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