Plywood hull shape definition

Looks pretty good to me.
Mathematically perfection may be required, but not for everything else in the world.

Perhaps the Vikings were really good math whizes???

 

What about the "kerf bending" approach where you make a series of v groove cuts to allow the plywood to conform to shapes? Anyone ever use this approach? Same approach used by the Divinycell flexible foam core panels. With the right plywood and a circular saw and simple jig, you can produce something like this:



This is how cabinet makers make a curved panel. Very simple, albeit slow technique.



Or even get fancy if you have a cnc laser cutter on hand.

 

I can assure you that any unfairness is due to the primitive building conditions and not the calculated dimensions. At that time, we were living in a farmhouse in northern NY. The length of the boat was dictated by the distance between the furnace and water heater in the basement. Hand tools only. Calculations are accurate to at least 1/32". Curves used were mathematically exact.

 

Kerf bending will cost you at least 1/3 of the strength and will make lots of cuts thru end grain. If you don't get that sealed perfectly you have just made a rot farm.

Very poor for a boat.

Even fiberglass would be better.

 

Yeah, I wasn't clear on the whole process.

Actually, you will want to FULLY fill all the voids with a suitable epoxy mix. The plywood serves as the form to acquire the shape, which is the tricky part. First, you would seal the plywood "form" with a penetrating epoxy and then fill all the voids with a lighter filler. Of coarse you also would want to fair it all out then glass over everything as well.

This would yield a true "composite" structure which will be highly rot resistant. The wood is not even as critical structurally this way. It serves mainly as the form to get the fancy curvature desired in a quite simple "craftsman composite", using very basic tools and some "goop" to trowel on.

Simply eliminating all the hard angles and using curves in all your structural members is going to do wonders for any design.

 

Kerfing plywood isn't a good idea, nor is what captbill suggests with penetrating epoxy in the kerfs, which isn't waterproof (not even close) nor very strong (in fact very weak). The net result of epoxy filled kerfs is heavier than the solid plywood would have been and you're not getting a good mechanical "purchase" with the goo mostly on the end grain. Do a simple test, kerf the crap out of some plywood, bend it good and fill with thickened goo. Next test it to destruction and see where your failures result. You'll pull out some end grain, but much of it will also be directly adjacent to the glue lines.

Sometimes kerfing comes in handy, but typically it's used on cosmetic applications, like a curved cockpit seat face or something, not a hull, where you're counting on every veneer, to contribute strength and stiffness to the hull shell.

Making developed hulls, with a serious level of accuracy require an understanding of the sheet goods physical attributes. With plywood, you'll have minimum radius requirements to stay within. You can cross over these a little bit (torturing), but not by much, before you prestress the panel to an unpredictable amount. This is key, predictability of the cylindrical or conical minimum radiuses possible. This requires experience with the materials involved and a database of physical properties (including minimum radiuses). With some experence, you can eyeball how much you can get away with or the belly some panels might make, around certain shapes, but this isn't very precise.

 

With a plumb bow, it is easy to get close to a minimum radius situation in the forefoot, but a flared bow and cut-away forefoot allow plywood to be clamped into place with minimal tension. I am always looking for easy-to-build design features. A topsides transition from bow flare to stern tumblehome is low-stress with the curvature distributed along most of the hull length. I use a conic projection at the bow transitioning to a second conic projection amidships and then to a parallel projection at the stern.

 

OK,. fair enough. Photos can be deceptive too. My eyes saw these things highlighted by the green lines.