Correcting Errors

On the left, my offsets lofted; on the right, Chapelle’s


(to start of project)

Winter is surely boat building season, or at least boat dreaming season. Over the last few days I’ve heard from three people making plans to start their own Melonseed projects: Guiseppe in Bari, Italy; Kirk in Colorado; and Bob in Hobe Sound, Florida. I completely sympathize, as I’ve been reading a lot about boats lately, too, while Winter makes itself at home. This is what it looks like here today (note how disturbingly diminished the wood pile is):


Like most builders starting a big project, they’ve scoured the web for any information available, just as I did, and found this site, and they’ve asked me to clarify some things.

Way back when I got started, I made some references, both here and on the Wooden Boat forum, about differences I found between Barto’s offsets and Chapelle’s, regarding what appeared to be errors in both. I never really explained what those errors were. To be honest, at the time I wasn’t sure if they were errors in the plans, or errors in my understanding. But all three are understandably curious about those errors now. At this point, I can say with some confidence that, indeed, there are errors in the plans.

If you aren’t building a boat yourself, what follows probably won’t interest you. In fact, unless you’’re building a Melonseed it probably won’t interest you; but for those who are, this is for you:

First of all, there are some very large, obvious errors in the Chapelle plans, though they aren’t apparent until the offsets are lofted. It’s pretty evident these occurred when the figures were transcribed, probably multiple times, before the final draft of the plans was made – all the measurements were copied by hand. Three measurements are off by a full foot, and this is quite obvious when lofted full size. The last one is a bit less obvious, but an 8 appears to have been read and transcribed as a 6, resulting in a measurement off by 2 inches. Here are the notes I made on the Chapelle plans:

Transom – WL9″ – says 1′ 6.125″ but should be 6.125″
Station #12 – WL 9″ – says 1′ 8.5″ but should be 8.5″
Station #11 – WL 12″ – says 9″ but should be 1’9″
Station #3 – WL 3″ – says 6.5″ but should be 8.5″

I made the corrections in a new table, and here is a copy of the corrected Chapelle offsets in PDF format:

Melonseed Offsets-ChapelleCX.pdf

Mistakes like these are very easy to make – I made several myself just copying the figures. Even Barto, who corrected the Chapelle errors in his plans, appears to have made a few of his own. When you know how complicated the process is for taking lines off a boat, it’s really surprising the figures are so incredibly accurate at all. Just ask Russ over at Hove To Off Swan Point. He watched and assisted as lines were taken off his beloved Sjogin for it’s first set of reproduction plans, and it’s quite an undertaking. And that project was done in the relatively controlled, comfortable workshop of Beaton’s Boatyard. I can’t imagine doing the same thing on a partially decomposed hull half buried in a marsh, which is how many of Chapelle’s boats were measured. It’s pretty amazing he and his team managed it at all, really, and must take a tremendous amount of dedication and determination to get it right.

The possible errors I found in the Barto offsets are relatively minor, but this makes them harder to see, even when lofted. In addition to setting the transom at a steeper angle than Chapelle, making a shorter boat, it looks like he made a few other small intentional changes, perhaps to make the boat easier to build in lapstrake. These are also noted:

Station #8 – WL 6″ – says 23.75″ but should be 22.75″
Station #7 – WL 6″ – says 23″ but should be 23.625″

It appears Barto may have purposely added fullness in the rear quarter around Stations #10 & #11, otherwise differences are minor, since each set of measurements is rounded to the nearest 1/8″ anyway.

If you loft Barto’s offsets yourself, you find odd bumps that correspond to the anomalies at Stations 7 and 8. Most of the other differences between the two plans amount to little more than rounding errors, up or down 1/8”. However, Barto includes a set of full size mold patterns with his plans, pre-lofted, and these small deviations, along with some other oddities, have all been smoothed out in those. There is one hollow place, for instance, near the Transom on the Chapelle plans (see below), that he reproduces in his offsets exactly, but this hollow is missing completely from the patterns, in which many subtle curves have been pulled straight and flat; again, perhaps to make the boat easier to build in glued lap. Given what a time consuming, though admirable, exercise lofting by hand is, I see no reason to use the offsets – the patterns appear to be correct, and everyone I know uses those anyway.

In any case, here’s a copy of the corrected Barto offsets:

Melonseed Offsets-BartoCX.pdf

The reason I know about these errors may seem a bit obsessive. I had both sets of plans on hand when I started. I got the Chapelle plans out of historical curiosity, thinking I might frame them, and the Barto plans to actually build from. But when I studied them and saw they differed, I couldn’t stop wondering why. It literally kept me awake at night. Which one was “right?”

