Fine Wood Water Craft Designer, Boat Builder Canoes and Kayaks

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Elements of Hull Design

About Eskimo Tom Artist, Designer, Boat Builder Canoes and Kayaks
About Eskimo Tom: Artist, Designer, Boat Builder Canoes and Kayaks

Design Elements

The objective here was to create a basic list of essential hull design elements for the new comer to contemplate when defining and requesting a custom canoe, kayak or other small watercraft from Fine Wood Water Craft.

Many folks do not have access to many resources for basic hull design concepts. The list is a long one, and it is not arranged in any particular order.

When I first began this journey, I had little knowledge, but I wanted to learn more. My aim was to learn enough to make a good buying decision when it came to buying a plan or kit. I say from experience that all the facts and figures in the world won't help you make the best decision. It will help steer you away from what you don't want.

Please keep in mind the following topics are discussed from the perspective of small, narrow, light weight human powered watercraft. I am not a trained navel architect and would lose any subjective argument on a boat design forum. Keep this point of view in mind as you read these Elements of Hull Design.

As for the endless debates over concepts of performance, stability and seaworthiness, it is my position that these are all relative and are defined by the unique perspective of the individual user based on their personal level of experience.


Size Does Matter

It does not matter if a boat advertises that it has the longest glide ratio, floats on air and doesn't require any paddler input to steer, if you can't move it from one place to another because it is too heavy or it will not fit in your storage space than the canoe or kayak is isn't ideal for you. If it takes two people to move, you can only go paddling when you have two people available. Keep that in mind as you move forward.


Displacement

In any conversation with prospective clients, one of my primary concerns is how many pounds will the boat be expected to carry. Your weight plus the maximum weight of your gear plus the weight of the finished boat should be at least equal to or less than the intended design displacement of the boat you choose.

Displacement is simply the amount of water expressed in pounds that will be displaced by the boat when loaded.

It’s a simple equation. At minimum, your boat must displace enough water to support you, your gear and its own weight in order to float.


Design Displacement

This is an expression of the optimum weight displaced intended by the designer for the boat to perform as designed. Obviously the boat will carry more or less weight than the design displacement however the boat will not perform as intended.

An excellent example is our Wenonah Minnesota II Kevlar canoe. It is an excellent BWCA troop ship and will carry a vast load between portages regardless of conditions. For its length, it is a breeze to carry on a portage.

However with only two light weight paddlers and no load, it is an unpleasant craft for casual excursions on windy days. Lightly loaded it sits well above its design waterline and presents too much freeboard for enjoyable paddling on windy days.

Can you paddle it on windy days lightly loaded? Yes you can and it can be paddled solo on windy days but based on experience, I wouldn’t recommend it to a friend.

Start with yourself. Are you big, small, short, or tall? Are you looking for something that will hold a lot of gear for a big trip or do you want a boat you can easily load and unload at the waterside for quick excursions after work and on the weekends?

Ideally the design elements you consider will deliver the most efficient hull for the kind of paddling most of the time.

An efficient boat will minimize the amount of friction created by the wetted surface area and wave making as it displaces water at the bow and replaces it at the stern.

On occassion you will find a figure called "pounds per inch of immersion." This number is useful in comparing designs. Lower numbers are typically associated with smaller narrow hulls. My favorite.

It is also a useful number in situations where you know you are possibly loading a boat beyond it’s recommend design displacement as it gives a guide to how much freeboard you will have remaining after loading.


Pounds Per Inch of Immersion

(PPI): describes the amount of weight required to sink the hull one inch over and above the designed displacement. It is useful to know when loading additional gear reminding the paddler of how much loading can take place before the hull's performance is noticeably degraded or downright dangerous.


Length

Length is measured two ways; Length at the Water Line (LWL) and Length Overall (LOA). Waterline length is where the rubber meets the road so to speak. All things being equal, the longer the hull on the waterline the higher the hull speed, to a point.

Added length can have a stabilizing effect and increases volume with lower penalties on wetted surface area relative to adding width. Boats with long waterlines may track (hold its forward course) better than a short boat.

