Looking for feedback on constructin process

I'm considering a variation of frame building a small skiff I am designing right now. I was going to create 4-6 stations for an 17-18 LOA and 6' bottom width, place them on a simple strong back and use the chine logs to form the shape of the vessel. The curves are not to complex for a single 1/4" sheet to conform, but would be difficult with a 1/2" piece. I am thinking that If I form the hull one sheet thick, then epoxy the second sheet on the outside can I use screws to pull the materials together like in cold molding. Let me hear what you think about this. Thanks, Adam

 

It is a waste of time and materials. 1/2" plywood bends fairly easily. Skiffs don't usually have any complex curves.

 

Adam, first the shapes have to be cylindrical or conically "developed" or you'll never get the plywood to bend around the forms. I generally agree in that 1/2" plywood is fairly easy to bend, but in a 17' boat, the curves maybe "quick" enough, that this would be way too thick to easily bend. Double planking with two layers of 1/4" is a common technique, though making the outer layer "suck down" with screws, isn't the best way to attach them. There's lots of ways to plank a boat and getting planking to "lay down" where you want it; as a rule forcing it usually isn't good.

Lastly, it might not be wise to reinvent the wheel. There are dozens of plywood building techniques, some more notable then others, but all proven. The "free form" type of building I think you're talking about has been done, but it takes a pretty skilled eye to get the shapes right. I watched a fellow do a whole 50' fishing boat like this, but he'd been doing it for 30 years, was taught by his father who'd done it for 50, who'd been taught by his father, etc. In short, if you have an impressive visual and spacial skill set, in regard to the shape of boats, then go for it. If you're like the rest of use, it might be a wiser choice to just get a set of plans, from a living, known designer.

 

The big question is why you need half inch ply ? 12mm is the sort of stuff you would use for lots bigger boats. This is especially heavy if you are also considering doing a bit of glass and epoxy on the outside for protection, as is normal.

You did say it was self designed, so it looks like you are setting yourself up for a seriously overweight build, especially using chine logs..

 

The hull design is derived from a "Harkers Island skiff" A quick google image search should give you the basics of a wide flat bottom sharpie style skiff with a very high bow and some flare. These were the work boat predecessors to the famous carolina flare now very popular with the sport fishing guys. The boats were typically built with juniper board stock for frames and planking and were often left the boat in water year around, so no glass, it just swelled up tight.

I talked to several local builders who still build these style of boats as well as much larger ones. I showed them my drawings of the skiff (styled after the old work skiffs) and told them my plan to cold mold instead of plank. The most popular suggestions was to use two layers of 1/4" marine grade. I ran the numbers myself on the design and all I really need is 3/8. But I could not find 3/16 marine grade, so I went with the 1/4". I considered 1 layer of 1/4" and increasing my laminate schedule but decided since I was cold molding I rather maintain the double crossing effect of two layers and decrease the laminate schedule and error on the side of overbuilding a little. Maybe that was the wrong decision, but I made it and moved forward.

Seems that 3 people so far think I'm a bit crazy. Oh well, being "normal" never got me very far

 

It would be nice if we had more details to go from; i.e., what do your "not too complex curves' look like. As mentioned, the curves must conform to a conical development. My concern is for the number of stations you propose. Four to six stations would give you a support about every 3-4 feet. Is that going to be enough to smoothly define the shape and solidly support your 1/4" plywood and additional layers? Flat surfaces need support to avoid slight waves, and curved surfaces need support to make sure that you are defining a symetrical shape.
Secondly, picture yourself epoxying that second sheet of plywood over the first. You slather the surface (or surfaces, you should really do both) then pick up the full size plywood sheet (is this going to be a scarfed 18' sheet?) and carefully and exactly place it over the first (what are you going to use to hold it in place temporarily?) Make sure you don't get any fresh uncured epoxy on yourself. Now, work like crazy to screw that second sheet down before the epoxy starts to cure. How much squeeze-out will you get? How many bubbles may you leave between layers?
The point I am trying to make is that you would probably be better off to put the second layer on in smaller sections. You referred to cold molding; that isn't done in a single large sheet. If you do the second layer in smaller pieces, why not plank it using 1/4" thick planks? A developable surface is not difficult to plank.
Good luck on your project!

 

Sorry, your most recent post wasn't up when I composed my last post. When you say 1/4" ply, that sounds like US doug fir; I would recommend you use imported metric ply. It is better quality, easier to work with, comes in various woods with different bending qualities and thicknesses. I agree with not using the increased laminate schedule; it is expensive and heavy. But one layer will really help seal and strengthen the hull. Now that I understand it will be a sharpie style, the curves involved should indeed be gentle; 3/8 (or its equivalent) should be able to take such bends, and the thicker ply will not require as many stations for support. If you decide to still go with 1/4" ply for the first layer, you could put on a second layer of 3/16 planking fastened (temporarily) with simple staples. The staples go quick and the holes are almost imperceptible when the staples are removed.

 

If you are building a Carolina skiff, they are several versions. I think you are referring to the ones that have a flared bow and some hook. Plywood will not bend to those compound curves, unless you cut it into narrow strips.

 

Posting your lines will go a long towards determining a proper construction technique. I think everyone has offered great advice, but for advice that "hit's the mark", a lines drawing (even a basic one) will assist greatly in getting you answers that are specific to your needs.

PS. If you're crazy, then this whole forum is packed with crazies. It's a great place to be.