cartopper scantlings
I just finished painting my Wa’apa and I’m already busy thinking about a proa.
Sailing the Wa’apa (without paint…I couldn’t wait), its clear how much nicer the proa tack feels than the trimaran tack. Stable, light, lovely.
So I’m starting to think about an 18’-20’ car topper. Hell, if Hugo can throw Proud Mary on his rack, I can throw just the vaka of a 20’er.
So, how do I make an ultralight vaka? Right now I’m thinking scaled down SRIR vaka with no pod (safety ama)—so narrow (12:1) with lots of lateral area.
Option 1: THICK SKIN: 6mm okume, glassed on a bench on the inside (ala PT Skiff) with 4oz glass, 6oz glass on the hull. Two stringers running the length of the boat, at the level of the self bailing cockpit floor. Have only 3 bulkheads (under the mast step, and at each end of the cockpit). That creates 4 equal 5’ long sections without frames. But there’s lots of strength in the panels. I like this option because it uses 5ply and not the 3 ply stuff you get at 4mm and below. I mean this is supposed to be cored construction, right?
Option 2: THIN SKIN MORE BULKHEADS: the same thing but with 4mm okume and 7 bulkheads (actually 3 bulkheads and 4 ring frames).
Option 3: THIN SKIN MORE STRINGERS: the same thing but with fewer bulkheads and more stringers.
what do you think?
Generally the more you can break down the panel size, the lighter you can make the structure, i.e. more frames and stringers means you can use a lighter skin. Also, the more frames there are, the lighter each frame can be. The trade off is that it is a more work to build more frames. Thinner skins will not be as durable (impact damage), but you don’t expect a light boat to be as durable as a heavy boat in any case.
As it happens, I’m currently designing a sailing dinghy that is to be built mostly from 3mm plywood. It has 3mm plywood frames and bulkheads spaced at 150mm along it’s 4.2m length. The seams will be glass taped but otherwise no glass will be used on the panels. The outside will be sheathed with Dynel for durability. The finished hull should weigh around 30kg.
I agree with Mal’s comments, but for durability, I would do 6mm THICK SKIN with no glass inside (just epoxy sealer coat) other than taped joints and glass outside.
You could also look at planking it in 250 or 300x6mm paulownia, which is two third the weight of gaboon, if you want to go superlight. Maybe then put in extra frames for good measure.
Whether you are balancing weight vs durability or weight vs cost and complexity, the only way to decide is to do a preliminary design for two or more alternatives and do reasonably accurate weight estimate for each alternative. I presume your weight target will be how much you think you can reasonably lift, with or without help as the case may be. Use the construction method that gets you to your weight target.
Weight is not just about the construction method. You also need to to think about how small you can make the hull, to reduce the amount of material required, i.e. how much can you reduce the freeboard and the hull beam, how will any foot wells or cargo spaces be constructed, what bits of the boat can you do without? For a multihull, how can it be broken down into managible pieces?
Skin on frame ala the old stick and tissue model planes would probably be the lightest, Monforts Geolite boats were the ultimate expression of that direction, and surprisingly durable with a ply bottom (in a SRIR type proa that’s not much ply).
I’d lean towards thin skin, a few stringers and frames to suit. 3mm ply, bottom with 2 oz kevlar inside, 4 oz sglass outside to just over the chines.
As Mal said, keep it rigidly minimalist, the stuff you don’t build weighs the least.
An expensive alternate would be foamcore, corecell w/ 2 oz kevlar, inside & out, 4 oz sglass outside, would have to run the numbers to see if it’s really worthwhile. FWIW EasyB 14’ canoe originally weighed in at 38# using 5 oz kevlar in lieu of the 2.
Lots to cogitate on.
This is super helpful! Yes, the parts you don’t build are the lightest!
It makes sense to me to throw glass (or Kevlar) on the inside to create a cored construction. Using peel ply, you don’t even need a fill coat.
So maybe the idea is 3mm with twice as many bulkheads(every 2.5 ft—not as close as Mal’s), also 3mm, and the stringers at the height of the footwell. Or, to your point Mal, is a footwell on this boat too much. Maybe its more of a beach cat hull, with little freeboard and a raised cross structure.
Next thing you know I’ll have convinced myself to use reverse bows…
Here’s another querry: has anyone built a boat with location tabs on the bulkheads? I’ve heard people talk about having tabs for alignment but have never seen a picture. Do you put a hole in the skin for the tab? Doesn’t that screw up the fairness of the skin? Or am I imagining this wrong?
thanks as always,
chris
It makes sense to me to throw glass (or Kevlar) on the inside to create a cored construction. Using peel ply, you don’t even need a fill coat.