So, to satisfy my curiosity, and get back to sleep, I entered all the offset values from both sets into a spreadsheet, and used a formula to calculate the differences. Voila! All the differences stood out clearly.


Places where Barto’s offsets differ from Chapelle’s.
(Note these are the differences left AFTER
the obvious errors in both plans were corrected.)

Well, sort of. I don’t think well in numbers, can’t visualize what they mean by just looking at them. So, later I entered all the values into a cad/drawing program where I could see what was going on. Ok, two different cad/drawing programs – one for boats and one for graphics: DELFTship, a Dutch nautical engineering program, and Adobe Illustrator.

DELFTship let me see the forms and rotate them in 3D, and I could toggle back and forth between the two plans. This showed clearly where there were odd bumps. The software also does all sorts of other crazy stuff, like calculates drag coefficients, theoretical hull speed, and whether curves can be rendered in stressed plywood panels, etc.. This made it easy to see that something was indeed wrong, but not what to do exactly to make it right. (At least, not in the free version of the software, which automatically fairs shapes to the ideal form, but doesn’t export a set of mold patterns or refined measurements.) I’m much more familiar with 2D graphics, so entering everything again in Illustrator let me see each station individually and stacked in place. Finally, it was easy to see what the final curves should be, and get their measurements.

Fortunately, this last step not only gave a means to fair and digitally loft everything to exact dimensions, but also to test modifications. I was able to add the crown of the deck, for instance, which is missing from the Chapelle and Barto offsets, and isn’t included in Barto’s patterns. Best of all, this made it possible to print each station pattern full size for cutting the molds.

Ultimately, aside from slight fairing, the only real change I made to existing forms was to the bow, adding a little fullness in the turn of the bilge for added buoyancy. Roger Crawford had said his first boats, built as exact reproductions of an old hull, tended to submarine into the backs of waves more than one would like, threatening to broach. A fairly common hazard in sailboats. He added some fullness in the bow to help resist the effect. I trust him completely on this, and added that extra fullness to my plans, as well. It only amounts to an extra 5/8” at a station or two, but you can see the difference in the picture at the top of this post. I was also able to preserve many subtle compound curves from the original, which can be reproduced in strip built construction, but are more difficult to achieve in lapstrake, as Barto’s version of the plans are intended.

Finally, there’s is also a funny hollow at the Transom around Station #12 in the Chapelle plans that I smoothed out a bit. Barto removes it entirely in his patterns, turning that curve into a flat line, though it still lives on in his offsets. The hollow may be intentional, giving a more pronounced wineglass curve to the Transom, but it also interferes with the flat run out the stern.

Barto’s straight line may not be as interesting visually, but undoubtedly makes a faster boat. Given the way lines are taken, and the acute angle of the hull here, a very small error in measurement can result in a large difference in numbers, so maybe this hollow is a just measurement error. Though my final shape is not as curvy as the offsets would indicate, it should make the boat faster and a little quicker to get on a plane, and yet retains most of the complex shape in the Transom that’s missing from Barto’s pattern. The difference may be hard to see in this reduced image, but it’s there, with Chapelle on the right and mine of the left:



Several people, including the three already mentioned above, have asked for copies of the final offsets I used. I can’t guarantee I haven’t made mistakes, too, but I’m glad to share them, and am posting them here for download:

Melonseed Offsets-CX2-Mine.pdf

Lofting the traditional way is not for the faint of heart, though, and my hat goes off to anyone who can do it. If there proves to be enough interest, I may look into a low cost way to have the patterns from these offsets printed out full size, along with plans for the unusual centerboard design (assuming it works) and the topsail rig (also, assuming it works). If enough people were willing to pay the cost of printing and shipping, that might be a possibility, though I expect requests to be few and far between. Just send me an email or leave a comment if you’re interested.

In the meantime, the Barto plans are readily available, and are quite good, even if they don’t include a centerboard. Builders who can work from the Chapelle plans alone probably have enough wherewithal to make adjustments as they go. Wood is inherently wise. Small errors tend to be self-correcting, particularly with lapstrake construction, so most of this discussion may only apply to people building in carvel or strip planks, and who are as tenacious as I am.


melonseed skiff, mellonseed skiff, melon seed, mellon seed   


33 Replies to “Correcting Errors”

  1. Right on Barry! I lofted the stations from the Smithsonian plans a few years ago and immediately noticed more than a few outliers. Once this sank in I just went ahead and tried to draw a fair curve. Some stations such as #12 revealed themselves to be a bit shy during the planking process. I planked with C-flex which is a springy fiberglass that will form a fair curve and just passed over the shy station. I regretted the lack of perfection but it really didn’t matter when all was done. I enjoy your site very much, can’t wait to see how the topsail works. I too have two daughters who have now become Melonheads and taken over old Dad’s toy. Regards

    Tuesday, December 21, 2010 – 09:20 AM

  2. Thanks, Tom! Funny thing about all this is: if it looks right it is right, regardless of what the tape measure says. The plans are more like guidelines. Seems silly I spent so much time trying to figure it out, really. Hope your daughters let you play now and then.

