15' Lapstrake Kayak.

Here is the next possible candidate in my progression of kayak designs and builds. The project is the genesis of a need to use some 3mm plywood that I have a small stack of and also to move beyond my one-sheet creations into more capable craft.



I'm aiming for light-weight construction and a miscalculation has left me with some ultra-thin ply/too thin ply for the original intended design. Lapstrake construction will allow me to optimize the properties of the this plywood to build a useful and light weight kayak.

One of my main concerns with using such a thin material is building a fair hull. In my last build with a slightly thicker ply, I found that the rigidity of the material was not sufficient enough to rely on it to fair properly by itself. Going with a thinner ply is going to emphasize this insufficiency. I self-alligning and/or supporting joint will be necessary in this construction. Several build techniques come to mind. 1) The "Lapstitch" technigue comes to mind, but requires a thicker ply and I will be looking to build without stiches. 2) This next method that comes to mind employs stringers bonded to the edges of the strakes to lend stiffness to the components and thus increase the likelihood of a fair joint while adding the the overall strength of construction. This method has promise. The edgewise stiffeners, if applied properly, can be planed to mate flush with the adjacent strake to form a filler block and faying surface for the joint. The stiffeners in this technique add sunstantial strenght to the construction and will contribute to the fairness of the joint, but they are not self-alligning. 3) A combination of these two techniques, a method I am considering, is to apply the stiffener slightly above the lower edge of each strake, such that it creates a "pocket" so to speak that the lower strake rests in and self-alligns.

Method three might require a backwards build where the strakes are added from the top rather than the bottom.

Getting back the the kayak, though, the planking is a modified lapstrake to get a pleasing appearance for the craft. The nature of a kayak doesn't promote a tradition lay of planking as there is relatively little curvature due to the kayak's long and narrow nature. I was also looking at simplified planking by trying to keep the planks as straight as possible. I've come up with a plan where every plank will fall within a 4.75" width. This allows me to rip my ply into 4.75" strips and join them as needed to get the lengths that I need for each plank. The lower planks are essentially straight while the uppers are allowed some curvature and two plank blanks will be joined at an angle to accommodate this curvature.



You can see that the upper two strakes are full length while the lower strakes blend into #2 as though there is a chine. I'm happy with this appearance and looking at the bodyplan there is really no chine. I'm thinking that the straight nature of the lower stakes will enhance the flow of water under the hull and may also aid in the inherit tracking of the kayak.



I feel like the hull is fat. It's max beam is a hair over 24" and waterline beam is just over 22". When I run stability numbers, it calculates less stabile than my single-sheeters and I'm having difficulty in believing that. The single-sheeter though has a waterline beam of over 24" and a draft of 8". The draft allows for a calculated CG about 2" above the waterline as opposed to this kayak that has a draft at 4" with a CG about 6" above the waterline.

Thats all for now. I'll be interested in your comments.

 

A single chine hull of the same overall dimension will probably have more initial stability

 

Why not build a jig with section molds? Is not doing so one of you requirements?

Ending planks on laps/plank seams is un-conventional construction. Does not mean it can't be done, but you will probably have to work out how to do it.

What do you mean by "allows for a calculated CG about 2" above the waterline"? Is that to maintain a desired GM?

Are you concerned about initial stability or overall stability? How are you comparing stability?

 

Your right. I'm attempting to build a more efficient hull and utilize the materieals that I have on hand. I've built the hard chine and would like to progress to a different hull form.

I do plan to use a jig and section molds, though I would prefer not to require a mold every 12 inches. I'm planning on 5 molds plus stem and stern profiles.

If you look at the section views, you'll see that there is really not a full chine(sharp bilge turn) anywhere. The "stealers" should land and blend rather nicely to the upper planks. Visually, I could not get an acceptable plank layout with full length below the turn of the bilge planks. Too much up-turn in the ends. Most likely, I'll plank the #2 plank, then the sheer plank, jump back the the garboard and plank and spile to the uppers. I'm not completely going to disregard a conventional garboard up plank job. Currently though, I feel like I need the #2 plank in position to define the shape of the lowers as they blend into it.

I use a standard CG of 10" above the keel line for my kayaks. My single-sheeter has an 8" draft so that puts the calculated CG at 2" above the waterline.
View attachment 62566

I run cross curves in Freeship and plug the values into a spreadsheet that I've written.



The RED line is the single-sheeter and the ORANGE is this kayak. As an additional reference, the BLUE line is my Mysticshore12 that is under construction.

Initial stability is comparable to the single-sheeter(initial slope of the stability curve), but ultimate stabiility is reduced by 30%(max. righting moment). It is odd that a kayak that is twice as long and less than 10% narrower would exhibit less total stability.

