or more, at least at the stern. The rest of the boat, except perhaps for the motor mount, can be inflated. The wider bottom aft accomplishes two things:

    1.) It reduces surface area loads. The lower the surface area load, the less energy it takes to get onto a plane (and to stay on it).
    2.) It reduces transom depth. Reducing this reduces the low-speed turbulence.

    Since this width is needed most at the stern of the boat, the bow portion can be tapered quite a bit. It can even have a pointed bow.

    The rigid bottom need not have a water-tight fit with the inflatable portion of the boat, as long as the inflatable portion has its own bottom. The inflatable portion can nest on top of the rigid portion and be simply strapped in place. The rigid portion would have the engine mounting structure. This rigid structure would have to be assembled on site.
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