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Thread: Design idea for a classic moth

  1. #1
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    Default Design idea for a classic moth

    Hi all,

    The recent thread on the differences between a laser and a moth had me thinking to send this out for your thoughts. Any critiques, observations on expected performance, thoughts on difficulty of build, etc. will be greatly appreciated. I live in Philly and there is an active fleet of classic moth sailors on the East Coast. I'm attracted to and intrigued by the Mistral for all the silver that design seems to pull in, but there are aspects of it, visually, that seem a bit severe to me. On that racing dinghy thread, you'll see a link to the earwigoagin blog, where you'll find renderings I made for the Mistral as research. (As an aside, my last name was buggered on the site, it's Slavinskas, and not Stanislav.) I basically took the lines of three models of moth, and slowly molded a combination of their sectional and fore-and-aft lines into what you'll find below.

    One thing about the Mistral, is that is has that tortured plywood look to it, so in my design I thought to add a chine. Along the chine, you'll see the sections run from having a hollow forward, to having a slight knuckle aft. Above the chine, the sections are straight, but below, I made them somewhat u-shaped, which may necessitate strip or cold-molded construction.

    There's less rocker here than in the Mistral, but perhaps there's too much of it still left in...? Do you think I'd have a hard time keeping the bow down?

    This may end up even tippier than the Mistral, but by much?


    Thanks again, and I look forward to your comments.
    Andy




  2. #2
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    Default Re: Design idea for a classic moth

    All Moths are tippy,extremely so.They also don't carry heavyweight sailors well but do respond very directly to the weight of the sailor depressing bow or stern.If I were seeking a classic Moth I suppose I would be looking at Shelleys,Duflos or Mistrals and of those the Mistral would be the easiest build because of the tortured ply construction.What makes you believe your design would have the speed to justify all the extra work of an alternative building method?With luck Richard Woods will be along to offer his views.

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    Default Re: Design idea for a classic moth

    I think you should look at the centers of buoyancy and flotation at several angles of heel.
    Just looking at the model, I would be thinking a more even rocker curve would give a better buoyancy distribution.
    The modern style has very few hollows in the bow but straighter waterlines back to B Max.
    SHC

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    Default Re: Design idea for a classic moth

    Quote Originally Posted by andrewdarius View Post

    There's less rocker here than in the Mistral, but perhaps there's too much of it still left in...? Do you think I'd have a hard time keeping the bow down?


    Andy
    Why keep the bow down? National 12s are sailed stern down to get on the plane when there is enough wind, but in light airs the crew moves forward to lift the run out of the water and reduce wetted surface area for displacement mode sailing.


    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Design idea for a classic moth

    Thank you for entering in. A year ago I looked at a used Mistral, could've had it for about 500, but I think some of you will know what went on inside when I considered, "What if?". I'm not a professional in the field, but have been building and restoring boats as a hobby for nearly 20 years, so the effort involved in construction isn't too much of a worry.

    Hopefully you will see the humor in this; most of us may walk down the dock in worn jeans and tired shoes, but reserve the sense of pride or celebration in the ride. That's why I'm in this game.

    Another way to look at it is; even if I built a Mistral, my relative skill as a sailor may keep me back of the pack anyway...

    Yes, the amount of rocker stays on my mind, and it would be a good study to look at buoyancy distribution at several heel angles. This is the stuff I need to read. Not afraid of the math involved; the only test I ever aced in college was in differential calculus.

    I wondered about needing to keep the bow down to get a little bite or edge in the water on windward legs. The mistral has quite a v-section to it forward, which may give it that advantage. I kept a bit of a v forward for this reason, not as sharp as a Mistral, but not entirely smooth either.


    Andy

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    Default Re: Design idea for a classic moth

    Perhaps this drawing will help with my question(s). I've rendered the submerged section of the Mistral, at left, and my working model, at roughly 262 pounds. The graph at the bottom outlines the submerged sectional area at each station. What can be gleaned from the shapes relative to one another?