But to answer your questions specifically, it's the total stability curve that has me perplexed based on the above comments. I use previous builds and their calculated stability curves for reference in new designs.

 

I too was looking at 1) lapstitch for a solo lapstrake canoe. However the rebate that you can get with thin ply is not sufficient to properly engage with the lower plank IMHO; I am planning to use 4 mm. Instead I plan to use a row of staples about 8 mm inside the lower edge of each plank - set at a slight angle - to engage the edge of the next plank. I haven’t tried it yet but it seems worth investigating. The staple holes will be hidden by the small epoxy fairing that will protect the exposed ply edge.

I used 2) chine logs in an earlier hard-chine build. It works very well and has a number of advantages. The planks are stiffer and stronger and are easier to handle without damage, this is particularly important if the developed planks have wide ends and narrow middles. They tend to self-fair and require fewer station molds to control the shape. They also provide handy attachment locations for things like seats, foot rests and suchlike. They greatly reduce the amount of sanding that epoxy fairing can require and add to the appearance of the interior of the boat. The method is self-aligning in one lateral axis but not in both.

I am also looking at a method to align the plank edges at a butted seam without using stitches. it actually takes advantage of the extreme flexibility of the material, allowing dry assembly of a rigid structure that is subsequently glued up, but it still has some challenges to resolve. In the meantime you may want to consider the stitch and glue method as an alternative.

On the stability issue, this depends on both the righting moment from the beam and shape factor of the hull, and the height of the center of gravity. In a narrower boat, particularly one with significant rocker, the Cog gets lowered which can result in more stability.

On my first successful canoe design I found it paid off to have a flat bottom just wide enough for my butt, with garboards flaring out to the greatest width (5-planker) I could get at the waterline. This boat was far more stable than a slightly beamier boat (3-planker) with a plain flat bottom, which floated higher in the water.

 

With planking of 6 mm or less you should consider some sort of lap support, be this a heavy fillet and an intentional gap or a batten glued in place. These materials are just too thin for sufficient glue area at the lap lands. Also you need fairly tight station mold spacing, unless you're using batten lap backers, as this stuff will sag under it's own weight easily.

In small craft such as this, it's often wise to use a single, flat garboard rather then two. It eliminates the centerline seam, makes for a slightly stiffer section, offers a flat floor and you gain displacement where you need it most. If this bottom panel is kept fairly narrow, it doesn't affect preformance at all and is a commonly employed trick by many designers. 3/4's of Bolger's small craft design incorporated this feature, round bilge or not, for several reasons, ease of building, stability, convenience of a flat floor, etc.

Another trick of these single garboard small craft, is to employ a substantially thicker piece of plywood in the garboard. It's stiffer, can take much more abuse, can offer some stability advantages, accepts fasteners better, etc.

 

I used both those ideas in this little canoe http://www.boatdesign.net/forums/projects-proposals/challenge-100-boat-12176-7.html#post195863 - you can also see the chine logs.

 

I feel that I've finally finalized the 15' lapstrake design. I'm attaching FS linesplan output. I've created a print out for all of the mold sections to be used on the strongback. I've also attached the PDF file that I sent to the printers if anyone is interested.

I've been working with Freeship to develope strakes that will fall into a 4" width, including lap, so I can rip my plywood into 4" widths and scarf as needed to get the lengths that I need. The lower strakes are developed nearly straight and the uppers have some curvature, but will still fit into the 4" width requirement. I'm eager to get this project rolling.

I also uncovered my stability controversy I was having with my spreadsheet. Seems I was pulling my displacement value for the one-sheeter from the wrong cell. I was picking up a high displacement value and it was giving me an erroneously high stability curve for the one-sheeter. Mystery solved.

 

It’s a pretty shape and will look and perform well IMHO. Low paddling effort at cruising speeds and agile, although perhaps not as fast as some kayaks flat out.

However, there seem to be a lot of planks and I don’t understand the rationale for 4" wide planks since none of them are likely to be straight.

The hull is so close to a chine hull already that I wonder if it is worth the extra building effort. I think it will be a troublesome build, requiring a complex building mold, handling delicate floppy planks especially in 3 mm ply, and a lot of marking and fitting; much better suited to a strip-build I think. If you went ahead I would recommend using stringers over the station forms to support the plank edges while you plane the bevels and gains (or rabbets) at the stems.

There is a lot of scope for simplification: the 3 planks on each side could be combined into a single sheer plank with very little change of shape. See note.

The bottom could either be stripped or the 8 planks changed into a Vee bottom, greatly simplifying the build. The only reservation I would have about that is the amount of force required to twist the wide bottom planks; a 3-piece bottom might be easier. That approach would be ideal for chine log construction BTW.



