Novice boat building question

Discussion in 'Software' started by rapscallion, Aug 31, 2007.

  1. rapscallion
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    rapscallion Senior Member

    I have freeship and I have designed hull and deck shapes I really like; I'. thinking about building them. What resources do I use to figure out how much structure I need to make the boat strong enough for the rig and all of the of the loads....
     
  2. alan white
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    alan white Senior Member

    Those scantlings are found by a combination of experience and formulae based on rules of thumb. In other words, boat scantlings are not generally engineered. They are arrived at by simple rules, adjusted to the particular requirements of the end-user.
    There is no steady sea state, and therefore no means to pin down the forces to be expected. "strong enough" is meaningful only when you add, "for such and such of a condition".
    The formulae you use to determine "how beefy?" has been derived from what worked in the average of all those conditions. From there, you can adjust the formulae to suit your particular expected conditions.
    Depending on the building method, there are plenty of books out there with reliable scantling formulae. Dave Gerr has written a few, and many other marine architects have written good texts as well.
    If this all sounds like alchemy, well, I guess I've made my point.

    Alan
     
  3. PAR
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    PAR Yacht Designer/Builder

    Before I go on to disagree with Alan's reply (a rare thing in most instances) I'd like to again voice my dislike of free or low cost "yacht design" software. Yep, you can develop some pretty shapes, textured and cleverly crafted surface models, but without an idea of what shape is a good one or how a slight change to a shape may affect performance, usability, stability, trim, etc. you're shooting in the dark. In other words, unless you've stayed quite close to a known, successful design, the software will not tell you if the "yacht" you've designed, will do as you hope it should, let alone float with the decks facing up, once someone steps on the rail to board her.

    Yacht design is an engineering discipline (several) and the structures are highly engineered. Yes, there are some guidelines and published "scantling rules" suiting the current flavor of the market (Dave Geer's book is an exception), but anticipating loads, stresses and the other variables does require a fair amount of study, plus a healthy dose of experience goes a long way too. On vessels with performance envelopes expected to fall within certain goals, this can be demanding from an engineering stand point.

    Rapscallion, the first thing I would do is determine what you need from the design and work this into a "design brief" with a priority list from most important to least, in regard to your desires. I'm just completing a 24' cutter design and the design brief for this boat (in order of client priorities) non-capsizable, trailerable dimensions without permits or the need for a big truck to haul her (dry weight is just under two tons), moderate D/L ratio (272), comfortable motion, displacement performance, about an 18 SA/D (got it done with 18.3), standing headroom (had to use a pop-top feature), less then 48" of draft (came in at 40"), good steering qualities - but not twitchy, inboard diesel, substantial bulwarks forward, rig preferences and the list goes on. Clearly this list shows the client had special needs for this boat (surrounding his wife mostly) and distinct preferences for the rig, build method, hardware, appendages, hull form, etc., in short, an experienced sailor.

    From the designer's stand point, the first thing I try to do is develop a general accommodations layout, using the design brief and see how much hull I need, to stuff all the desires into. This gets me in the ball park and I then work up a hull shape, again with the brief in mind, that makes a big enough whole in the water to safely support this load, with a reasonable margin of safety. At this point I've preformed little more then quick sketches, usually the old fashioned way, on paper with a pencil, which I find much faster then software. I then use rough guides, that I've developed for weights of different construction methods, to determine a reasonable displacement and it's on to "Convoluted, Concessions in Compromise" - tough choices time. This can often be a hair pulling exercise, as the client wants shoal draft and standing headroom in a 20' schooner that of course doesn't look "boxy".

    The quick answer to your questions Rapscallion, would be to pick up a few of the many design texts available. "Elements of Yacht Design", "The Design of Sailing Yachts", "Elements of Boat Strength", "Yacht Design and Planning" (for the nostalgia). A quick tour of the book store on this site can offer many of these titles or your favorite book seller.
     
  4. alan white
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    alan white Senior Member

    No disagreement, I think. There is a lot of engineering that goes into a boat's design. My comment had to do with the aspect of design that produces scantlings. In a wood boat, for instance, does your computor tell you what the frame dimensions will be, or the hull materials/thickness? Or of FG, the schedule of layers, type of glass, and resin type?
    A navel architect will trust the computor to tell him what the stability curve looks like, rigging loads based on geometry and known values, but little more than rules of thumb for scantlings can be written into the program anyway.
    What I mean is one would certainly accept as truth the weight distribution of a hull when the computor told you, but if the program told you that the transom ought to be so-and-so thick plywood, you might say, "That's what YOU say, but you never saw that wave that I did off Hatteras that August morning six years ago!"

    A.
     
  5. PAR
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    PAR Yacht Designer/Builder

    I do use software that provides some options for scantlings. Mostly these are area and mass calculations. An example would be steam bent frame spacing changes, without gaining weight or loosing strength. The options would could include, different planking thickness, molded and siding dimension changes, etc.

    I do have to admit, I rely on "known" (specifications that produced structures, not too light nor too heavy, for the application, etc.) combinations for scantlings, but I've been at it a while. On designs where a performance goal is desired, I will review the "usual" scantlings and see if improvements can be made or on rough service craft, I'll look at ways to beef up the structure without too big a weight penalty. Does this mean we agree Alan?

    In the end, I find it's very easy to over build the structure, producing a "burdened" craft. This is especially painful in small boats, where weight distribution, hard points and other issues can bite you in the butt.
     
  6. rapscallion
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    rapscallion Senior Member

    Thank you all for the insights. I picked up several of the books mentioned. I want to build an 8 meter racer/cruiser and I want it to be strong and safe, but light and fast. I have a laser 28, and I love the boat. I would like something like the laser, but with a lifting keel so I can launch it from a ramp. That type of boat simply isn't in the market at them moment. I'm a chemist at a composite firm, so I can get epoxy at cost. I just need to design a boat that I can afford to build and one that won't fall apart on lake Michigan.
     
