In my previous article, I discussed the features and benefits of using DuraKore as a core, especially for hobbyist 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 asked for 13mm thick x 300mm wide x 2.4m long which needed to be cut together to make planks longer than the 9.2m hull length.

The bevels were made by machining a male cone into the 1.5mm thick hardwood veneer at the end of a DuraKore plank, and a female cone into the plank’s hardwood veneer that would be glued and bonded to the male cone.

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 strip of sample plank 50mm wide and fasten one end of the plank to a bench with the majority of the plank hanging over the edge of the bench. The bevel joint should also be a fair distance from the edge of the bench. Slowly add weights to the end of the board until it breaks. If the joint is a good quality bevel joint, the break will have occurred elsewhere along the length of 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 into hardwood veneers.

The DuraKore supplier sells an accessory that fits on a circular saw to easily do this. However, a jig 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 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.

The 205 and 206 hardeners require a mix ratio of 5 parts resin to 1 part hardener. Hardeners 207 and 209 require a 3 to 1 ratio and solid state 6 to 8 hours.

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 higher temperature climates. Its useful 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 quantities, to save time mixing the material all day, but this resin generates heat once the hardener is added and agitated, and with larger volumes of epoxy in the pot, 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 must be thickened to glue the bevel joints so that it does not run out of the joint before it cures, and the West system provides additive powders to allow 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 canister and not the dumpster. My wife and our neighbors kept 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 jig 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 needs to be. Those finished planks were just under 40′ long for the 9.M hull, which was fine because it’s important to stagger the bevel joints as the hull is planked, and scrap will occur because of that.

Those long planks then had to be mostly cut to 50mm wide so that the planks could be fitted over the male mold frames to form a round bilge core. In fact, what it really has are many small spines, 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 supply of strip planks, it was time to start installing them into the form.

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 mold frames.

The first plank was important as where it would sit along the hull would depend on how well subsequent planks would sit 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 that it would not sag and also fill the gaps in the edge joints. The next plank was lifted and put into place, making sure that the grooved joints of each plank were not aligned with each other, and only then screwed into 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 runs of the temporary mold frames.

This edge bonding process continued for approximately 6-7 weeks after business hours until the entire DuraKore core was finished. A battery-powered screwdriver made this job easier for me.

As the planking progressed, it became apparent that I would have to stop in the area I was backfilling 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 shovel. helix and resist sit flat against the edge of the mold frame.

I then had to fit a new 25mm board along the highest point of the convex curve along the waterline, stem to stern and bolt that board down as my new glue-on edge. The next 25mm plank was glued at the edges and fitted to the bottom edge of the new plank, and I continue to plank towards the previous planking area, slowly increasing the width of the planks to fit the curve, thus filling the gap length elliptical that was left. As the gap closed, I found that I had to write the end of the planks so they would fit against the bottom plank, and as the elliptical void closed, each subsequent plank got shorter. You can see the photos showing the decking on my website.

Once the entire hull core was in place, thousands of screws were removed and the hole filled. The glued joints were lightly sanded taking care not to remove any hardwood facings. Hand sanding was the safest method as machines tend to dig too easily.

My next article will be on the fiberglass layout in the main hull.