Note: a stringer glued down the inside centerline of the sheer plank while it is flat results in a slightly convex outer surface when it is bent. It would reduce noise, and result in very little weight increase. The stringer thickness requires some experimentation as this is a rarely-used technique so there is not much data on it; I would start too thick and plane down to suit. You may find 2 stringers are needed to get the shape you want.

I, FreeShip, if you remove the inner creases from the sheer the planks the development tool will calculate the distortion required to make the sheer from one plank, distortion numbers up to around 2% are do-able in my experience, but add a trimming allowance around the development edges for the first plank. Just an idea to test how open-minded you are! I doubt this trick will work on the bottom planks though.

 

Hey, Terry. How goes?

Here is the plank development from FS. You can see that the lower planks are almost straight. The uppers show small amount of curvature, but still within 4". The exposed plank will be 3" or less. I like your concept of the stringer mid panel and may try to incorporate something like that in another build. Sounds like a version of tortured plywood.

I've not done a lapstrake and this is certainly divergent in the realm of lapstrake design. I'm hoping the lower laps will provide some directional stability with out the use of a skeg. Thusly, straight is good. Since this build is somewhat experimental, I'm considering the use of 3/16" 5 ply subfloor for the strakes and foregoing with the stringers in preference of a more conventional lap for the moment to confirm build viability. I will also not be sheathing and will only use glass tape on the inner seams that I feel need the added strenght. The keel line will most certainly be SAGed, though I am considering a central rub strip at the keel line.

It would be an easy transition to a chined hull...if simplicity was the entire goal. I'm trying to get the appearance and strength of a glued lap build with, hopefully, a simplified build through the use of nearly straight plank of universal width. Lastly, while I have developed the plank in FS, the ultimate plank shape will be determined at build time as each strake is applied.

 

This gets more and more interesting. I must admit I am surprised how straight all the plank developments are, on what seems a nice curvy hull. Neat trick!

I discovered the mid-panel stringer trick by accident; I put a chine log and inwale on a flat sheer plank and got a concave flare curve, the opposit of what you want. Vertical curve depth was 3/8" over 12" width, plank was 4 mm x 10.5', Horizontal curve width was about 12" corresponding to a beam of 24" if it had been a double ender - it had a transom so was beamier than that but not relevant to this discussion. The chine log and inwale were, if I remember correctly, 3/4" x 7/16" thick. If I were doing this I would start with a 1" x 3/4" thick stringer and see if it made the plan view bend at a level of force I felt comfortable with, planing down the thickness until it felt right, then I’d check the vertical curve. You’ll probably have to taper the stringer ends since the sheer plank curves all the way to the ends. Use a glue that doesn’t have a tendency to creep!

However, you will lose the lapstrake look if you use this trick . . .

Laps are not taped or sheathed as far as I know, too much of a hassle. You will probably have to use fairing compound on the inside of the laps to get glass tape to sit down and stay put; I haven’t done that but to minimize weight build up I’d use micro-balloons. I question whether taping the seams is going to add anything however; a lap joint is very strong if it is wide enough - about 3x plank thickness will make it stronger than the ply.

I’m not sure how the bottom laps will affect performance, it is tempting to wonder if turbulence across the laps will create drag. This design is an adaptation of J. Henry Rushton’s Wee Lassie, the laps likely cross the water flow direction, but it is a highly respected design, so I don’t think it’s as important as all that.

Good luck!

Afterthought: the Wee Lassie was originally lapstrake, part of my adaptation was to use butted seams so there are no laps visible in that pic, but you can see where they've been!

 

Now you've done it!

Now you've gone and done it!

I've completely rethought my whole build.

I was laying out my forms yesterday from my full size print and it was really sinking in how small(widthwise) my strakes are going to be with this craft. All sorts of other thoughts also started bouncing around in my head: the time and effort in this build, proof of concept build vs. quality build, Ancient Kayaker's ideas on alternative constuction, alternative constuction.... I guess a few hours of mundane tasking will make any mind wonder.

Terry, thanks for your thoughts on stiffened, convex, concave panels! You are correct. This design is not far from a developed panel, S&G build. The more I think about a lapped build with this dimension of material, the more concerned I become about the effort vs./and quality of the result. During my mental ramblings yesterday, I hit on an idea that will give me the look and strength of a lapstrake, but still keep the effort and simplicity (I think) of a S&G build.

