Planking alternative advice

[grousehunter]

I am lofting a thirteen foot E. Schock design from Mystic's collection which is drawn up for solid, half inch planks, laid carvel. It is a vee bottom daysailer with pieced together frames, solid keel, half-decked, about 100sf of sail (knockabout rig). I'd like to swap in plywood and am hoping that everything else can be left alone. In Gerr's book, the Sn is actually so small for this boat that the tables are difficult to use but he does give the general statement that plywood may be substituted and the thickness may even be reduced. I happen to be a fan of Schock and do have some other plans of his for similar boats in which plywood is specified and it appears that the boat would be just fine structurally (I intend to use his most often specified thickness for his small vee-bottoms - 3/8 inch). The other concern is whether the design allows for strictly developed surfaces which would make plywood a viable option from an assembly standpoint. From reviewing Schock's own analysis of 'plywood compatible' designs (excellent article in an old Sport's Afield Boatbuilding Annual), this boat again, can take the ply just fine. The parallelism among both the upper and lower hull section lines appears to be compatible. The boat may even have been designed intentionally with developable surfaces to allow the builder the option. Although the frame spacing is clearly arranged for solid carvel planking (eleven inches compared to 14-18 for other daysailers between ten and eighteen feet OAL which specify plywood), I would leave frame spacing alone, let her be a bit overbuilt, erring on the safe side.
Comments?
Thanks all,
Ed

[kenjamin]

You can build a heavy over-built sailboat if you want to but modern designs in glue lap ply are much lighter because they use 1/4" quality marine ply and in some cases for 13' boats, do away with the frames altogether. It just depends on whether or not you want a sailboat that accelerates well and thrills you or do you want a boat that looks like you want it to and floats but is a slug to sail.

[wizbang 13]

I'm with Kenjamin, don't overbuild a small boat. I have (among others) a 1/4 " ply , 50 mph /40 merc job,that has no frames, fastenings,chine log ,stem or keel. It gets abused (goes airborn and lands HARD) more than any sailboat, but holds together. ( well, there screwed stringers on the bottom)

[grousehunter]

The extra weight from four more frames built from 3/4 x 2 sticks might be about sixteen pounds guys and the plywood has about the same bulk density as the relatively heavy species of wood that I would be using if I went with solid planks.

I was looking for comments on the likelyhood that an 'apparent' developed surface within a design in which solid planks are called out, could be built with 3/8 inch plywood and perhaps what other aspects of the lines and construction should I look at when considering such an alteration.
I appreciate the concern however.

[kenjamin]

Just make sure you don't leave any plywood end grain showing. Cover it with trim or thickened epoxy. Keep in mind that there is a wide range in quality and weights of 3/8" marine plywood. For what you're doing Ockume is lighter and will bend easier than the 7 ply 3/8" solid Sapelle that I used on the bottom of my 20' Caledonia Yawl. Good luck with your build!

[wizbang 13]

16 pounds is a lot of weight in a 13' boat. I don't know what that other thing,develpoed surface within a design, even is!

[mmd]

Have at 'er, Ed. Not every boat benefits from scraping every possible pound off of it. There is a benefit from inertia, too. If you are concerned about the ability to skin it in ply and you don't have the skills to figure this out from the plans, don't buy your ply or planking material until you are about to work with it. That way, after you have the boat in-frame, you can buy a sheet of thin, cheap plywood (flooring underlayment?) and lay it on the frames to see if it will go on cleanly. If not, go out and buy planking stock. The plywood will fine a use somewhere in the shop...

[Varna]

It seems to me you have miscalculated the weight of 4 additional frames-16# is surely too high. .75x2" and a 5' or so beam is not much lineal of frame stock. Given the original was likely spec'd to oak frames and W Cedar-which is light planking material relatively-it seems you will have little actual weight diff using .375 ply. Careful listing and maths will tell the story.
To my mind it is still valuable to keep the frame count minimal for several reasons in the ply version. One is less down the road maintainence regards sanding/painting. another often overlooked but a nicety is a bit more open working room while building-depending on your method. When doing a glued-up hull build more room for clean-up of aerodux or west sys is a nice thing to have.
And the value of lighter for the same strength is never to be devaued-this one I have been coming around to as I have always benn an 'over-builder'. But age is showing me that light is good, better. Schock is great, have fun building.