    The Mistral has a much more evenly distributed underwater volume. The shift in crew weight will significantly alter these shapes, how can that be read into performance?

    Forgive me if I come off as another hack. I don't mean any disrespect to those who read these drawings as childs' play. This may be something more often discussed at BoatDesign, but my internet presence resides here mostly.

    Your input is greatly appreciated.


  7. #7

    Default Re: Design idea for a classic moth

    Your waterline at 265 lbs. shows that you are not floating on the full 11 feet allowed, i.e. a floaty design of approx 10 feet waterline length. You could: 1. Reduce the waterline beam. or 2. Reduce the rocker. or 3. Whittle away at both until the waterline gets back to the full 11 feet. (As in the Mistral which you show sitting down at the transom about an inch or so, which actually matches the Mistral's sailing trim.)

    I apologize for butchering your last name. I actually checked the spelling when writing the post but when the brain wants to do something, well it just goes ahead and does it. I've corrected it.

    RLM

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    Default Re: Design idea for a classic moth

    Thank you,

    I'll massage the lines a bit.

    There really isn't any way to simplify the construction method over the Mistral. I'll keep reworking it with the waterline and wetted surface area in mind. I wouldn't be doing this except for how open the local fleet seems to be for experimentation. It's a great way to bring others into the fold. What I find especially encouraging, is when you post pics on your blog of a starting line, and out of 20 boats, only five are of the same design.


    Andy

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    Default Re: Design idea for a classic moth

    So, stepping back a bit; I thought to do a study of the sectional shapes of the Mistral below the waterline. The sections are very triangular, with considerable rocker. I haven't sailed a Mistral, but it seems, on quick observation, that with these sections, it sails mostly in displacement mode...correct me if that's wrong.

    On top of, and alongside the triangular sections, I drew u-shaped sections that carry the same area and waterline beam per section. Given the same sectional area along the length of the boat, this should also be of similar displacement, but lessen wetted surface. Again, please let me know if that's just plain incorrect.

    Construction is definitely more complicated, but is there any performance to be gained/lost through keeping the original rocker, but making the underwater sections u-shaped?

    Is stability compromised from decreasing draft?

    Is there a good source for underwater sections and waterlines for racing dinghies?






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    Default Re: Design idea for a classic moth

    YES .. the UK sailors have been very good at creating archives of boat shapes.

    In particular the National 12 website has a range of drawings. Also the UK Cherub class. Make sure you look at the UK Cherub website as the other national sites have very little hull shape information.

    Best wishes
    Michael Storer

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    Default Re: Design idea for a classic moth

    Quote Originally Posted by andrewdarius View Post
    Is stability compromised from decreasing draft?
    If the water-plane is the same shape and beam, and flare is the same above the waterline, stability will be pretty much the same.
    Flare is the tricky bit as your U shaped sections will have slightly less flare than a triangular one.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Design idea for a classic moth

    Yes, the flare indeed is affected. I kept the sheer of the Mistral, took away the triangular underbelly, added the new rounded sections, and then connected those sections to the sheer with a straight line. Below shows a comparison of the two. (The line running through the Mistral is not a diagonal, but the centerline of the plywood panel). The drawing on the right is the modified version. The Mistral waterline is included in this modified version. By making the underwater sections rounded, she ended up sitting a bit higher out of the water by a bit. The waterline is theoretically narrower, with less wetted surface.

    I wonder if the addition of a chine, negates any benefits from making these alterations? If you can see in the drawing, the chine begins as a knuckle, becomes hollow amidships, and then goes back to a knuckle at the transom.




    Here's a rendering of the modified hull.




    ...and then, just because this is addictive, I tweaked the topsides, pinching the hull forward, and gave it a bit of tumblehome at the stem. These are small, and light boats, does anyone think there may be a benefit to having a narrower, less buoyant bow? Would/could that help in a chop? The chine ends up more pronounced; does this negate any benefit?