    Tuesday, December 21, 2010 – 06:31 PM

  3. Adding my 2 cents here. Having built from the Barto plans, I can attest there are some anomolies and omissions in the plans. I traced the supplied full-sized frames onto velum and used those to cut out my frames. Then I did some fairing using a long batten to take out some of the high and low points. Even then, there were some places where the strips just wouldn’t fit the frames. In these spots, I just let the wood do the talking and find its own natural curves. I’m talking a maximum of around 1/2 inch for four or five strips in a couple of places before the strips started laying true to the frames again. I figure the wood would find its natural curves and there is nothing wrong with that. Ultimately the proof is in the result. Katydid floats, sails, and has joyous curves. So all is well.

    For those who are interested, my build photos are at” rel=”nofollow”>http://picas Not nearly as poetic as this wonderful site, but I hope it’s useful for those who are diving in.

    Happy building to all, and to all a good night…..

    Tuesday, December 21, 2010 – 10:58 PM

  4. Interesting. Tom, sounds like your “shy” spot on Station 12 may correspond to what I call a “hollow.”

    And Tony, I wonder if the spots you found were along the turn of the bilge in about the middle of the boat, where I found bumps, or in those wide spots in the rear quarter. Nothing jumps out in the patterns as looking out of place, so now I’m really curious. Do you remember? (I STILL go to your site to see how you did things – it’s the gift that keeps on giving.)

    Fortunately, except in extreme cases, wood that isn’t forced will find a natural fair curve. My assumption is always that I made a mistake somewhere, and let the wood be the guide, but it sure makes you scratch your head and use pirate words until you work through it.

    Tuesday, December 21, 2010 – 11:55 PM

  5. How to view TT’s Melon Seed build on Picassa ?

    I recently purchased a MS built by Phil Maynard. I sail out of Sebago Canoe Club, Carnarsie, NY on Jamaica Bay.
    I am interested in MS history particularly means and methods of building MS’s.
    I learned of Barry Long’s activity from Dave Lucas.
    Barry’s activity and blog are wonderful.
    I tried unsuccessfully to open TT’s link ” http://picas%20<a%20href= " to images of build by TT.
    Can anyone be of assistance getting me a link ?

    Saturday, February 19, 2011 – 10:40 AM

  6. I am really interested to follow your modified version of the plans and molds. I bought Barto’s but I never lofted in my life. I want to start building the boat, wich it is my first naval project. I think your boats are by far the loveliest application of the model.
    However, I am in Argentina and I don’t know of you found enough interest for printing the plans. If it’s not the case, could I buy from you a digital version to have it printed in a plotter down here?
    Best regards!

  7. hey, I’d be interested in maybe getting a set of patterns if you’re still offering them and depending on the cost. A digital copy would work too, I can print them here.
    Thanks much,

        1. Ah! Sorry, I didn’t see that you had responded to the newer comment I had posted. I read through the blog again to find my original post. Disregard! my apologies.

  8. Hello. I really like your work. I want to build a boat as you. It will be my first boat. It is possible to get you the templates for the construction of the hull of the boat? I live in Russia. Thank you for your any reply.

  9. Your post is very straightforward and informative, even to a guy who has only dreamed of building a melon seed. Somewhere in my journey I discovered the melon seed and immediately feel in love with the beautiful curves and the power behind it all. Now I still dream of building a melon seed while I own a copy of American Small Sailing Craft( somehow I thought I would be able to pull up workable lofting numbers from the book) the numbers for the offsets are barely legible to my eye sight! Any way thanks for your information and keeping my interest. The funny thing is I am buying a house off of Little Egg Harbor and so much so I want to sail there, “let it be in a melon seed!”

  10. Hi Barry,
    This is Alfred from Australia. I married a lovely American girl (been together for 13 years) who wants to move back home. Should be moving to the States early next year. I have fallen in love with the melonseed and I am determined to make it my first wooden boat build. I appreciate the time and effort you have put into clarifying all those discrepancies in the offsets. I would be absolutely over the moon if you could send me your illustrator file. I intend to make a scale model to familiarize myself with the boat while I am waiting for a green card.

