BYYB Forums

While waiting for a garage (my excuse not to commit myself to building the Super Skipjack), I am building a study model in 1:10 scale. Progress is slow, as it competes with reading all the good stuff from this board, watching the videos, and a several other of my hobby projects, not to mention regular work... I am in no hurry. So far I have made a keel (slightly modified) and cut the pieces for the hull. I have the drawings for both the Super Skipjack, and the Weekender, and I can't help noticing some differences.

For example, the aft end of the bottom panel is straight in the Super Skipjack, but has a curve in the Weekender. Same for the bottom side of the transom. I seem to recall people having problems bending the transom in place, so I assume the Stevensons have simplified the construction here. The top of the transom is curved, as is the aft end of the deck, in both boats.

Another part that I have trouble visualizing is the lazarette bulkhead. In the Weekender its bottom edge is straight, but in the Super Skipjack the bottom edge has a 1-inch raise in the middle, as if the flat bottom should have a unnatural curve there. The top edge is curved the proper way, to give the deck some camber, in both boats.

I guess all this comes clear when I get to the point of putting the pieces together, but I have wondered long enough, so I thought I'd share my observation. Not that I really need an answer...

The bottom of the lazarette is curved so that when you install it there the bottom will meet up flat with the bottom of the hull, i also made the opening larger im going to have my tiller over the deck so i wanted better access to the lazarette for storage

Greetings Heikki,

(11-09-2011, 01:29 PM)Heikki Levanto link Wrote: [ -> ]Another part that I have trouble visualizing is the lazarette bulkhead. In the Weekender its bottom edge is straight, but in the Super Skipjack the bottom edge has a 1-inch raise in the middle, as if the flat bottom should have a unnatural curve there. The top edge is curved the proper way, to give the deck some camber, in both boats.

Actually, the bottom edge of the Weekender lazarette should have the same goofy curve as the Super Skipjack. If you cut the Weekender lazarette from the dimensions in the plans with a straight lower edge, the bottom corners of the lazarette will pull away from the hull bottom as the curve is bent into the panel exposing a portion of the ends of the stringers. I found that annoying. But as construction progresses you discover that the bottom corners of the lazarette get buried under the seats leaving only the center 11 inches showing, and the joint is quite tight along the center.

The problem is that the lazarette is installed inclined from vertical, and forms a partial oblique conic section, specifically an oblique partial cylinder. There is a whole branch of mathematics dedicated to unwrapping 3D shapes into flat plates. If you roll up a piece of paper into a cylinder and then cut off the end at an angle, when the paper is unrolled you will find that the cut end is no longer straight, but instead follows a sinusoidal path whose amplitude is proportional to the obliqueness of the cylinder. The following examples are complete cylinders, one vertical and one oblique.



If you chop out a small portion from the center of each you will see the difference between a vertical and an inclined lazarette bulkhead. Of course the development of the oblique cylinder is upside, but you get the picture.

Finally, the curvature on the top of the lazarette should be more severe than the bottom as the top curve has to accommodate both the obliqueness of the lazarette and the camber of the deck.

Cheers,
Tom

(11-10-2011, 05:54 PM)tombayus link Wrote: [ -> ]Actually, the bottom edge of the Weekender lazarette should have the same goofy curve as the Super Skipjack. [...]

The problem is that the lazarette is installed inclined from vertical, and forms a partial oblique conic section, specifically an oblique partial cylinder.

Thanks, Tom! I had considered that the lazarette bulkhead will be curved to match the rear deck edge. And I had considered that it will be installed at an oblique angle. None of these alone did explain the curve, but of course, in combination, they do.

I suspect that the transom is theoretically more interesting, having a straight edge at the bottom (both transom and bottom edges are straight), and a complex curve at the top (both transom and deck have a curved edge). I have a feeling that this may not be theoretically possible, but that the plywood can be forced into such a shape, when the curves are small, and be all the stronger for it.

Greetings Heikki,

(11-11-2011, 12:29 AM)Heikki Levanto link Wrote: [ -> ]I have a feeling that this may not be theoretically possible, ...

Yes, theoretically, it is impossible. Substitute a sheet of paper for the plywood and you will quickly discover that conic sections, which include a cylinder, are easy to roll because the curvature is in only one dimension. Now try to roll the ends of the sheet in opposite directions. If you look closely what you get is a pair of opposing triangles hinged together with their bases across the center of the paper. The other edges of the two triangles will hinge into conic sections with the curvature perpendicular to the edge. I used the word hinge because the paper will crease where the curvature changes. So, in "theory", as you bend the top of the transom into a curve with the bottom of the transom fixed to the straight rear edge of the hull bottom, the shape that should form is a triangle with the apex just touching the center of the curve of the rear of the deck, and the upper corners of the transom bending away from that central triangle. A plywood sheet can be bent into any shape that includes a series of triangles whose vertices are coincident with the edges of the plywood. Or at least that is how the mathematics works. In practice, those creases will smooth out a bit and might not be so visible in actual plywood provided the overall curvature is slight and the triangles are narrow.

Cheers,
Tom

You can force plywood into slight compound curves, but they must be slight. Conical or cylindrical shapes have to be employed in all sheet goods (plywood, cardboard, sheet metal, etc.) designs or it'll kink. In fact, the curves are controlled by the amount of "strain" along the edges. This is why you'll never see a plywood boat with dramatic curve along the joining edges.

The simple solution is to not force the plywood into shape, but to cut a shallow arch into the bottom panel. This will remove the strains along the face of the panel and you'll have the curved transom too. You will not harm anything at all, by shaving a few inches off the outboard edges of the bottom panel. I make this modification of flat transom boats all the time and it's pretty easy, once you figure out what plywood will and will not do. Forcing plywood to accept a difficult set of curves, preloads the panel, often within a few percent of it's elasticity or yield point. I've seen panels bent like this that failed hours or days later as the fibers in the panel let go one by one, ultimately reaching a fatigue point. Twisting panels like this we call "tortured". And the name truly applies. The last one I did that I tried too hard on, sounded like a shot gun gong off in the garage, several hours after I'd bend it over a jig. I thought I'd gotten just lucky enough, but during diner, it went bang.

Not that yours will do this, but preloading panels like this, especially if using something other then really good marine grade stock, is just asking for a lesson in "developed" surface dynamics. Again the easy fix is just to cut a sweet arch (smooth and fair, plus any radius will do) on the aft edge of the bottom panel. On a Weekender size boat, I'd use about a 3" "belly" for the arch, to get a modest curved transom and a 6" belly (deepest point of the radius) for a dramatic one. Of course you'll have to cut the side panels back a bit to match the material you've removed from the outboard edges of the bottom planks, but this should be obvious, once you start drawing for the arch on the bottom panel.