Florida Offshore Multihull Association
Floor Fix
By
Ron Butler
What do you do when IT happens to you? Your seldom used high pressure floor erupts into one giant zit and there’s no oxy crème around that will fix it. I mean when your dinghy has been out of the bag less than a dozen times and the rest of the time it’s been stored inside a garage, this kind of thing shouldn’t happen. Should it?
The
problem
But what are your choices? A new high pressure floor costs over $700. Of course, the rest of the dinghy is like new so maybe it deserves another floor but at $700 a errrr… pop? When my friend Dale's floor erupted it was time to explore options.
Ok, PVC floors ain’t worth the powder, so what are the other options? These inflatable high pressure PVC floors cannot be repaired. Well, the only reasonable answer is a new plywood floor. Sure there’s PVC foam but it’s way heavy and too flexible OR you could lay up some foam and glass boards; nice and light but time consuming and expensive. Plywood seems to be the best choice.
OK you say, but since I have an air floor where do I get patterns for a plywood floor? Well, first of all, don’t even bother asking the manufacturer. They contend that they don’t have a wood floor to fit. On the other hand,West Marine sells a wood floor for their West brand dinks but they cost over $500. So we come down to rolling your own. The good news is, the folks at West Marine were customer service oriented enough to let Dale trace a set of floorboards that they had in stock for an inflatable that was virtually identical to Dale's boat. Of course, you could just find a friend with a similar dink and trace his floorboards. We like our friends at West Marine.
This is what our patterns of the first two boards looked like, traced onto cardboard.
Cardboard patterns
Next the patterns were traced onto 3/8 thick luan plywood. Now this stuff isn’t marine grade but it’s the right thickness and it has no voids (or at least very minor ones). Also, it’s five laminations thick so it’s plenty stiff and light.
The final boards look like this with the joints installed.
The other way to get a pattern is to simply use the deflated high-pressure floor. It’s useless for anything else anyway. The photo shows the boards aligned on the old deflated floor. They’re a pretty good match. Once you’re done with the old floor as a pattern, just slip it under the old Ford to catch the oil drips.
New boards overlaid on the old floor
Ok you ask … how come five boards instead of four? Well, Dale wanted the boards to fit inside the inflatable duffel bag for storage and transport. Four boards won’t make it. Also there’s supposed to be a slight upward camber to the bottom in the forward sections of the dink and the extra joint allows the flat plywood to conform to the bottom curvature of the keel bladder more easily.
After cutting out the plywood sections, you need to sand all the edges nice and round. No sharp edges or splinters allowed here.
The next step is to construct the joints between panels. Dale used flat aluminum plate that he through bolted to the plywood. Screws just pull out after a while … through bolting is better. The aluminum strips are 1/8 inch thick by 3 inches wide and as you can see in the photo they don’t extend all the way to the ends of the panels. These also need to be sanded smooth on the edges. Rounded off corners are essential. Again, no sharp edges allowed.
Now then, to assemble the boards there is one last step. The through bolted aluminum strips will cinch down so tight that the adjacent board can’t fit into the slot formed by the two aluminum struts. This is because the bolts compress the plywood slightly but just enough to keep the next board from fitting in easily. There’s a couple of ways to deal with this. One possibility is to shim the top aluminum strip so that it forms a slightly wider slot opening. Another solution would be to taper the edge of the adjoining board so that it fits more easily. We chose a third way. We bent the top aluminum strips slightly so that when firmly bolted to the board, the entrance slot formed is wider than the adjacent board.
Band-aid optional
As you can see in the picture two adjacent pieces of aluminum don’t lay flat. The slight bend keeps the edge separate enough to allow easy joining of the floor panels but provides a tight fit when the panels are fully engaged.
The next issue is how to through bolt the plates without sharp edges ...i.e. nuts protruding into the bottom of the dink. The solution is to use stainless barrel nuts like those used to install windowpanes where the inside nuts show. That way you get a nice smooth Phillips head showing on both sides of the joined surfaces; very clean and neat. Burrs are easily sanded off.
