Duet – new construction

It’s been an interesting few weeks building boats using epoxy resin and vacuum bagging, we’ve learnt a lot and the quality of the finished product is improving.

I still use a laminate stack of two layers of 200 gram carbon twill sandwiching a layer of 200 gram carbon/kevlar, but the vacuum bag is really forcing the layers together and squeezing out the excess resin. This produces a lighter boat but without the additional polyester resin from the wet layout process, some of the stiffness has been lost.

Rather than simply adding another fabric layer, I have implemented a frame structure within the hull to add strength and stiffness without adding much weight.

The initial design incorporated a gunwale shape which creates a strong rim around the cockpit. This is supported by the three tubular carbon thwarts across the width. Next is the flange which runs along the inside of the cockpit. This supports the cross braces for the seats and footrests. Addition longitudinal rigidity is gained from the carbon tape along the central join.

Duet geometry


Below the flange there is nothing. This was fine using polyester wet layup, but not with Epoxy vac-bag. We don’t want it too stiff because it will break if the boat is dropped or hits an object in the water, so some flexibility is required.

So I took a leaf out of the Wenonah design and implemented a series of ribs.

After a couple of trials on the demo boats, I decided that the optimum number of ribs is twelve, spaced at 300 mm intervals within the cockpit hull area. They are not required beneath the decks as they are stiffened through the design geometry, but they also retain enough flexibility to absorb impacts.

Rib material requirements


I cut out 50 mm strips of Sorix and 100 mm strips of uni-directional carbon fibre. Soric is a core material with hexagonal cells.

Soric core material


The cells do not absorb resin, but the outlines do. This creates a very strong and stiff framework. It really is amazing stuff and I now use it to thicken and stiffen the gunwales, on the seats, and for the ribs. I also use peel-ply to ensure a consistent surface finish.

Material preparation


The ends of the Soric strips finish just under the flange.

Soric measuring and positioning


The uni-directional strips extend beyond the flange, up the side of the hull, but the ends are tidily encapsulated within the flange support.

Uni-directional carbon measuring and positioning


The centre join is formed by a 50 mm strip of carbon/kevlar and a top strip of bi-directional carbon. The ribs are sandwiched between the two thus increasing the strength.

Ribs complete


Once the ribs have gone off, a platform is laid into the hull to support the installation of the flange materials. These comprise of:

Flange materials and design


This right angled flange bonded to the inside of the hull really improves the rigidity of the boat. It tidies up the ends of the ribs and of course supports the footrests and seats.

Flange construction


Coupled with the seat and footrest cross bars, the boat is truly ridged. There is still a little bit of flexibility between the ribs to help absorb impacts.

Cross bracing


The epoxy resin and vacuum bag construction method also improves the quality, but it is massively more expensive than a polyester wet layup.

I also use Soric along the gunwales to thicken, stiffen and strengthen. It doesn’t look as nice because the pattern of the Soric matrix comes through to the surface due to the vacuum pressure, but it is far more efficient and at the end of the day, it is a racing boat.

In summary then, I now have the boat construction I wanted. I believe it uses the best materials in the best way, but in the least quantities. It’s jolly expensive but we diggers are worth it!

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