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    Default Re: Design idea for a classic moth

    Yes, still at it, but I thought to be a little scientific about it. I used Freeship to import the lines of 3 boats; a boat designed by a local moth sailor and blogger Rod Mincher, the Mistral, and lastly my own. I'll insert them here in that order, as I believe the numbers show my hull to be the least stable, or the most sensitive to crew weight, though it does minimize wetted surface and waterline beam among other things.

    There's a lot to digest here, and if anyone has the energy to enter into these numbers I'd appreciate the education. And as always, I don't mean to insult the craft of our NA's, I'm simply very interested in the process. I sank each hull to the same displacement.

    Here's Rod's boat, which has been referred to as his Tweezer design.




    The hydrostatics for Tweezer;




    Here's the Mistral;



    The hydrostatics for the Mistral;




    And lastly my own design, referred to as GG2 for now;




    The hydrostatics for GG2;


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    Default Re: Design idea for a classic moth

    Here are the waterlines at 15 degrees of heel for all three.

    Tweezer;



    Here's the Mistral;






    Here's my GG2: Note, there is a chine.


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    Default Re: Design idea for a classic moth

    Nice! Is the hollow allowed in Classic rules?

    I'm not a designer but I've been lucky enough to interview or correspond with many of them. If you're worried about the chine's effect on handling, you could try introducing curvature (from recollection, a corve of 12mm chord or so will do) in the sections just underneath the chine, to soften them.

    A few of the top designers (including Bieker and Andy Paterson, I think) have played with wave-piercing bows but found that in monos they introduce handling issues, although I can't recall exactly what they were.

    If you're a less than expert sailor (as you say) then it could be worth checking the immersed shape at higher angles of heel. Difficult boats like 12 Foot Skiffs are designed specifically to avoid downflooding etc when they are well heeled. Your shape appears to be fairly symmetrical when heeled, which anyone who sailed one of the early Magnum types will know to be a good thing! They were very wedgy in plan form and developed so much helm through heel that the boat would just bear off if it heeled to windward, regardless of what the rudder did.

    One last thing - is there any reason why people tend to go for the elliptical sections rather than the more prominent (ie brutal) flat floors of Int Moths, N12s, NS14s, MRs etc?

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    Default Re: Design idea for a classic moth

    I believe the rules allow 1 inch of hollow, aft of the daggerboard. I figured it was worthwhile to reach at that as a limit. I'm certainly game to continue massaging these lines.

    I'm in the middle of Bethwaite's books, for now yet in the weather and wave sections...very curious to see what he writes on hull shape.

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    Default Re: Design idea for a classic moth

    Alternate views of the latest idea;




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    Default Re: Design idea for a classic moth

    Chris249, thank you for the inspiration to push further. I've inched along the design spiral a bit, and tried something a bit more severe, if that's the correct word. Potentially easier to build, but it seems from the numbers that it could be even less stable than my 2nd iteration. We'll call this one gg3. Any comments are welcome. There's been a bit of talk over Bolger's work lately, and maybe this one enters into that realm...






    Waterlines when heeled 15 degrees



    Alternate views;




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    Default Re: Design idea for a classic moth

    With Moths,the stability is principally dynamic as the faster you go-the more stable the boat becomes.They fall over left to their own devices when unoccupied.How is the developability of the panels?It certainly looks more easily built than previous iterations of the concept.

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    Default Re: Design idea for a classic moth

    If that boat is intended for a light weather area, maybe, but if in average to strong winds I'd straighten her up from widest waterline to transom, stability and speed.

    John Welsford
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    Default Re: Design idea for a classic moth

    You have a good point to wonder if this is developable. There's quite a bit of twist there, and perhaps a bit too much.

    The latest inspiration was the i550, perhaps you can tell.

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    Default Re: Design idea for a classic moth

    Thank you John. My anxious, old stomach most likely wouldn't let me head out in above average winds, but this thought will be good to insert into the next round.

    The tweezer design has a much flatter run, but word has it that the Mistral still comes out in front. I've pondered over the amount of rocker in the Mistral compared to other popular designs. It seems severe, but apparently still quick.

