Scaling up a sailing dinghy - sanity check

Discussion in 'Boat Design' started by Cheapster, Aug 19, 2019.

  1. Cheapster
    Joined: Aug 2019
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    Location: Denton Tx

    Cheapster New Member

    I've been wanting to build a wooden boat and have been searching plans for a little while now. I've pretty much settled on the Farr 3.7 since I really like the looks and, unlike a lot of others I like, plans are available. This boat would just be for me having fun as there is no class association that I'm aware of in my area and I'm not really concerned about being class legal or even racing in general. So what I'm really thinking about is scaling that 12' boat up by 25% to a full 15 feet. Gets me some waterline length and also makes for a bigger cockpit which would be great for taking a friend out on the lake.

    Now, just because I don't plan to race doesn't mean I don't enjoy going fast! I would scale up the rig as well as the hull and at that size I compared my proposed "Farr 4.62" with some other performance dinghies including the Flying Dutchman, 505, Laser (My current boat) and a few others. From what I can tell, it seems the two most important features are waterline length and sail area. I can see no other explanation for the FD and 505 still being in the relatively low Portsmouth handicap numbers in spite of being designed in the 1950's! If hull shape were tremendously important, modern computer designed hulls would have obliterated those boats already, yet that doesn't seem to be the case...

    Not completely anyway! Also looking through that Portsmouth chart, I've noticed that the RS boats in particular seem well ahead of the curve. And interestingly, as far as I can tell, the Farr 3.7 scales up to be very close to the RS 700 in length, beam and general hull shape, although the hull shape is really hard to compare with random internet pics. Even the sail shapes and areas are similar, with the scaled up boat going a tiny bit over.

    So with all that in mind, I'm not trying to kid myself into thinking I'll build a boat that will keep pace with the RS700, but scaling up that little boat is looking like a very viable plan to me at the moment. Seems like potentially a very fun boat. But before I try going too far down this path, I'd appreciate if you guys actually experienced with boat design might care to weigh in regarding where I've seriously erred in my assumptions or any pitfalls I should watch for as I pursue such an experiment.
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Scaling up a boat plan more than ten percent is almost always a no-no.

    You say that you are not particularly interested in speed but then refer to Portsmouth numbers that imply potential boat speed.

    The FD, for example, is a very light weight boat with a lot of sail area, has athletic crews who use the trapeze to keep it upright. In the right conditions, with a skilled crew, it is wicked fast. Sure enough it has been around for a good many years. The designer knew what he was doing and probably had no computer programs to help with the design. Knowledge of what makes a fast boat has been with us for a very long time.

    Computer programs are wonderful tools but that is the extent of it: Tools. No computer yet devised can design the perfect boat. People do that. They input design parameters and the computer quickly does a mountain of arithmetic for the user. The designer may then adjust some of the lines to get better result from the math that the computer has spit out. We humans know how to do the arithmetic but the computer can do it in a fraction of the time that a human with a calculator could manage. Just imagine a time when we did not even have those nifty calculators......the designer had to do the calculations the hard way: Pencil and paper. Even some of the old time guys or girls produced exceptional design work that resulted in exceptional boats..

    For your use, I would suggest that you use a proven design for the type of boat that you envision. The Windmill or Snipe comes to mind. Both of those are practically antiques but they both perform very well and have a fair turn of speed along with a potential aftermarket when you get the itch to build a bigger or more sophisticated boat. Plans for either of those boats, and a whole bunch of similar ones, can be had for a mere pittance.
     
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  3. Cheapster
    Joined: Aug 2019
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    Location: Denton Tx

    Cheapster New Member

    Thank you for the thoughtful reply!

    Actually though, I said I do want to go fast, but I'm not all that interested in racing. Seems as I had feared, that scaling is probably not such a great plan. I'm pretty familiar with design, though I haven't done a ton of it. But I'm a serious CAD/CAM guy having been a continuous MasterCAM user for the last 20 years or so. I've made bunch of stuff over the years, not just for my machine shop customers, but I've also made several bass guitars for myself, that I fully modeled in MasterCAM and cut on the CNC router. All the glue joints were done by hand of course, and are perfectly tight. Also have done many pocket knives, model airplanes, hotrod and motorcycle parts...

    Anyway, I'm not trying to brag, just saying I'm reasonably crafty and have a lot of capability for actually bringing ideas to life. I mean the sexy stuff, not like making yet another ashtray in shop class or whatever. Unfortunately the Snipe and Windmill designs don't really catch my eye or capture my imagination. Especially since we had a guy with a Snipe in the sail club when I was a kid and I don't even know if he ever even FINISHED a race, lol. Really gave me a poor impression of that boat.

    Maybe I'll just scale the 3.7 up by 10% or work on finding plans for a Contender or just go for some kind of catamaran. The cats are fast no doubt, and probably way more practical for me but I'd really like a sexy planing boat. One that's faster than a Laser. Why it has to be faster than a Laser I don't know. I guess it just feels kind of backwards to put all that effort into building a sexy boat only to have it end up slower than the one I bought for a hundred and fifty bucks...
     
  4. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

  5. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    The Farr is a trapeze boat. I suspect you'll still need to add trapeze even if scaled up.

    Check out the Yachts and Yachting forum. despite the name its almost 100% UK dinghy sailing site. and a Farr3.7 is regularly mentioned

    And as others have said you'll need to be athletic just to keep it up, never mind sail fast. The Contender is a much more stable boat (I've sailed one) than a RS600 or RS700. Currently I sail a RS Aero which is a lot of fun to race but definitely not a "slop along placidly" boat. A lot quicker and lighter than a Laser for same length and SA.

    You might find my Zest and Zeta not to mention Strike 15 of interest

    Richard Woods of Woods Designs

    wwww.sailingcatamarans.com
     
  6. Cheapster
    Joined: Aug 2019
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    Location: Denton Tx

    Cheapster New Member

    I will check that out Richard, Thank you. I didn't see the Zeta or Strike on your website, do you have links you can share for those? Sorry I'm not too interested in the Zest with the wings, but I have given some thought to your Pixie catamaran and may possibly go that way. Or BlueLightning has a couple designs I like, namely Hadron and Heatwave. Or maybe I could the Farr 3.7 unaltered. Or the Contender... I haven't actually emailed anyone yet but I've unsuccessfully searched around for plans on that boat.

    Or.. or..? Yeah I'm a mess, sorry.

    Maybe by Winter I'll actually make up my mind.
     

  7. Richard Woods
    Joined: Jun 2006
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    Location: UK, USA and Canada

    Richard Woods Woods Designs

    There are several home built Hadrons in the PNW. Mike Scott has one, sails at Whidbey Island SC, often on their FB page. You need to be big to sail one though. Certainly over 80kgs. I was talking to Keith Callahan a couple of regattas ago.

    The Zeta is here Sailing Catamarans - Zeta 14ft singlehanded trimaran http://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25/458-zeta-14ft-singlehanded-trimaran

    the Strike 15 here Sailing Catamarans - Strike 15 trimaran performance daysailer http://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25/223-strike-15-trimaran

    and videos here sailing trimarans - YouTube https://www.youtube.com/playlist?list=PLT7PbPvOm8lzv_qjPJuA80kU1ugLfABgi

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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