    Kind Regards Alfred

        1. In the meantime I am giving it a go myself mainly for learning purposes. I’m not sure how to use the diagonals because they don’t seem to intersect so far. Thanks for replying. Happy travels.

  11. Hi Barry
    Love your work and what a great site
    Would it be possible to get the illustrator files digitaly I am down in OZ and have access to a large plotter

  12. Barry, Just starting to strip my MS here in Gainesville, FL. I thought I was fairly meticulous in transcribing Barto’s plans to my forms. Centered them on strongback using chock line… I go to attach the first strip and define the shear line and guess what. It’s pretty wavy from side to side and up and down. Double checked all the measurement and they seem pretty close. So I unscrewed the strip from several of the forms and frittered with it until I had a more natural flowing line. It ended up about ½” above the notch on #s 2 & 3. I thought it quite interesting how all the two dimensional renderings I had studied failed to prepare me to comprehend the complexity of this thing in three dimensions. To define the deck curve I actually made a protractor that was 109 7/8” long. Got the radius by solving for the internal triangles and angles… It was a little more accurate than the stick method, which I also tried. Anyway, I was curious to hear if you had heard of such difficulty with the shear line? BTW I’ve studied your and Tim Crawford’s blogs like they are Torah. Thanks. When I first opened Barto’s plans I said to myself, “Is that all there is? Where are the instructions?”

    1. Charlie, the offsets and plans are indeed more a set of “guidelines” than exact recipe. Part of this comes from rounding errors in the measurements that compound each other. Part if it is inevitable variations in cutting and assembly – small deviations can make a big difference. Ultimately, if you adjust until things look right you’ll be okay, more so than if you try to force things to exact measures that may not actually be exact.

      Barto’s plans are ten times more complete than Chapelle’s one sheet lines and drawing, but still leave a lot to the builder. The plans don’t do a good job of defining the sheer. For such an important line you are best to go with what you see that looks right.

      And there is nothing at all for the deck crown. I ended up drawing out everything in illustration software, plugging in the numbers and then tweaking until they looked fair. And still did not fit that first sheer plank to the notch, instead letting the wood show me what was right. Rest assured if you start out close and then adjust until fair, all will turn out OK. Once the hull is planked, you cut everything else to fit that. This is especially true of the transom. I know most builders have to cut and fit that and shape and adjust until that’s right – the measurements of that are only approximate. Be sure to account for the thickness of the wood – at that sharp angle the difference in the front face and the back is rather dramatic.

      And glad the information here is still useful to people. Thanks. 😉

      1. Thanks ever so much for your response. You have articulated what is my sense things. The most difficult part of my shear line problem was to ensure both sides were symmetrical after getting one side how I liked it. CM

  13. Your videos and a few outside pushes got me to put a strong back and molds together. I’m sourcing a nice piece of wood for my transom this week. You have a great resource here and I want to thank you for providing it.

  14. Ok I am “lofting” the offsets onto Inkscape and I have some questions. What are the Diagonals A&B used for? Do you have a measured drawing of the transom and can I have it please? Do you have measured drawings for the bow stem? Yes I am attempting to do this without official plans – just the numbers you have provided and images of the original drawings. Any other info you can give would be appreciated.

  15. Hey Liam. The diagonals are an extra set of checks, most useful for setting up the forms when lofting with traditional methods. Since these boats were measured by hand in the field, often under adverse conditions, a single measure can be off significantly. Add up a several small errors and you get a big one. By providing multiple “best guess” approximations from different angles, a builder can work out the average “true” measure by compromising between various conflicting ones. Not as necessary when using modern methods, but still good to keep in mind. Every measure is actually rounded up or down from the true value, and even then errors can creep in. If you start building and something doesn’t make sense to put it where the measure says, often better to adjust to what does make sense as you go.

    I found it nearly impossible, even with the best drafting software, to get an exact pattern for the transom. Couldn’t even get close. I had to throw the first one away. The only thing that worked for me was to put up all the molds and use a batten to trace a pattern on an oversized blank, either wood or cardboard. Bear in mind there will be a difference of more than an inch between the inside face and the outside, due to the extreme angle and the thickness of wood used.

    Same, but not as bad with the stem. I used battens to figure out the extreme forward point in space where the point of the stem seemed to look right, then clamped a sheet of door skin in place and traced a pleasing arc from there to where it meets the keel. Even then, there was some springback of the stem curve after lamination, and it ended up an inch further forward than I measured. Not a problem, but was a head scratcher until I realized what was going on later.

    Essentially, all these measurements should be considered a good starting point – more “guidelines” than absolutes.

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