Sea Kayak Design Considerations [29]

By Norm Sanders

See also an analysis and reply by John Winters

Watercraft design is more of an art than a science. Bill Foster, a master craftsman and wooden boat builder in Hobart often told me, “If she looks right, she is right.” (Boats are always “She.” The reasons why are too complex to cover here.)

As human beings, we all have an innate sense of what looks right. Put a squat, flat bottomed rowboat on the beach next to a sleek kayak with sweeping sheer and see which one draws the crowd.

Sheer, the line of the deck from bow to stern, traditionally starts high at the bow, dips amidships and then climbs back to the stern at a point lower than the bow. There is a reason for this. The bow is high because it must climb over waves. The midship area is low to allow access to the water by paddles or oars. The stern is then upswept to rise over following seas.

Many modern power boats and yachts have flat or even reverse sheer and work quite well (except that they may be wet in a sea.) However, they don’t LOOK right. Most sea kayaks have moderate sheer. Whitewater kayaks have pronounced reverse sheer which makes them very wet in the surf. They punch through waves rather than lifting. A big bonus with sheer is the fact that the bow and stern want to float to the surface when upside down. The kayak tends to initiate Eskimo rolls by itself — leaving the paddler to furnish the finishing touches. Rolling is also easier in a kayak which has a low aft deck and a cockpit roomy enough behind the seat to lean well back. The secret of effortless rolling is to keep your centre of gravity as low as possible.

Another design consideration is the location of the greatest width of the boat, the beam. There are three basic choices:

  1. Fish-form — greatest beam forward of centre.
  2. Symmetrical — greatest beam at centre.
  3. Swede- form — greatest beam aft of centre.

Boat design is a massive collection of compromises. Fish-form is fastest, but hard to handle without a rudder. (Have you ever seen a fish without a tail?) Symmetrical is easier to steer and slightly slower. Swede-form sacrifices a little more speed for a lot of directional stability.

The shape tracks well and is easy to turn, the obvious result of sitting further aft with the paddle. Wherever the greatest beam is, it shouldn’t be too large. Beamy boats are slow, hard to manoeuvre and difficult to roll.

The next consideration is the cross section of the hull. The theoretical best shape is round, like a log. This would have the least wetted surface and thus the least drag. It would also be unstable (like a single scull rowing boat) and require constant balancing with the paddle (or oars.)

Most Australian kayak designers have simply flattened the round shape on the bottom to gain the necessary stability. Many American designers are now tending to favour hard chines, a return to the original Inuit shape which was dictated by strips of wood covered by seal skin. Hard chines increase the wetted surface marginally, but return a major dividend in terms of tracking. They also act like surfboard rails which help manoeuvrability in breaking waves.

Yet another consideration is deadrise — the amount of “V” measured upward from the keel. A deep V hull with a lot of deadrise will track well and have less wetted surface, but be initially unstable. Again, compromise is necessary. I find that deadrise of about 12 degrees works well.

Stability is a very slippery concept. Most sea kayaks are designed to attract newcomers to the sport and favour relatively flat-bottomed shapes which feel “safe” to beginners. However, these designs often have poor secondary stability and can suddenly capsize when leaned beyond a certain point. Experienced kayakers prefer a design which emphasises secondary stability over initial stability. A moderate V bottom with hard chines feels a bit tippy at first, but gains stability when leaned and feels predictable even with the coaming in the water. Secondary stability is increased further if the deck joins the hull in the typical Inuit fashion along a stringer rather than being rounded off and joined further down. The additional flotation at the deck line is quite noticeable.

Seat height is another very significant factor in stability. High, comfortable seats may feel great when the kayak is sitting in the dealer’s showroom but can raise the centre of gravity enough to become dangerous in a sea. Even a difference of 2 cm can cause a radical change in stability. The closer you can get your posterior to the bottom of the kayak, the better.

There is still another hull characteristic to consider: the keel line from bow to stern. For many years, racing kayakers and canoeists thought that a flat run was the most efficient. Now the trend is for the bow and stern to be raised in the form of rocker. Rocker yields less wetted surface and thus reduces drag. Pronounced rocker also allows the bow to lift more easily over breaking waves. However, the main advantage for sea kayakers is the additional ease of turning.

