Light Can A Eureka Be Built:
An email arrived from from a couple of serious canoe users with many years of experience. They were looking to build the lightest plywood canoe possible and had found the Eureka site on the net. They asked me whether I thought a 14 to 16kg finished boat would be possible.
I generally tend to be quite careful with weight questions because lightweighting a boat is a matter of putting every step of the building process under the microscope. Get lazy in one or two areas and it is very easy for extra material or heavier material than expected to creep into the building - resulting in the boat being much heavier than expected. The result is disappointment for the builders (and the designer)
Despite my cautious approach the customers decided to go ahead to build. It turned out that they had a background in lightweight model aircraft - great attention to detail through the whole project - this Eureka looks great and the weight is right on target.
How it was Achieved:
Over a series of emails the customers and I went over various strategies. Basically I drew on my Balsa Canoe experience where I had attempted to push everything to its limits and learned a great deal about what it is possible to get away with.
CP brought a number of building techniques and methods that I didn't know much about to the building process. Their engineering thinking was clear too - they knew what to do to reduce material but not sacrifice stiffness in the structure.
Plywood - thickness was dropped from the common 6mm and reduced to 3mm. There was a slight problem with this in that the bottom panel went into a slight hollow. Something we had also noticed with Biting Midges boat built of 4mm (here). However the hollow on both boats was only slight and made gentle transitions into the rest of the surfaces of the boat - so would have little or no affect on performance.
Fibreglass - a thin layer of 100gsm glass was used on the bottom three ply panels to eliminate the risk of the ply splitting - plywood made up of three layers is prone to splitting along the grain of the outside veneers. A thin layer of glass moves the Neutral Axis out of the middle veneer so lateral bending loads are shared with the glass. A "crows foot" weave glass was used - this is a particularly flat weave of glass reducing the amount of resin used to fill the weave
Resin - the customers used a method I hadn't heard of to reduce the amount of epoxy left in the glass. They used a toilet roll to suck excess resin up. As the outside layer of the tissue becomes saturated with resin it is rolled off and thrown away. Basically a disposable mop. They used "perhaps 1 to 2 kg less than specified in the plan.
Timber - the lightest locally available timber for gunwales, inwales and other timberwork is Paulownia. It is a lower density than Western Red Cedar - the previous best choice for lightweight timberwork. The Paulownia is perhaps a bit less dent resistant than the cedar so they made the decision to use Cedar for the external gunwale
Timber Scantlings (dimensions) - were pushed toward the minimum with a list of sizings that I thought would be OK in light of the Balsa Canoe project. The big thing I had learned there was that the conventional sizings are much bigger than needed - at least for flat water use. Diagram below shows gunwales and inwales being clamped in place (detail from plan).
Gunwale - could go down to about 22 x 15 with the long edge horizontal.
Inwale - could go down to 10 x 20 with the long axis vertical.
Inwale spacers - are the actual key to the stiffness of the sheer structure - so they could be widened to move the inwale further in toward the centreline - the spacing between them could be extended slightly.
Details - Another great idea was the way the seats were made simply by drilling holes in the seat frame and weaving some cheap washing line cord through the holes. Drill the holes nicely (and stagger them - one forward and one back so the timber won't "tear along the dotted line) and it looks great. And saves more weight over plywood seats. The builders noted too that this style of seat keeps the paddler firmly in position even when the boat is heeled or is bouncing around in waves.
The builders ended up taking their new boat for a short paddle and then life intervened and they ended up being too busy to hit the water for some time. Then this ...
Finally a chance to go paddling in the Eureka. This week we have done 3 X 10km and 1X 8km paddles. On both river and lake. Mostly calm conditions, just a little wind on the lake on one occasion.
It seems very stable but we can see where inexperienced people could get caught out. Because of the chine the boat has several stable positions of heel. It is very happy to sit on the chine between the base and bilge boards. This is probably about 15 degrees of heel. To some people this may cause them to abandon ship. Also a thought re the comments on the forum. With the straight ply bench seat as per the plans, once the boat heeled the occupants backside will slip to the lower side thereby causing a capsize. In most paddling boats foam is stuck to the seats for the dual purpose of comfort and keeping the bum from slipping. You can move around in the Eureka with no problems.
Directional stability is OK for the boat that it is. It has a fair amount of rocker so it is easy to manoeuvrer so obviously not the most directionally stable boat but the compromise works well. It certainly isn't all over the place. Again beginners may have trouble going straight, but that will be the case in any canoe. In the limited amount of wind we had, the directional control was good. We achieved reasonable speed with effort. 1 1/2 hours for 10kms this was taking it easy and stopping occasionally to drink and take pics.
A couple of people were very flattering regarding the boat, so it obviously has the looks.
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