First Boat Design (11' Pram)

Discussion in 'Boat Design' started by Rakali, Sep 12, 2015.

  1. Rakali
    Joined: Sep 2015
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    Rakali Junior Member

    Working on my first boat design in Carene. It's meant to be an easy to build (consistent angles, no rolling bevels) simple sailboat. I'd like it to be stable enough for a beginner sailor, fast enough to plane, and roomy enough for dinghy camping. As it's my first design I'd love some feedback from more experienced designers. The Mirror dinghy was my inspiration although I'd like a more open interior in the finished design. I made the first few images by importing the 3D DXF from Carene into SketchUp and texturing it.

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    Last edited: Sep 12, 2015
  2. JSL
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    JSL Senior Member

    Nice 'big mirror'. Should sail nice but will not plane. Camping...? learn to travel lightly (sleeping bag & backpack) and you are fine.
     
  3. sharpii2
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    sharpii2 Senior Member

    How much is this boat supposed to displace?

    Without this figure, you can not figure anything else out.

    As a camp cruiser, I can suggest an interesting layout.

    You could have a deck that just eight inches or so above the lowest point of the hull. Under this deck, you can store an impressive amount of gear, especially heavy stuff, such as water bottles, canned food, and anchors.

    The bow could have a bulkhead about two or three feet aft the bow transom. A fore deck with a hatch could cap this. In this space could go lighter things such as boom tent, sleeping bag, and clothes.

    This alone will not be enough to give the hull rigidity. You will need some web frames for this, maybe two to four per side, more or less evenly spaced. I'd make them as deep as the side decks are wide, about four to eight inches.

    The rear most frames would be covered with a lengthwise bulk head on each side. This bulkhead will enclose emergency flotation, as the bow one would. Each of these would need an inspection plate.

    The rest of the frame bays, one or two of them, I'd leave open. I'd install hangin hooks under the side decks there to hang pouches which could hold easy to reach tools and maybe your lunch.

    In lighter winds, you would sit on the low deck. In heavier ones, you could perch on the side decks.

    The centerboard or dagger board can be moved off to the side some, maybe six inches to a foot, so you won't always be tripping over it.
     
  4. Rakali
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    Rakali Junior Member

    Thanks for your reply. Why won't it plane and what could I change to help it to plane?
     
  5. JSL
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    JSL Senior Member

    I am not sure what speed you are after but planing would be about 9 knots if you had an outboard fitted... or sufficient sail(which I doubt.) To achieve this you need to straighten/flatten the buttocks & get rid of the rocker. This will also move the LCB (and the LCG) aft a bit which is desirable in a planing hull.
    Other members may have some good info for you.
     
  6. Jammer Six

    Jammer Six Previous Member

    Looks nice to me!
     
  7. fredrosse
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    fredrosse USACE Steam

    Looks to me that the transom is not right shape, should have sides almost parallel to the aft section lines if you want to avoid tortured shapes. Also check forward sections.

    Try DEFTSHIP or FREESHIP, good beginners boat software, and will show developable plywood forms, you need that to have a reasonable build process.

    Good luck, I know you could spend years learning about small boat design, but the simple hull you are proposing can get you going OK, and you are probably willing to accept something less than ideal here. Looks like good advice from others replying here.
     
  8. peterAustralia
    Joined: Mar 2006
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    peterAustralia Senior Member

    seems a very nice boat, looks pretty right to me
    If u want to design a boat for the fun of it, thats fine.
    If possibly you were of the opinion, I have to design my own, because there are no designs out there, I wont agree. Here is a list of 390 different rowing boats, am sure you can find one that suits

    http://www.tacking-outrigger.com/rowboat3.html
     
  9. messabout
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    messabout Senior Member

    A guess at the displacement where you have the waterline is not more than 100kg. The lines will be perfectly alright if the weight of the boat, its' gear, and the skipper do not exceed 100kilos. You have drawn a conventional pram that will serve you well if you do not expect too much from it. Under sail and a moderate wind you are not likely to see much more than 4 knots. It will probably row well enough to get you where you are going if the distance is not far. It will have all the power it can use with a 2HP Honda or similar engine.

