This is a method used to replace the bottom of a boat overnight for a sailing regatta.
Often it can be easier to replace the whole bottom plywood or other panel of a boat than to mess around with lots of small repairs.
In this case this OzRacer dinghy had been built with a very brittle plywood 4mm thick. It worked fine everywhere but feet went through the bottom. So replaced bottom in a day including epoxy coating for sailing the next day.
If only my building mate Peter had my catlike agility!
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Step 1: Mark All the Framing Positions
Use a fine drill to drill from the inside of the boat adjacent to all visible framing. Turn boat over and draw outlines of all the framing.
Step 2: Get Out Your Router - Safety Glasses
Method 1 - If no fasteners use a router
Set router to the plywood panel thickness and go over all the framing locations.
In this case the router cut was set to 4mm - same as plywood thickness. Stop and check every few minutes to make sure the router bit is not slipping out gradually making the cut deeper than necessary.
FINISHED! This stage
Method 2 - If fasteners it is more laborious - which is why we don't use fasteners in modern epoxy construction.
Cut alongside all the framing using a jigsaw.
Skim the surface of the remaining ply to expose fasteners
Chisel around them enough to wind them out with vicegrips or pull them out with a hammer.
Step 3: Most of the Bottom Ply Will Fall Down - Any That Doesn't Can Cut With a Saw.
The bottom ply will fall down as the last cuts are done. But sometimes there are parts that remain connected. Just cut the remaining tabs with a saw.
Step 4: Belt Sander Time to Get Everything Smooth
Go over the bottom with a belt sander to bring everything to the same level.
The belt sander is a very violent tool - you cannot let it stop in one place ... just keep sweeping until you see clean wood.
Step 5: Make Sure the Boat Is Untwisted!!!!!
This is really important as the boat will be easy to twist when the bottom is removed. So get the twist out now or when the new bottom goes on twist will be there forever.
Put a piece of straight wood at one end of the boat and sight against another straight line at the other end of the boat while standing beyond the end of the boat. Chock up the boat until any twist is gone.
Step 6: Mark Inside of New Ply With Framing Positions
With this boat the ply can be screwed into position - we use temporary drywall/plasterboard screws which will be removed later and a cordless drill.
Turn boat over and trace around the internals.
if you can't trace around the internals before putting the ply down mark the remaining outside of the boat with where the internals intersect with the hull skin. This can be transferred to the plywood
Then remove the ply and drill pilot holes on approx 300mm (12") centres in framing positions. Refer to the hull if not sure.
Step 7: For a Tidy Job ... Masking Tape.
Before the bottom goes on we mask the surfaces adjacent to the glue areas. Don't put the masking tape exactly on the edge but a couple of mm (1/16" approx) from the glue surface.
We use a one hit method which saves most of the sanding using epoxy.
Don't work in direct sun. Read material directions.
You can't stop this halfway through - but you get 3 days work done in one intensive session and don't have to sand three times!!!
- We put a first coat of epoxy (eg West System, RAKA, System 3 or other) on the inside of the new ply
- When it is tacky put on a second coat.
- When tacky put on third coat
- Immediately mix up glue by adding the high strength glue powder (not lighweight filler powder) to the resin to make something like a mayo thickness or a little thinner.
- Put it in an icing bag and place the glue on the framing quickly. If you leave the epoxy in the bag it will go off in 5 minutes with smoke. But get it out on the framing first quickly before smoothing it with a plywood offcut. Once out on the framing you will have 30 to 40 minutes to work.
- Put bottom panel in place and wind in the screws that were in before into the same holes
- Put screws in all the pilot holes and stop tightening when the epoxy oozes out - epoxy works best with light clamping pressure.
Step 8: Clean Up Inside
Best tool is a 25mm (1") thick piece of plywood sharpened like a chisel edge using sandpaper on the table.
- Turn boat over
- Use chisel tool to lightly push excess glue onto the adjacent masking tape.
- Remove masking tape.
Step 9: Filling Holes on the Outside
Image shows the wrong method! Better methods below.
Method 1 - no time to spare
Bottom can be trimmed with a plane and corners rounded. In general we don't round the corner on the back of the boat to get good water release when sailing.
When glue starts feeling a little hard when you tap it with your fingernail then you can pull some of the screws.
Give the panel a bit of a pull - not tooooo hard to make sure it is stuck down. If doesn't yield then you can pull all the screws.
Mix epoxy and lightweight filler and fill holes with piping bag cleaning off with a putty knife. Three coats epoxy we on wet like the inside of the ply and it will be ready to sail when the epoxy feels hard when drummed with your fingernails.
There will probably be small dimples where the holes were filled - buy hey ... you are sailing!
Method 2 - More time and a better job with minimum labour.
Look at the drawing above. Filling holes for minimum sanding means creating a minimum of surface area that needs to be sanded AND filling the holes first shot.
This means the filler SHOULD NOT be spread out with a putty knife - it is too much area and the timber will suck up resin leaving a dimple.
So use the lightweight filler powder and epoxy mixed up to be like peanut butter and pipe a blob the same diameter as the hole being filled but raised above the surface.
This has much less surface area and sands in seconds.
When filler is cured finish off with a sander with a hard plate or sandpaper and block before doing the epoxy coating.
See more quick repair and boat building problem solving at our FAQ on Storer Boat Plans.