Best info source for stretching a hull

Discussion in 'Boat Design' started by Delaney, Jan 25, 2009.

  1. Delaney
    Joined: Jan 2009
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    Delaney Junior Member

    How can I get the scoop on how to best stretch a hull:?: IS there a particular book that would help?

    I am interested in stretching a hull after the vee in the front where the cockpit starts and the hull is flattening out.

    :idea: Is it as easy as adding the desired distance, about 2 feet, spread out among the last few frames, 3-5 of them? Will this keep the proportions pretty much in line? I could always add some additional support stringers between the frames longitudinally for support if needed.


    I am kinda of clueless here, but have great confidence in the minds of those in the forum. Thanks.:)
     
  2. daiquiri
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    daiquiri Engineering and Design

    If it is a constant deadrise design, you should have no problems in modifying the structure, within certain limits.
    If it is a variable deadrise hull, then things get a bit complicated.
    Can you post some dimensional data of your boat, and some pic or drawing maybe?
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The best method is to increase frame or station mold spacing to the desired length. There are practical limits to this technique of course. Much depends on the design you're building, which we don't have any information on, yet.
     
  4. rasorinc
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    rasorinc Senior Member

    If your plan has 8 frames just move them equally 3" further apart to give you 24" put an extra side and bottom stringer on each side if you want to feel better. Only respace frames aft of the end of the stem
     
  5. Delaney
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    Delaney Junior Member

    Here is the hull I am considering. It is Weston Farmers Sundance and it is the rear flatter portion that I want to add two to three feet on for more fishing room. Comments
     

    Attached Files:

  6. rasorinc
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    rasorinc Senior Member

    start spacing at the 4th frame from the transom and add the stringers I suggested and you should be OK from what I see. 2' --3' I would start at the end of the stem that would be frame 6 or 7 from what I see. add extra stingers on bottom and sides--they do not have to be wide. 1& 1/2" wide for the extra ones should work out fine.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Adding to the end of that hull will produce some weird (read ugly) lines at the aft end of the boat. It will also alter the handling of the boat substantially. Just figuring out what to do with all that tumblehome will be a pain in the butt.

    Frames 2 thru 8 are on 24" centers, with 7" hanging in the transom (ignore the transom). Station 1 is 21.5" and station 2 is 14", so without even spacing you'll have to do some math, but it's not hard (it'll work out around 26.5" for the 2 thru 8, 2.1" for station 1 and 1.2" for station 2). Personally I'd just change the station spacing on the loft floor, then redraw the frames with 24" spacing (you'll need a couple extra of course), maybe even cramping them up in the eye's of the boat (likely for pounding loads). My new station spacing is a close, but quite rough estimation. They will not produce a "fair" set of lines.

    Is there an "Altering Yacht Design Plans for Dummies" book available? Nope, you just have to understand how lofting works and problem solve your way through or select a 19' to 20' design that's better suited to your needs. Now there are books on lofting and some cover stretching a hull like this.

    Done properly, by respacing the station molds, you'll have a slightly finer hull form, which will improve mileage and performance, plus the looks will remain the same. Problem solving is what boat building is all about.
     
  8. Delaney
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    Delaney Junior Member

    Thanks for the reply. My other thought was to scale it up 10-15 % which would give me a little more width also. From what I understand that just involves a lot of math, but keeps all the dimensions the same.

    What do you know about CAD? :idea: Once a plan is on CAD can it just be scaled up automatically by pushing a few buttons:?:
     
  9. apex1

    apex1 Guest

    That looks like a good idea..............:idea: but it is´nt.
    Yes it is nearly as simple to scale the drawings on CAD. But the result is not what you need, and the design does´nt work. You scale up everything and thats a desaster.
    Follow PAR´s advice.......... he knows what he´s talking about!

    Regards
    Richard
     
  10. PAR
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    PAR Yacht Designer/Builder

    If you put the lines drawing into CAD you can respace the stations to suit your desired length. Respace the station centers only, don't be tempted to physically "enlarge" the boat. You'll find this doesn't work like it would seem. 15% additional length is the maximum I'd consider lengthening this design.
     

  11. Bob E
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    Bob E some day

    I'm more into smaller craft like canoes or marsh boats but have read a bit about lengthening a given design. If you have evenly space stations you can just add and inch or two to the spacing between each. The resulting hull should be fair if the original design was. As has already been pointed out, the new hull is also a little finer if the boat was lengthened.

    What has not been mentioned is that the two feet in added length is effectively not the smallest part of a boat but instead is the largest two feet of the hull (the two feet where the boat has maximum beam). In a larger boat such as yours (compared to a canoe) I would want to be sure that everything is still strong enough. You may be increasing the length by 10% but the weight of the boat and its carrying capacity is increased quite a bit more than that 10%. If the boat is already lightly built, all kinds of things may need to be beefed up.
     
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