Building
Selecting a Kayak.
Below is a list of considerations and an explanation.
Click below for a comparison table of our kayaks
The tables will open in a separate window so that you will not lose this page.
METRIC
IMPERIAL
Kayaks and canoes seem very simple boats but small differences make a large difference to the performance and the handling. This characteristic is used to produce boats suitable for different conditions.
1. Use to which the kayak is to be put.
2. Beam and Cross Section
3. Length
4. Wetted Surface Area
5. Volume
6. Rocker
7. Hull Form
8. Rudder or Skeg
9. Deck and cockpit
10. Prismatic Coefficient
1. Use to which the kayak is to be put.
You should try to have a clear idea of the type of kayaking you will be doing. If you intend shooting down waterfalls in narrow rivers you will need a different boat from one used for sea expeditions. In general, the former should be short, round bottomed and disposable and the latter should be long, roomy and hard chined.
2. Beam and Cross Section
The maximum width or Beam affects a kayak's volume, stability, and speed. You must consider the width both at the waterline and overall. Closely related to beam is the cross sectional shape of the hull which may be flat, round, vee, or a combination of these shapes.
In order to carry equipment shorter kayaks must be beamier than longer ones. However, there are disadvantages to the extra width: they are usually slower than narrow boats; longer, heavier and more cumbersome paddles are needed to avoid hitting the edge of the boat; wide boats are often more difficult to paddle in a straight line without continual correction. Between 50 and 63 cm is best for a single sea kayak. A double may have a beam as large as 89 cm.
Beamier boats are not necessarily more stable. There are two elements of stability: initial stability and ultimate stability.
Initial stability is the type of stability a flat-bottomed kayak with a wide waterline beam has. A boat with high initial stability doesn't feel as though it is going to capsize but, if it is tipped over too far, it capsizes without warning.
A boat with high ultimate stability, on the other hand, feels as though it is going to capsize if you so much as raise one eyebrow. Such a kayak will have high flared hull sides. It may be tender at first but as it starts to heel over more of the hull is immersed and its resistance to capsizing increases. This low initial stability actually helps a skilled paddler control the boat by allowing it to be leaned easily when turning and when in rough seas.
Round-bottom, vee-bottom boats and boats with very narrow waterline beams usually have lower initial stability. Those hulls with flared sides and high volume tend to have high ultimate stability. A simple way to increase the stability of a kayak is to lower the centre of gravity by lowering the seat.
3. Length
The length often determines the maximum speed of the boat, how much it can carry and its’ stability.
There are two separate lengths that must be considered for every boat. The first is the length overall, or LOA: this is the distance between the two most distant points on the bow and stern. The length on the waterline, or LWL, is the more important length measurement: the LWL is the distance between the immersed ends of the kayak when it's normally loaded.
Most benefits of length are due to a long LWL, and not to a long LOA. A boat with a LOA much longer than its LWL is said to have long overhangs. Long overhanging bows and sterns are often added to a design for cosmetic reasons. Some paddlers feel that long overhanging bows are beneficial in rough waters. But similar advantages can be achieved with a moderate overhang by increasing the volume in the bow. These long overhangs, particularly if they are overly upswept, can add considerable lateral surface area that makes a boat more difficult to handle in strong winds.
A boat's top speed is related to its length. A boat creates a bow wave where it cuts through the water and a stern wave as the water comes together again at the boat's end; in order for waves to move faster they must be farther apart. A kayak’s theoretical top speed in knots (a knot is 1.15 mph) is roughly 1.34 times the square root of the LWL. This speed is called the boat's hull speed. Actually for brief periods when surfing down waves a kayak can attain higher speeds. Even on flat water a kayak, like many boats with very narrow hulls, is capable of speeds slightly higher than its theoretical hull speed.
However, the limiting factor in speed becomes overall drag that your limited power must overcome and not theoretical hull speed. Once the kayak's LWL approaches 17 feet or so you'll gain little or no speed by lengthening it, unless you are a very powerful paddler. Racing kayaks can have 21-foot waterlines, but most people rarely paddle fast enough to achieve hull speed in a kayak.
