Thread: 10 kg Folding Sea Kayak
30th May 2010, 09:04 AM #1
10 kg Folding Sea Kayak
I'm not sure if it counts as a "wooden boat", but some of it is wood, and I'm planning to replace more of the HDPE with wood to make it lighter.
The beginning of the idea was to make a full size, expedition capable sea kayak that would fold up and fit in a bag and be light enough to take backpacking and or fit everything needed for a kayak trip within the 20-25kg airline weight allowance. Everything I need for camping can be got down to 9-10kg excluding food, fuel and the kayak, so the kayak needed to come in at around 10kg all up. I stumbled across Tom Yost's website, which provided huge amounts of inspiration and information. Yostwerks Kayak Building Manuals - Homebuilt kayaks by Thomas Yost The problem was, all his folding designs with aluminium stringers and HDPE cross frames were at least twice my weight goal.
I started to think about carbon tube stringers and cross frames of marine ply, just making the corners where the stringers have to snap fit to the frames from HDPE. A look at carbon tube sizes and weights and a bit of calculation and it seemed the weight goal was possible in a 17 foot/ 5.2m sea kayak with PVC skin.
Some design issues quickly became apparent - I couldn't just line things up on a strongback, cut stringers to length, bash them flat and screw them into position, the tubes would have to terminate in sockets in the end frames. Some precise angles and lengths would need calculating and cutting, and every piece for the main frame would need to be drilled and finished to final dimensions before assembly. In the end I had to make my own design software (it ended up as a monster excel spreadsheet), and then figure out the design.
It all came together in the end, very pleasing to see the frame fit together and tension up to the designed rocker and then float on it's lines. Weight came in a fraction over 10kg, I have plans to replace most of the HDPE in the end frames with Paulownoa/ply /leftover carbon tube laminated up and make a lighter cockpit coaming so I think 8 and a bit kg is possible. Not as light as Micheal Storer's balsa canoe, but this one feels as though it could be loaded for an expedition and taken end over end in the surf without breaking. (Haven't tried this yet)
Quite a process and time frame to completion of the project, I now have three kids so the next project has to be a bigger boat! I'm currently in the early stages of building a Micheal Storer Goat Island Skiff and have sworn to myself I won't try and completely redesign it!
A few pictures are below, I can post some more with some more info on construction if anyone is interested.
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30th May 2010, 09:49 AM #2
Ian, Thats' a fantastic effort. I'm currently working on a Yost design in the shed a Sea Otter-R so it great inspiration. I might give up and copy yours cos thats actually more like what I want!
Are you only using 3 stations and do the CF tubes 'snap' in? Is the tubing 3/4" , it looks a bit bigger. The two longitudinal wires (?) are they to place the frame in tension for stiffness? How does that work?
The PVC looks thin- I'm thinking of using 500g/m2. How did you do the coaming?
30th May 2010, 10:11 AM #3
What weight PVC skin did you use? I'm guessing that the surface area of the kayak is around 5 square metres, so the skin must be a significant part of the weight. Did you consider a nylon or polyester skin? Or don't they work well with a folding kayak?
I have a SOF the same length that is the same weight, but it doesn't fold. It is a Yost construction style with 12 mm ply forms and Paulownia stringers. However at the moment it still has the 0.1mm thick clear PVC I put on as a temporary skin. It has lasted 15 months and a few hundred kilometres, which is better than I expected.
It looks as if assembling the kayak needs one young male helper and a young female supervisor. Was your third child keeping lookout so your wife didn't catch you building a kayak inside the house?
That is an impressive piece of work.
31st May 2010, 06:33 AM #4
More details on construction
Thanks for the interest.
Skin is 600g/m2 on the hull, 400g/m2 on the deck. There are zips most of the length of the front and rear deck, the assembled frame goes in through the rear zip opening and then everything is tensioned and zipped up. There are 5 longitudinal cords that are tightened up to maintain rocker and rigidity, I might be able to reduce that to three cords.
