strip planking v ply

With epoxy you don't need a perfect fit. If there's a 1/16 gap on the outer side btw the strips epoxy'll fill it.. with other glues in the past this didn't work..

 

So it is important to lay out the cabinetry and built in furniture, bulkheads etc. to take the places where frames would otherwise need to go, making them redundant.

 

Exactamente socio!

 

It's been hinted at, but not nailed down, the essential difficulty with using a strip planked method as a panel, rather then a compound curved area, is that the strip planks have a huge majority of their strength oriented along the longitudinal axis and virtually none cross grain. This means the panel will easily break in this direction, very likely along one of the strip seams. Someone here has made a kayak/canoe with this method and though sheathed, it's cross grain strength would be considerably less then a similar panel of marine plywood. Since it's light and has low loading, it'll hold up okay, but not nearly as well as a plywood hull of the same dimensions.

To get back to the original questions, yes, strip planking can be done much cheaper then plywood, if you only count the wood used, but this isn't a fair comparison as the plywood is pre-processed. This "processing" makes for a stiff panel that is pound for pound stronger the steel. You have a fair bit of work to do to the strips to have the equivalent strength, which can be considered part of the cost equation. These things should be considered for a fair comparison.

What strip planking can do is compound curves easily, but plywood can not so there's another rub. Plywood is used on designs with straight sectional lines, but strip planked hull forms are highly curved. This feature alone means you can't substitute the two different methods for each other easily. You can strip plank with plywood, but it defeats the reason for employing plywood in the first place, large panels. You can strip plank on a chine hull but you end up with a heavier hull once you accommodate for the strength issue and use thicker strips to compensate.

In the end, plywood (really thin plywood) can be used on round bilge hulls, but you lose the advantages of plywood. You can use strips on a chine hull but you have to increase strip thickness to compensate which makes a heavy boat.

So, if you have a plywood design, build in plywood. If you have a strip planked design build in a typical round bilge technique like; one of the many molding or strip methods, sandwich core, etc. and leave the plywood for what it's best at, which is covering large areas with a single hunk of processed wood product.

 

That's the best advice - with plywood, someone has gone to the trouble to make a big panel; at least have the decency to use as much of it as possible in one piece!

Just to nit pick for accuracy - plywood should be used for designs that developable surfaces, which doesn't mean the sections are straight, but that the panels have parallel or radial ruling lines that are straight. The frames, especially up forward on the bottom panel will be curved.

 

You nailed it downed much better than I could have.

 

Huh? I'm currently reading Dave Gerr's The Elements of Boat Strength, from which I quote:

I plugged in some rough numbers for a boat I'm interested in building (55' LOA, 18' Beam, 10' depth (not draft)) and came up with frame spacing of 7'. He also specifically mentions planking being stronger than plywood, although I'll have to thumb back through to find the exact statement made (it wasn't a blanket "all planking is stronger than all plywood" deal, but something more specific which I can't quite remember right this minute).

I'm new to all this, and just curious where this information fits in...not trying to pick a fight.

 

You're not using the Geer statement in context, which defeats it's use. Most strip planked and monocoque plywood builds, have widely spaced frames, if they bother to use any at all. You're frame spacing may or may not be correct, though judging by the 7' dimension, you're not talking about "frame spacing" so much as athwartship divisions, relatively equally spaced along the length of the hull shell (of what ever build method). This is a wholly different engineering approach then a plank on frame comparison. In fact, what you are describing is the way bulkheads and interior furniture are arranged in a homogeneous hulls hell such as that employed by GRP builds.

It's very important to speak apples and apples, with these types of comparisons.

 

Actually, the 7' dimension came directly from Gerr's frame-spacing formula (Formula 9-3, FWIW)...so unless he mis-labeled his formula, I think I'm referring to it correctly. He actually calculates bent-frame spacing here for use in carvel-planked hulls, but goes on to say that sawn frames on chine hulls are spaced farther apart, but not more than 36" OC.

Then in Formula 9-5, he says what I quoted above and goes on to say that frame spacing for strip-planked hulls can be "even farther apart," and that they may even exceed 36" OC.

Again...not trying to pick a fight, but I would like to get it sorted out. I'd been planning on using plywood for a hull, but after reading Gerr's wood construction chapters, I'm waffling on it. He seems to be a proponent of strip-plank construction, going so far as to call it his "construction method of choice, by a slim margin."

 

You're not doing the apples to apples thing that I mentioned and if you're not, it's a futile discussion that isn't worthy of participation. You have a 36" qualifier, but you insist of following a generic chart for a 84" spacing on a different construction method. Hummmmm, what's the worst that could happen. It's important to have at least a grasp of the engineering fundamentals in regard to structural approaches or more importantly differences. Some of these are interchangeable, others, such as in this case they are not. In other words "scantling rule 9-3" is the bent frame rule for carvels and it comes with two pages of qualifiers (understandably) rule 9-5 is the strip planked frame spacing, and again uses many of the "qualifiers" such as planking thickness increases for S/L over a specific amount, etc. Your comments suggest a plywood build, so we're back to the apples and apples thing again.

 

For the third time: Not trying to pick a fight. Trying to understand. Either I've made a comparison somewhere that I'm unaware of, or you don't understand what "apples to apples" means. Allow me to paraphrase my responses and logic thus far:

Formula 9-3 Frame Spacing gives bent-frame spacing. Rather than "two pages of qualifiers" it lists exactly five conditions under which the spacing is to be modified - one of which is that no frame spacing is to exceed 36".

Formula 9-4 gives the formula for adjusting frame siding and molding to allow convenient frame spacing and is irrelevant for our purposes here.

Formula 9-5 Strip-Plank Frame Spacing does not give a totally different formula for spacing frames as you seem to indicate above ("...you insist on following a chart for 84" spacing on a different construction method."). In fact, it modifies the spacing given in Formula 9-3 (4x bent-frame spacing) and then lists exactly three qualifiers (not sure what your definition of "many" is, but for me, 3 ain't it). One of those qualifiers is that frame spacing may exceed 36", and another (again) modifies the bent-frame spacing 4.6x for hard-chine hulls with sawn frames.

So. I have neither a 36" qualifier, a chart for 84" frame spacing, or a different construction method. What I do have, as I said, is a question about an earlier response that suggested that strip planking required closer framing than plywood construction since, as I also said, Mr. Gerr seems to say exactly the opposite. (Qualifier to Formula 9-1 Plank Thickness: "...plywood hulls are hard-chine, so use hard-chine frames and frame spacing." Qualifier to Formula 9-3 Frame Spacing: "Single-sawn frames on hard-chine hulls are spaced 2.5x farther apart than bent-frame spacing...[not to] exceed 36 inches.")

Clearly, "not to exceed 36 inches" (plywood) is closer frame spacing than "4.6x bent-frame spacing; may exceed 36 inches" (strip-plank).