Retro-fitted Hydrofoil with adjustable angle of attack

Hector Wong, Falmouth Marine School

Synopsis

This project takes an alternative approach to the designing and utilisation of hydrofoils in

the marine industry. Currently, the use of hydrofoils could be viewed as limited and not fully

exploited as it could be. The main focus has always been on medium to large sized vessels.

Unlike the current view, my approach will have a greater emphasis on using hydrofoils on

smaller vessels and even recreational crafts.

The project involves designing retro-fitted hydrofoils with an adjustable angle of attack and

a mounting mechanism for kayaks. The analysis of kayaks of different sizes, as a parametric

study. Calculating the lift coefficient for set parameters. The consideration of different types

of hydrofoils and their variance in arrangements. Optimizing the amount of surface area

required for different weight categories using the formula for lift. Analysis of 3D-Cad

models. Investigating the ergonomic aspects of designing a consumer product. Finding a

balance between making a viable product for consumer use and not compromising the main

function of the hydrofoils as an engineering solution.

Author’s biography

Currently studying FdSc Boat Design And Production – Falmouth Marine School

Introduction

The purpose of this paper is to investigate the incorporation of hydrofoils on kayaks. It

involved the analysis of different types of kayaks and their suitability for the application. The

study was undertaken because despite the presence of hydrofoils on kayaks, they are

typically suited to calm water environments (lakes/slow flowing rivers). One of the reasons

for the undertaking of the study originally developed in an earlier project, which involved

the incorporation of pedals and a gearing system on kayaks, which was undertaken in an

attempt to increase the kayak’s potential speed. The other motive for carrying out the

project was the desire to add another element to kayaking, maximising potential speed, and

minimizing the physical effort required, in addition to the current categories surfing, play,

and touring kayaks. It would allow people that are interested in kayaking but are not

physically fit enough to participate in the activity, as they would not need to exert as hard to

maintain a steady pace as they would on a normal kayak.

In essence, a hydrofoil lifts the vessel/kayak higher above the water surface, effectively

reducing the amount of resistance.

In the process of finding a method in which it would be possible to increase the potential

speed of a kayak, the idea of reducing the amount of resistance acting on the kayak came to

light. Principally, at a certain speed, the amount of lift that is generated by the hydrofoils

would be equal to the total weight of the kayak and user, therefore lifting the hull out of the

water. As opposite to increase the power (energy) to move faster, the use of hydrofoils

decreases the drag, which allows for a better utilisation of the power (energy) required for

the movement of the kayak. The addition of hydrofoils would provide a more stable ride in

rough water conditions.

When any vessel travels on the water, a considerable amount of energy is used to disperse

the water that is in front of the vessel (by pushing the hull through it). Therefore, by lifting

the hull/part of the hull out of the water, there is less resistance, since the amount of drag is

only that caused by the hydrofoils. (Ray, 2009) In comparison to the foils found on an

airplane, the hydrofoils are much smaller, because the density of water is about 1000 times

greater than that of air. “The density of water is nearly 1 g/cm3. Therefore, the specific

gravity of a substance is equal to the density of that substance, so long as that substance is

either solid or liquid.” (Dr Duray et S. Martel, 2006) The higher level of density also means

that the foils do not need to travel close to any speeds of the average airplane before

generating enough lift to raise the vessel out of the water (Ray, 2009).

Hydrofoils have been adopted for use on watercrafts for more than 100 years. They can be described as an experimental progress at the early stages of their initial designs. They are used on small to large engine driven vessels ranging from such as sailboats, powerboats, commercial passenger ships. In addition, they are also found on human powered vessels, which are typically used for recreational purposes, such as kayaks, canoes, water scooter, windsurfing boards. Hydrofoils have also been widely used on products that are strictly classed as sporting equipments essentially, such as water skis, air chair, and wakeboards. Fully submerged foils are not in contact with surface waves, hence potentially providing a more comfortable ride in rough conditions. However, as the design forces the hull to raise above the water, making it airborne, hence not being self-stabilizing. As a result, vessels need to have an independent control system which is capable of adjusting the angle of attack of the foil surfaces in order to keep the hull at a specified height above the water and a control course.

The widespread use of the technology was developed from the initial discoveries in the

application for hydrofoils, which at the time was purely to increase the speed of vessels,

without requiring an increase in the amount of installed power. Advancements have made it

possible to use hydrofoils to be used for recreational purposes, even developing into a

recognised sports in some countries, with sports such as Hydrofoiling, that involves

performing stylish mid air manoeuvres, using the lift generated from hydrofoils to propel

the person in the air.

The foundation of the project involved a comparison of the different ranges of applications

of hydrofoils on vessels at the moment, which highlighted the differences and similarities in

relation to the different uses respectively.

