This is an archived message posted to the Kayak Building Bulletin Board. If you would like to ask a question about building kayaks, you can post a new message.
Hello Rick:
There is one aspect of the primary/secondary stability discussion I did not see mentioned. I read about it in Nick's book. It has to do with the desirability of high primary stability.
Consider two boats, one high primary stability, the second one lower. When sitting in the boats, either underway or stopped and the water is relatively flat, the high primary stability boat will naturally feel more solid, and the second boat will feel less so, twitchy perhaps. The "solidity" that you perceive is from the hull's design that keeps your torso perpendicular to the water's surface. You tilt a little and the water and the boat "push back"--it feels stable. The second boat doesn't resist your initial movements as much, it's shape displaces the same amount of water when you're vertical as when you're a few degrees to one side or the other. Without the boat "holding" you upright as strongly, you could feel less stable. Flat water, remember?
Now, if the water has some chop or some waves to it, the percections will quickly be reversed. Here's why: The same forces that you perceive as making the boat feel stable, that is, keeping your torso perpendicular to the water, still act on the boat. But now, the surface of the water is no longer parallel to the horizon and what you'll sense is that the boat is pitching around, being tossed about by the waves. Definitely not a stable feeling! But the second boat, the one that felt looser, the one that didn't seem to mind whether your torso was vertical or tilted some to one side or the other, will now feel more comfortable. This is because it's hull shape will not resist the movement of the water underneath and around it.
This may give the impression that the first boat is easier to remain upright in. Perhaps, but often the water is not very flat and in those stiuations, the boat with the less well defined primary stability will ... "roll" with the motion of the water, and fling you about far less.
The secondary stability becomes more prominent in moving water for this reason.
Ed Valley *****************************
This is an archived message from The Kayak Building Bulletin Board.
Guillemot Kayak - High
Performance Wooden Sea Kayak Designs
