Kayak design parameters.

[philSweet]

LP. Been there, done that. 16' plywood skiff on I 95 North at 2am at 75 mph, on an overpass, in the rain. Honda civic tried to pass me on the acceleration lane but was deterred by bridge inspection machinery he didn't see. Cut in hard behind me and took out my trailer. I was being passed by a semi and my trailer got stuffed under the semi, which took out the axle, sheared the springs and popped the tires. civic exited the off ramp and I dove into the sawgass at 75mph. Best wetsand job I've ever done. That hull polished up like abralon 2000 pads in the hands of experts.

I once loaded the same skiff at a very busy ramp and had to move it about a quarter mile before I could take the mast down. Left the rudder pinned down. Whole boat supported by its chin and its rudder. No worries, it was bigger than it needed to be anyway. Murphys law- the only time in 25 years that I leave the rudder down is when I drag it 1/4 mile.

The abrasion resistance of 9 oz cloth and epoxy is really remarkable provided the underlying material can take the point loading without deforming.

[LP]

It's been a while since I posted about this project. I've been working the 1/8" deck vs. the 3/16" deck dilemna. Two sheets vs. 3 sheets. etc. etc. Rather that fight the curvature of a thicker deck and/or use a third sheet of thinner ply, I've opted to use an inverted "V" deck. This has netted me other benefits as I can now cut all major components from the two sheets of 3/16 (4 mm) ply. I can get the canted frame now without piecing it together and I was able to orient the deck grain to radiate out and aft from the central deck joint. I've attached the flat panel layout and a DXF file of the same if anyone is interested. There are also three mold sections shown that could be cut from cheap stock. I haven't done it yet, but I would/will trim the corners of the molds to allow for fillet space during assembly.

I've also been working up a three sheet version that I can take up to 15.5 feet. It will have the curved deck that I like because I can justify the third, thinner sheet in the larger build. I was working a 14' concept, but it seems wasteful to not fully utilize the materials at hand. I'll post here at a later date.

[cthippo]

Which size is this?

[LP]

CT,

This is the 12'er. I made accomodations to fit everything into a set of scarfed 4 x 8' sheets of 4 mm ply. I've got it spec at 290 pounds displacement. At 330 lbs, it sinks an aditional 3/4". My numbers show it more stable at 330 lbs than at light displacement, 220 lbs. I show a downflood angle between 40-45 deg. at heavy displacement.

If I was going to max displacement consistently, I might consider transitioning to a 14' LOA for more reserve stability.

[cthippo]

Sweet, thanks!

[LP]

I've started construction of the 12'er.

Mysticshore 12 gets started.

[LP]

I'm trying to pull together the final elements of my next kayak build. This one is for my wife to get her out of the first 7.5', single-sheeter that I build. She love the little thing and floats in it like a cork.

This next one is going to be 13.5 feet and will incorporate some design developements over the last one. Some intentional, some out of necessity. As mentioned earlier, the 4mm ply I purchased was thinner and more flexible than anticipated. That lead to some issues with fairness as the hull of my 12footer was stitched and glued together. In the next build, the 13.5 footer, I was planning to go even thinner to try to save on weight. My plan was to use stronger laminates. (S-glass vs. E-glass or maybe even some kevlar) The crux of the story is that I purchased enough 3mm ply to build two more kayaks. The 13.5 footer for the wife and maybe a 15 footer if I could squeeze something useable out of the other two sheets. Well, the 4mm proved to be overly flexible and now I'm looking at possible methods for reducing the flex in the 3mm ply to avoid the fairness problems associated with 4mm build.

I'm borrowing an idea from one of our other contributers. Ancient Kayaker, aka, Terry Haines likes to pre-glue his sheer clamp into place on his plywood sheets prior to bending them. I wasn't sold on the idea when I first read it, but after dealing with the 4mm ply, I'm convinced it has a place in light-weight boat building. I had a combination of 5 molds and bulkheads in the 4mm build. Without any additional support, I had a hard spot at each location. Even after springing my sheer clamp into place (1/2" X 3/8" pine), I had hard spots at each mold/bulkhead location. It wasn't until I'd sprung in crosspawls (sic) every 2 feet that I felt I had enough control on the sheer that I could even think about glueing the sheerclamp into place.

My plan is to pre-glue the sheerclamp into place on a flat surface. I will do the same with at the chine, though with an added twist. It's going to be a split chine with "halfies" on either side of the joint. Additionally, the chine piece on the sheer/side panel will be set back a minimum of the material thickness. Just for good measure, when I draw up the side panels, I may add a 1/4" extra on the lower edge and setback the chine strip the same amount. This will create a recess (see diagram) that the lower panel can rest inside of and provide some alignment qualities. The chine strip on the lower panel will be setbeck the thickness of the upper chine strip.

Thinking about it though, the lower chine strip may not be needed and may cause difficulty as this joint would need to be assembled wet and would need to be worked one difficult and frustrating session. Two options; increase the setback to create a channel the epoxy could be brushed/flowed into and topped with thickened epoxy or use no lower chine strip to leave the gap under the upper chine strip accessible to thickened and unthickened epoxy.