Whitewater boats with a great deal of rocker are very easy to turn indeed, but can be frustrating when trying to paddle in a straight line. Some sea kayaks are equipped with rudders or skegs to help with directional stability. This comes at the cost of additional drag — about 10 percent for the typical flat plate rudder. Skegs also create drag and turbulence around the skeg box.

The best of all possible worlds, good tracking with easy, rudderless turning, is possible, even though the two sound mutually exclusive. I feel the way to achieve this is through a moderate V hull with a pronounced keel, combined with hard chines and a sharp, skeg-like stern and lots of rocker. The chines, keel and sharp stern keep the hull tracking in a straight line until the paddler exerts a turning force. Then… the kayak turns.

Paddling downwind is where tracking ability is most important. In general, boats will lay broadside to the wind when not under sail, paddle or power. If the paddler wants to head further off the wind, the kayak will try to return to this position, or sometimes round up even higher.

This is particularly noticeable in fish-form kayaks where the long sweep of hull behind the cockpit acts like the tail on a weather vane. Swede-form kayaks behave better, with the bow tending to head more downwind. This is because the paddler’s weight is well aft. Downwind tracking ability can be enhanced in any kayak on extended trips by loading heavier items in the aft compartment.

If the kayak is Swede-form and the bow is high due to pronounced sheer, the kayak may actually lay naturally almost downwind which means that the paddler can use far fewer course-correcting sweep strokes and concentrate on forward progress.

Kayakers can test their own craft for rounding-up tendencies by simply paddling out on a day when the wind is blowing 15 to 20 knots and then drifting without skeg or rudder.

The next consideration in kayak design is length. I tend to favour lengths in the vicinity of 16 feet. Many feel this is too short to carry enough supplies for overnight trips. I find it adequate. In addition very few kayakers actually take extended trips and pay a large penalty in weight and convenience for the mostly unused cargo capability when day paddling.

There is a commonly held belief that longer kayaks are faster. In theory, this is true. The effective top speed of any non-planing watercraft is a function of the waterline length and can be calculated in knots as 1.34 times the square root of the waterline length in feet. (For a 16 foot, 4.8 meter kayak, this is 5.36 knots or about 10 kph.) Try to go any faster and the boat is being forced to climb out of the water on its own bow wave. The energy required becomes very large, until the boat lifts enough to plane. Not even the legendary Dave Winkworth is powerful enough to get a kayak to plane on flat water. Very narrow hulls, like those on a Hobie Cat, are exceptions to the equation, but still require more power than a paddler can muster to really get moving.

So, longer kayaks are faster, if the paddler has enough power to overcome the increased wetted surface and the attendant drag. For us mere non-racing mortals, the additional length means more resistance, more weight, less manoeuvrability and less acceleration.

At the speeds which most of us cruise (6 khp or 3 knots) all commercially available kayaks show much the same resistance figures in the Sea Kayaker Magazine reviews. For instance the Coaster by Mariner Kayaks is 13’5″ long and has a resistance of 1.9 pounds at 3 knots. The racy Looksha II by Necky Kayaks is 20 feet long and has a resistance of 2.0 pounds at 3 knots. (Incidentally, both designs have hard chines.)

Less powerful paddlers reported that they actually found it easier to stay with the pack in the rudderless Coaster than in larger designs. However, if the pack happens to be racing down the Hawkesbury, things are radically different. Resistance for the Coaster at 6 knots is 17.5 pounds versus 11.1 for the Looksha. The Looksha was conceived with speed in mind. The designer says: “Looksha II was designed as a rudder-controlled boat with no compromise intended.” Frankly, I find the thought of putting to sea in a kayak which requires a rudder quite frightening.

Construction techniques for kayaks could and do fill books. As far as the paddler is concerned, hull-deck joins and bulkhead installation cause the most problems. Some paddlers could have built a new kayak with the time they spent just trying to track down leaks. Most common are the leaks caused by the use of the H-shaped vinyl strip used to hold the deck to the hull for glassing on the inside with tape. It is very difficult to get the tape into the ends of the boat, leaving the vinyl to keep out the water — which it doesn’t do very well. Running a line of Sikaflex between the vinyl and the hull can sometimes help. Some manufacturers put a gelcoat strip along the join on the outside, which is mostly cosmetic. It is far better (and more expensive by about $100) to run tape around the outside of the join as well as inside.

