Directional stability of sea kayaks

One of the most frustrating aspects of paddling a kayak is that of making the thing go in a straight line, especially downwind. The boat tries to round up, and the paddler must make continual correcting strokes. Frustrating, and also fatiguing, and fatigue has a bearing on safety.

The writers of sea kayak books never explain how fins and rudders work. I suspect this is because they don’t understand the dynamics, although the problems are obvious enough. Unfortunately the naval architecture texts usually offer little help, and the only one I have found of any value is Kent, KL, Ships in Rough Water, T Nelson & Sons, 1958. (Perhaps this is not surprising: bearing the scale in mind, kayaks are always in rough water.)

The dynamics

static

The diagram shows three Centres; the Centre of Gravity, the Centre of Lateral Resistance (ie. the hull’s resistance to being moved sideways), and the Centre of Effort (through which the wind can be considered to act). Left to itself, a kayak will drift sideways, because all three Centres are close to each other. (I use the word ‘Centre’ advisedly. In reality it is the locus of the centre as it moves about with passing waves or gusts.)

Things change when the boat is under way and Bernoulli takes a hand. In particular, the CLR moves forward; exactly how far is uncertain because it seems no-one has done the sums for kayaks. Some work with ships (Kent, 1958) suggests that it may be very near or at the bow.

upwind

Into wind, the CE will also be forward. With the two Centres close to each other, the yawing moments are small, and it’s easy to keep the boat straight.

downwind

Downwind, the CLR will still be forward, while the CE has moved aft. The situation can be made worse in following seas by the bow burying, the stern being in the air, and so on. With the stern on the crest of a wave, it may effectively be moving astern because of the orbital movement of the water in waves. With the two Centres widely separated, the yawing moments are large and the boat wants to broach. (This is also dangerous in power boats: go too slowly in following seas and the rudder can be ineffective because of the orbital movement of the water.)

waves

Control methods

Ships control their directional stability with rudders, and it takes considerable skill on the part of the helmsman to maintain a course in rough seas. It was reasonable to follow ship practice and fit rudders to sea kayaks.

But a rudder does not cure the problem. The kayak is still directionally unstable, and the rudder allows one simply to correct the symptoms. To do that, it must be turned one way or the other, and that creates drag, and the less well balanced the boat, the more the drag. As well, there is the mechanical complexity of the rudder itself and the tiller bar or pedals, and the hazard of the hardware on the stern.

What is needed is a means of restoring balance between the CE and CLR, by moving one or the other. One can either move the CE forward (e.g. with a sail (but that can introduce some other problems)), or move the CLR aft with a fin.

fin down

The fin is the simplest solution: by moving the CLR aft, close to the CE, balance is restored. There is only one moving part and one control line. The kayak will have a different ’feel’ from that of a rudder craft. It will yaw about a bit on waves, but hold its course without any steering effort. It’s a case of setting the fin for the conditions, easy with a bit of practice, and paddling normally. (Provided you don't muck things up by carrying a pile of stuff on the aft deck.)

(The Aleuts used a different scheme in their baidarkas: they moved the centre of gravity fore and aft with ballast rocks. Modern kayakers also trim their craft in this way, but with expedition gear, not rocks.)

Caveats

The fin works best on hulls with some rocker. It is easier to stabilise a manoeuvrable hull than to steer a very straight-running boat. Voyager is 459 cm long, and its fin has also been used successfully in Selkie, Weekender, Ice Floe, Pittarak, and Nordkapp. There are many other fin-equipped boats on the market: Skerray and Capella to name only two. I wouldn’t waste my time fitting one to a Mirage because of its lack of rocker.

Voyager fin

Voyager fin in ‘full down’ position. The fin is 38 cm long and 11 cm wide, and the extra blade shows the pivot slot and the hole for the knots in the control and shock cords. The curve (lower right in the picture) is grooved to act as a ‘pulley’ for the cords. Control is through a single cord, cleated alongside the cockpit. The diagram shows the internal layout and general dimensions.

Voyager fin diagram

There is some loss of internal space with a fin, and access to the stern may be more difficult. However, the advantages outweigh the disadvantages.

Reference

Kent, KL, Ships in Rough Water, T Nelson & Sons, 1958

Addendum

A version of this paper was printed in the February 1996 edition of Sea Trek, the magazine of the Victorian Sea Kayak Club, and contained the footnote

‘2. The word ‘steer’ is used here for convenience. The rudder is not for steering, but to trim. Sea kayaks are steered with the paddle, like all kayaks and canoes.’

The second and third sentences of this note have been quoted (without attribution) by Paul Caffyn, who is a strong advocate of rudders after his circumnavigation of Australia in 1982. Before fitting a rudder to his Nordkapp Caffyn used a detachable skeg. This device was either on or off, and in common with other on/off devices had two effects: too much or too little.

Caffyn is therefore not comparing rudders with infinitely adjustable fins.

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