The idea is to still build with 4" roughed out strakes, but to create lapped panels on a flat surface. I will fine this some more, but I figure a 3/4" lap is sufficient and would correlate to dimensioned lumber to be described in a moment. I would forego with any plank splicing and simply butt mulitple strakes to get the length I need. I justify this by staggering butted joints and by the fact that the central planks in a panel will have planks overlapping on either edge and the edge planks with have some form of additional support (shelf, chine doubler or keel doupler). At this point, I will add mid-span stiffeners at the lap locations. My initial sizing would be as you suggest of 3/4" x 1". I'm not sure of your orientation on these, but I would have them molded with the 1" dimension. This I feel would give the greatest shift to the centroid and as such, the greatest amount of convexity. Lastly, I would add doublers or possibly a layer or two of glass tape to all of the interior butts though this could be done in concert with previous steps.

Additional thoughts:
The lapped width could be varied to get a diminishing plank width(stem to stern).
The planks would be layed out dry, taped(adhesive tape) on one edge, the upper plank flipped over, epoxied and flipped back and left to cure. Sounds simple anyways.
The garboard panels could be lapped or made from a single thicker ply panel.
The shelf, chine douple and keel douplers should be thinner and of lower modulus than the lap stringers for maximum panel warpage. 1/4" initially. If a thicker shelf was needed, it could be laminated thicker once the hull was shaped.

Once the panels are built, the kayak should go together like any other S&G though the panel bending forces will be much greater, but hopefully within reason.

I will need to do a test panel before committing to this style of build. I have a scrap panel of 3mm marine ply that will serve nicely.

Is this open minded enough?

 

Making flat sides from lapped planks is common in dory designs, in the days before ply wide planks could not be obtained readily. However, in this case I’m not sure what you will be saving; it might be lot of risk and trouble to achieve a lapstrake look. These are my suggestions and definitely do a test panel first if you wish to proceed:

You can get the lapstrake look without the stringers if you can live without the convex sides by leaving out the stringers. If you do that the lap width need only be about x3 ply thickness.

A 3/4" lap is quite wide, but maybe justified with the stringers since the joints will be stressed by bending. Reinforce the butt joints well, they will be under tension.

The stringers will make the assembly very stiff, you may have to plane them down to make the bend. If you decide to add chine logs to the outer edges of the assembly before you bend it, kerf them every inch or two almost full depth - kerfs are slits cut across the grain - so they bend without stretching the ply, or they will neutralize the convexity.

 

I believe that I can add shelf and chine doublers and still get the convexity that I want. I believe that care needs to be taken in choosing doubler thickness . I may just need to go with a ply doubler equal to the panel thickness. Kerfs would defeat the purpose of adding the additional material. The key will be to add just enough to make the edges self supporting and self fairing. The difference in thickness between the edges and the stringer should provide the bending moment.

Some pics of the test panel build:

Everthing in position for epoxy. Though not very visible, there is packing tape on the work bench to keep the panel from sticking.

Wetted with epoxy and soaking in.

Saturating the butt ends.

Everything in place and clamped and weighted.


This is the first two of three strakes. I will most likely glue the butt blocks on the next round. There will be no reason not to glue them initially. I am building this test panel full scale and will be very curious about the results.

 

Happiness: Ripping a 16' board into smaller pieces inside a heated shop in January in New York.

I glued the third strake yesterday and ran out this morning to the HD. I spent $20 on a 16' 2x10 of close-grained douglas fir. All so I can get 2 1"x3/4" stringers for my test panel.

Cutting and milling the 1"x3/4" stringers, I realized they are really too hefty for thus build. I trimmed them to about 5/8"x3/4". They are still too much for this build, but I'm going to leave them for now to see how the panel will respond to these dimensions. The panel I am mounting to is full size, but still less than the width of my surface planer. This will give me the option to reduce the height dimension as necessary.



As things stand right now, the strakes are assembled and the stringers are glued and clamped while the epoxy cures. I was having trouble getting the stringers to lie net to the lapped panel due to variations in the workbench top and my improvised work bench extension. I started shoving shims under the edges where I was seeing gaps, but finally used a couple of cut-offs from my stringers and shoved them under each longitudinal edge for the full panel length. It worked great and faired out the bumps in the bench and the gluing went well afterwards. All of my epoxy use so far has been with unthickened epoxy. I figure flat surfaces should fair and fay with out the use of thickeners. It's simpler and so far, successful.

I trimmed the fore and aft ends of the panel so the stringer will show proud. When the time comes, I plan to attach line to the ends of each stringer and draw them in via Spanish Windlass. I'll be very curious to see the natural curve of the panel and will also be eager to manipulate the curvature through variation in stringer thickness. The bow and stern have higher curvature than midship so a tapered stringer thickness will be suitable for multiple reasons.

If this test panel work out as desired, I'll be looking for ways to reduce the material used in the stringers. There is a very good chance that I'll be looking at a "T" section stringer. I could build it up from multiple sections or rip it out of a solid piece of dimensioned lumber. The primary reasoning is to reduce material in the structure to keep weight to a minimum.