Thoughts on Rocker

Article One:   Seen from above, most canoes and kayaks taper to a point at each end. But now look at the same boats in profile. Some rest square on their keels from stem to stern. Others lift up at both ends. Some rise only a little; some rise a lot. Either way, this rise is called “rocker.” To see how it got this name, just put a canoe or kayak with a lot of rocker on a level floor. Then press down on either end. See? It rocks back and forth! And that’s not all. Like the breadth of its beam and the shape of its bottom, rocker, too, affects a boat’s performance.
Not Your Grandpa's Rocker
Canoes and kayaks with large amounts of lift at the ends are sometimes known as “banana boats.” They pivot quickly, and they’re wonderfully easy to turn. That’s welcome in whitewater, and it can also be helpful on narrow, obstructed streams, even if there’s not a rapid anywhere in sight. But it’s not so good on windswept lakes. Keeping a highly-rockered canoe or a whitewater kayak pointed the right way on a windy lake requires both strength and skill.

On the other hand, boats with straight keels—boats having little or no rocker, in other words—are happiest when they’re going point to point in a line as straight as their keels. They’re most at home on large, open bodies of water.

Understandably, flatwater paddlers and sea kayakers gravitate toward straight-keeled boats, while whitewater boaters (and surfers) choose highly-rockered craft. But what about folks who want one boat for all conditions?

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Article Two

Having rocker in the keel line achieves several goals in reducing drag:

1. It enables carrying the semi-circular cross-section – that which has the least amount of wetted surface – deep into the ends of the boat and therefor maximise the minimisation of drag that this cross-section affords. I say ‘maximising the minimisation’ rather than ‘optimising’ because I want to emphasise how it does this. Boats with a flat keel line with little rocker only have a semi-circular cross-section through a short length of the middle of the boat, but it quickly turns into a U-shape as you follow the lines into the bow and stern.

2. A keel line with proper rocker also represents a boat with no hard ‘corners’; it’s a boat that maintains parabolic lines without any flat sides. Having places that disrupt the parabolic arc of a boat’s lines – places where the sides of a boat run parallel to each other (below the water line) – increases drag. Many of the newly designed boats that I see out there seem to have this issue. They have finer (more shallow) sterns and/or bows, but those arrise from rather abrupt turns in the hull. If you look along their length, you’ll see that the parabolic curve has ‘corners’ in it rather than one smooth arc.

3. The rocker allows a boat to have greater draft mid-ship for the same weight displacement and wetter surface vs boats with flat keel lines and flatter cross-sections. Greater draft in a boat – meaning greater depth below the water line – is desirable for reducing drag.

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More Articles

 

http://www.sailnet.com/forums/sailboat-design-construction/79173-hull-rocker.html

 

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/distribution-rocker-40440.html

 

Originally posted 2011-12-05 06:54:17.

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