Secondly, a comprehensive parametric study was undertaken on a wide range of kayaks,

which range from river kayaks to all rounder kayaks. The data gathered included Size,

Length, Width, Volume, and Weight. All the different data types were used as input when

designing the hydrofoils themselves. Using the gathered data, the average characteristic of

the kayaks were calculated as shown in Table 1.

Type Size Name Manufacturer Length (cm)

Width (cm) Volume (ltr) Weight (kg)

Riverplay Small Ammo Pyranha 207 63.5 218 16

Riverplay Medium Ammo Pyranha 219 65 242 16.7

Riverplay Large Ammo Pyranha 226 68 288 17.5

Riverplay Small Z.One Pyranha 249 63.5 180 16.1

Riverplay Medium Z.One Pyranha 257 65 210 17.1

Riverplay Large Z.One Pyranha 265 68 262.8 17.9

Riverplay Medium Varun Pyranha 208 63 221 15.2

Riverplay Large Varun Pyranha 220 66.5 229 16.2

River Running Medium Karnali Pyranha 257 65 280 21

River Running Large Karnali Pyranha 260 66.5 303 22

River Running Small Burn Pyranha 238 64 238 17

River Running Medium Burn Pyranha 245 65 279 19

River Running large Burn Pyranha 253 69 304 20

River Running Small Fusion Pyranha 294 63.5 250 18.5

River Running Medium Fusion Pyranha 312 66 308 20.8

Intro - TG Master Pyranha 266 64 225 14.5

Club - Approach 9

Dagger 274 64 - 17

Club - Approach 10

Dagger 305 71 - 18

Touring Kayak - Solo RTM 330 70 - 20

(AVERAGE) 257 66 252 18

Table 1

The difference in the weights of kayaks in relation to their size length/size, was one of the

variations that had to be taken into account before completing the design specification.

After further analysis/research, it was clear that a range of different hydrofoils would be

required to suit the different needs of different users (in terms of weight limits).

Several different shaped kayaks where re-created as a 3D model in a CAD program, which

made it possible to view how the hydrofoil would possibly function and look aesthetically on

the drawing board, thus making it possible to analyse the ergonomics of the potential

designs. Another complexity involved the use of adjustable controls for the hydrofoils,

which allows users to adjust the angle of attack of the foil. In particular, the incorporation

of control mechanisms for this application is particularly challenging, firstly due to the

weight constrains that makes it impractical to use heavier components, even though they

could be better suited in terms of reliability. Another limitation is the actual amount of

space that is available to house the components, for efficiency and performance reasons,

any attachments/mechanisms must be as streamlined as possible. Most importantly, unlike

other solutions/modifications performed to a specific vessel, this application need to be

compatible with a range of different kayaks, each with ever so slightly different shapes,

characteristics, size and other factors, because it is being retro-fitted.

Based on the results, a decision was made on the type of hydrofoil configuration that was

used, the balance between performance and ergonomics was considered. There are two

main hydrofoil configurations, which are Surface-Piercing foils and Fully Submerged foils.

There are a range of different foil/strut arrangements that are suited to different types of

ships and their uses. The main factor that influences the choice of the hydrofoil

arrangement is the distribution of weight, for instance ships would be suited to a

conventional or canard, providing that 65% or more of its weight is being supported towards

the bow of the ship. However, it is not principally applicable for the kayak hydrofoils

because the weight is distributed fairly equally, and could vary depending on the sitting

posture of the user.

Because these product will be used by water sports enthusiasts in general, the assumption

was made that they might not have a great deal of mechanical knowledge/ hands on skill,

therefore it was crucial to keep the design as simple as possible, in order to minimize the

number of potential problems that could occur, without compromising the functionality of

the product in doing so.

In addition, the ease of manufacturing and repetition were considered to ensure that the

design would not be excessively complicated nor difficult to mass manufacture.

Then a laminate schedule was drawn up, for all 3 sized hydrofoils. This provided a more

accurate estimate for weight of the hydrofoils, in addition to the cost of production.

Table 2 shows the 3 different weight categories/restrictions, that the 3 different sized

hydrofoils have been designed to be suited to respectively. The 3 different sizes were

classed into 3 ranges, Small, Medium and Large. The calculations are based on the weight

categories that include the weight of the kayak, with an allocation of up to 20 KG for the

weight of the kayak. In summary, the foil area dictates the amount of lift that can be

generated, the heavier the weight, the greater force, thus requiring a greater foil area to

produce lift.

Table 2

Table 3 contains the area of foil area required was calculated using the equation for lift, see

equation (1). Table 4 contains the values and converted values that were used for the

calculation.

Weight (KG) Surface Area Required In Total (m2)

Surface Area For Each Foil (m2)

90 0.4685 0.2345

110 0.5727 0.2862

130 0.6768 0.3384

Table 3

Conversions

L= (90)( 110)(130)Kilogram Force (882.5985 )(1078.7315 )(1274.8645) Newton

CL= 1.54 1.54

P= 1027 Kilogram M3 1027 Kilogram M3

V= 3 Knots 1.543333332 Meter/second

Table 4

The hydrofoil layout consists of 2 hydrofoils, therefore the total amount of surface area

required would be allocated equally between 2 hydrofoils.

