Plywood Construction Details

Hi all,

assuming a small flat bottom boat made from plywood where the bottom and side plates are screwed/ glued together with a stringer. The side plates are thinner than the bottom plate. I was thinking about the proper way wether the bottom plate should extend over the side plate (case B in drawing) or the side plate extending over the bottom plate (case A). Intutive I would go for the side plate extending over the bottom plate (example A) so the thin plate would have more supporting area.

Is there THE way to go for problems like this? If not, what would you recommand and why?

 

B is proper, if you ask me. For boats, the side panels exert a force down when boats take air and land on waves and you want the strength on the bottom to support the sides.

But either way probably works.

The hull side to bottom connection in a foam boat is more critical.

@Boat Design Net Moderator will fix the title.

 

B is the " usual.". Before epoxy, a rebated hardwood batten might be glued between the bottom and side plywood to protect the end grain OR an external chine log. Epoxy and cloth will do that far better today.

 

Thanks for the replies, @fallguy your explanation is actually quiet logical.

Same boat, same question, other location on the hull: should the transom, also made from plywood, extend over the side panels or the side panels over the transom? My guess would be sidepanels extending rather the transom plate. My explanation would be that the transom could be seen as a frame on which the sidepanels exert a force on.

 

Ply overlap usually the transom.

 

I would extend hull past transom as skara says.

 

Tanks for your replies and also thanks for renaming the thread @Boat Design Net Moderator

 

Hi mc_rash, at the stern you can do both, together, with the transom fitted within the sides as a frame like you said, taking the side and bottom forces, and then afterward a sheet of ply or veneer can be glued right across over the top, to give a seamless stern, and like on one of my boats, hide anything that doesn't look perfect. Also acting as an extra water seal, stern reinforcement, and taking any damage from outboard motor clamps etc.

 

"Small" is pretty subjective, are we talking about an 8' pram or a 20' boat that will see a lot of banging around getting it on/off a trailer?
I like to bend-in a chine in two pieces laminated with epoxy.
The thinner individual pieces can take a fair curve with much less effort and then the edges of both bottom and sides can be protected.
The outer chine piece can then be rounded-off to a nice radius for easy glassing, and the inevitable bangs and dents will not bother the plywood.

 

Actually, as seasquirt pointed out, it is a construction detail. For most skiffs (plywood or otherwise), the boat is assembled upside-down with the transom and stringers on the building frame. So the sides overlap the transom then the bottom overlaps the side. In river keelboat and wooden barge construction (i.e. cross planked flat bottoms) they are built upright, the keel, bottom, and frames are set up first; then the stringers and finally the side planking which will overlap the bottom, transom then overlaps the side.

And, as has been implied by rangebowdrie, in large plywood construction the bottom, sides, and transom may be composed of several thinner layers. In this case they can interleave, giving a better joint. See Glen Witt's Boat Building With Plywood or other plywood-epoxy building texts.

 

B would be my choice. Plywood on frame boats are normally built upside down, so you build the frame and set it up on a strongback, plank the sides, plane off the plywood to the rolling bevel you planed the chine log to and then plank the bottom and plane off the bevel to the topside planking. After filling all the fastener you would usually sheath with epoxy and glass but on some small boats they would install a solid lumber spray rail at the chine which may just be screwed and caulked on and that would cover the end grain of the bottom planking. Structurally it probably does not make a difference, especially if you are glassing it (and you should) but there is generally an assembly sequence that makes sense.

 

Small in this case will be around 10 ft - actually one could call it very small! It will be moved on and off a small hand-moved trailer from the house via a bad, bad street to a channel a few hundred meters away.

I do not understand your bend-in chines. Perhaps you can show a section drawing of what you mean?

 

@jehardiman and @Steve W although my question was meant in a structural way your answers also point out the way you build the boat. Never thought about that but since now I will!

 

Perhaps a translation error.
Nothing special, you fasten the chine to the stem and simply bend it around the frames or forms and fasten it at the transom.