The edge assemblies (note crease in top plate)
The next problem is how to make the chine reinforcement. The chine rail joins the floor panels into a single rigid floor along the outboard edges of the center three floor panels. The cross section of this piece is a little tricky but can be made several ways.
The simplest way would be to bend a flat piece of aluminum over at about 180 degrees allowing for the thickness of the plywood to fit between the folded halves. This tough to do without special bending equipment or fatiguing the piece. Another solution would be to buy used channels … difficult to find but may be available at your local inflatable repair shop provided they maintain an inventory of used parts. Our local shop doesn’t sell used gear. Another would be to have them machined out of aluminum or starboard … expensive. Dale chose to make his own out of plain old southern yellow pine (read 2x4).
The cross section looks like this. This cross section was also traced from a West Marine dinghy.
Cross section of the chine rail
Fortunately, Dale is very skilled with a table saw and was able to machine these himself. I didn’t ask how many tries it took to get a good one but knowing him, the first one probably came out fine.
Now there’s another issue with wooden chine rails like these. Do you see how thin the lower lip is? What if the lower strip splits or cracks? After all, it is the thinnest part of the cross section at about 3/8 inch. It could crack and leave a nasty splinter to tear the rubber boat bottom or worse the air chamber. Therefore some reinforcement is needed. Of course, the lower limb could have been made from a strip of aluminum screwed to the bottom of the strut (picture the strut with the lower limb removed) but we chose a different way. Dale epoxied some fiberglass mat to the strut. This idea presents problems of it’s own.
For strength, the glass should reinforce the inside of the slot as well as the outside edges. This makes for some tricky lay-up work.
The lay-up schedule
This was accomplished by setting up a “mold” where the wooden rail becomes the core material and is pressed into the wet glass mat. The illustration shows the layers of stuff needed for this operation. Board one is a 2x4 with a ¼ inch groove and a ¼ batten glued into the slot and protruding enough to more than bottom out in the slot of the chine rail. The newspaper layers (2) were added to make the fit as tight as possible once everything was in place. Next a layer of clear plastic (3) was added to keep the epoxy glass from sticking to the plug. Note the wooden strips stapled to the edges of the clear plastic. (also, notice the marking pen alignment marks) Then wet glass (4) (pre-cut to size) was added and then last, our wooden chine rail (5). The whole mess is then clamped together with the plastic wrapped around the rail and allowed to cure.
The picture shows how it goes in dry form.
Dry fit
Clamps are added down the length of the rail as required to keep things tightly pressed together. The wrap of clear plastic squeezes excess resin out so that the resulting chine rail is tightly covered in epoxy glass with no excess runs or blobs of stuff. This is how the finished chine rail looks compared to a bare wood one.
Plain wood vs. a laminated rail
All that’s left now is finishing the floorboards and chine rails, then assembling the dink. Here again there are many choices. Epoxy paint, gel coat, varnish, are all viable choices. Dale chose to use two-part polyurethane paint mainly because a friend had some white and black left over from another job. That meant he could get light gray by mixing just a dab of black with some white. Best of all it was free! Probably the two-part polyurethane will hold up better than ordinary varnish or paint; we’ll see.
Final assembly (note non skid patches)
Performance of the boat with plywood floors seems about the same as it was with the air floor. The 9’ 6” dinghy will plane out with a 6 hp Yamaha and Dale’s 230 lbs. plus a little cargo. Although Dale likes the air floor better, (it’s lighter plus it’s softer to fall on) the plywood seems a reasonable low cost alternative to keeping an otherwise useful dinghy in service.
Sea Trial
If Dale had it to do over again he would not buy an inflatable made with anything other than Hypalon. He won't buy another high-pressure floor made of PVC either. Despite the weight saving, they’re just too expensive when they don't last. Remember too, that a great brand name doesn't necessarily equal great product.