Walnut - a 7-foot dingy

Discussion in 'Boat Design' started by kvsgkvng, Feb 22, 2012.

  1. kvsgkvng
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    kvsgkvng Senior Member

    Hello everyone!

    Here I show the design of a small dingy. There are no options in the size of this boat, it must be within 7'x4'x1.833'

    I used freeware Delftship to create this model. Then I used CAD to unfold the panels. The white rectangle is the standard 4x8 plywood sheet.

    I also attached the model file and the DXF export with all data and completely unfolded panels.

    Would you please take a look at this boat and kindly let me know if this boat is seaworthy? If yes, I would continue thinking how to add a habitable cabin and some sails.

    Thank you for your time and feedback.
     

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    Last edited: Feb 29, 2012
  2. die_dunkelheit
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    die_dunkelheit NA Student

    Maybe it's in another one of your posts, but I have missed it, what's the intended purpose of the design? I ask because of the mention of a cabin.
     
  3. Petros
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    Petros Senior Member

    Cute.

    It would probably float okay, how low do you intend to sit in it? that would make more difference on such a small boat than changing the hull shape. I would consider making the transom wider to allow a little more interior room and it will increase the static stablity a bit.

    A cabin on such a small boat? Go with a boom tent, or a self supporting fabric canopy that can be folded away.

    There are lots of similar designs available, many for free, why go with a new unproven design? I suspect some of those floor panels at both bow and stern might be a bit tough to warp into shape. If you want to experiment than that okay too, but be aware of the pitfalls of developing a new design, might take several builds to get it "right". With something this small it would not be that costly, but you have to enjoy the creative process of building, and rebuilding, or it just becomes a chore.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Without a SOR for the design is difficult to offer an opinion about how well it fits it's needs. On such a small craft, I'd be looking to maximize it's initial stability, by flattening the midship section a bit more. This will benefit the stability curve, offer more accommodations for the crew and the ride will be better at displacement speeds. I also agree with Petros in that a fatter transom would be helpful across several areas, but again without knowing her SOR, these are arbitrary guesses at best.
     
  5. lewisboats
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    lewisboats Obsessed Member

    My first impression is that that forefoot will play hell with tacking...I would reduce it somewhat.
     
  6. kvsgkvng
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    kvsgkvng Senior Member

    Walnut 5 as suggested

    Thank you all for your opinion and constructive advice. I tried my best to accommodate all suggestions. As the result, I have "Walnut 005" for your review. It is boxier and is surprisingly more stabe. Developability, lofting, displacement, resistance seem to be fine.

    Here are the answers to raised issues:

    Q. What's the intended purpose of the design?
    A. I plan to use the boat in lakes, rivers and in protected coastal waters with occasional moderate waves. I plan to navigate major rivers and occasionally make passages near coastal shores. In general I plan to use it for moderately prolonged periods of time, most likely for several weeks at a time.

    I would like to outfit this boat with a sail rig of some kind, a small lead ballast, and/or some sort of electric accummulator instead in the middle and fit somewhere a centerboard. Supplies and other things would be stashed on each side. I also plan to have some sort of very small outboard engine and small solar panel on the cabin roof.


    Q. How low do you intend to sit in it?
    A. I plan to sit in semi-reclined position, on cusion placed on the floor and shift my weight from side to side to compensate for tilt.


    Suggestions:

    (a) I would consider making the transom wider...
    (b) I also agree with Petros in that a fatter transom would be helpful across several areas...
    I did widen the transom and the boat appears to be more stable.



    (c) My first impression is that that forefoot will play hell with tacking...I would reduce it somewhat
    I think the boat is so small that changing the course would not be a problem. Of course, the forefoot could be little less steep.


    (d) SOR - I could not find any unique definition, my most probable guess, that this term has something to do with the purpose of the vessel.

    Perhaps this little "vessel" is not even near to those shown in the galleries, but I think it is more dear to me as I can build it myself! The only thing is to watch out not to build a fluke. That is why I again humbly ask for the opinion of people who know this subject.

    Thank you.
     

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    Last edited: Feb 29, 2012
  7. bntii
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    bntii Senior Member

    Take a look at Geoffrey Nightingale's Dinghy Ownership - Mainly for the Non-Racing Man

    It's as fine a book as one is likely to find on dingy sailing and cruising. I have an older copy from the late forties, but the book is now available as a reprint.

    The book is well worth a look before you build for the ideals contained.
     
  8. die_dunkelheit
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    die_dunkelheit NA Student

    SOR= Statement Of Requirements

    Going just a bit longer would make it much more comfortable for the intended purpose.. You plan on overnighting in it but there won't be much room to lay down in something that short, make it say 10' and you've got something a bit more doable. Staying on it a week? Portapots only get so small, and will probably be one of the bulkiest things on the boat. Colman makes a pretty little one, but it's not exactly a "marine" portapot. I know that a lot of the lakes around here have a minimum requirement regarding portapots or they won't allow you to overnight on the boat. The smallest of your choices are a 3.5 gallon bucket and a fitted toilet seat (link) or a slightly more specialized version with the same idea (link) but with both you'll have to use these bags (link) to solidify your poo.

    And unless you're okay with ham sandwiches for a week or two you'll need some cooking apparatus. I've used a one burner propane stove before and they're quite small.

    It's a fun idea, but take into consideration if you will be able to fit a week's (or two) provisions in such a small boat. (which is the food, water, portapot, cooking apparatus, sleeping bag, etc.)