A third possibility is to laminate the lower chine strip to the edge of the lower panel and setback the upper chine strip the thickness of the chine strip and the lower skin. This way I get edge thickness on both panels for fairness, thicker chine material to allow for a greater chine radius and fairly good epoxy accessability in the assembled joint.

I'm also considering the use of doublers at frame/bulkhead stations. There is some slight hardspotting in my 4mm build at frame stations. This build process reminds me a lot the process I use to see at the aircraft factory in the metal bond shop where they would laminate multiple layers of pre-engineered shapes to get the most possible strenght for the weight. I have toyed with the idea of laminating two layers of the 3mm together. The inner layer would be like a skeleton of the panel perimeter with crossmembers at frame and bulhead stations. I'm not so sure this is an efficient use of material.

Lastly, my scarfs on the 4mm worked, but I'm concern with the strenght in a 3mm ply scarf. There is little room for error. I've pretty much decided to use butt joints with plywood doublers on the inside. The areas where the joints have little curvature and the doubler will act to stiffer the hull laterally at that point. I may work some taper into either edge of the doubler to reduce any tendency to produce a hard spot longitudinally.

[cthippo]

Sweet new project, LP!

Can't wait to see this one in the water.

[ancient kayaker]

How's it going? I like the idea for the rounded chine; I have wondered about doing that but I haven't actually tried it yet.

BTW I used butt joints with ply doublers on my first canoe and did not see any hard spots on the finished hull, probably because the extra stiffness from adding the chine log and sheer clamp to the plank while flat swamps the extra stiffness of the doubled ply. On a kayak, nobody can see the doublers if the joints are located under the decks, which also keeps them away from the highest stress areas around the cockpit.

However since then I have done scarf joints on several boats and it isn't difficult, I tried various tools but a sharp low-angle block plane is just as easy and almost as quick as using a power tool like a router or belt sander.

[JRD]

Hi Terry,
How is your ply and cedar kayak build progressing? I was following your progress with interest a few months ago, and seemed to have missed any recent reports you may have posted. It would be great to see some photos of the finished product when you are done.

Cheers, Jeff

[ancient kayaker]

Originally Posted by JRD

Hi Terry,
How is your ply and cedar kayak build progressing? I was following your progress with interest a few months ago, and seemed to have missed any recent reports you may have posted. It would be great to see some photos of the finished product when you are done.

Cheers, Jeff

It hasn't moved since April; I had an arthritis flare-up in the Spring and couldn't do much of anything. Finally a couple of weeks ago it was diagnosed properly and I got some treatment that actually worked. Now I am slowly working my way around the yard and house catching up with all the jobs I let slide for 6 months; once my conscience is clear on that lot I can get back to the boat (named Rose Lee after the famous stripper).

It was coming along pretty well back in the Spring and is looking good sitting in the workshop; I'm really looking forward to finishing it. There's another boat there (a single sheet canoe) that is almost done, just needs seats, breasthooks and varnish, I'll get that done while I'm there.

I keep eyeing the local lake longingly every time I take the dogs out for a walk, but I am not strong enough yet to venture out so testing the boats in the water will have to wait. The arthritis was an aggressive inflammatory type and I have lost some muscle mass that I have to get back - unfortunately the condition burns protein not fat otherwise I'd be in great shape!

[LP]

CT, Thanks, man. I'm afraid that since my last post, my focus is starting to shift towards doing a lapstrake. I have a stack of 3mm ply that is an excellent candidate for such a project. I've been exploring build methods and a lapstrake kayak would make a nice introduction to the method. We'll just have to see where the synapses take us.

Terry, Good to hear that you are doing better. That's a good bit of information regarding the doublers. I am finding that, despite my best efforts, I'm getting hard spots on my current build where bulkheads and frames meet the hull. Future builds with thin ply will find doublers under frame, too! An alternative would be to float the frames and bulkheads on the chine and sheer stringers and fill the void later with a material of choice.

On my second single-sheet kayak, I radiused the chine edge down until it met the fillet material on the other side. This gave me the greatest amount of radius possible and left full fillet strenght. It's pretty amazing how much radius you can get from 3/16" ply with this method. Of course, an exterior sheathing of glass was applied to the hull. My fillets on that hull were fairly generous, too. As the chine angle diminished fore and aft, I did not go as deep into the chine.

A radiused chine (with plywood) will lock you into a paint hull, too, IMHO. I like bright and in some ways bright is easier than paint. I always have a debate right before radiusing the chine. Typically, it involves form(esthetics of paint v. varnish) and function(water flow lines around the hull). So far, function is ahead 2-0.

[JRD]

Terry, pleased to hear things are improving for you and look forward to seeing progress in the shed coming along again soon!

[joseph sigley]

we have just build a new sit on top kayak mould that we are selling would you like to see the mould we have built if so send me an email at info@j-s-composites.co.uk and we will send you some pics and prises