In my previous article, I discussed the features and benefits of using DuraKore as a core, especially for hobby boat building, and why I chose to use this material over foam and Western Red Cedar to build my Grainger 9.2M Trimaran.
In this article I will talk about how DuraKore is supplied and what work I had to do to make the strip boards for the project I was building.
DuraKore is supplied as planks, and for this project I ordered 13mm thick x 300mm wide x 2.4m long which had to be cut together to make planks longer than the hull length of 9.2m.
The bevels were made by machining a male taper into the 1.5mm thick hardwood veneer at the end of a DuraKore plank, and a female taper into the plank’s hardwood veneer that would be glued and bonded to the male taper.
A 1 in 12 taper provides a stronger joint than no joint if produced correctly.
The test to test the finished joint is to cut a 50mm wide strip of sample plank and fasten one end of the plank to a bench with most of the plank hanging over the edge of the bench. The bevel joint should also be a considerable distance from the edge of the bench. Slowly add weights to the end of the plank until it breaks. If the joint is a good quality bevel joint, the break will have occurred elsewhere along the plank.
A taper of 1 in 12 means the length of the taper will be 1.5mm x 12 = 18mm long.
The amount of balsa cut from the female edge will be at least 18mm deep, and the inside edge of the hardwood veneers will taper to the 18mm deep balsa cut within the female joint.
The male edge is simply machined with a taper that extends over 18mm on hardwood veneers.
The DuraKore supplier sells an accessory that fits on a circular saw to do this easily. However, a template can be made to do the job. I bought the attachment.
Once I had prepared a long, flat surface that I could attach 5 DuraKore planks to, I mixed up the glue.
The epoxy resin I chose to use was 105 West System made by Gougeon Brothers, Inc.
It is a transparent, light amber, low viscosity epoxy resin that can be cured over a wide temperature range to produce a high strength, rigid solid that has excellent cohesive properties and is also an excellent moisture barrier.
There are two types of hardeners formulated for use with 105 resins.
205 and 206 hardeners require a mix ratio of 5 parts resin to 1 part hardener. 207 and 209 hardeners require a 3 to 1 ratio and 6 to 8 hour solid state.
I used 205 hardener, which is used primarily for general bonding, barrier coating, and fabric application. It was also formulated to cure at lower temperatures and produce a rapid cure that develops its physical properties at room temperature. Its useful life is from 9 to 12 minutes at 22 degrees C. And in solid state from 9 to 12 hours.
206 Hardener is a slower hardener and provides a longer working time, especially when working in hotter climates. Its shelf life is 20 to 25 minutes at 22 degrees C.
Special pumps can be purchased to dispense the correct amount of resin per full stroke of the resin pump and the correct amount of hardener per full stroke of the hardener pump.
The temptation is to mix larger batches, to save time by mixing the material all day, but this resin generates heat once the hardener is added and stirred, and with higher volumes of epoxy in the container, the longer and shorter the reaction. the useful life. Before you know what has happened, your hand is hot and the epoxy is hardening in the pot.
The epoxy needs to be thickened to bond the bevel joints so it doesn’t run out of the joint before it cures, and the West system provides additive powders to allow for this. 411 powders are suitable for this.
I mixed about 4 pumps of resin and 4 pumps of hardener and while mixing I added the powder until I got a peanut butter consistency.
Mixing containers can be purchased, however I preferred to put my money in the can and not the dumpster. My wife and our neighbors saved plastic milk bottles and other suitable containers for me. My supply of milk bottles was crazy at times. I cut the top off of those to make suitable containers.
Five planks were glued together and laid flat along a flat floor and straight edge template to cure. This was important as the finished plank needs to be straight, otherwise the hull will have a lot of bumps and gaps to fill, making the fairing job bigger than it should be. Those finished planks were just under 40 feet long for the 9.M hull, which was fine because it’s important to stagger the bevel joints as the hull is planked, and there will be waste because of that.
Those long planks then had to be cut to mostly 50mm wide so that the planks could be placed over the male mold frames to form a round bilge core. In fact, what it really has are many small ridges, which are hardly noticeable, and disappear completely once the fairing is finished.
Around the waterline area bilge areas I had to reduce the width of the planks to 25mm and a couple to 12mm to get around the tighter radius.
Once I had a good stock of strip planks, it was time to start installing them in the mold.
First, I made sure the helmet could be removed from the mold frames at a later date by applying electrical tape to the edge of all the mold frames.
The first plank was important, as where it would be placed along the hull would depend on how well subsequent planks were placed around the curvature of the hull. I screwed the first board into the deepest point of the concave frame curve for each frame, and with a bit of trial and error it became apparent where it fit best.
The edge of the first board was covered with thickened glue so it wouldn’t run down and also fill in the gaps at the edge joints. The next plank was lifted and placed in place, making sure that the grooved joints of each plank were not aligned with each other, and only then was it screwed to the temporary frame.
I also found it necessary to screw plywood battens through the planks to keep the edges flush in the areas between the temporary mold frame runs.
This edge gluing process continued for approximately 6-7 weeks outside of business hours until the entire DuraKore core was finished. A battery-powered screwdriver made this job easy for me.
As the decking progressed, it became apparent that I would have to stop in the area I was filling in at the time, because it became impossible to fit the long planks around the bilge curves, as the plank was beginning to twist like a spade. helix and resist sitting flat against the edge of the mold frame.
I then had to lay a new 25mm board along the highest point of the convex curve along the waterline, bow to stern and screw that board in as my new gluing edge. The next 25mm plank was glued at the edges and snapped to the bottom edge of the new plank, and I continue to plank towards the previous plank area, slowly increasing the width of the planks to fit the curve, thus filling the long gap. elliptical that was left. As the void closed, I found I had to type the end of the planks so they could fit against the bottom plank, and as the elliptical void closed, each subsequent plank got shorter. You can see photos showing the decking on my website.
Once the entire hull core was in place, thousands of screws were removed and the hole was filled. The glued joints were lightly sanded taking care not to remove any hardwood coating. Hand sanding was the safest method as machines tend to dig in too easily.
My next article will be on the fiberglass layout in the main hull.