To me it makes no sense to add a relatively heavy and expensive exotic fabric to one side of a plywood panel. As a core material, plywood is heavy and generally not thick enough to give useful separation between the skins (if skinning both sides). Skinning one side will add little additional stiffness relative to the cost and added weight, just use thicker plywood if extra stiffness is required (or more frames).
If you use plywood as a core for high modulus fibre skins, the plywood takes almost none of the load and the skins have to be designed to take all of the load, therefore the plywood, which makes up the majority of the weight, has no structural benefit.
My philosophy is to use the plywood as a structural member. Wood is a relatively low modulus material which must flex a bit before it takes up any significant loads, hence you need to reduce the panel size (the span between each frame or stringer) so that the deflections are not significant relative to the overall size of the hull. Wood is a beautiful material. Let it do its job. If you want to use exotics, use a proper thick low density core so that you get the full benefit of that material.
On the inside you don’t need peel ply or a fill coat, particularly w/ 2 oz kevlar, it’s pretty fine texture already. weave of cloth inside has a little nonslip value also (not much use here but good on a canoe).
On this size boat a footwell may not be of much use if your shoe size tends towards the large side.
It’s instructive to mentally go thru assembly and a few test sail and look at ‘everything’ around you asking “is this necessary?”
My only experience with locating bulkheads was to predrill a couple of holes in the skin and one in the bulkhead to thread a cable tie for typical stitch and glue construction, worked fine.
Cheers,
Skip
Just wondering, what about rigging the Wa’apa as a proa? Are the pieces too heavy to cartop?
Just wondering, what about rigging the Wa’apa as a proa? Are the pieces too heavy to cartop?
Fair question!
1. I either want to go the deep asymmetric vaka route (like SRIR) where you don’t use rudders up wind, or go with dagger/rudders (like KBBC). Modifying my Wa’apa for those is not practical. I’ll decide with models.
2. I think I want to try a boomless dierking/gibbons rig like Proud Mary—stepped on the cockpit sole. And there’s a bulkhead right where that would go on my Wa’apa.
3. My wa’apa works! It sails great! It would be nice to sail it while another build eats up my garage.
4. The wa’apa was my first boat. I can do twice as well on the next one…at least! I learned a lot of lessons about epoxy/ply craftsmanship that I’m eager to try on a lighter boat.
5. I want a bit more load capacity.
1. I either want to go the deep asymmetric vaka route (like SRIR)
I can`t find anything that can be shorted to “SRIR”, so I must ask what is SRIR?
Cheers,
Johannes
KBBC is the Kahana Bay Beach Cruiser
http://proafile.com/forums/viewthread/243/P15
SRIR is Sweet Rhode Island Red
http://proafile.com/forums/viewthread/292/P60
sorry for the jargon!
Thanks Loumanen!
It is ovious once you mentioned Sweet Rhode Island Red.
If I was to build as ligh as possible I would go for a thick low density foam-core with CF or FG skins. It is very easy to add strength where needed, like a carbon fiber “belt” around the gunwale for a greater global stiffness.
I am very impressed by the extreme stiffness even simple styrofoam/polyurethane-glue/alu-foil or paper laminates gives.
As mentioned before, things that is not there does not add any weight. The greatest weight savings comes from removing everything non-essential. With a lighter boat there is less need for freeboard - which gives less windage and resistance in waves - less sailarea - smaller mast and so forth….
(Picture below from boatdesign.net)
Cheers,
Johannes
I agree with Johannes, Why not use flat cored panels that weigh far less and can be had for the same money, or less, than quality marine ply ,
Puncture resistance
Puncture resistance
Two ways to design a puncture resistant structure.
1: Thick high strength steel. Heavy, hard to work with, expensive, needs heavy duty machinery. It seems like I am the only one stupid enough to go this way with proas.
2: Design the structure to flex to absorb energy by spreading the energy over a large area.
This is where modern foam-core construction really shines.
With an inner stiff and strong load-bearing carbonfiber skin taking all the global loads you can have an outer skin of ripstop polyester or nylon weave in an flexible epoxy. This can take enormous impacts without breaking or rupturing. In a small boat (less then 35 feet or soo) the stiffness from a very thick (30 - 100 mm) foamcore composite is enough, even though the outer skin is flexible. This kind of foam core design is very easy to repair. If the foam gets crushed, the outer skin will probably be intact. Once ashore again it is easy to cut out the damaged part, sand down the edges, glue new foamcore, shape and cover with new rip-stop flexible nylon or polyester weave.
Cheers,
Johannes