Length does not only affect hull speed. If two kayaks of different length but similar design are compared the longer boat has several advantages over the shorter one.
· The longer boat won't pitch as violently in steep waves.
· The longer boat will be more stable than the short boat.
· The longer boat will have a higher volume and will hold more gear.
· The longer boat will go in a straight line, or track, better.
On the other hand, there are some disadvantages to longer kayaks.
· The longer boat is less manoeuvrable.
· The longer boat is heavier and more expensive.
· The longer boat will be harder to store and transport.
Single sea kayaks with a LWL of 16 to 18 feet seem ideal. Double sea kayaks work best with a LWL of 18 to 22 feet. Flatwater kayaks are typically shorter, as are, very wide kayaks. 14 foot kayaks were originally designed to fit inside the luggage compartments of Victorian trains but this is no-longer a consideration.
4. Wetted Surface Area
The wetted surface area is the area of hull surface below the waterline. At low speeds the friction of the hull's skin is a greater source of resistance to forward progress than are the waves formed by the hull's forward motion. So a long narrow boat with a high theoretical hull speed but a large wetted surface will require more energy to paddle at low speed than a short fat boat with a minimal wetted surface. Paddlers sometimes buy long kayaks thinking they'll be able to go faster; for short bursts they will, but at normal touring speeds they may actually end up moving slower than they would have in a shorter boat with less wetted surface area.
Boats with very low wetted surface areas have cross sections resembling a semi-circle; these hulls have very low initial stability. In extreme cases, such as flat water racing kayaks, they can only be handled by expert paddlers. Touring kayaks hulls must be flattened out to increase initial stability, and, with it, wetted surface area. Hard chined boats and flat-bottomed boats may have slightly higher wetted surface areas, than multi-chine or round bottom kayaks. However, they also tend to have larger load carrying capacities and better handling and that may be more important on long trips.
5. Volume
A kayak's volume may be expressed in cubic feet or litres, but more often it is simply referred to as high, medium, or low.
Volume is influenced by a kayak's length, beam, prismatic coefficient (see later), and depth or deck height. A high volume kayak is one that will hold tons of gear and a big paddler.
· A high volume boat it needed to take camping gear and supplies.
· A high volume boat is usually slower and heavier than a low or medium volume boat
· A high volume boat is drier and more comfortable than a smaller volume one.
· A high volume boat, with its higher bulk out of the water, is more difficult to control in strong winds than a lower profile lower volume boat.
Physical size is also important in choosing a kayak's volume. Smaller build people can feel uncomfortable or "lost" in a high volume boat and may also lack the weight to lean such a boat. Larger paddlers often feel cramped in a low volume kayak. Unless you plan extended trips or are particularly heavy or tall there is little reason to choose a very high volume kayak.
6. Rocker
A rocker is the upward curve of the kayak's keel line over its length. If you place a kayak with pronounced rocker, such as a whitewater kayak, on a flat floor its middle will touch the floor while its ends (at the water line) will be several inches above the floor. A boat without any rocker placed on the floor will touch the floor over most of its length.
· A kayak with a large rocker will turn easily but be difficult to paddle in a straight line.
· A kayak with little rocker will be difficult to turn but easy to paddle in a straight line.
Paddling in a straight line, or tracking, is more important than turning ability in touring and in sea kayaks. To strike a balance between tracking and turning most sea kayaks are designed with a small amount, say 50 – 70 mm, of rocker.
Longer boats, boats with vee-shaped hulls, and boats with very fine ends can have more rocker without compromising tracking. Rocker improves a boat's handling in waves. It's a common misconception that adding rocker makes a boat slower: actually, modern flat water racing kayaks often have a considerable rocker.
7. Hull Form
The kayak designer must also decide where to place the boat's maximum beam. If the widest part of the kayak is at or near the midpoint of the boat's length the boat is said to have a symmetrical form. If the maximum beam is forward of the midpoint it has a fish-form hull; if it is aft of centre the boat has a swede-form hull.