Length above the water matters little for flat-out speed. In the real world it must be assumed that the waterline will change with the load and wave conditions. What was once above the water will soon enough be submerged in wavy conditions.

On hulls with over hang (longer above the waterline that at the waterline) the waterline length increases as the hull sinks into the water with the added weight of gear and large people. This is not so on boats with plumb (vertical) bow and stern profiles.

Much of a hulls drag comes in the form of skin or surface drag as the water moves across the wetted surface of the hull. Taken to extremes, extra length becomes a liability, not an advantage as the surface drag on a longer boat becomes excessive.

The combined displacement of the paddler, gear and boat along with the intended or desired speed is useful to help determine a desirable waterline beam and length.

A difference of six inches can make a difference both on the water and in the garage if storage is tight. A short boat requires less material and weighs less making the boat easier for the smaller paddler to carry.

Boats that are difficult for one person to load and unload off the car will be used less often than one that requires two people to move. Where storage space is at a premium the short boat will have the advantage.

They are also more useful in narrow creeks and rivers as they are easier to turn. It is possible to design a short boat that tracks well.

Prismatic Coefficient

The prismatic coefficient is the ratio of the underwater volume of a hull to a rectangular block of the same length and cross sectional area of the hull.

In simple terms it is an expression of how fine or pointy the ends are and useful for comparing one design to another.

A lower figure means a full midsection and fine ends, higher figures mean fuller ends. The lower the Cf, the finer the hull at the waterline. The coefficient given is at the intended design waterline and typically changes at different displacements.

If the ends are too fine and they don’t flare out above the waterline, the ends of the hull will not lift much in an oncoming or following wave. The front of the hull will plunge into the trough of a wave after surfing down the side.

This is less of an issue in a closed boat like a kayak. The deck will shed the wave in a kayak however in an open boat like a canoe this causes serious problems as the hull fills up in heavy waves.

Bottom Profile (Viewed from the front)

The canoe's front view and see the cross section it presents to the water. A flat bottom is an indicator of great initial stability, meaning you could easily stand in the canoe to fish or whatever.

This sounds like a wonderful safety feature, but it does have numerous drawbacks. For instance, a flat bottom tends to follow the water surface and will roll with every wave that passes. Couple this with very poor final stability, little freeboard and you have a boat that may flip over in rough water.

A flat bottom doesn't present much lateral resistance and can skid sideways in a stiff wind.

A canoe with a rounded arch bottom should have better maneuverability characteristics. Yes, it will have less initial stability and will feel "tippy," but if the sides are flared it will have something to rest on when it is leaned.

In other words, it will have great final stability, a key element in freestyle canoeing. The rounded arch hull also sits deeper in the water when not leaned, giving it good lateral resistance and acting like a keel to help maintain course.


Rocker

Look at a hull from the side and observe its profile or shape lengthwise along the bottom or keel line. A boat whose keel line curves or sweeps up at the bow and stem is said to have rocker. The amount of curvature is measured from a level baseline running the length of the hull. One can have more rocker at the bow or stern.

In general, a hull with a lot of rocker makes it easier to pivot or turn at its midpoint. Significant rocker forces the center of the boat to bear more of the weight. As most of the weight is centrally located, the ends of the boat will have little bearing in the water.

With little lateral area at the ends of the hull it is easy to turn, especially on top of a wave. This is very desirable in a whitewater boat where quick maneuverability to avoid rocks is necessary.

Most whitewater canoes have a banana profile for this reason. In these boats, high rocker is combined with blunt or full ends to help the boat rise up on waves.

The Prospector is a good example of compromising rocker and volume in situations carrying heavy loads in varied whitewater and flat water conditions. Designs like the Prospector develop significantly more rocker when leaned as the bow and stern pops out of the water making them more maneuverable than their length alone would indicate.

BWCA tripping canoes are expected to primarily glide straight for a long time and will have minimal rocker to improve straight line tracking. Think of an arrow and the quills at the ends. Significant rocker located primarily in the bow of these boats significantly reduces unnecessary wetted surface area and thus resistance.


