First time poster seeking advice

Discussion in 'Boat Design' started by mbowser, Nov 23, 2015.

  1. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Agreed Tad and the stem curve in the profile of the first chine will be impossible to close. Another solution might be to put in a partial chine in the garboards forward half. This would also remove much of the remaining entry hollow too. This partial chine would just blend into the garboard around station 10. To do this you could simply slit the plywood in the forward garboard, which will create a long pie wedge shaped gap, that can be filled and taped over (assuming a taped seam build). I (and plenty of others) use this technique to get odd shapes (like Carolina flare bows for example) in a predominantly developed hull.
     
  2. mbowser
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    mbowser Junior Member

    Thanks, that makes perfect sense. I never even thought about how hard it would be to bend a panel in that area.

    I moved the first chine down and that did seem to flatten out the garboard bottom panels so there isn't as much twist. Does the attached seem more appropriate or will this just make for an uncomfortable ride in any kind of chop?
     

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  3. mbowser
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    mbowser Junior Member

    Thanks PAR,
    I think I understand what you are talking about in theory, but I am going to have to sit down with some scissors and construction paper to get a better idea of how this would work.
     
  4. Tad
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    Tad Boat Designer

    Yup, that's better. I don't want to tell you what to do but I would drop the first chine more aft of the stem, to take the hump out of the profile. Then I would raise the forward ends of the second and 3rd chine so they run out fair in profile....just for looks and to fine up the bow a bit.....

    Making a paper model, or a few of them, and/or a bigger one in doorskin, is an excellent way to get a feel for these shapes.
     
  5. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    I'm with TAD on the shaping. It might seem odd, but 'dropping' the chine helps eliminate the bow hollow in plan view. Also a little playing with the forward rocker can make a difference, just a few mm can have quite an influence. TAD has given you some very helpful pointers towards a more modern underwater shape which should work well.

    When you come to build, it is important to have enough temporary framing to hold the planks where you want them. They don't have to be screwed, pinned or anything just enough to stay where they should be. Weights, gravity, clamps whatever to keep accuracy. It is normal to butt join the front part of these planks and when the angle is sufficient change to a normal overlap joint.

    If you have a conventional hog, and keel capping, you will find you need to measure quite a bit when yopu shape the hog to get the forward part of the lower (garboard) planking to go exactly where it is designed to be.

    A stiff card or thin plastic sheet model will teach you a lot about what you can get. It is possible to get a very, very, small amount of 2 plane bend in ply, it is not as rigid as a steel sheet, but try and minimise the stress.

    I'd probably revert back to a vertical transom, partly because it is much easier to Datum the build stations in reality on the floor, partly for an easy rudder hanging. Not really a big deal but makes it a bit easier for a first build.
     
  6. mbowser
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    mbowser Junior Member

    Feel free, having only built a few small, simple dinghies from plans, I don't have any feel for how much a plywood panel can be manipulated.

    I'm not sure what you mean by the 'hump in the profile' but I dropped the chine further aft on this iteration (see attached). Do you think that is enough? The odd thing that is showing up on the developability check is that when I radius the forward part of the chine it gets angry, but if I remove the radius it seems fine. It appears to me that this section would not be a compound curve and should be able to be manipulated but I'm not sure.

    Thanks again for everyone's help, it is much appreciated. If I can arrive at a design that seems reasonable (from a functional and buildable perspective), I'll move to a prototype phase and build a scale model (I like the doorskin idea).
     

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  7. mbowser
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    mbowser Junior Member

    That probably makes sense, I like the look though. I'm going to keep it for now and if it proves to be too much added difficulty when building a scale model, I'll go back to vertical.
     
  8. mbowser
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    mbowser Junior Member

    So I made a bunch of the suggested changes such as dropping the forward chine further, flattening the transom and just generally cleaning up the design. Thanks again for all the input.

    I'm using Freeship! plus and decided that I would develop the plates and make a scale model. I had a 4x4 sheet of 5.2mm luan plywood and scaled it to use the full length (about .19) and built a model to get a feel for how difficult some of the bends would be.

    I probably did this the hard way, but couldn't figure out a better way to scale the model (I didn't see any scaling feature in FreeShip!), so I laid out a grid where 2 9/32" = 1 foot and then used the developed plates to mark off the panel point intersections on the grid. Then I used a batten to draw a fair curve for each panel and cut it out on the bandsaw. I tried to be reasonably careful, but I only had a few hours last night and just wanted to get it done. I knocked the big high spots off with a rasp, but didn't do much in the way of fairing the panels.

    Then I stitched it all together with zip ties and to my amazement, it actually worked. The model actually looks like the drawing and went together surprisingly well.

    So I guess I'm looking for what a next step would be. Is there a way I can predict the boats behavior given what I have done so far? Obviously a rig will need to be designed and that's another unknown at this point and I have hydrostatic calculations, but they are pretty much greek to me (I am studying up though).