The main tubing is 19mm ID/21.4mm OD/1.2mm wall, the ferrules are 16mm ID/18.8mmOD/1.4mm wall made by C-Tech Carbon Tube - Round - www.carbon-tube.com in Auckland, NZ. There are 5 four metre long stringers (keel, 2 chines, 2 gunwales). Each is made of 4 one metre sections with a ferrule glued in one end and shock corded together. (For the Americans 19mm is about 3/4 inch, so OD of main tubes is a little bigger than 3/4 inch)
The end frames are made of HDPE and are/need to be quite strong in all directions. There are 4 intermediate frames of 12mm marine ply with a few bits of WRC laminated on where necessary. Ply frames are at the footrest position, front and rear of the cockpit and middle of the rear part of the kayak. It might be possible to design to eliminate the middle rear frame, but with the asymmetric swedish form (LCB is 54%), it's needed to hold the shape back there, and in any case helps support the rear deck in a re-entry following capsize if you don't roll. The ply frames "snap" to the carbon stringer to some extent, at the moment they are held together with velcro straps where necessary, but I plan to make two small trianglular HDPE snap connectors for each corner and screw them on either side of the ply. Details/pictures soon.
The cockpit rim is laminated from 12mm and 4mm ply and is hinged so it folds in half.
Bill P - I'd be prepared to send you a table of offsets (Similar format to Tom Yosts) if you wanted to have a go at this or a similar design. Be aware it's built around me (70kg, 175cm height), and is quite narrow (52cm beam, but waterline beam of 43.3cm which is quite narrow, tippy). It's easy for me to make adjustments to LOA, beam, waterline beam, position of footrest cross frame to suit, so if you are interested let me know your leg length, weight and desire for stability and I can fiddle things accordingly. Also, what build method are you thinking of - copy of mine with carbon, non folding wooden or aluminum tube following Tom Yost more closely.
More with pictures soon, probably on Wednesday morning.
31st May 2010, 07:13 AM #5
Total surface area
I just checked the numbers on total hull surface area, it's 4.74 m2
Multiply the hull and deck areas by the respective g/m2 and it comes out at 2.53 kg, add about another 500g for zips, velcro, flaps over same etc and the skin is about 3 kg. Haven't put it on the scales separately to check. Not sure if (presumably) coated nylon or polyester skin would work for a folder, I guess it would if you wanted an ultra light day-tripper.
Folding Kayaks by FirstLight make the lightest/most high-tech commercial folding kayaks I have come across, they make their skins out of an unsupported urethane material which is probably very good - presumably durable enough with care if they are trusting their commercial reputation to it and stretchy as it has no polyester fabric in the structure so easier to skin. I'd guess it can be glued with the same heat activated glue that we are gluing the PVC with, this PVC glue is urethane based I believe. I haven't found a source for this kind of urethane fabric. Am away from a big city so haven't had the opportunity to go around and look at fabrics to decide if they're suitable, may have to do some looking and requesting samples online or ask the firstlight people.
31st May 2010, 02:12 PM #6
Iím after a folder for multi day touring that can be taken on a light aircraft and can be made for a reasonable cost. One that can be packed/unpacked reasonably easily for portages too.
Thanks for your offer of a special design, Iím thinking the Sea Otter shape & lines perhaps suits my needs more so than say a West Greenland Qajak style. Tom quotes <13.6kg for the Sea Otter, which until I saw yours I thought was very good for a home built.
I reckon your frame design is superior to Toms. The tensile strings would make that hull very stiff and I like the way they Ďtieí the whole frame structure together. I havenít been comfortable with just shock cord & snapping into the stations with the only Ďhardí attachment at the stems.
I would be keen to see photos of your end stations, the tension system and your coaming. The stems areas beyond your end stations appear to have no chine or gunnel support just the ridge & keel- is that OK?
Iíts early enough in my project for me to change tack. The kids are after a chook shed anyway, so I might just pause my folder and reconsider.
31st May 2010, 07:37 PM #7
I agree that the Sea Otter design would serve your needs well. My design was the result of a long process and ideas I wanted to try out, and I'm happy with the result, but if you got out on a trip the differences would be more a matter of personal tastes than anything objective. There's actually very little difference in performance at cruising speed across the whole range of boats designed with cruising/touring in mind.