Beam

Beam is simply a measurement of the hull’s width and is measured several ways. Beam at the waterline is the important measurement. Beam is also measured over all and in canoes it may also be measured at the gunwale along the top edge.

When the beam at the gunwale is less than beam overall it indicates the boat has tumblehome where the sides “tumblehome” which help make paddling easier especially on wide hulls as the paddler has less width to reach the water.

The width or beam of the hull influences stability, volume and wetted surface. Generally, a wider hull is more stable especially in calm water. Jump into a 21" sea kayak, then into a 36” beam canoe and compare.

Inexperienced paddlers feel more secure in a wide stable hull so this is a consideration if you are new to paddling. Too much width has penalties however.

While the wide hull with a flat bottom will feel more stable in calm conditions, as waves grow in size, the wide flat hull will follow the wave profile casing the hull to lean dangerously when it hits the boat side ways and the person and cargo tips away from the wave.

Kayaks are generally faster than canoes of a given length because they are typically narrower. There is less resistance pushing water aside and less surface area. Canoes are wider than kayaks for a good reason.

Paddlers in canoes sit on seats four to eight inches above the bottom significantly raising their center of gravity. To counter this increase in the center of gravity, the beam needs to be sufficiently wide enough to maintain a comfortable level of stability.

Kayakers sit just above the bottom of their craft and may actually sit below the waterline. Since the center of gravity is so much lower, the hull does not have to be as wide to experience a comfortable level of stability.

The added width means more surface area, increased drag and has a negative impact on speed.

Width carried from the center out to the ends of a hull can improve stability. A hull that is very narrow at both ends can be rater unstable when both ends are riding wave crests and the center of the hull hovers over a trough. There is little keeping it upright.


Depth

The primary measures of depth are at the bow, center and stern. Increased depth especially at the center translates to increased volume and capacity given the same length and width.

Increased depth helps to minimize taking on water from the effects of waves and spray. Freeboard is the vertical distance of the sides from the waterline to the sheer or gunwale.

A general rule is to have a minimum of 6 inches of freeboard. More freeboard than necessary will be more susceptible to wind resistance, but to little will be more susceptible to waves.

Wind from the sides or quarters can affect steering and promote rolling and tipping. A sailing canoe needs additional freeboard when heeled over by wind on the sail and mast. Depth is a critical dimension in a kayak or canoe as you need to be able to get in and out of the boat safely and ideally your feet will be positioned comfortably for the duration of your paddling.

However in situations where the hull is leaned over significantly, tumblehome taken to an extreme will reduce the final stability as there is less volume available to resist or push back against the water.


Waterline Area

The area enclosed by the LWL, typically expressed in square feet. Water plane area is the horizontal patch of surface that is formed by the intersection of the hull at the designed waterline with the surface of the water.

The Water plane has an affect on wave resistance, stability and pitching in waves.


Center of Bouyancy

(CB) is the center of buoyancy as an imaginary focus of all vertical forces that keep the boat afloat.

Often called Lateral Center of Buoyancy (LCB); it is the center of the underwater volume of the hull and can be expressed as a distance abaft the forward end of the LWL, or aft amidships, or as a percentage of the LWL from the bow end.

If the boat is to float on her designed waterline, the center of gravity (CG) must be in line vertically with the CB, both fore and aft and athwartship. If the two centers are not in line the boat will change trim, and so change her underwater shape, until the new CB lines up with the CG.

For example; if your boat is floating perfectly in trim and you add 50 pounds of gear aft you will move the center of gravity of the boat aft.

The hull will sink by the stern and the bow will come up until the underwater shape changes enough to move the CB over the new CG. The same applies athwartship. With luck, the CB and the CG are both on the centerline of your boat so she floats level, without any heel angle.

When you move to the starboard rail you move the CG off centerline to starboard, so the hull will heel until the change in underwater shape moves the CB vertically above the new CG.


Longitudinal Center of Balance

LCB is the distance from the start of the waterline to the center of the volume the hull displaces. The center of the displaced volume is called a 'centroid' but LCB is just the x direction component of its coordinates.

