substitute strip planking for plywood build

Greetings-
New to posting, been reading for years. I bought plans for a small sailor named Scamp a few years ago and I've since retired meaning I'm fixed income. Scamp is a single chine boat, plywood and fiberglass, 12 feet long.
I've done 4 strip planked hulls in the past and wonder if anyone has an opinion on building Scamp strip planked? Problems I see are weight, no keel (it's a side board boat) so no strong back in the design as it is now. Bottom is flat and all of the strip planked boats I've done are conical. I would spring for a plywood transom.
I have enough strip planking and fiberglass to do a large house! THat's why Im interested ($$$). Thanks very much for your input.

 

On a 12 feet long boat, strip planking would be a fine substitute for plywood, assuming the plywood was going to be glassed on both sides.

 

Thanks RWatson, that's what I figured

 

Scamp is a hard chine design, making it much less desirable to strip plank. As a taped seam build, the planking goes on fast and the hull shell is together quickly.



The real advantage to strip planking is the ability to have compound shapes and a round bilge hull form. Scamp could be redrawn as a round bilge, making a strip planked version much more suitable as a build method. If I was going this route, I'd fair out the pram bow, into a pointy one, if just to make it more aesthetically pleasing. Most folks like to see a pointy end on the front of their boat.

This said, you still could strip plank a Scamp, though it would take longer to build and scantlings would need to be revisited, by someone that's made this type of conversion before.

 

only if you laid the planks on one by one.

If you laid out the strips in a flat surface, glued them together, and even glassed one side - they could be cut and stitched like plywood.

I would also suggest some advice on the size of the planks and glass layup schedule for a quality job

 

Making planking out of little strips on a bench, seems as tedious as laying them one by one on a building jig, plus some (like carrying them to the building jig, trimming and aligning them). I also don't like the idea of testing the strip edge glue lines and inner sheathing, as they get bent into shape. Sounds like pre-loading the sheathing and glue lines unnecessarily.

 

Laying and gluing on a flat surface would be easy. It requires no clamping, and no difficult pressure angles. It would ensure a totally flat surface on one side. The other side could be belt sanded uniformly.

Depending on the strip thickness and finish, one layer of glass would put no more preload on a panel than plywood, maybe less.

 

I once did a simple calculation on the cost of replacing marine plywood with cedar strip planking.
Assuming the same thickness of wood, and the same glass on each side, the strip planking is significantly cheaper. Assuming your labor is free!

No comment on the physical strength comparison, but I am talking about thin glass sandwich (wood core) where the glass primarily determines the strength and stiffness.

 

In this size you are going to be hard pressed to match the weight of plywood and still have the same impact resistance.
i.e. your boat will not be as bashable as ply and could also be heavier.
Since strip gives you fore and aft strength use double bias cloth on the outside and just uni at 90degrees on the inside.
If you only have 0-90 cloth lay it at 45 degrees.

 

why bother changing something as critical as the primary structure? why not find something similar in a strip built hull, you can always modify the deck and cabin to make it match the scamp if you like the look.

I am with PAR, I would make it have a proper prow front.

 

You know i've seen these pronouncements, but not really data.
Experience is worth a lot, but I don't have it in this case.

Is there anything showing an impact damage test for light ply vs light strip plank?

The modulus of glass is so much higher than wood that I would expect the glass to have a signifigant benefit as far as strength and stiffness, even in the typical 6oz single ply weight (even with just plain weave glass).

 

The data is available and a typical single skin 'glass hull has a lower modulus then plywood of the same weight, up to a point (size). In small craft, plywood predominates for several reasons, cost being a big one, but ease of working is another big one too. A single skin 'glass hull of the same modulus, will be heavier then a plywood hull. As far as stiffness is concerned, 'glass is way lower than plywood longitudinally and slightly lower at 90 degrees to plywood's outer grain. This is why 4 mm kayaks are lighter than the 'glass versions of the same thing. The same thing was found in older wooden racers (sail and power), when these where converted to 'glass construction. The wooden versions where lighter and the still active classes require the wooden boats to carry a weight to compensate the 'glass competitors.

Strip plank can suffer from similar issues, compared to plywood, in smaller sizes, unless carefully designed (which most aren't). If you take the time and work out panel support and integral stiffeners, a well thought through strip can be lighter then a taped seam plywood build of the same size, strength and stiffness. Most don't go to the bother on small craft and there are limits as to how dainty the strips can be, before treating is as a cored build, rather than a sheathed strip.

It's also important to compare apples to apples. It's just not reasonable to put a taped seam plywood build up against a sandwich type of strip build. There are at least a half a dozen distinctly different strip plank build types, some are miniature plank carvels, while the other end of the spectrum is really a strip core composite, with the skins bearing much of the load. It's these "composite" style of strip builds that can easily rival the lightest taped seam plywood build. Typically the sheathings will be 2/3's the total laminate weight of a single skin 'glass build and often coupled with a very light, nearly sacrificial wooden core, who's role is to just separate the skins.

Again with these types of builds (composite strip) skin thickness can be an issue, so the core needs to pick up the slack, making smaller builds heavier, for simply practical reasons. Your scantlings calculations (for example) might suggest a 15' day boat with a 3/4" core can live with a 12 ounce outer skin and be strong enough, but can you live with this in a bay full of oyster beds?

Comparatively, it's very difficult to compete with taped seam or glued lapstrake builds in smaller sizes. In most every regard, plywood wins, strength, penetration resistance, stiffness, cost, easy of application, etc. When structures get bigger and more complex, clever engineering can make this gap narrower and eventually, you do get to a point where it's more economical to use single skin 'glass, a man made composite (foam, honeycomb, etc.) or alloy.

 

PAR, with all due respect you are talking section modulus (EI) not modulus (E)

When I have been talking about strip planking I have been talking about strip core "composite" when I go by your definition. I personally think it is completely reasonable to compare taped seam plywood to strip core "composite" planking. My "core" of choice for very small boats is cedar because of its density.

I have a stitch and glue plywood kayak with 6oz cloth inside and out. Its clear to me that the glass significantly stiffens the plywood. To make a perfect comparison (in one respect) I would have to put together panels with the same core weight since the glass epoxy is the same. Since the cedar is significantly less dense than occume plywood, the cedar core will be thicker. I expect that to be stiffer and stronger than the ply (with glass). I'm getting ready to make a strip planked glass covered kayak.
I'll try to make sure to make comparable panels to do some informal tests.

 

The obvious question? Have you talked to John Welsford about it?

Scamps are now available in grp

Richard Woods