Boat Building Project – Grainger MTB920 – Installing Duracore Strip Planking

In my previous article I discussed the features and benefits of using DuraKore as a core especially for amateur 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 planks 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 lengths which had to be scarfed together to make planks longer than the length of the 9.2M hull.

The scarfs were made by machining a male taper onto the 1.5mm thick hardwood veneer at the end of one DuraKore plank, and a female taper into the hardwood veneer of the plank that was to be glued and joined to the male taper.

A 1 in 12 taper provides a joint stronger than if there was no joint if produced correctly.

The test to prove the finished joint is to scarf up a sample 50mm wide strip plank, and clamp one end of the plank to a bench with the bulk of the plank hanging out over the edge on the bench. The scarf joint should be a fair distance away from the edge of the bench also. Slowly add weights to the very end of the plank until it breaks. If the joint is a good quality scarf joint, the break will have occurred somewhere else along the plank.

A 1 in 12 taper means that the length of the taper will be 1.5mm x 12 = 18mm long.

The amount of balsa cut out of the female edge will be at least 18mm deep, and the inside edge of the hardwood veneers tapered towards the balsa cut-out 18mm depth inside the female joint.

The male edge is simply machined with a taper that runs out over 18mm on the hardwood veneers.

The DuraKore supplier sells an attachment that is fitted to a circular saw to do this easily. However a jig can be made to do the job. I bought the attachment.

Once I had prepared a long flat surface on which I could join 5 DuraKore planks together, I mixed the glue.

The Epoxy resin that I chose to use was 105 West System manufactured by Gougeon Brothers, Inc.

It is a clear, light-amber, low viscosity epoxy resin that can be cured in a wide temperature range to yield high strength, rigid solid which has excellent cohesive properties and also is an outstanding moisture barrier.

There are two types of hardeners formulated for use with 105 resins.

205 and 206 hardeners require 5 part resin to 1 part hardener mixing ratio. 207 and 209 hardeners require a 3 to 1 ratio, and solid state 6 to 8 hours.

I used 205 hardener which is mostly used for general bonding, barrier coating and fabric application. It was also formulated to cure at lower temperatures and to produce a rapid cure that develops its physical properties at room temperatures. Its pot life is 9 to 12 minutes at 22 degrees C. And solid state 9 to 12 hours.

The 206 hardener is a slower hardener and provides a longer working time especially when working in climates of higher temperatures. Its pot life is 20 to 25 minutes at 22 degrees C.

Special pumps can be purchased to dispense the correct amount of resin per 1 full stroke of the resin pump, and the correct amount of hardener per 1 full stroke of the hardener pump.

The temptation is to mix larger amounts, to save time mixing the stuff all day, but this resin generates heat once the hardener is added and stirred, and with larger volumes of epoxy in the container the greater the reaction and shorter the pot 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 glue the scarf joints so it would not run out of the joint before it cured, and West system provides additive powders to enable this. 411 powders are suitable for this.

I mixed about 4 pump strokes of Resin and 4 pump strokes of hardener together, and while mixing stirred in powder until I got a peanut butter consistency.

Mixing containers can be bought, however I preferred to put my money into the boat and not the rubbish bin. I had my wife and our neighbours saving plastic milk bottles and other suitable containers for me. My supply of milk bottles was insane at times. I cut the tops off 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, or else the hull will have many humps and hollows to fill causing the fairing job to be a larger job than it should be. Those finished planks were just short of 12M long for the 9.M hull, which was fine because it is important to stagger the scarf joints as the hull is being planked, and wastage will occur because of that.

Those long planks then had to cut down to mostly 50mm widths so that the planks could be laid up over the male mould frames to form a round bilge core. In fact what you really have is many small chines’s, which are hardly noticeable, and disappear completely once the fairing is completed.

Around the water line area bilge areas, I had to cut the width of the planks down to 25mm and a couple to 12mm in order to get around the tighter radius.

Once I had a good stock of strip planks made it was time to start installing them onto the mold.

First I ensured that the hull could be removed from the mould frames at a later date by applying insulation tape to the edge of all the mold frames.

The first plank was important, as where it is placed along the hull would depend on how well the following planks laid up around the curvature of the hull. I screwed the first plank into the deepest point of the concave frame curve for each frame, and with a little bit of trial and error it became evident where the best fit was.

The edge of the first plank was coated with thickened glue so that it would not run off and also fill any gaps in the edge joints. The next plank was lifted and fitted into place, making sure that the scarfed joints of each plank were not aligned with each other, and only then screwed to the temporary frame.

I also found it necessary to screw plywood battens across the planks to hold the edges aligned in the areas between the spans of the temporary mold frames.

This edge gluing process was continued for about 6 to 7 weeks outside work hours until the entire DuraKore core was finished. A battery powered screw driver made this job easier for me.

As the planking progressed it became evident that I would have to stop on the area that I was currently filling in, because it became impossible to lay the long planks around the bilge curves as the plank was beginning to twist like a propeller blade and it would resist sitting flat against the edge of the mold frame.

I had to then lay a new 25mm plank along the highest point of the convex curve along the waterline, bow to stern and screw that plank on as my new edge to glue to. The next 25mm plank was edge glued and fitted to the lower edge of the new plank, and I continue to plank down towards the previous planked up area, slowly increasing the width of the planks to suit the curve therefore filling up the long elliptical void that was left. As the void was closed I found that I had to taper the end of the planks so that they could fit against the lower plank, and as the elliptical void was closed up each subsequent plank would become shorter. You can see the photos showing the planking on my web site.

Once the whole hull core was laid up, every screw was removed thousands of them and the hole filled. The glue joints were lightly sanded flat taking care not to remove any hardwood veneer. Hand sanding was the safest method as machines tend to dig in far too easily.

My next article will be about the fibre glass lay up on the main hull.