It's thought that fish-form and symmetrical-hulls are more efficient than swede-form hulls, but the difference, if any, is slight. You've probably noticed that many modern kayaks appear to be swede-form. Because a boat's bow overhang is usually much longer than its stern overhang the maximum waterline beam of a symmetrical-form boat may be aft of the centre of its overall length, causing it to appear to be a swede-form. Some designers prefer to place the maximum beam aft of the cockpit so that the paddle will more easily clear the deck. Also, many paddlers seem prefer the appearance of a swede-form boat.
8. Rudder or Skeg
In heavy winds and seas rudders are invaluable. They are also useful in doubles and in heavily loaded singles. However, any kayak that relies on a rudder to handle properly is unsafe. Rudders and skegs shouldn't be used as a remedy for poorly designed hull.
A skeg is a rudder that is fixed and cannot be used to steer. It improves tracking in rough conditions. A removable skeg is the best type so that it can be used to balance a boat in rough conditions and shipped when not needed.
Rudders can and do fail so they should be considered a convenience and not essential.
9. Deck and Cockpit
The deck is an integral part of a kayak's design. It adds enormous rigidity and strength to the hull. The deck must have sufficient camber, or curvature, to accommodate the paddlers knees, feet, and gear. Camber increases a kayak’s volume and allows the deck to shed water quickly and keep the paddler drier. A kayak with a cambered or peaked forward deck is easier to Eskimo roll. However, decks that are too high increase windage and make the boat more difficult to handle in extreme weather conditions and a high aft deck may hinder leaning back to roll.
A few plywood kayak decks are composed of flat, as opposed to cambered, sections. Such decks often have a tendency to flex.
A kayak's cockpit must fit the paddler; it must be both snug and comfortable. Cockpits and seats can be customised by gluing in closed-cell foam padding. The cockpit opening should allow the paddler to enter and exit the boat quickly and efficiently. However, it must not be so wide or long that the paddler can't brace his or her knees under the deck. A key-hole shaped cockpit is often used because it can be long enough to allow easy entry and exit while still providing a place to brace one's knees.
A sea kayak's coaming must be strong enough and low enough to sit on when performing a wet re-entry. If the kayak is to be used primarily in calm water without a spray deck then the coaming may be higher to help keep the paddler dry.
10. Prismatic Coefficient
Another measurement related to a boat's beam and cross section is its prismatic coefficient, A kayak's "prismatic" describes how full or fine ended the hull is. A boat with a high prismatic coefficient has its volume distributed along its length causing its ends to have a "full" shape. A boat with a low PC has its volume concentrated near the centre and its ends finely tapered. The prismatic coefficient is the ratio between the volume of displacement (how much water a loaded boat displaces) and the volume of a prism that has the same length as the LWL and the same cross sectional area as the widest part of the submerged portion of the hull. Most kayaks have a prismatic coefficient of between 0.45 and 0.60. The importance of this number is in that a boat with very full ends, a high prismatic coefficient, will tend to "push" its bow and stern waves farther apart and towards the ends of the hull. The farther apart the waves are the faster the boat is capable of moving. Of course, when the prismatic coefficient gets too high the waves get too big and too much energy is required to push them; we then have a barge. A kayak with very fine ends, a low prismatic coefficient, won't make efficient use of its length to achieve a high hull speed because the bow and stern waves will be too close together; however, such a boat may be very efficient at low and medium speeds. Many designers don't actually calculate a kayak's prismatic coefficient; instead they rely on experience and a good eye to draw the proper balance of fullness and fineness in the ends.
A kayak's ends must also have enough buoyancy to stay above water. When boats with low prismatic coefficients are paddled in steep waves their ends may tend to dig into the faces of the waves. This can result in broaching when paddling downwind and makes the boat difficult to control in other circumstances. In such boats the hull should flare and volume above the waterline to increase its reserve buoyancy.
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Fyne Boat Kits
Unit 5, Station Yard, Burneside, Kendal, Cumbria, LA9 6QZ
Tel: +44 (0)1539 721770 Fax: +44 (0)1539 721 770
E-mail: info@fyneboatkits.com
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