Stiffeners glued to deck

Posted by Paul

I have been pondering the various options of how to make the bridgedeck stiffeners. After I had bought the rectangle tubes at $38 each for 3 meter lengths for a total of $190 James suggested it would be cheaper to make the stiffeners out of 100mm foam rather than tube. I did get the cost of 60mm round pipe, it was $58 for 6 meters so the whole job would end up about $80 more than round pipe but I figure that is more than offset by the saving in coving material without even counting how much easier the rectangle tube will be to work with.

I was going to cost the option of foam and take the tubes back if cheaper as I don’t intend to run any wiring or sheeting lines down the tubes so they don’t need to be hollow. But in the end I ran out of time to get the foam costed so went ahead with the tubing. The only reason I mention it is because it highlights just how many solutions there are to the same problems. Round tube, rectangle pvc pipe, foam, in the end the choice is up to the builder, the end results are the same.

Before I got started on the rectangle pipes (I was still deciding which way I was going to go) I used the time to prepare the materials I would need, such as cutting strips of uni. The plans call for 2 layers of 40mm 450g uni, but Warren gave me a part roll of 1000g uni so I cut 50mm strips (because the top of the rectangle tube is 60mm I figured I could use 50mm tape) and will only use one layer on each. I will end up with 50mm strips of 1000g as opposed to 40mm strips of 900g (2 x 450g) so I will have about 15% more strength than the plans call for but with little added weight. This is a contentious claim by the way, because the plans don’t actually specify how many stiffeners to use, so at worst by over strengthening 4 stiffeners I will have about the same strength as 5 stiffeners of specified strength. The same will apply to the glass I use to fix it all down. The plans say 50mm glass overhang either side of the stiffener but because of the rectangle tubes, 200mm glass will not give me 50mm overhangs, only 40mm, so I will use 300mm (I don’t have any 250mm) and will have about 80-90mm overhangs. I figure its better to err on the side of over strength.

To cut the uni strips is easy, just roll the glass out to length, start your cut between 2 strands and ensure you stay between those strands the entire length. Once I had cut them to length I rolled them up on a piece of ply as an axle ready to wet out either on the job as I go (my preferred method) or on a wet out table before rolling it up again to roll out on the job. The outside strand may unravel but as I have cut to 50mm I don’t mind too much and will clean that up as I wet it out. I am not sure I work fast enough to wet out on a table an roll it up and get it onto the job before it can go off, I think it will be safer for me to wet it out as I go.

So once I decided to go ahead with the tubes I started by sanding the tubes which was easy considering each side is flat with only the curved edges to sand around. The reason for sanding them is so that they are keyed ready for glass to stick to them, without sanding the glass could easily delaminate. This would not cause a major structural problem but would be a cosmetic problem and a nightmare to fix later. All plastic moulded product also has traces of mould release used so the plastic product is easily removed from the mould but will also cause fibreglass resin to not stick well to it, another reason to sand it first. I imagine the rectangle tube would be easier to sand than a round tube.

Next I set the saw fence and set about cutting them exactly down the middle. The first attempt was not too good and I ended up with a few major chips out of the side. It wont be a problem as I have about 200mm more than I need so this will end up as an offcut but I made sure I had the pipe well secured before I cut the rest and the results were much better. As I was getting ready to cut the pipes I could not find my safety goggles (I found them the moment I finished cutting!) so I decided to wear my scooter helmet. Funny looking but effective.

Once I had all of the tubes sanded and cut down the middle I gave them a good wipe with a damp (with water) cloth to remove sawdust. The pipes are statically charged from the rotating saw blade (and maybe the sander too) so you have to give them a good wipe so the glass will glue to a clean but keyed surface.

I then set the tubes out in place and marked them so I could run glue lines down for the pipe sides to be glued to the deck. The method I used to get the glue lines down was to use a scraper with a glob of glue on and let it run down the scraper as I moved it along the lines leaving a thin line of glue. I contemplated using the piping bag method but in the end decided that the extra work of filling the bags would offset the time saving of getting the glue down, the only advantage I could see is that maybe the thickness of the glue line may have been more consistent.

As I went with the glue I used a weight on top of the stiffener to keep downward pressure on the pipe so that is made good contact with the deck as the glue set. In the end this glue is just there to keep the tube in place so that I can squeeze filler into the right angle to cove into a nice shape for the glass bed. I guess being 2 edges it was twice as much work as if I had just glued a round pipe down but that was and will be more than offset by the time saved by the flat surface for sanding and glassing as wall as the much lesser amount of filler I will need to mix and apply.

So now that the stiffener tubes are glued down I will now have to cove the right angles and then wet the uni strip down each one and glass them with the double bias to properly fix it all to the deck. I hope to get this all done during the week so that I can spend next weekend fairing and painting the under wing ready to glue up into place. I am hopeful of getting it glued up before I go to the boat show.

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