Blog Archive

Feb 6, 2007 Waterwings

I got a call from Tim Anderson inviting me to come out and watch him and some of his friends playing with water-wings. Water wings is not what they're called but that's what I think of them as. If you want to see them in action, go to this website. The interesting thing about them that is not apparent from this video is how much energy they take to keep moving. Perhaps this shouldn't be surprising. These things require you to keep 200 pounds of human suspended above the water besides making you move forward. But when you're there, you hear the huffing and the puffing and realize that it's quite a bit of work.

Feb 4, 2007 Curragh Model

I finally finished the curragh model the other day. And here are some pictures. The complexity of the building process and the number of parts isn't any greater than what's involved in building a kayak. If anything, it's simpler than making a kayak. The main difference is that a curragh is wider and heavier than a kayak. Beam is around 4 feet, ribs are bent from blanks that are about 6 feet long. Length is about 17 feet, same as an average kayak.

Here's a detail of the bow to gunwale joinery. Gunwales aft of the joint are straight. Gunwales in the bow are sawn to shape. There is no sophisticated bending of wood here.

Here's a view of the model with planking in place. The originals were completely planked with the edges of planks touching each other. This had to drive up the weight of the boat considerably. But the planking wasn't watertight. It still needed to be covered with cloth and tarred. The planking in the originals was nailed to the ribs. I didn't want to work with tiny nails, predrilling holes and so on for the sake of authenticity and chose instead to space out the planks far enough so I could lash them to the ribs with string.

And here is a closeup of the bow. I used willow ribs here because they were easier to manipulate than the sawn oak which needed to be soaked first and then heated to be pliable. There's a little bit of an optical illusion in the picture here. The ribs are the dark parts and the light parts are the couch in the background. The eye tends to reverse the two.

I don't usually build models of kayaks. For one thing, it is almost as easy to build a full scale kayak as it is to build a model. The main advantage is that a scale model isn't quite as big a storage problem and is cheaper to build. Other than that, it isn't something I'm partial to. For one thing, the model won't really tell you how the full scale thing will work.

Feb 4, 2007 Greenland kayak hydrodynamics

Dave Browne built a Greenland kayak with me. The ends of this boat were fairly sharp and the beam was fairly narrow at 21 inches. When we took the boat out for its maiden cruise, I paddled next to Dave and was able to observe its behavior in detail. One thing I noticed for the first time on the Greenland boat is that the bow wave didn't start until about the point where the chine intersected the waterline. In other words, the bow wave didn't start at the bow, but about 18 inches back. The reason this happens is that the bow is so narrow that it doesn't push any appreaciable amount of water out of the way. It isn't till the boat starts to widen about 18 inches farther back that it pushes enough water to create a wave.

This raises the question why you would want to have a long overhanging bow that doesn't displace much water and adds little bouyancy to the boat. Worst case, this sort of construction makes the boat heavier and makes you buy more wood than you need to. The answer that offered itself was that the sharp bow was an evolution to deal with ice.

I remembered that back in Wisconsin, when I paddled around ice, I could drive the stem of my Greenland kayak into a half inch thick sheet of ice and that the stem would crack it neatly. The crack would propagate a few feet ahead of the stem and the ice would be gradually pushed aside. So I suspect that the narrow bow of the Greenland boat is not so much designed to deal with water as it is designed to deal with ice. The bow also works in thicker ice where you can wedge it inbetween floes and gradually push them apart without putting a huge amount of pressure on the skin right at the bow.

The question then arises, what function does a sharp overhanging bow serve when you don't have to deal with ice in the water? In flat water, I suspect fairly little. Some people might actually be better served with shorter, more blunt bowed boats. The main advantage would be lighter weight and less windage at the cost of some good looks. However, in rough water, the upswept bow would offer some reserve buoyancy and prevent waves from washing over the deck and crashing into the chest of the paddler. The kayaks of the Bering sea have seemed to solve the problem of waves in a different way. They keep the bow low but put a center ridge on the deck so that when waves come over the bow, they slide down the sloped deck and never reach the paddler. The hulls of Bering sea kayaks also have more rocker than Greenland kayaks to make it easier for the low bow to lift over waves.