15' Lapstrake Kayak.

Clamping might have been easier to do when glueing the stringers to the plank edges if the stringers had been glued to the planks one at a time then glued together - I don't have enough clamps so I cut short pieces of plastic pipe.

 

Today's clamps are paint cans, paint thinner cans, boxes of screws and anything else that weighs anything at all. Once I figured the method it was a piece of cake. Being in a time crunch didn't help either.

 

Well, all went bust. I had almost 12" of bow in the test panel and all was well. It really only had a fraction of an inch of "belly" at midship where most of the bow was concentrated. I was trying to work in a little more bow by assisting with a little foot pressure and "pow". There was some squiggy grain in one of the stringers that let go and then the panels failed and then the other stringer let go. Oops! I knew the stringers where too big for other reasons and was expecting a little more compound curvature with them. I didn't figure they would fail from the compressive forces in the bend. I'm wishing now that I had backed off before failure. Se la vie!

I may try to salvage the panel by epoxying it all back together so I can mill the stringers down and try again. I'm dissappointed with the failure and also with the minute amount of lateral curvature induced by the stringers. I'll have to rethink my plans a bit. I didn't even get any pictures before the whole thing exploded in my face!

I'm still intrigued with building a lapped panel kayak just to see if the appearance will be pleasing or not. I'm talking with my daughter about building a kayak together that will be hers. This would be a good opportunity to try a lapped panel on a smaller project.

 

Sounds like the idea is not as good as I had hoped. In my case the panel was a plain piece of ply not a lapstrake assembly; my guess is the laps have stiffened the middle of the ply so it is less able to accept a compound curvature. Reducing the stringer thickness is going to reduce the "belly" still further, I expect. I can also get stubborn over things, but I try to remind myself of this quotation:

"If at first you don't succeed, try, try again. Then give up. There's no use in being a damn fool about it." - W.C. Fields

 

I hit on an idea last night/this morning to induce a little more lateral curvature. Most all of my kayak incorporate a bump rail of some sort at the sheer. My current method has me put in a small sheer clamp to fair the sheer and provide some meat for my clamping screws while the epoxy cures during deck installation. This is followed up by the bump rail that covers the exposed edge of the deck and adds additional strenght to the sheer.

I could do away with then interior sheer clamp and mount the bump rail/sheer clamp on the outside of the hull prior to bending the topsides. This way the "rail" will be pulling the top edge of the panel inboard for additional curvature. Additionally, a sacrificial rail can be mounted at the chine to pull that portion of the panel inward, but will be removed once the bottom panel is fixed in place. An added benefit of the sacrificial chine rail is that it can provide the "meat" again for the clamping screws for the bottom. Fewer stitches, fewer headaches and less blood.

Thats it for now. I have some stringers to plane down, test bends to do and some rails to make and install for the next test.

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Additional thought now that I have all of my materials planed to thickness. The original stringers on the panel are just under 1/2" thick and this imparts a fair amount of flexibility in the panel. There is about 5" of bow in the panel when supported at the ends; reverse bow if supported from it's inboard side. I am considering, and will most likely, attach the o/b stringers in this state so that when the correct curvature is applied, the pressures applied by the stringers will induce the most possible belly in the panel. The question becomes, how much reverse bow do I want to build in the panel? I may limit it to about 6" or I may just let the panel take a natural shape from the weight of the panel and the necessary clamps.

I'm off to make some pvc pipe clamps.

 

I think that would help increase belly curvature, but the assembly will get stiffer since the force to compress the sheer must come from somewhere.

 

That is the $100 question. If I were a full blown engineer, I might be able to calculate the tension forces in the sheer and chine rails. I'm left with trial and error. I think that I will go with the max reverse curve and hope there is enough panel to salvage for another attempt if the first proves disasterous.

 

I am a retired engineer of the electronic persuasion not mechanical. A few things rubbed off from half-a-lifetime hanging around mechanical engineers, but this one requires more analytical ability than I have . . .

 

All glued and waiting. I've got about 6-7" of reverse curvature in this lay-up. Fingers are crossed.

 

FYI when introducing curvature during the glue-up, if the stringer is on the inside of the curve clamping requirements can be reduced by this simple trick.

Glue the stringer to the ply while flat and clamp the ends and middle of the stringer, clamp fairly heavily at the ends then form the curve. The stringer on the inside of the curve is under compression while the ply is under tension, so the stringer presses against the ply without needing any lots of clamps.

