

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  watertight 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
This is simply the distance from the rear of the
cockpit to the paddler's center of gravity. This picture illustrates how
to find a paddler's center of gravity.
You may want to add a couple inches to the CG to get the Cockpit
Aft to CG in order to make room for the back rest. Just be sure to
account for that extra spacing in the overall cockpit length as well.
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.