The others are VCB - vertical center of buoyancy and TCB - transverse center of buoyancy.

The center of buoyancy as an imaginary focus of all vertical forces that keep the hull afloat. Its counterpart, the LCG, is the focus of all loads that push the boat down. Under normal conditions and in the absence of other forces LCB and LCG are in balance or equilibrium.

%LCB is what often distinguishes whether a hull is 'Fish' or 'Swede' form. Fish form hulls have LCB less than 50% of LWL while Swede hull forms are more than 50%. Swede hull forms displace water more efficiently, reducing the effect of wave resistance and are therefore faster, especially at higher cruising and racing speeds.

Smaller pitching motion in waves, good handling in following seas (waves coming from the back) and drier ride are few other benefits of Swede hull forms.


Center of Flotation

(CF) is the center of the waterline area and is the pivot point about which the boat changes trim, much like the pivot in the center of a teeter totter. On normal sailing hulls the CF is somewhat abaft the CB and, like the CB, is expressed as a percentage of the LWL or a distance from either the bow end of the LWL or from amidships.

Of course, as the boat changes trim, due to added weights at one end or the other, the LWL shape changes, so the CF will move slightly.


Vertical Center of Buoyancy

Vertical center of buoyancy is the vertical coordinate of the displaced volume centroid. Hard chine and flat bottom boats have lower VCB than rounder or elliptical hulls. Higher VCB has positive effects on secondary stability.


Stability

Unlike large vessels, canoe and kayak stability can be more a matter of perception than scientific evaluation. The definition can be rather subjective depending on the experience of the person paddling. What feels stable to the experienced paddler will likely feel “tippy” or unstable to the new paddler.

In its simplest terms stability is the boat’s tendency to stay level with the surface of the water. In an aid to the public, the press and designers breakdown stability into components described as primary or initial stability, secondary stability or final stability.

Stability is strongly influenced by the hull’s side profile. Any flare added above the waterline will increase secondary stability. It is possible to have low primary stability and high secondary stability and vice versa.


Secondary stability

Boats tendency to “push back” when leaned over, the resistance to tipping over. As the hull is leaned, it presents a different waterline profile and the area below the waterline likewise changes.

Designs with high secondary stability often end up having more hull area in the water when leaned countering the leaning effect. The differing shape can also help “carve a turn” when leaned over sideways.

Low primary stability makes a boat easier to paddle in rough conditions such as high waves. A wide stable hull will want to stay flat relative to the surface of the water, a dangerous situation when sideways to a cresting wave causing the boat to lean forcing the center of gravity over causing a possible dunking.

It requires more force to keep a wide stable boat upright in this situation. A less stable round bottom hull with less initial stability actually is more stable in this situation as it’s easier for the paddler to correct and keeping the boat upright in an on coming wave from the side.

Round bottom boats tend to provide the least wetted surface to the water keeping friction down between the hull’s surface and the water. Simply stated, final stability is the point at which the boat tips over.


Keels

Historically on canoes a keel was a slender, strong lengthwise piece of wood located at the centerline along bottom with a rabbet or groove cut lengthwise on both sides to provide a sturdy foundation to nail planks.

It serves to reinforce the bottom to support loads. Keels typically protruded below the boat, in some cases being deep and narrow in other cases being wide and shallow.

They promote straight line tracking by guiding water flow and add lateral area underwater to help resist sideways drifting in a wind. They also took abuse providing a point of contact for normal wear and tear.

Keels are useful in sailing canoes to help reduce leeway or drifting sideways while under sail. Composite plywood and epoxy construction make them less necessary for strength.

To aid straight line tracking a shallow vee at the base of the hull will mimic the tracking effect of a keel. Too much Vee increases wetted surface area however adding drag. A protruding keel does contribute to increasing surface area resistance.

A keel is not suited for river travel as it typically will be the keel that gets caught on rocks. It slows down or stops the bottom of the boat while the water pressure keeps pushing the upper portion promoting a capsize in moving water. It is also annoying in lakes with lots of obstructions near the surface


Hull Shape

When you paddle you push water around the bow or front of the boat, under it and out the back or stern. Consider an airplane, car or truck as they move through the air. An aerodynamic shape creates less turbulence and friction requiring less energy to move forward. Each is designed for a specific purpose. The same is true for boats.