My own preference is to fasten the deck to the hull at the gunwale. The deck has a downturned rim which clips over the hull. This creates a lap joint which is then glassed with tape on the inside. It does not leak — and is very strong… Strong enough to dent the bow of any Pittarak which hits me in the surf.

Bulkheads are another weak point. Many fibreglass boats use thin, lipped bulkheads which are pushed into place in resin. Water has a pernicious ability to find channels under the bulkhead. The best bulkheads are glassed both sides with tape. Bulkheads should be placed as closely together as possible to limit the volume of water which can enter the cockpit when swamped. The aft bulkhead should be located just behind the edge of the coaming so that the cockpit empties completely when the kayak is turned over and the bow lifted.

Many experienced paddlers favour using the forward bulkhead for a footrest rather than relying on adjustable pegs. A bulkhead is always there for firm support when you need it during rolls or rough conditions. Rudder pedals are spongy and, like footrests can give way under pressure. A footrest which collapses in the surf could allow the paddler to slip forward into the hull and become trapped under the deck upside down, which could ruin his or her whole day.

So far the discussion has concentrated on basic hull design. Other important considerations are hatches and deck lines. In their haste to sell kayaks to novice paddlers, dealers extoll the virtues of large hatches for easy access to compartments. In America, the sales pitch goes: “Large enough to take your two-burner Coleman Stove.” For the kind of paddling most Yanks do, this is fine. However, take one of these easy access hatches to sea and the water starts pouring in.

I feel that there is only one hatch for a sea-going kayak: The Valley Canoe Products (VCP) 7 inch round hatch. It never leaks and seals so well that it actually bulges upward when the air in the hull heats up in the sun. (Provided the hull itself is airtight.) There are some imitations floating around, so it is important to buy the genuine Pommy VCP article. Some may argue that a 7 inch hatch is too small for gear. To them I say: “If you can’t get it into a 7 inch VCP, you shouldn’t be taking it.” It is certainly big enough for that most essential of gear, a Trangia. Long items can be a bit of a struggle however. I solve this problem by mounting a VCP hatch in the aft bulkhead. I tie a line to one drybag and push it towards the stern. Then I load the rest of the gear into the hatch. To unload, I remove the gear I can reach and then pull the string. The bulkhead-mounted hatch has an added advantage in that spare paddles don’t have to be removed to access the aft compartment.

Deck lines on a sea kayak are a must. However, there is a debate over recessed versus bolt-on deck line fittings which has been raging for years. Recessed fittings are supposed to be less damaging to human bodies and other kayaks during rescues. They are probably cheaper to produce, being moulded into the hull, but are difficult to repair if they fail. Bolt-ons are stronger and can be placed wherever the owner wants them. I have always used bolt-ons and have never noticed any ill-effects. I did occasionally clip my knuckles on a pair on the foredeck which I had mounted too far aft. I simply removed the offending deadeyes and filled the small bolt holes.

Some kayak makers fasten the deck lines to massive stainless steel fittings at the bow. This heavy lump of metal acts like a battering ram in the surf. The best way to attach the deck lines is the method I first saw on an Arctic Raider: A loop of nylon webbing over the bow.

Given the list of design prejudices outlined above (and they are prejudices, open to lots of argument) it should come as no surprise that my optimum sea kayak is 4.7 metres long, has dramatic sheer, hard chines, moderate deadrise, and a Swede-form shape. Others will no doubt disagree and I am sure their comments will be welcomed by the Editor.

As for me, well, she looks right, so she is right.

Norm Sanders, AKA the Old Sea Dog, started his seagoing career in 1935 on Lake Eire, Ohio at the age of three, paddling a raft constructed by his father out of a packing crate and four innertubes. (Contrary to popular belief, he was not found drifting down the Mississippi on a mat of bulrushes as a new-born baby.) Norm Sanders has since built two racing multihull yachts, a single hull ocean cruiser, three Grand Banks dories and three sea kayaks and is the designer of the Inuit Classic.. In 1974 he crossed the Pacific in a 29-foot fibreglass sloop. When his crew jumped ship in Fiji, he sailed single-handed to Hobart where he bought and campaigned a Huon Pine ocean racer. He has now arrived at that pinnacle of sea-going sport, sea kayaking. He wonders why it took so long.