    Andy

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    Default Re: Design idea for a classic moth

    Not a designer and not really even knowledgeable but, if impressions are of any use, here are a couple;

    I think if you put so much rocker as you have in your earlier drawings, coming off the plane will be like hitting the brakes, and it'll be harder to lift onto the plane.

    I think if you pinch that bow you'll end up with too much curve and create a handling nightmare unless you're planing constantly. We'd all like to be planing constantly but there are transitions to manage ...

    Even though there are already loads of dinghy designs around, I think it's great that people like you want to explore all the possibilities and go through the nuts and bolts of creating a craft.

    Rick

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    Default Re: Design idea for a classic moth

    I think Moths are too wide to benefit from the Dreadnought bow. I do like the idea of a flat area to plane on.

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    Default Re: Design idea for a classic moth

    Johnw,

    I think I'll alter the bow, as it seems a bit too pinched in some views. It's causing a bit of the excessive twist in that top strake too.

    There are a few pointy dinghy's out there that were inspiration. I'll admit to adding certain design elements without knowing the full extent of their sailing characteristics. I suppose that's one reason I'm sketching in public.

    I seem to remember that you were playing with a few designs of your own. Have you made any recent developments?

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    Default Re: Design idea for a classic moth

    Well, I've been playing with a sail training boat with a trad rig:

    http://forum.woodenboat.com/showthre...prit-rig/page4

    I haven't really discussed the hull on that thread, but the idea is to give the boat a narrow enough waterline to make it feel lively, while having flare and a chine above the waterline that cause it to stiffen up quickly as it heels. Very different from modern racing dinghies, where you want the flare well above the waterline so waves won't slow the boat. The bottom will be a warped plane design, reminiscent of some powerboats before the deep V came along.
    Last edited by johnw; 11-16-2017 at 10:12 PM.

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    Default Re: Design idea for a classic moth

    Quote Originally Posted by andrewdarius View Post
    Johnw,

    I think I'll alter the bow, as it seems a bit too pinched in some views. It's causing a bit of the excessive twist in that top strake too.

    There are a few pointy dinghy's out there that were inspiration. I'll admit to adding certain design elements without knowing the full extent of their sailing characteristics. I suppose that's one reason I'm sketching in public.

    I seem to remember that you were playing with a few designs of your own. Have you made any recent developments?
    The drawing board is occupied with a design for a client, but I'm in the midst of a design and build for my own use, with several others building as well. Long Steps is a sail and oar long range beachable cruiser, my blog here has quite a lot on her. http://jwboatdesigns.blogspot.co.nz/
    Next one up is a sailing canoe.

    John Welsford
    An expert is but a beginner with experience.

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    Default Re: Design idea for a classic moth

    I think many pre-foil Mothists looked upon the dreadnought bow with scorn and derision. I think it was tried and found to lack enough buoyancy to sail down wind in anything above moderate breezes.
    On the other hand, it is a way to avoid the hollows that inevitably appear in plumb stemmed boats with flare. Somewhat counter intuitively, moving the stem head aft helps, but at the same time, you should increase the volume low to compensate for what you are deducting up high. I don't think this is a one to one deal.
    Moths are pretty heavy displacement boats because they are so short, so increasing the Prismatic in this way should be OK. I also suggest you consider several inches of stem immersion and one inch of stern immersion for fundamental hydrostatic calculations. Usually designers would like to be too big than too small, because it is a whole lot easier to add weight than remove it. Nothing is worse than seeing your new design floating below its marks.
    Small racing sailboats tend to like to be a bit small for their intended displacement. Bethwaite postulates something like 75%, for his skiff designs. This certainly works on those boats that can always be powered up to planing speeds with bigger rigs, but I think that is too much for boats that have to operate across the wind ranges with a single sail plan. I try to draw the hulls at various trim states that represent how I think the boat will be trimmed in different conditions and points of sail and then compare hydrostatics, see these trend and compare that with how I thinkj the boat has to move in waves etc.
    SHC

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