Disagree that my frame design is superior to Toms, most of the changes I made were a result of the decision to use carbon tube. It's difficult to join at the ends without fatally weakening it so the ends had to terminate in sockets in the end frames without any drilling or screwing of the carbon tube. This requirement opened up a whole can of worms in design, as to hold the tubes securely the sockets had to be at 2 to 3 times the tube diameter, which meant they had to be cut at precise angles, which meant figuring out some software to calculate the angles and a jig for the drill press to drill them... All stringers were left at 4 m length, but the end points vary due to the different curves of the gunwales,chines and keel stringers. (For this design, the chines finish 7.2mm closer to the centre at each end and the gunwales 8.5mm closer to the centre at each end than the keel stringer. This needs to be sorted with shims on the corners of the end frames or making the chine and gunwale stringers those amounts longer than the keel stringer. Lots of complication and extra weight in those end frames which isn't necessary if you have aluminum which can be screwed in at the ends. I've got some ideas as to how to make those end frames lighter but they won't be much simpler to construct, so I'd be inclined to stick to Tom's plans at the ends.
I could eliminate some cross frames and the deck ridges due to the inherent rigidity of the carbon tube that you perhaps couldn't if it was aluminum.
"The stems areas beyond your end stations appear to have no chine or gunnel support just the ridge & keel- is that OK? " That part seem fine for strength, forces beyond those end frames are minimal except for straight compression. The rear stem/rear cross frame is involved in tensioning up the skin once the frame is in place - more on this later.
My advice would be to stick with the Sea Otter design, and the overall frame construction methods of Tom's unless you want to go the carbon tube route. I wouldn't worry about the strength/security of the way Tom puts his boats together, he's built a lot of these kayaks and lots of people have put them to some serious use.
Changes I would consider worthwhile:
Make the cross frames of 12mm ply and just put triangular HDPE snap connectors on both sides of the ply at each corner. Cross frame weight will be halved, and you'll have enough HDPE for 5 or 6 kayaks left over. HDPE connectors on both sides of each corner will mean you can use longer screws that go right through the ply and bed into the other connector so they will stay in place. Others have tried putting HDPE connectors in the corners of the ply frames screwed into one side and the screws just pull out of the ply/everything becomes loose quite quickly. I'd put the screws/HDPE in with epoxy glue, it won't stick to the HDPE but it will help bed the screws into the ply. With double the connector power and triple the connector width at each corner rigidity/security should be higher and weight /cost much reduced as ply is much cheaper and half the weight of HDPE.
Tensioning strings are worth the trouble, I think, and do add to the rigidity without adding much weight. I needed them to maintain the rocker due to how I tensioned up the skin to make it smooth and fair on the bottom. Tom commented on how smooth and tight I'd got the skin for a folder, more on the secret to this in the next installment...
1st Jun 2010, 10:16 AM #8
Can't wait for the next installment Ian.
Thanks for validating my design choice- I think I I'll stick with the Sea Otter, Yost contruction. Maybe with a couple of full length stringer cables in lieu of the two rear deck tubes.
I take the point about the density of plywood vs HDPE, theres a kilo or two to be saved there. However I have some HDPP ex industrial cutting mat which I got for less than the cost of ply and I intend to that use for my frames.
If I was starting from scratch I would go ply/HDPE clips for frames as you suggest which would be stronger, lighter & cheaper.
Your tension system would preclude any need for spring clips, taping of the CF tubes? I'm keen to see more of this system.
1st Jun 2010, 05:03 PM #9
Well done Ian, terrific effort. Buying carbon tubing when you cannot be sure of the outcome takes determination and self belief - something I lack!
Thought you might enjoy reading this article in Duckworks about a 10kg sit on top design and build.
Duckworks - A Boat Named Alice
2nd Jun 2010, 08:48 AM #10
"Determination and self belief"
Hi Brian, good to hear from you. "Determination and self belief" - maybe, but I also had a fair idea of what I didn't know, and the journey I'd have to take to get there.
One big plus of the folding skin on frame kayak method is if you stuff up the design, go too far out on a limb in some direction, you can always take what you learnt, redesign, pull the frame apart, make some new frames from ply, put it together with a new skin and try again, so it was knowing that that I spent $1000 on some carbon tube. (Please don't tell my wife!) It's also possible to make cheap cross frames, put them in temporarily with tape, wrap it all in polyethylene sheeting and duct tape and give it a try on the water before finalising the construction (though I didn't do this)
Spend $1000 or more on ply, timber and epoxy, glue it all together into a sailing boat, put it on the water and find it doesn't work and there's not much you can do with the result. If you do come up with something new and it actually works really well, then if you do more looking, you'll probably find you've re-created something that someone else has already designed!