The shape of the boat as it displaces water creates drag. Where the boat meets the water is called wetted surface area. More wetted surface area means more friction and more paddling effort. So the primary concern is the shape of the boat underwater looking at the boat from the bow, side and stern.

A round bottom displaces the least water, is fastest, however it’s the least stable. Multi-chines are somewhat more efficient. If all prominent dimensions and ratios are held constant, the multi-chine boat will have about 3.17% less wetted surface and be about the same percentage faster at cruising speeds than a comparable single chine hull. They will have the same top speed, however.

A flat bottom displaces the most water, has a greater load capacity. It will tend to be slower and offer the greatest stability in calm water. At one extreme, think of a rowing shell. Without the paddler working the oars it can hardly remain upright.

It carries a light load and is extremely fast. At the other extreme is a barge. Its flat profile will carry an extremely large load but is slow. The semi-elliptical or shallow arch shape offers a compromise.


Side Profile

Any profile which flares out over the water can provide better secondary and final stability to a point. There is more boat available to resist the tipping momentum. It also helps deflect water from wave action.

Too much flare makes the boat wider and likely will interfere with paddling when taken to an extreme. The side profile of the hull can generally be broken down into slab or flat sided, flared sides and tumblehome. Tumblehome can be considered the opposite of flare in that the sides at the gunwale curves back toward the center of the boat, it “tumbles home.” Tumble home allows the maximum beam to be placed at the waterline allowing higher initial stability.

It is possible to have flare just above the waterline and tumblehome at the upper most edges. It is a desirable feature on wide boats to reduce width at the gunwale making it easier for the paddler to get the paddle in the water without sacrificing volume. The curve of the tumble home has a structural benefit in that it helps stiffens the hull.

One thing that makes leaned turns effective is it typically shortens the waterline length and putting the hull on it’s "rockered" side.

This is why wide boats tend to respond better to leaned turns because they effectively have more rocker when they are leaned. Wide full hulls tend to be harder to lean. Width increases buoyancy and adds resistance to leaning which equates to stability for some folks.

It’s wise to choose a boat as narrow as you are comfortable with.


Asymmetrical vs. Symmetrical Hulls

Historically canoes have been symmetrical, that is the front half of the boat is identical to the back half of the boat. This is beneficial in situations where a tandem boat is used solo. A solo paddler can sit “backwards” in the front seat and have reasonable control of the canoe when paddling alone.

A fine entry line which widens slowly, cutting water quickly and moving it aside slowly, gives water more time to get out of the way and replacing it at the stern. Such a hull profile is easier to paddle on calm, flat water. Pushing water. Bow, stern, entry line and exit line of the water. dividing the water at the bow. A sharp pointy bow parts water easier than a blunt fat bow.

Most kayaks and many newer canoe hull designs are asymmetric. The front or bow half of the boat is narrower than with the widest section aft of center. This “longer entrance” parts the water slower than a fuller bow section.

At the stern the asymmetrical boat is made “fuller” in an effort to keep the stern from being pulled down when paddled hard.

Too fine a bow will not support much in the bow area. In heavy waves a fuller bow is desirable to keep the bow from plowing into or under waves especially in some kayaks surfing waves or following seas, too fine a bow will plow underwater.

An arrow-like shape promotes speed but may inhibit maneuverability especially under sail. This may be tempered by the designer with some added rocker at bow and stern.

Thin arow like bow shapemay plunge or provide little lifting ability in large waves. A raised bow may help the boat lift over waves, but only if the bow has enough buoyancy to provide the lift. Adding flare above the waterline adds fullness providing additional capacity and reserve buoyancy.

Putting this buoyancy or fullness at the waterline would make the craft stable but slower as the paddler and hull plows through the water.


Bow and Stern Profiles

When the hull’s length at or below the water line is close to the length overall, that is near vertical, the bow is called plumb. A plumb bow and stern provides the maximum length at the waterline.

