Question -Laying Grid on Marine Grade Plywood Sheets

[freeboatrsrce]

Hello,

I am building a 10ft Row Boat and will need to epoxy two 8ft x 4ft sheets of plywood together so the overall length is 16ft. After this is done, I will be laying out a square grid 6in x 6in so the developed plates can be measured and cut by using the grid. This will be done on marine grade plywood. Has anyone had experience with this? The best method I can think of for laying out the grid is using a long straight edge with a long T-Square, and then lightly marking out the grid with lead pencil. Has anyone tried this before? The lead pencil marks will need to be removed after the pieces have been cut. I figured on just sanding them off with an electric sander hoping this will remove the pencil marks completely. Any suggestions would be appreciated.

[LarryMcI]

The lead pencil residue will not impair adhesive properties. Sanding-out lead (graphite) can be troublesome, since the dust will re-distribute itself during abrasion. If only for aesthitic value, why not use a water-based felt tip marker (yellow pigment) then fade your grid using spot lamps after it's not needed?

[alan white]

Really, you should mark on paper, get it right, and then transfer the shape to the plywood. I wouldn't trust a T square unless you test it first by reversing the top. The two test lines should not diverge.
The best way is to make accurate tick marks along all four plywood edges.

[missinginaction]

I don't know how involved you want to get here, but two ideas come to my mind.

1. The more expensive idea is to use a lazer to project your grid lines into the ply. I use my rotary lazer for all kinds of projects. Light always travels in a perfectly straight line.

2. A less technical idea might be to simply get some masons twine and snap a chalk line on the ply every six inches. Any store that carries masonry supplies should be able to supply you with line and chalk. Drive a small nail every six inches at the perimeter of the ply, pull the line taught between the appropriate nails and "snap" your chalk line. The chalk will come off easy with a damp cloth. If you want the chalk lines to be a little more durable a neat trick is to spray the lines with a little hairspray this will "set" them but still enable you to clean them off with a damp cloth. Just steal a can from your wife.

Regards,

MIA

[Wayne Grabow]

I agree with Alan White's post. I set up a sheet of plywood, well supported, on saw horses, used approx. 3' by 6' sheets of building paper aligned with the bottom edge of the plywood, then drew out all offsets measuring up from the bottom edge and laterally from a center line which I drew on each sheet of paper using a 4' drywall square. The paper was clamped to the plywood, the bottom plywood edge was checked for straightness, the drywall square was checked for squareness, all vertical heights were measured at both right and left edges of the view, and the result was excellent. My shop is temperature and humidity controlled; the construction paper was acclimatized for months beforehand; and dimensions were periodically rechecked. No changes occurred.

I drew out 14 cross sections and a bow profile. The full-size frames were constructed directly on the drawings, and then the drawings were cut out with a scissors to use as patterns for a final check. The patterns are still available if I need to recheck anything, and were used to help shape a hull cradle. The paper patterns could also be flipped to the reverse side and placed on a roughed-out frame to check for right-left symmetry.