I didn't do any prototyping as described above, I just built my kayak complete, put it on the water but I'd done a lot of computer modelling so I knew where the waterline and longitudinal centre of buoyancy and such like would be. I also downloaded line drawings or pictures of every kayak in the range of designs I had in mind, overlaid them with what I'd come up with and analysed where mine was different and what the consequences would be.
A kayak is also much easier than a sailboat, the kayak is purely a displacement hull, there is only so much that you can vary and get wrong.
For those who feel inspired to follow the amateur self design route, I'd still recommend caution. I'd agree with everything that real designers such as Michael Storer, Par, Ross Lillistone have said on this forum and elsewhere about the pitfalls of designing or modifying designs yourself, it's much more complicated than you think. On the other hand, all these guys started somewhere (Par at age 10 I believe).
A sailing dinghy has to work in both displacement and planing modes and transition between them, and there are the issues with lateral balance and the way things scale with size. Stability scales with the forth power of size, displacement hull speed with the square root of length, foil lift with the square of speed... There's also all the structural stuff.
If you understand all these things and lots of things I haven't mentioned/don't know about and have built and sailed half a dozen boats yourself and start from something existing that already works, and enjoy a challenge and the possibility of failure, then go for it!
I did see the article on the boat named Alice, I thought the construction out of lots of very thin tube made it very complicated and fragile, (but was dictated by a source of cheaper tubing?). Was his down to 10pounds? Mine uses only 5 longitudinal stringers which are quite solid at more than 3/4 inch dia. When I figure out simplicating the end frames and cockpit combing, it could be built quite quickly and easily. My aim was to build the strongest expedition capable boat I could at around the 10kg mark rather than the lightest possible kayak. I can stand all by weight on any one of the cross frames, and I'm sure it would be OK going end over end in big surf as long as the end didn't hit the bottom. (I'll post some pictures once I've completed sea trials at St Clair beach, Dunedin)
Have to go now,
3rd Jun 2010, 05:54 PM #11
One big advantage of a folding skin on frame kayak is you can experiment and alter the frame one component at a time - you could proceed as you plan to, then later borrow a cutting board from the kitchen when your wife isn't looking, replace one of your frames with my ply - HDPE constructions and see if it's lighter/stronger/worth he trouble.
Will try and post pictures and address some of the other ideas/issues on Saturday morning.
All the best with the Sea Otter.
3rd Jun 2010, 07:15 PM #12
Ian, thanks for your full and detailed replies.
I have plans for Hugh Horton's Bufflehead sailing canoe. Here is a blog of the design in build using 4mm plywood over frames, planks fitted edge to edge then inside kevlar coated and outside carbon coated.
Stacy Smith's Photos - Bufflehead 50-50 Sailing Canoe | Facebook
I have been thinking that to build her as a folding kayak might be possible following many of Tom Yosts methods.
My question is, that often Tom's designs have little rocker, and then when loaded on the water the rocker takes shape. If you are building to a set of plans for plywood with a fixed designed rocker how would one best approach the issue of rocker increasing as she is loaded.
5th Jun 2010, 08:55 AM #13
Most of Tom's designs have little rocker, but that's inherent in the style of the design and use. They're not designed to do a slalom. If you got in one and tried to paddle it in a circle, it would have the turning circle of an oil tanker. However, with these hard chine designs, if you tilt the hull on edge with you hips, suddenly you have a rockered hull shape in the water and it turns quite easily. Best of both worlds - straight tracking when level, if you're paddling forward and need to turn to the right you edge the hull towards the left chine and it carves a nice turn to the right. Turning on the spot similarly with even more edging supported hopefully by the sweep stroke. I'd guess most of his main designs would bend only an extra cm, two at the most when you get in them.
His Sonnet designs (hybrid inflatable/minimal frame) are different, with minimal frame, they are built with no rocker/sheer curve and bent to an appropriate rocker in the water.
