The last of the one sheeters.

Here is my latest delving into design and building. It's more of a teasing, but it occupies my few moments of freedom that I get. My initial goal was to build a complete kayak out of a single sheet of 4x8 ply. After my first effort, it became quite apparent that the design goal was not going to produce a viable kayak. In keeping with the one-sheet ideology, I acquiesced to the idea of all hull components being produced from a single sheet of ply. The attached flat-plate diagram shows the lay-out of such pieces.

Scantling-wise, this design, as printed, would need to cut from 1/8" ply. All hull pieces would be cover both sides with 6 oz. cloth. The deck would only need cloth on the exterior surface. The curvature of the forward deck become exceedingly rigid while the aft could stand additional reinforcement if it was intended to be weight bearing. The cockpit ring is laminated front 5 layers of 3/16" ply. 3 at 3/4" wide and 2 at 1 1/2" wide to form the lip. The cockpit ring is very strong and can withstand over 200 pounds situated at it's fore and aft extremes.

The problem with my design criteria is that I am limited in both overall displacement and achieveable stability. Stabilitywise, at 200 lbs. diplacement, I believe that it is comparable to a stable racing kayak. At 100-140 lbs. displacement, it becomes very stable and docile.

In progressing with the development, I concluded that I could build a stronger, lighter (maybe) and cheaper kayak if I would increase the hull scantling to 3/16" and only glass the exterior. An added bonus was that the 3/16 ply was much more rigid and was easier position in the stitching process. In order to maintain the deck curvature though, 1/8" ply is still required. I may consider a peaked forward and that would take me back the the single sheet idea, but the curved fore deck is one of the appealing features in the design. Ultimately though, the single-sheet design idea is more of an academic exercise as it only produces a produce of limited use.

None the less, I wanted to take the project to some sort of completion. Today's photos are a representation all that has transpired since inception. The last epoxy building coat was applied to the topsides today and tomorrow I'll start building the coats on the bottom.

The initial float test was done sans deck and skeg. I was concerned about directional stability due the the amount of rocker in the design and my fears in that regard were confirmed. She moves very easily through the water because of her slight wetted surface at 7 1/2' LOA. Certainly not desireable for a long distance cruiser. I see a couple of advantages with such a short waterline. She fits very nicely in the bed of my short-bed pickup. My other single-sheeter has a flat bottom section and at 23 lbs., she could easily be straped to my back for a hike into a remote fishing lake.

Ultimately, if I were to build another "shorty", I might only limit myself to the 8' length of a sheet of ply to allow myself the pleasure of building in useful stability and displacement. The 8' limit might even become negotiable if it meant the craft would be better suited otherwise.

My next one? 12' at the very least. I want one that will be fun for me too!

 

Nice looking ‘yak! Due to the material required for the deck it’s not practical to build a kayak - at least not for an adult - from a single standard sized sheet of ply. A decent canoe can certainly be made; I have a 10-1/2 footer single sheet canoe waiting to be finished.

12' is a nice length IMHO, easy to car-top, decent turn of speed and agile enough for exploring narrow waters.

Depending on how you treat your boat I question the need for glassing; I have a canoe made with 3 mm marine ply that is several years old. It gets used more than any of my boats, and although it was not glassed - just 3 coats of latex house paint - it is holding up just fine. The routine of glassing inside and out is a hold-over from days of inferior plywood IMHO, and of course if the boat is for white-water or it's a ribless cedar-strip boat it is essential.

 

Terry,

Thanks for your interest. I think I was underestimating my own personal sitting CG in my stability calculations. I was expecting a little more stability with this second model over my first. Granted, I haven't done a side-by-side comparison yet as my deck is only now installed and my sitting position more defined. I've attached PDF lines plans for both my current build and the 12'er that is on the 'board." You'll notice a very deep "V" for a kayak in the shorty. I was trying to lower my CG in the little vessel, but I'm afraid it also increases centerline bouyancy.

These little kayaks were a proof of concept for more than just the single-sheet idea. I'm working out some design concepts for my personal understanding. I think that the scantlings in the short models can also be used in the longer model.

I'm very intrigued with the strip method that TXRIVERRAT is utilizing. I'm going to do some testing to see how thin I'll be able to plane the strip panels. I like the idea of being able to custom plane the plank thicknesses for each strake. I would most likely glass the inside surfaces prior to assembly for fear of splitting an unglassed panel during hull glue up. I might end up glassing prior to final milling as I am completely leary of milling an 1/8" thickness.

I also like the solid wood aspect where I can radius the chine joint and still maintain a surface finish suitable for bright finish. I also like no hard fastenings in the finished panels. I'll be eager to start with the new project.

 

The CoG height of a typical adult sitting flat on the ground is about 8", I understand. It always surprises me how low that is but I suppose the lungs are mostly air and leg bones are massive. I use FreeShip to design or evaluate the hydrostatics of my designs; among other things it will provide the metabolic height. For a canoe, so long as that is at least 8" of so above the CoG I find stability is adequate. It is hard to estimate without a computer or a lot of effort and is effected by a Vee bottom and rocker as well as waterline beam.

The river rat’s method is nice to look at and quick to build. From a purely utilitarian point of view I think marine play is easier and it does not need glass for normal use which I like, as I don’t really want to use glass and a gallon of epoxy if I can avoid it. If you glass before cutting the plank panels you may find your tools lose their edge quickly, and you should take precautions against the dust.