    Is there any value in fully building the model and tank testing?

    Any additional advice would be greatly appreciated.
     

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  9. PAR
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    PAR Yacht Designer/Builder

    There's no "Cliff's Notes" to understanding yacht design. The same would be true about empirical observation in a tank. Simply put, if you don't know what's going on, it's a hunt and peck proposition at best, so how's your guessing abilities? Pick up a book.
     
  10. mbowser
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    mbowser Junior Member

    I figured; I've been reading Skene's Elements of Yacht Design (with calculator by my side), but it isn't a quick read. Can you recommend another?
     
  11. PAR
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    PAR Yacht Designer/Builder

    None of them are quick reads and there are dozens of texts on the subject in general with many specialized versions. Gutelle's "The Design of Sailing Yachts" might work for you.
     
  12. philSweet
    Joined: May 2008
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    philSweet Senior Member

    To scale for a model in Freeship, go to the transform tab, choose scale, and enter the global scale in all three coordinatates ie 0.19. Then go to the intersections button and delete all the waterlines, section lines, buttucks, and diagonals and create new ones at some appropriate spacing, like 4". Then SAVE UNDER A NEW NAME.

    When doing stitch and glue, Its better to use lap joints as much as possible. At some point near the bow, you will have to transition to a butt joint on the upper planks. But I hold on to a lap joint out to 45 degrees or so.
     
  13. mbowser
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    mbowser Junior Member

    Thanks, I knew that had to be a feature somewhere, but I didn't see it. I'm sure my cut plates would have been much more accurate. Oh well, I guess I'll have to save that for next time.

    Ok, I assumed that each chine would be a butt joint, wouldn't lapping the panel increase the panel's bent radius enough to cause problems or do you bevel the receiving plank's edge to eliminate the difference? Also, are you cutting gains at the transition areas?

    All told I spent about 3 hours building this model up and I wasn't too concerned with tight joints, knowing that there was a lot of error in my scaling process. I just wanted to see that the built shape approximated the designed shape, and I feel like that was a success.

    I like working with FreeShip! (slowly getting more comfortable with it) and enjoy spending time designing, but now I'm at the point where I need to decide if this is a design that I want to go forward with and spend money building. I don't mind wasting $10 here and there on luan plywood and some zip ties to build a few models, but I don't want to spend thousands on a design that will be a dud. I'm slowly getting up to speed with the basic mathematical principals of this field, but I know it would take a lifetime (or more) of learning to fully understand it.

    I guess I'm looking for someone with a studied eye to validate (or invalidate) this design for it's intended purpose (as PAR stated: "A performance oriented raid boat"). There are a lot of great existing designs out there that would satisfy my needs and I may end up with one of them, but right now I'm having fun learning the fundamentals.
     
  14. gonzo
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    gonzo Senior Member

    Glue lapping is time consuming and requires a lot more skill than simply taping. With taping, gaps get filled with thickened epoxy. I like the design. In your place, I would build it, put an outboard on and go for a ride. More fun, cheaper and educational than tank or computer testing.
     

  15. Petros
    Joined: Oct 2007
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    Petros Senior Member

    you have a decent design, typicaly minor tweaking on a hull this size will results in minor changes in behavior. More important is the design and placement of the sails, rigging, dagger board and rudder than detail changes in the hull lines.

    Consider that the sails are like the engine in a race car, and rudder and keel are like the suspension, if you spend all your effort designing the perfect race car chassi, and throw in any old engine, wheels and tires you have available, performance will still be very poor.

    I did my first stitch and glue plywood design last summer of a 14 ft dingy sailor, where the lower side panels came together at the bow looked very similar as your disign. I can tell you I had a lot of trouble getting it to close right there where free ship shows red. I ended up altering the design during the build, used a slightly wider peice of wood on the prow (it had a small flat there already, but it ended up being twice as wide, from abotu 1.5" max about half way up, to about 2.75"), and I used a small block of wood that I sculpted to fit right in the base of the prow, and cut the plywood back a bit. Glued in it with lots of filler, and than sanded all the corners off when it cured. This avoided having to force the plywood to fit right down to a near point with a small compound curve. I also put a small and short curf or dart in the plywood there to help it bend. with wood is was easy to make it work, but it did seem like I should have been able to make it all come together, but I fought with it too long and ended up using a filler block of wood there. It was the first time build of a new design, everything else went remarkably smooth, so this was not too bad a set back.

    Good luck with your design, it should work well. Go ahead and build it once you are happy with the lines, you will not be satisfied if you chicken out and buy plans, you will be a lot more satisfied and proud of a hull you designed and built all yourself.

    Another good book that does not go too heavy in analysis is "The Nature of Boats" by David Gurr. Gives a good qualitative discussion of why boats are designed a certain way, trade offs, and offers some simple design rules and relationships that are enough to design a decent sailing dingy.

    good luck.
     
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