Of course a sailing craft needs a lit more rocker than a paddling craft if you are ever going to tack it.
Depending on the mix of design/skinning material/skinning method it's possible to have exactly the opposite problem to it gaining too muck rocker as it is loaded - how to maintain the desired rocker and stop the hull hogging.
I think it would be possible to build the Bufflehead as a folder following Tom Yost's methods. The main thing you would have to pay attention to would be managing the forces from the sail and lee board and the righting moment to keep the boat/sail upright. I would be inclined to make the cockpit combing the main structural element of the boat - maybe from 50 mm wide 4mm ply sections with aluminium tube epoxied/glassed along top and bottom edges and the sections fitting together with ferrules on the tubes. Mast partner in the front of the cockpit combing, a beam that clamps across behind it to carry the leeboard. Maybe a couple of stays from this leeboard beam to the mast. Will try to draw and scan a picture of what I have in mind. Have a look at the Klepper website - they have been building folding kayaks with sail rigs for a long time now, Franz Romer sailed the Atlantic in one in 1928, Hans Lindeman also did the Atlantic in one of these in 1956.
There must be someone on the Solent with a Klepper with a sail rig you could look at.
You asked about designing to deal with the extra rocker when the canoe is loaded up - strange to tell, I think you will have the opposite problem if you build the Bufflehead with it's upward curving sheer line One thing you notice with all the comercial folding sea kayaks is they are built in a "two icecream cones" shape. Straight sheer line, highest in the middle and deck ridge sloping downwards towards each end. Extreme in this regard are the First Light designs.
This is for good reasons - the bottom of a sea kayak is not a developable surface (at least in one piece), so if you want to cover it in a single piece of fabric, you need stretch and or wrinkles and or darts. The secret to a smooth underwater surface is longitudinal stretch. (If you try to stretch the material around the kayak by pulling it up tight on the gunwales, you can't get enough to smooth out all the wrinkles unless you use the light unsupported clear PVC's, but the % stretch necessary in the long direction is less) If the ends are cone shaped, the stretch along the bottom matches the stretch along the deck and the structure stays balanced/undistorted.
The traditional greenland kayak shape
comes from thin but relatively wide gunwale planks which form most of the structural strength of these boats. Tie the ends of the gunwales together, push them out in the middle and flare them at the top of the gunwales and the ends will curve upwards in a nice sheer line, and the structure will hold it's shape against a lot of force.
Do the same shape in aluminium or carbon tube, and all the stringers are curving up towards the ends, and they want to be straight. Put some significant longitudinal stretch along the bottom to smooth out the skin, and they'll straighten and want to curve the other way, which is bad however you look at it.
(1) Design as all the comercial ones do as "two icecream cones" so the structure remains balanced under tension.
(2) Stick to the Greenland shape and don't put any longitudinal stretch in the skin. Put up with some wrinkles in the hull skin, try and keep them in the area between the chines and the gunwales so they don't affect water flow too much and smooth them out with a glued dart or two as necessary. (I think this is Tom's approach)
(3) Stick to the Greenland shape but counteract all those forces trying to make the kayak bend the other way with lots of tension in cords along the deck and in an approximately parabolic shape from the bottom of the stems front and rear with the high point in the middle. (Or use inwales curving this way as is documented somewhere in Tom's sight)
(4) Use more stretchy material than the easily available PVC with polyester weave. Have a link to the people who manufacture the skin material that First Light use, I think they're in Germany.
Have to go now, more later.
5th Jun 2010, 10:22 AM #14
As I think Mik has said in regard to his balsa canoe, end to end forces on a lightweight hull in the water are quite distributed, create less bending moment than you would think. However, moments and torsion forces from the sail to foils to movable ballast (you) holding the boat upright are much greater once you go the sail route.
6th Jun 2010, 05:42 AM #15
Pictures on Flickr
Hi Everyone, I'm just figuring getting a photoset of the kayak construction up on Flickr. I might end up linking to the individual photos here with some more description/information, but in the meantime, I think this link will get you to the photos I am adding with some description alongside them.
Folding Sea Kayak - a set on Flickr
Most of the photos are high enough resolution to zoom in on if you want to see any details.
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