I imagine you could assemble the flat panels and pass them though a thickness planer, by try it on something unimportant first! If the boat is glassed both sides the wood in the center is no more than a core, and the only thing about it that counts structurally is its thickness, so don’t plane off too much. If weight reduction is the objective then it might be wiser to use a less dense wood . . .

 

Awesome little boat, LP! I like it.

Too bad my size extra-fat *** would never fit in it!

 

I did a calculation once of my own sitting CG with a bathroom scale and moment arm calculations. I no longer remember the exact figure, but am going to check it again shortly. Freeship uses KN(KeelmetaceNter) so for my kayak development, I've just been using 12" above the lowest point in in the "keel" as an overall CG. I have been neglecting kayak mass as it is 1/8 +/- my mass. I then develop righting moment curves from the cross-curve function in Freeship. It makes sense though to apply a CG/M comparison, though. At a glance, my little boat has about a 4-5" difference. The twelve footer is sitting around 7.5".

My thought was to machine both sides fair, glass one side, knock(sand) off the high points and machine the other side if I needed further thickness reduction. You're right though about it becoming a core where thicker is stronger, though heavier. I'm falling back to where I was with the first build with thinner core and full sheathing. A thicker material (where I'm at now) with only a single exterior sheathing offers more bang for the buck. I'm thinking ribs and then back to ply again. Guess I've gone full circle now.

Thr only reason to go really thin is to allow for the deck curvature. The hull panels, if glassed beforehand, would get their interior surfaces glassed while the deck pieces would get their exterior surfaces glassed.

 

Thanks, man!

 

Flipped and coated.

I'm hoping you can see the detail of the entry and exit of the hull at the chine. I was able to work in some hollow in those areas the give some nice butt-lines near the centerline of the kayak. It actually was a necessity in working the deck panels into the same piece of ply.

The skeg might be over kill, but I can always make it smaller. I can already tell that I need to cut come rake in it as has already become apparent that it likes to bump into things.

 

You have a fair amount of rocker and it's a short boat so it's tracking will benefit from the skeg, and you can always cut it down if it's overkill . . . looking good!

 

nice little project. If you had not made the short hull such a deep V, it might have been a lot more stable.

If you are going longer (more sheets) I would suggest going to just under 16' (8' plus 8') in length. IF you are going to use it as an actual sea kayak (long distance traveling), the length gives you better tracking, and more stablity, plus a bit more storage volume at not much more cost since you will be into two sheets of ply anyway.

 

Righting Curves

I'm sure I had looked at less of a "V", but did not have any early versions in that configuration. I took a few moments to work out a rough plan for such an option. (See Linesplan.) It reduced the draft by about 2.5", but according to my calculations, stability is also diminished. (See stability curves.)

"A" is an initial config. (Blue)
"A1" is the build config. (Green)
"A1SD" is the Shallow Draft version with a shallow "V". (Yellow)
"B" has a slightly narrower beam with more flair to the sheer strake. (Orange)
"A1a" has an even deeper "V". (Red)

I think the trend is interesting with "A1", "B" and "A1a", though it makes complete sense. Overall, it's a progressively deeper "V" with no major change in max. righting arm, but the curve is shifted further to the right with each progression.

 

Stability improves as the CoG gets lower so anything that lowers the load helps, like rocker or a vee-bottom, provided you sit low down. My first home-built boat was very narrow with a Vee bottom but I had too big a space between it and the seat, almost vertical sheers and no rocker all of which made it too unstable.

 

Exactly, thus all of the V and all of the rocker. The trick is to get the CG down low without a comensurate increase in displacement down low, too. My seat is as about as simple as it get. A trapazoid piece of ply that lays down in the V bottom, filleted and taped. I overestimated the width of my back side and it might have cost me a 1/2" in height. It dosn't sound like much, but in a craft this size, every fraction counts. I've been debating whether or not to rip it out and resize it.

 

In a craft of that size 1/2" is a significant percentage of stability. My larger solo canoe has a metacentric height of 14.5" and an estimated CoG of 10" loaded so 1/2" for that is about 11% . . . maybe double that for your boat.

 

Two weeks on the road. Whew! It took three days to fly from Dallas to Buffalo on Delta! Rant!!!!

I couple of pics. This is my seat configuration. Three trapazoids. The third forms a floor under the forward edge of the seat. In the future I'll make it triangular to form a floatation chamber under the seat. The second picture shows the scale of the seat width. 7" is too wide. I'm debating removal of the small section, extending the forward section down and aft and adding a smaller narrower after section.

In the first picture, you can see the ring frame placed behind the seat. The intent is to attach a shock cord to the bulkhead at the back of the stowage area and to the seatback panel, apply a seal around the perimeter and let the shock cord provide the force to hold it all in place.

I need to decide on my seatback cushion. I was considering an old style ski belt, cut to fit the width of the seatback. One of the old plastic covered relics. I had one and threw it away years ago. The closest thing I could find was this.

http://www.overtons.com/modperl/pro...&i=11831&r=view&aID=600H2C&cID=SHOPPING_11831

I like that I could position it where it would provide a nice lumbar support. The excess could be stategically placed to provide knee cushions. The added benefit is that it all provides floatation. It would be nice to find a source for the raw material as opposed to cannibalizing a finished product. I've also been eyeing "pool noodles". Any other ideas? Maybe when I finalize a real kayak plan, I'll invest in a commercial product.