Big Hole plywood repair photo sequence below – a fast professional fix
Fasteners in traditional construction are the ONLY way to go – whether metal or trunnel. But once moving over to glued construction there are certain advantages in eliminating the fasteners as far as possible.
We’ve made a bit of a career of it over the last 20 years or so – probably a few thousand boats if you include kits and classes and plans. Up to around 50ft.
No problems at all.
In general if using modern methods I would be choosing materials that glue reliably as far as possible and eliminating the fastenings.
FAQ to solve boatbuilding problems – lots of tips and tricks for using plywood, epoxy and more
Lower Maintenance because of less deterioration.
The three main reasons are
1/ It eliminates the fastener plugs that end up imprinting the paint job after a few years. Darker and red boats are the main culprit but I’ve seen plenty of lighter colour boats do this too (we are in Australia – so think hot).
2/ Fasteners by necessity have to cross glue lines so provide a way of water getting through the structure. Without them even when there has been localised damage – it has stayed localised.
3/ Eliminating Fasteners makes repairs quick and simple.
The last reason is the ease of repairs – you can remove large amounts of boat along really complex lines with a router preset to a particular depth.
The router can be set to the plywood thickness or if cold moulded it can be done to the depth of each successive layer so each layer overlaps further onto the existing structure than the previous.
Then glue new parts in using temporary formers (or not if the remaining structure has enough support for the new bits being glued in). It is a bit amazing to rip the bottom, deck or side off a racing boat after a prang and have a new section glued in for racing the next day – sure it won’t be painted – just epoxy sealed – three coats and there will be one very tired crewmember the next day (he’s the one who has to finish the outside coats – the inside ones are done immediately before gluing the patch in place). But having no fasteners makes it really quick to remove the damage.
This example shows a very small boat, but the method works just about as quickly for much bigger boats too.
Overnight Repair for an OzRacer sailing dinghy that Peter put his foot through
(he doesn’t have my catlike agility!)
Actually it was a series of problems from building our budget OzRacers and Oz Geese. Two boats that cost us $350 each and sail really well.
We had found some budget ply – a hardwood ply 4mm thick (5/16″). I’d used bottoms of a similar thickness previously and found they were OK – but this plywood was really brittle and had large voids – you could see light through it on a sunny day and the voids looked like red laser beams shining through the ply.
EEEEEEEEEK
Anyway we tried to ignore it and hoped for the best. The bottom of the boat had already broken away from the centreboard case logs (red arrow below). So we decided to replace the bottoms with 6mm gaboon ply which was lighter in weight than the ply we were replacing.
So I replaced one bottom one day and the other boat’s bottom the next – so we still had a boat to sail each day – Luxury!!
Since this time the boats have been sailed extensively with no problems at all.
So here are the pics.
| The hole in the bottom – but wait – there’s more… |  A couple of days before the bottom had delamined away from the centrecase logs and the blue arrows shows the torn veneer from a piece of string that was laminated into the ply!?! |  From the inside of the boat I drilled holes adjacent to all framing that the bottom was glued to so I would know where to run the router |
 The router was set to the 4mm thickness of the bottom ply. As I routered around the framing and filleting that supported the floor became visible. |  After more routing – elapsed time was around 15 minutes by this point. Fast and furious. I can’t complete all the routering because of the protrusion of the bottom runners. |  So I have to finish off by trimming bits with the japanese saw and then chiselling any remaining ply off the underlying framework. |
 I have run over the gluing surfaces with a belt sander to get rid of any other protruding bits. You can see the centrecase clearly and at the far end is the mast step framing. |  Before the bottom goes back on I check the twist of the boat by laying a straight edge across the middle and eyballing to make sure the bow are stern are running parallel. I blocked up the corners of the boat to make sure everything was true. |  I dry fit the bottom by screwing in place with plasterboard/drywall screws. Then flip the boat so I can mark all the positions of the framing – I won’t be able to see it from the outside of the boat when the bottom goes on permanently so my plan is to drill guide holes |
 Marking the edge of the sidetank fillet. This join doesn’t have any support from framing timber – it is epoxy filleted in place. |  I’m marking the width of the chine log on the edge of the ply so I know where to put the screw to put the wee beestie back together. |  When the bottom is glued on then screwed down excess glue will ooze out either side of any of the framing – I masking tape on the sides of all the framing and fillets so the ooze doesn’t make a bad mess. We are now 50 mins in. |
 I missed a pic in this sequence.
Before the bottom goes on I coated what would be the inside of the bottom with three coats of epoxy applied wet on wet so the inside of the bottom will be epoxy sealed at the same time as the gluing is finished. While I waited for each coat to go tacky I make and eat my lunch. As the final coat goes on the ply sheet I apply glue mix to all the hull framing then start screwing the bottom in place. Time – 1hr 40 mins. |  One area that was a little problematic were the areas where the buoyancy tank faces were originally filleted to the bottom. It is easy to use temporary screws to hold the bottom in place when there is timber beneath but you can’t drive a screw into a fillet!!!
So I put battens across the boat – if I didn’t have Peter’s long clamps I would have screwed the battens into the chine log. Then I pack under the batten to press the bottom against the fillet surface. Could just drill holes and use cable ties.
|  Now I can flip the boat over – as the bottom is screwed in place the hull is untwistable – these boats are really rigid now.
I go around with a chisel edge piece of timber and clean up any glue ooze then remove the masking tape. The inside of the boat is ready to go sailing. |
As it was a warm day I was able to turn the boat over after about 7 hours (8pm) and plane the ply sheet flush with the edges of the boat and radius the edge slightly.
If wanting to glass tape the chines or fibreglass the whole bottom follow these links
Glass taping and epoxy sealing the bottom of the boat – simultaneous method.
Fibreglassing larger areas
I then masking taped the painted sides of the boat and do three wet on wet epoxy coats to seal the outside of the boat. That’s another couple of hours (the temperature has dropped) but I watch a couple of shows on TV between coats and cook and eat dinner.
The last step is to wait another couple of hours until the final coat has gelled off and fill the screw holes.
An alternative method that Pat from Duckflat uses is to fill the holes first making them completely flush. The normal downside of this is that the timber soaks up some of the epoxy in the filler and you end up with a dent over each filled hole. BUT Pat waits until the filler has become a little rubbery and then does the bottom with three wet on wet coats of epoxy. The liquid epoxy fills the dents. I haven’t had a chance to try it yet – but it sounds good.
Next morning I sand the filled holes flush and put the boat down by the river.
Here is a picture of one of the repaired boats taking the world sailing speed record for PD Racers – 9.2 mph which is also the record for OzRacers. It was rough that day and no problems from the bottom!