I always test it dry to make sure the pressue is OK, not too heavy and not too light; a piece of paper between the stringer and ply should be held firmly but not enough to tear it when it is pulled out. If I am using a water-based glue like Titebond III which sets up much faster than epoxy, I moisten the surfaces to be glued to give me more time.

 

I am completely surprised! After the first catastrophy, my expectations for this endeavor dropped tremendously. However, I am fully satisfied with the results.

I was admittedly hesitant to start the bending process this morning, especially with the reverse curvature that I had built into the panel. By pressing the panel flat, I was already seeing some lateral curvature. Progressing into the bending, I saw the lateral curvature continue to increase. I took things slow and kept a close ear on for any sounds of impending failure from the panel. All went well with no sounds of stress fracturing.



Maximum belly in the panel is 5/8" across a 9-10" panel. Very respectable. I put a strip of masking tape across the panel to emphasize the curvature. The spanish winch system worked well combined with a couple of cross-spawls between the panel and winch line to push the panel in the right direction. Moving the spawls around helped move the longitudinal and lateral curvature to varying positions.

More notes later. Just wanted to post some early returns.

 

Wow! That looks great!

I'm amazed at the look, it appears more than 5/8" and certainly enough for the look of the boat.

I hope the bending force is not excessive; the boat will have to be built strongly to avoid the dreaded "exploding boat" syndrome

 

I agree fully! With this successful test, I was wondering what to do now with the test panel. I think I need to double up the windlass line and continue the bending process to at least 24" of bow or failure so I have an idea of the stress limit of the panel. 2X bow factor should be fine. Less than 2X and I may consider glass tape on the middle strake as I would anticipate a blown out there. Of course, the failure may occur within the stringers under compression as happened before and that would require a different remedy.

The deck and bottom panels will stabilize the side panel edges (chine and sheer) so additional support, if necessary, will need to be mid panel.

I will need to manipulate the strake lay-out as my initial ideas look to be a little bit off. I was aiming to have the sheer strake slightly wider than the other two at midship. From the pictures, I think it looks excessively wide and the bottom a little too narrow. The chine stringer most likely make the lower strake look even narrower and the appearance should change in the final build when it is removed after bottom installation. There is a bit of bow in the sheerline of the flat panel that I will flatten a bit to narrow the appearance of the sheer strake.

The laps in the test panel are 3/4". I was considering a reduction to 3/8", but fear that may reduce the lap strength too much. Exploding boat syndrone anxiety expressed. 1/2" may be the ticket. The build stringers will be molded 1/2" and sided 3/8" and tapered prior to assembly, I think. It looked like there was slightly reduced belly in the vicinity butt blocks. I plan to cut the blocks slightly undersize, widthwise, to avoid any lateral compression forces that might try to flatten the panel in their vicinity.

 

Taper your butt blocks, much like a scarf on the fore and aft edges to relieve some of the hard spot. It'll still be there, but the transition to and from will be better.

I must have missed something in this thread. What is the object of the lapped panel? One of the best things about a glued lap build, is the removal of edge set, which is common is traditional lap builds and often hard to eliminate, in spite of attempts to avoid it.

 

Hey, Paul. Good to see you drop in.

I may have a misunderstanding of edge set. I was thinking that it was the unwillingness of a plank to take the twist needed to wrap around the hull according to the desired plan. Traditional plank or plywood plank. I figured it would be the same. Is it that the glued lap with ply can take a smaller scantling and can be twisted more easily?

I'm following through with an idea that came about as the result of this thread. I like the appearance of a lapstrake hull, but have not endevoured to build one, yet. Terry offered an insight (posts #9 and 11) that has me inspired. A lapped hull with the simplicity of taped seam construction, but with compound curvature. A fusion hull form that uses elements of glued lap, stitch and glue and tortured plywood construction techniques. My test panel was to prove viability of the project and the elements are starting to fall into place. The side panels and garboard panels will be lap glued flat. Longitudinal stringers will be added in the appropriate places and manner. All of the pieces will be brought together via screws, wires, clamps and straps. I see the effort to build only slightly more than a stitch and glue and the skill level required to be well below what would be requires for a true lapstrake.

The end product will still have a chine, but for the most part, it will be below the waterline and mostly hidden. The sidepanels (topsides?) showing lapped seam with a nice compound curvature.