Many wilderness tripping canoes have a plumb bow and stern to achieve the longest waterline possible to attain the highest hull speed and capacity for a given length.

The boats typically have a long flat run from bow to stern with a slight uplift at either end to aid in turning.

Historically the old traditional designs which were based in part on native designs featured big re-curved profiles at bow and stern.

High recurved bow and stern profiles are partially a requirement in traditional birch bark construction.

High recurved ends in modern designs simply allow the ends to be pushed around more by the wind and add weight.


Weight and Strength

The heavier boat will be more stable for several reasons. It will sit lower in the water which can give it a wider waterline depending on design. Having a wider water plane has a positive contribution to initial stability.

Sitting lower in the water lowers the center of gravity again creating a sense of more stability.

Ideally build the lightest boat conditions permit given the situations you regularly encounter while paddling. If nothing more, a boat that is easy to carry from the car to the water will get used more often than a heavy one that’s difficult to move. Loading and unloading the boat to and from the water is typically the extent of my portaging today. I have no qualms about renting an ABS boat when whitewater paddling.

Would you like it lighter? Reducing the wood thickness works. For every 1/16" of reduced wood thickness you save 0.145 pounds per sq. ft. For every ounce of glass reduced you save 0.026 pounds per sq. ft.)

If you intend to use your boat for serious tripping, I suggest you do not reduce the thickness of the wood or the glass unless you are a careful paddler.

There are limitations, making the boat too light sacrifices strength. The quickest way to reduce weight is to reduce materials and the quickest way to do this is less length, width and depth. A shorter, slimmer boat uses less materials and weighs less.

Next comes the main structural items. In canoes this would include gun wales, decks, bulkheads, seats. Use a foam pad for a seat instead of a cane or molded seat. Styrofoam or other closed cell foam for bulkheads are light weight and provide additional flotation.

Consider thinner wood and four ounce glass instead of six ounce glass. Special lay ups varying the size and weight of glass can structurally reinforce key areas.

Just because you need more abrasion along the bottom requiring heavier glass doesn't mean the whole boat has to use the heavy lay up. Heavier glass holds more epoxy.

More or less coats fill coats of epoxy affect weight and strength. A sufficient level of protection of the wood will greatly enhance the durability of your hull, this is important inside the hull especially where you sit or stow gear can deliver as much abuse on the floor of a canoe as the outside.

Mud and gravel on shoes, collecting firewood, tackle boxes, shuffling camping gear will readily abrade a finish.


Aesthetics, Harmony and Balance

While some commercial designs are great, the advent of mass production efficiency in the factory takes precedence. Some design compromises are made to facilitate the limits of the material and the production method used.

This is evident in the blunt shapes found on the ends of many plastic boats. Sharp or fine ends are complicated to produce and release from molds.

Production efficiency and a broad appeal to cater to a market large enough to recoup development costs. Tooling and marketing expense are the design aesthetics you purchase. Plastics however contribute unique sheer strengths and strength reinforcement.

Adding wood trim to a plastic boat may make the boat appealing to some, but in my opinion, it just seems even more unnatural.

A combination of forms in infinite variations no matter how functional must still be visually pleasing. A wood hull is aesthetically beautiful and when functioning properly offers harmony, balance, grace and a beauty all its own.


Lining Off

The lining-off of planking is a design consideration and is not difficult if care is taken. The lines are made attractive to the eye with careful judgment providing a fair and tapered run of planking the length of the hull.

The rule of thumb principles are to use wide planks on the garboards and at the sheer and to a lesser extent the sides, but to have narrow strakes to go around a firm bilge.

Multi-Chine Hull Hard-Chine Hull

A skilled designer can design excellent craft in either category.

In general, if the two designs are similar in other respects, multi-chine boats, with more efficient hulls will be somewhat quicker. Folks looking for that last amount of speed choose multi-chine hull.

Paddlers who like quickly carving turns, consider a hard-chine shape.

For a given length and beam, a hard chine hull will offer more volume.