First Boat Design (11' Pram)

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

  1. Rakali
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    Rakali Junior Member

    Follow Up

    Thanks for all your comments. I came across this planing pram the GV13 (it's a powerboat I'm hoping that won't matter)

    http://bateau.com/studyplans/GV13_study.htm?prod=GV13

    I'm trying to adapt my design to be a bit closer to it. One thing I notice is the aft section is much more boxy so I widened my transom. The deadrise is the same as mine (10 degrees) so I see no reason to change that.

    Can anyone suggest other changes I might try and also let me know what I might be sacrificing by making these changes?

    Thanks,

    Matt
     
  2. Rakali
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    Rakali Junior Member

    Update

    I found a better drawing of a GV14 and revised my lines a bit (GV14 above the redline my new lines below. Full drawing in Carene below that).

    The transom is now lower and wider. To my eye the only significant difference now is the entry at the bow. Not sure if that will effect things dramatically.

    What say you? Will it plane? Also what am I sacrificing with these changes?

    [​IMG]

    [​IMG]
     
  3. messabout
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    messabout Senior Member

    Now the boat will plane. The keel is straight from about midships all the way to the transom. It has become a planning boat while the first drawing showed a displacement boat. The significant difference is the line of the keel aft.

    The boat in the new drawing will sail but it will sail poorly because the transom will be dragging when sailing flat or heeled. The unwanted reality is that you can not have it both ways.

    You can compromise some of the design features so that the boat will plane and still sail fairly well but you will be giving away some performance in both regimes.

    Almost any of the several computer programs will draw your boat and cough up dimensions, ratios, area curves, prismatic coefficients, skin area, wetted surface, etc. What the program can not do is design the boat for you. The same ol' same ol': GIGO. You have to tell the program what to do, it does it. It is up to you to input the features that are as near as possible to being appropriate for your design aims.

    Are you sure that you want your boat to have a forward transom like a pram, garvey, or Jon boat?
     
  4. Rakali
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    Rakali Junior Member

  5. sharpii2
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    sharpii2 Senior Member

    I think what passes as "planing" for most sailing dinghies is actually Semi-displacement speed.

    The shape you originally drew would probably reach that, given enough sail area (SA).

    It might reach eight to ten knots when lightly loaded, with the crew perched on the side deck and sufficient amount of SA and wind.

    The bigest problem I see is inadequate displacement.

    Tyo can pretty much count on the boat weighing at least 50 kg (110 lbs). I imagine you weigh somewhat more than that.

    I would aim for a displacement of at least 150 kg, so there is a tight but reasonable weight allowance for boat and crew.

    It is difficult to have a planing boat of that size which can plane while loaded down with crew and camping gear. And when it's not planing due to such load, its sailing speed might be slower than a pure displacement hull of the same size, load, and SA.

    Scows tend to plane quite readily due to thje enormous lift of their wide, flatish bow sections. You can check out youtube videos of puddle-duck Gease. They are puddle-duck racers with stretched out hulls.
     
  6. Rakali
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    Rakali Junior Member

    What I meant to say was....

    My notion is that the boat would only plane when sailed unladen and probably sailed solo. I'm not expecting it to plane with crew and gear. I know that would be asking too much.

    I just want the hull shape to be enough of a compromise to be stable first and foremost and fast when the occasion allows for it.
     
  7. PAR
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    PAR Yacht Designer/Builder

    If stability is higher on the priority list, the ability to plane off will be greatly diminished. Once you get the shape right with full plane hull forms, it's simply power versus weight. So the simple way is to estimate what it'll take to get your boat to plane off and provide sufficient sail area. For example 100 sq. ft. of sail area is about 2 HP, so if you think your little boat can get up and scoot on 2 HP, than 100 sq. ft. of area is the ticket.
     
  8. Rakali
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    Rakali Junior Member

    Thanks to everyone for your input. I have come full circle in my design and am back at the first design I posted. I now feel planing isn't something worth sacrificing for.

    I may still reduce the bow angle and lengthen the waterline as was suggested.

    I'll keep updating as this project progresses.
     
  9. peterAustralia
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    peterAustralia Senior Member

    ahem

    I guess your stubborn and want do design your own unique boat. There are a lot of existing boat designs out there. Earlier I gave a link to a list of 390 different rowboat designs.

    Your design is about 5ft in beam. (just under), what about a Skylark 10 or Skylark 12 from Selway Fisher (12.0 x 5.0 ft) weight comes out at 100 pounds according to their website

    Very similar to what you are planning. Also there is piccup pram by Jim Mickalak, plans cost $23 (11.0 x 4.5 ft)

    http://duckworksbbs.com/plans/jim/piccup_pram/index.htm
    http://www.selway-fisher.com/Skylarkup13.htm#AR

    save yourself heaps of time and go with a plan, build a really nice boat, enjoy yourself, get on the water. I kinda feel I am pushing water uphill on this one though
     
  10. Rakali
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    Rakali Junior Member

    Peter I really appreciated the link you sent with all the many designs. I already shared it with some friends. The reason I'm designing the boat isn't stubborness. The boat is part of a larger project I'm working on to design an easy to build, cheap, shippable (standard UPS size) boat. This design is part of a proof of concept for the project. Although there are 390 designs on your page I'm fairly certain none of them meet the exact parameters for this project. They do however provide an excellent design resource for an inexperienced designer such as myself. Thanks for your help.
     
  11. peterAustralia
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    peterAustralia Senior Member

    ok
    fair comment, I will leave you to it.
    Aside, if you or your friends want to take a copy of the list, and put in on another website, please feel totally free, I am looking for a place to put the list longer term, and stop my bandwidth being exceeded every month
     

  12. se7ven
    Joined: Sep 2015
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    se7ven New Member

    Hi every one.
    I make the hull of ship by rhino (creat surface). then i want to import to Shipcontructor for expanding, but i don't know to do it. please help me.
    I cant find where to post my message show i post here. so sorry
     
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