4,8m Skiff

Discussion in 'Boat Design' started by GraemeR, Aug 11, 2013.

  1. El_Guero

    El_Guero Previous Member

    What is GA? I think General Aviation, and I gotta think that is not right in this context.

    Thanks

    Wayne
     
  2. GraemeR
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    GraemeR Junior Member

    General Arrangement

    Graham
     
  3. GraemeR
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    GraemeR Junior Member

    Here is a developability drawing with stress points near the bow

    I as going to make a card model once it got further iterated up the spiral.

    Graham
     

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  4. LP
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Have you done a calculation to see where your CoG is going to be?

    According to your print out, the CoB is 7% aft of midship and that doesn't sound bad unless your loaded boat puts it elsewhere. The picture of your previous pram looks like it is floating bow down, but that could just be the photo. It's not a good angle to really see it. I only say this because it's looking like your headed in the same direction again with this design. The Center of Gravity calculation with tell you if you are on track for a balanced boat. On a boat this small, you could get away with a weight and balance of occupants, gear and motor. Estimating boat weight at midship would put it close enough for general purposes.

    I would be tempted to pull the S out of the chine and make it all convex in a shape that suits other requirements. I think you need more displacement forward and modifying the chine will do this. You also have a large amount of submersed transom. For rowing or sailing a free transom would be much better, but displacement and CoB requirements may necessitate a partially buried transom.

    Good Luck and have fun!
     
  5. michael pierzga
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    michael pierzga Senior Member

    I can never see the shape of a boat when its computer drawn.
     

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  6. LP
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    LP Flying Boatman

    I forgot to say the most important (imho) part.

    Go into the "TRANFORM" menu and pull up the "Lackenby..." screen.

    Don't do anything here, other than to look at your forward and aft prismatic coefficients.

    Your total prismatic is at .79. This is good for a planing craft, but way too high for displacement modes. I suspect your aft prismatic is upwards of .9 and your forward is way down around .3-.4. You should be looking at something over all in the .5-.6 range. .54-.57 more specifically, but the first range will be an improvement. Typically, you'll have a higher Cp to the aft so you have that part right.
     
  7. GraemeR
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    GraemeR Junior Member

    Ah ha aft prismatic is 1.05. With tweaks fore is 0.68

    I'll bring both down.

    Many thanks

    Graham
     
  8. LP
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    LP Flying Boatman

    And then there is something else. ........

    To get accurate Cp readings, Midship needs to be defined at the location of the largest section area. This also can vary, but should be in slightly aft (5-10% ish) of the fore-n-aft middle (LWL in most if not all cases) of the boat.

    Technically, it's not possible to have a Cp over 1.0. Locate the peak of your "curve of Section Areas" and define this location as you mid ship section.

    Click "PROJECT", then "PROJECT SETTINGS", then select the "MAIN DIMENSIONS" tab from the dialog box and enter the value in the Midship location box.

    This will automatically change your Cp values to usable values.

    As you start moving things around, your midship section will move also. If you use the Cp values to help shape your hull, you'll need to go in and redefine your midship location to match the peak in your curve of sections to keep it expressed accurately.
     
  9. GraemeR
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    GraemeR Junior Member

    This makes a big difference, I think I will be redesigning for a few days.

    Many thanks

    Graham
     
  10. Martin B.
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    Martin B. Junior Member

    4.8m Skiff

    Hi Graham,

    Whilst you want to design your own "wheel", watch out, you may get a triangular one :eek: Round ones have already been designed.

    Your original Post indicated something like an open dual/multi purpose boat. Have a look at Michael Storer's designs http://www.storerboatplans.com - specifically the 4.73m Goat Island Skiff. This craft with a big history of home builds, has a hullshape rather like that shown in your earlier Post, is virtually the length requested and a sail plan with which you are familiar.

    [​IMG]
    [pic from Storer's website]

    For home build Threads see http://woodworkfoeums.com/f169/.
     
  11. GraemeR
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    GraemeR Junior Member

    Gents,

    I've made some changes to meet the Lackenby criteria. (See Cp fore and aft).

    The length has been increased to raise displacement without increasing the draft further.

    I still have the Rabl calcs to do, but does this look near to the right ball park, before I press on further please?

    Many thanks
    '
    Graham
     

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  12. LP
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    LP Flying Boatman

    You should define some section cuts. Three buttock sections would be good equally spaced from centerline. Typically, hulls are divided into 10 section lengthwise from along the LWL. I put my waterline cuts fairly close below the waterline to see what shapes are taking place there and wider apart above. These lines help to define and visualize the hull. Then if you turn off the colors in the lines plan, everything is a lot more clear.

    You carry your chine really high for a long distance aft. Do you have a purpose for doing so?
     
  13. GraemeR
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    GraemeR Junior Member

    Thanks for your comments. I'll add some cuts.

    I raised the chines aft to reduce the aft Cp. Not sure if it's the best way ...

    Graham
     
  14. Petros
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    Petros Senior Member

    I finally located the old magazine with Great Pelican plans (after two days of searching). I have scanned and attached the whole article. Now that I look at it again it is not complete detailed plans, most of the plans they publish in the old Mechanix Illustrated were complete, just small size. But there is enough information to study the design, and likely build one for yourself if you are used to figuring things out for yourself.

    If I was building one I would keep the lines the same but use a dagger board rather than a swing keel, and likely modernize the rigging details. but lots of these boats have been built and are well loved by their owners, I still see them around the Puget Sound area. They are remarkably roomy inside, with a roomy cockpit too. I have considered it since a roomy cabin on a small boat makes it much more useful in the local damp climate. Of course you can leave the cabin off if you like. I have also seen this built with an aft and forward cabin, center cockpit and full conventional junk rig. If you need a higher quality scan, PM me your email and I send them. The forum limits the size of the attachments so I had to resize them down.
     

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  15. GraemeR
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    GraemeR Junior Member

    Petros

    Many thanks for your efforts,

    I'm busy looking at different designs and comparing the data.

    Many thanks

    Graham
     
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