  7. alan white
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    alan white Senior Member

    Sounds exciting. Lifting keels aren't seen too often out there. Yet, except for the intrusion into the interior, they are extremely practical and effective for ramp-launched boats that can still stand up to a breeze without resorting to flat bottoms and wide beams. The road-legal 8 1/2' width limit (many US states) works for sailboats upwards of 24 ft, but over that, seriously limits designs that depend on form stability (excepting Bolger-type designs with absolutely flat bottoms). All the more reason to get some real ballast down low. A J-boat hull might be a good departure point.

    A.
     
  8. rapscallion
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    rapscallion Senior Member

    I was thinking about starting with a T8sc as a starting point. I wanted to start with a stealth 8m, but all of the links I had to that information are inoperative. I also like the Ross 830 hull shape.
     
  9. PsiPhi
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    PsiPhi Newbie

    I'm a newb, and all I know about boats is what I have learned here, but now I get to disagree with one of the senior regular contirbutors and I know what I'm talking about.
    PAR, you're wrong [hey, this is fun] about free software, some of the best software in the world is free software (so is some of the worst, yes I know). Open Source projects get developed by software engineers who write programs for the same reason you build boats, and generally without any commercial pressure, inept managers, or political sub-agenda's screwing things up.
    I don't think Freeship is the problem, I think knowing how to use it is, and judging from what I have read by the time anyone knows how to use it properly they probably don't need it anymore anyway :rolleyes:
    I haven't used it because I wouldn't know what to do with it, wouldn't know a good design from a bad design. I played with Hulls, all the designs look pretty but it doesn't mean they'll float!
    Now, if you pay many of hundreds of dollars for software you will get help and support from the vendor, and maybe that's what is needed, but again, that's not the softwares fault - you pays you're money and you takes you're choice.

    Software is a tool, just like a chisel or a drill, you have to pick the right one for the job (and cost is a part of that decision) and learn how to use it, but isn't that half the fun of a hobby like this?
    Good luck with your project, and despite what I say listen to these guys advise, it's one thing you'll get for free that is worth having :)
     
  10. PAR
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    PAR Yacht Designer/Builder

    You're correct PsiPhi, in that software is a tool. The basic issue I have, is the current trend in developing these "tools" to generate lots of numbers, that the user doesn't understand, on a shape the user doesn't know, but suggesting it will be effective as a craft meeting the design goals and the pretty elements that can lull an unsuspecting user, into thinking they've designed a boat. In fact they'd designed a pretty picture and a list of mathematical data derived from things they don't comprehend. This is when it gets scary. I know of at least a few web sites, where plans are available, but a disclaimer (in very small print at the very bottom of the last page) suggest these plans were generated by a person that has limited (if any training) and that the "grain of salt" should just be eaten.

    Would you get in an aircraft designed by the bus boy at the local greasy spoon? What if the aircraft looked really nice and had innovative features and a fancy paint job? How about a ride in an elevator designed by the same lad, to the top floor of the Chrysler building? Okay, now how much father from shore then you can swim back to, will you risk your family, in a boat designed by the same fellow?

    Personally, I find this less then ethical. Boats, air craft and elevators in tall buildings, place people in an unnatural environment. It's not natural for folks to fall from 30,000 in the air, or plummet the length of an 80 story elevator shaft or swim back to shore while several miles off the coast. It's not reasonable to expect survival and why we insist on some sort of qualifier(s) for these types of engineering endeavors.

    If you can't qualify your rational, data and or design elements with a substantial, understandable reply, then you should honesty suggest you are an amateur and that all of your design efforts should be treated with the whole salt shaker, not just a single grain.
     
  11. PsiPhi
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    PsiPhi Newbie

    Yep, I agree entirely, I think.

    I guess the real point is not how much the software cost, but who developed it, why, and whether it's up to the job - and that last is not necessarrily a function of how much it costs to buy.

    The first thing I do when I buy a computer is delete the Window partition and install Linux. It's free software, but better than most you can buy for the job it does.
    I just had an echocardiogram recently - If the doctor had been using freeware I think I'd have gone elsewhere :)
    It's horses for courses. I personally use a lot of freeware, but then if it crashes and burns all I loose is a few hours (or days) work, I don't drown and nobody dies a horrible death.

    My advise: use freeship to make your designs and desires, then engage a bona fide yatch designer to turn them into a real craft. People do it all the time with houses and architects, and if it's a big project the cost is probably only a small percentage of the overall cost, and eventual value, of the boat.
     
  12. fcfc
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    fcfc Senior Member


    Do you know http://www.finot.com/bateaux/batproduction/beneteau/firstclass8/fstclass8_ang.htm (now superseded by http://www.beneteau.fr/UserFile/File/Voile/gammes/Monotypes/First_class_7-5/I_FC7.5_GB_230708.pdf ) or http://dixdesign.com/26didi.htm

    These may not legally apply to you, but may give a strong point on details to engineer beyond hull shape:
    http://imci.org/userfiles/other/downloads/RSG Guidelines 2007 en070525.pdf
    http://www.eagle.org/absdownloads/listdetails.cfm?id=112
    http://www.abycinc.org/standards/toc.cfm

    There are also (not free) ISO papers:
    http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_tc_browse.htm?commid=54258
    12217-2 (stability) 12215-5 (scantlings) 12215-6 (structural details) 12215-8 (rudders) 12215-9 (appendage and rig)
     
    Last edited: Sep 4, 2007

  13. rapscallion
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    rapscallion Senior Member

    actually, my Dad just bought a first 8, I haven't sailed on it yet..
     
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