    Maybe take a look at the Dinghy Cruising Association or the US Wayfarer Association Cruising page for ideas.
     
    Last edited: Feb 23, 2012
  9. kvsgkvng
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    kvsgkvng Senior Member

    Do I need a dinghy -- to be or not to be?

    Thank you for the suggestions. Most likely, I stated my far-reaching goals in regards to the "dinghy cruising." In reality, it may end up as a fishing trip on the nearby lake.

    One more thing, the main purpose of this exercise is I going after the knowledge, experience of the new and ability to make it happen.

    I could have bought a decent size sailboat, as I have enough for that, and start right there...

    If this project happens to be what I expect, I think it would become a tender for my next project.

    So, is this boat "no good?" Should I continue posting the progress and more details?

    Thanks everyone.
     
  10. die_dunkelheit
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    die_dunkelheit NA Student

    I'd reduce the forefoot a bit, which would give it a fairer entrance angle among other things. The bow sticking out with such a thin profile would make it act like the forefoot on a full keel (in my mind at least) and that would make it harder to steer or at least make it turn a really big radius for it's size, to me you want a tender to turn on a dime... On the note of it maybe being a tender, it is a wise move to build a smaller boat before your bigger project, especially of the bigger one will be built using similar methods. That way you familiarize yourself with the process. Back to the forefoot, lessening it would also make the form easier to develop from ply.
     
  11. kvsgkvng
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    kvsgkvng Senior Member

    To my untrained eye this looks fine

    Here is a rendering of the "Walnut_006" dinghy. Sail is controlled by the main sheet and furler rolling. Solar panels for lithium-ion battery is placed on top of the rigid cabin. I wonder if I made any principal error(S).
    Thank you, all.
     

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    Last edited: Feb 29, 2012
  12. die_dunkelheit
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    die_dunkelheit NA Student

    Before you go with an aftmast rig read this thread Aftmast Rigs
    I'm not saying that an aftmast is bad in any way, heck it may be the smallest boat to use one which carries a little bit of bragging rights I guess, just do your homework on it. I know it's just a render, probably a rough one, but the clew can't come all the way to the back of the boat.
    Being that small of a boat going, the opposite direction, with a cat rig may be best.
    Can't really tell from that angle, did you reduce the forefoot?
     
  13. PAR
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    PAR Yacht Designer/Builder

    There are obvious and less so reasons, a very small percentage of boats carry the aft mast rig. At this point, your best course is to gain some hydrodynamic and sail dynamic education. Gutelle's "The Design of Sailing Yachts" is a good place to start in regard to the sailing aspect. The hydrodynamic portion could be satisfied with several texts, but "Principles of Yacht Design" by Larsson would by the usual choice.
     
  14. Petros
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    Petros Senior Member

    I like it a lot, go ahead and build it. You will learn a lot and a boat that size will not cost much to build.

    I would go with a simple cat rig sail however, far less likely to give you trouble. Or even a single junk rig (perfect for simple singled handed sailing).

    If your intention is to use it for a tender later, I would make the whole cabin easy to remove, use stainless over-center latches or even a row of stainless screws down each side. IT would be more useful as a day sailor/fishing boat and a tender that way. When you want to go overnight you just attach the cabin. Or go with my original suggestion, a folding soft top or boom tent.

    One of the fun parts of building your own design is to experment with new ideas, and with a boat this small it will not take long nor cost much to build. Go ahead and build it as you see fit, incorporating the suggestions that you like.

    Good luck, have fun.
     

  15. kvsgkvng
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    kvsgkvng Senior Member

    Walnut is evolving with generous help of people in this forum

    Hello, everybody!

    Thanks for kind word and direction in which to go. I was thinking a lot about this dinghy and whatever I do, 7-foot boat is extremely limited. I was listening to the suggestions and one recommended to lengthen the boat to 10 feet. Oh, well I scaled it to 12-foot long by 6-foot wide thingy. On the picture it looks very, how to say it, harmonious. In my engineering experience there is a saying “if it looks good then most likely it will work fine.”

    I am aware that I need a lot of knowledge to pickup in order to even think that I may try to design boats. But I am not sure I would like to go this way. What I want is to have something to work on in the future, with your help good people, who might kindly point to me my gross errors.
    I did go through the “aft rigging” thread and there are many things positive and many things negative about placing the mast at the aft. What I learned from that info is that it works. I don’t care if I could squeeze 0.14 mph if I use this and that. Most important to me that it works, looks good and is easy to handle.

    I decided to use this layout because in my mind when someone is in semi-reclined position in 7-foot boat it would be very inconvenient to work on the mast which is at the bow. That was the only decisive factor in mast placement.

    Now I see that this location looks cool, works OK and is easier to handle. So, what should I be worrying about else?

    As a structural engineer well versed in composites I can easy take care of the mast with proper analysis and design. At this point I just believe (without numbers to prove it) that the mast is going to be OK. In extreme case I could provide three stays, two on the sides and one on a small outside boom facing backward.

    Statically, all the centers are pretty much in the middle of the boat. Those stability curves seem to be up high there. The pictures look nice. However before expending /time/effort/money/ I would be most appreciative if you readers would voice your concerns. I mean did I make any bloopers?

    So, making it easy as pressing a button in the pole, is this boat going to live or die?

    Thank you all for taking your time and helping me in this quest.
     

    Attached Files:

    Last edited: Feb 29, 2012
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