    The boat is a displacement boat. Asking it to plane is not realistic. You might force it to plane with a 6 or 8 HP engine. It might even plane under sail in a half gale. If it did plane it would be a bit scary.

    Your best deal will be to take out a new sheet of paper and draw a 14 foot Sharpie or something similar. You can get the sharpie to plane under sail, if the sail is big enough and the wind is up. The longer boat will be far more friendly as a camper, it will go faster than the shorter boat with the same wind conditions. A longer sharpie or flattie is easier to build, It will carry more weight, and it won't be as easily stopped when you ram it into a large wave or wake.

    If you are just going to splash around in small bodies of water the pram will do just fine. If you have longer distances in mind, then the bigger boat is clearly superior.
     
  10. SukiSolo
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    SukiSolo Senior Member

    If the Mirror was your inspiration, worth remembering where it came from and the predecessors such as the Cadet (junior pram dinghy). Mirrors' plane OK under sail but are weight (crew) sensitive, they are OK under motor I'd say 4Hp max, and row OK too.

    The much more V'd entry helps the Mirror shape through waves compared to a full pram bow, however it might help to keep a bit more volume forward low down. This makes developing the panels a bit harder, just needs careful thinking. If you look at more modern designs (mainly very slightly longer development class boats) you may get a clue as to where to push the design forward a bit. The rocker will be more led by the total weight you want to carry, and if you wish to use it in rough water. Might be better to lengthen the W/L and accept a slightly less angled pram bow panel.

    If I were building it, I'd probably use 5mm, maybe 6mm floor ply with at least two stiff floor battens each side, maybe even a third if you have no side tanks. The Mirror gets a lot of stiffness from the I beam of the side tank ply panels. It also gets some stiffness from curving the bottom panels quite hard. Be interested to see how it may be braced and /or buoyancy tanks. Note also that the Mirror has a substantial upper transom construction to brace any outboard motor ie full width not just a local pad.
     
  11. kerosene
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    kerosene Senior Member

    The tool you are using gives very blunt tools for controlling rocker. It doesn't allow change in deadrise etc. Its quite limited.

    Its such a simple boat that I wonder why do you want to design it yourself.

    (btw fredrosse its just that one view where transom looks square. Its perspective in that particular view that makes it look messed up.)
     
  12. Jammer Six

    Jammer Six Previous Member

    I understand exactly why he wants to design it himself.
     
  13. kerosene
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    kerosene Senior Member

    I do too but its not necessarily smart. And my point is that its such a generic shape that the pride of saying its my design is somewhat mellow.

    I don't know why I am replying to you - as I know your posting history.
     
  14. DCockey
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    DCockey Senior Member

    Simple but nice.

    Transom shape looks fine to me.

    It's worthwhile checking the waterlines for several displacements, though the CG location is very dependent on where the occupant(s) sit and any gear is stowed. To avoid the boat feeling "tippy" the chine should be submerged for a reasonable distance, say a third of the length or more.

    The satisfaction of doing something depends on where you start from. A pram like this might be a trivial exercise for someone experienced in designing boats, though I've seen some simple designs by folks who claim to be experienced which didn't look as nice as this one.

    A simple shape is a good idea for a first design. Too many folks show up on this forum wanting to design a complex boat without any design experience.
     

  15. PAR
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    PAR Yacht Designer/Builder

    With some minor modifications to this hull, you could get her to plane off, though the elementary understanding of the subtle differences, in hull form and volumetric requirements, should be understood. I too think all budding designers should start at this scale, though typically with a baseline hull form that meets their fundamental SOR, which this one doesn't. Simply put, use a suitable pattern to model around and continue to study what makes things work, so you can see the differences. Maybe put another way, you're trying to design a Camaro, but starting with a Taurus as the baseline model to extrapolate your information from. Start with a Mustang instead, you'll have a much better chance of success. Lastly, as has been pointed out getting the "centers" right on small craft can be critical, because there's no real margin for error. Build a database of similar designs and look at their design decisions, particularly the volumetric and hydrodynamic portions of each example. You'll probably have to work out some of it yourself, but the exercise will highlight similarities, that can become handy when you're faced with the same decisions on your little boat.
     
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