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!

Seat development

I’ve now got quite a collection of racing kayak seats in my quest for a better canoe seat design. One thing that has struck me is that they are all pretty much the same size and shape. OK there are a few tiny differences but the manufacturers seem to assume that one size will fit all racing kayak paddler’s bottoms.

Once again, compare this to the cycling world where there are literally hundreds of saddle designs, shapes, sizes and materials, plus a whole host of different options for lady riders.

The discerning canoe racer however is more demanding, and much prefer a tractor seat type design, plus they are seated much higher.

Anyway, I set about modifying my original effort and recast it to have more volume at the back.

Increased surface area

The picture shows the size compared with a “standard” racing K1 seat.

Next I needed something on which to mount it. I have made a number of seat supports using timber and fibre glass, but they take so long to shape them to support the seat pan.

As it happened, two new seats from Nelo had just arrived. These are a rather different and innovative design in that the seat is supported by a cast metal frame.

Nelo K1 racing seat

The seat pan is secured by five rivets. So I drilled off the rivets and separated the cast metal “spider”.

Nelo seat “spider” support

Together with some stainless steel bolts and some plastic spacers, I secured the new seat pan to the frame.

New seat pan secured

I had just got the Darkness Duet back for a few days, so I bolted it into the rear of the C2, and John and I took it out from Pewsey Wharf.

New seat in rear of Duet

I’m showing it compared with a K1 racing seat. The marks around the edges are where I used pegs to hold the two layers of foam whilst the glue set. At this point I was reluctant to cut the seat runners down.

Well at first it felt weird because it was different to what I was used to. On the return from Wootton Rivers I’d forgotten about the new seat and was quite used to it after the hour.

I’ve now made a second one and secured both to square profile aluminium tubes ready for testing. I’ve also trimmed the runners.

New seats ready for testing

I’ve mounted them on timber supports so they should sit on the flanges with just the timber drilled to math existing holes.

I now need to get them tested.