How to: Kayak Design Fundamentals
 

Kayak Design Fundamentals

There is much more to consider than just good looks when designing a kayak. This page is a brief overview of some key design characteristics and definitions that you should be aware of before jumping into the design process.

Definitions:

Deck - top half of kayak Hull - bottom half of kayak Bow - front of kayak Stern - rear of kayak Cockpit - contains paddler seat, foot braces and backrest Bulkhead - water-tight inner wall dividing kayak into compartments also acts like a brace for sturdiness Coaming - lip surrounding cockpit for spray skirt connection Length (LOA)- overall length of kayak from tip to tip Length of waterline (LWL)- length between bow entry point into water and stern exit point from water
Water Line - where water intersects kayak hull Draft - depth kayak sits in water when design displacement equals target displacement. Beam overall (BOA)- width of kayak Beam of waterline (BWL)- maximum width of kayak where kayak intersects water at designed draft Design displacement - volume or weight of water displaced by kayak sitting in water at a given depth. Target Displacement - total weight of kayak fully loaded. Weight of kayak + weight of paddler(s) + weight of cargo. Sinkage - amount of weight required to sink kayak an additional inch. Flare - difference between beam of waterline and beam overall. This helps determine initial and secondary stability. Kayaks that have low flare have high initial stability but low secondary stability. Conversely, kayaks with high flare will have higher secondary stability but will have lower initial stability. Initial Stability - the measure of a kayak's resistance to being tipped from the upright position. Secondary Stability - the measure of a kayak's resistance to continued tipping throughout a roll.
For an illustration of initial and secondary stability, consider the differences between a regular chair and a rocking chair. The regular chair feels very stable while you sit in it. Due to high initial stability, it has a very high resistance to tipping back a small amount. However, once the chair is tipped onto two legs, it has very little resistance to further tipping. Due to low secondary stability, it can very easily be pushed the rest of the way back. The rocking chair, on the other hand, feels somewhat unstable when sitting at rest in it. A person who has never sat in a rocking chair would not have much reason to believe the chair won’t just roll over backwards. This is an example of low initial stability. However, as the person begins to tilt the chair back, they will discover there is more resistance to the tilting the farther back they go. In fact, the rocking chair is very difficult to tip over backwards. This is due to high secondary stability. Therefore, the effects of initial stability are felt at little or no tilt in the kayak while the effects of secondary stability are felt at greater angles of tilt. High initial stability has obvious advantages for beginner kayakers who intend to paddle in calm waters. Unfortunately, if the waters are not calm, that high initial stability will cause the kayak to become more sensitive to waves and choppy waters causing the kayak to become very difficult to handle.
The kayak on the left has low initial stability and is able to remain upright in waves. The kayak on the right has very high initial stability which causes it to remain level with the face of a wave. The kayak on the right would not be considered seaworthy. Another aspect of kayaking to consider in relation to initial stability is turning. When turning a sea kayak, advanced paddlers will tilt the kayak to the side to make turning easier. If the kayak has too much initial stability this will be more difficult to do and will result in a kayak that is difficult to maneuver. For short kayaks or whitewater kayaks this will not be an issue because they can easily be turned with a single stroke of the paddle.

Paddler Measurements:

1) Length of paddlers legs - used for determining position of front bulkhead 2) Foot size - used for determining deck height in front of paddler 3) Length of cockpit - must be long enough to allow clearance for paddlers legs 4,5) Fore and Aft depth of cockpit - must provide enough clearance for paddler to sit comfortably 6) Cockpit aft to paddlers center of gravity - positions paddlers center of gravity in line with kayak LCB

Hydrostatic calculations:

Length to beam ratio = LWL / BWL Wet surface area to wet volume ratio = WSA / Wet Volume Theoretical top speed = 1.34 * Sqrt(LWL) Midship = Start Of Waterline + LWL/2 Midship symbol - Indicates the longitudinal midpoint between start and end of waterline. Longitudinal center of buoyancy (LCB) - Longitudinal centroid of the displaced volume measured from the start of the waterline. %Longitudinal center of buoyancy (%LCB) - LCB with respect to the LWL Longitudinal center of flotation (LCF) - Longitudinal centroid of the waterplane measured from the start of the waterline. %Longitudinal center of flotation (%LCF) - LCF with respect to the LWL Hull form (Fish form and Swede form) - Determined by the %LCB %LCB > .5 = Swede form %LCB < .5 = Fish form When the LCB is ahead of the midship symbol, the kayak is Fish form. When it is behind the midship symbol, it is Swede form. Kayaks that are Swede form have better handling characteristics and displace water more efficiently than kayaks that are Fish form. (.49 to .55) is generally thought of as a good range to try to keep the %LCB within for kayaks.

Coefficients:

Prismatic coefficient (Cp) = Wet Volume / (LWL * Ax) Where: Wet Volume = Volume of submerged section of hull LWL = Length of water line Ax = Max wet cross section area Prismatic coefficient is a measure of the fullness or fineness of the hull. Higher Cp's mean the submerged ends of the hull are wide, while low Cp's mean the ends are narrow. The range of Cp's for kayaks is 0.45 to 0.65. Typically, a higher Cp within this range will result in less drag due to the more efficient way the kayak will displace water as it moves. Block coefficient(Cb) = Wet Volume / (BWL * LWL * Draft) Block coefficient is also equal to Cp * Cm. Midship coefficient (Cm) measures the fullness of the maximum submerged cross section of the kayak.
Waterplane coefficient (Cwp) is a measure of the fullness of the waterplane. A seaworthy kayak with good secondary stability will have a full waterplane.