In the appendix, Fig 1 shows a basic CAD rendering of the submerged hydrofoils attached to

a kayak created from a sketch. It also shows the surface area of the foils for the different

ranges respectively. This first sketch made it easier to visualize the task ahead and aspects

Weight Categories (Actual) Weight Categories (Guideline)

Inc (Average Kayak Weight) User Weight

70-90 KG 50-70 KG

90-110 KG 70-90 KG

110-130 Kg 90-110 KG

that must be considered before making any major decisions in either the mechanical or

ergonomic design aspect.

Unlike a vessel that is propelled using water jet drive/propellers, additional consideration is

required, mainly because the foil arrangement must not be on either side of the kayak, as

that would interfere with the area that is required for paddling.

The hydrofoils and mounts were designed to fulfil certain functions as a product, the first

being its mechanical function, which is the basically ability to produce lift, being the main

and most important function. Its measured is easily quantitatively in terms of figures and

data, which can be calculated/estimated through calculations and the use of simulations.

Other considerations include the ergonomic aspect for the design, which is important as it

needs to be a user friendly product, however this can’t have precedence over the

mechanical function. This on the other hand, is harder to calculate/quantify as it involves

taking into account human physiology, biomechanics and anthropometry, which was

considered by basically putting yourself in the shoes of a potential user of the product.

Therefore, the underlying challenge involved finding a balanced design that did not

compromise one function for the other.

The final design for the control mechanism to adjust the angle of attack of the hydrofoil is

shown in Fig 2.

The hydrofoil wings are attached to the mounts by a round support that is pivoted in the

middle as shown in Fig 3. This allows them to move/rotate in order to change the angle of

attack. The circular support will have a sealed ball bearing to ensure that it rotates

smoothly. The ends of support will be sealed with a rubber gasket, keeping the water out of

the internal sections, which reduce the frequency requiring to constantly reapplying

lubrication. It is crucial to be able to alter and have a variable angle of attack, in order to

respond to changes in sea conditions, weight, and average speed.

It fundamentally uses an applied force to alter the angle. A stainless steel cable in a outer

cable housing is attached to the front of the hydrofoil wing is connected to the base of the

hydrofoil mount, which is along the mounting arm to the top of the kayak, and then fed into

a gear lever that allows the user to change the amount of cable available thus force the

hydrofoil have a steeper angle of attack. The stainless steel cable will be lubricated to

ensure the least amount of friction, to provide a smoother travel within the outer cable

housing, the lubricant will also aid in displacing water. Support selves fitted to the end of

the outer cable housing will ensure that water does The gear lever will be an index shifting

system, basically meaning that the gear lever’s control has discrete stops. With each of the

stops corresponding to one angle/position for the hydrofoil. This makes it possible for the

user to change the gear and be able to alter the angle of attack of the hydrofoil accurately

each time, as opposed to friction shifting mechanisms that would require the user to adjust

and make sure that the hydrofoil is in the desired angle. Thus being a major setback, as it

would be considerably difficult/impractical to needing to keep looking at the hydrofoil wings

each time the angle was altered, to ensure it had changed to the correct position.

The index shifting system would have 5 different discrete stops, allowing the user to have a

choice of 5 different angles of attack. With each change in shifting adding 5 degrees to the

angle of the hydrofoil continually. Therefore, having a range from 0 - 25 degrees, in a fixed

increment of 5 degrees per stop/gear.

At the rear of the hydrofoil wing, there is a coiled spring attached, this keeps the hydrofoil

steady when it is at 90 degrees (idle). Fig 4 shows the hydrofoil wing when the cable is

pulled by the gear lever. As the cable pulls the front of the hydrofoil wing, the spring

extends uniformly, therefore when the cable is released the spring would compress and

return to a lower angle of attack or its idle position if the cable is released fully.

Fig 5 shows the hydrofoil and the control mechanism, when the hydrofoils are flat. With the

coiled spring compressed. In this position/angle, it produces the least amount of lift.

Fig 6 shows the hydrofoils and mounts attached to a kayak

The final stage of design was to find an effective and simple method of locking/attaching the

hydrofoils securely to the kayak. For ergonomic purposes, the locking mechanism has to be

placed on the top of the kayak. Furthermore, it could potentially be vital as it would not be

possible to drag a kayak with the hydrofoils attached over solid ground. Therefore, some

users might choose to attach the hydrofoils after it has been placed in shallow water. This

also allows the user to detach the hydrofoils if one/both were to be damaged whilst

kayaking. The initial design for the control mechanism was located on the next to the

hydrofoil wing itself, hence being inaccessible once the kayak was submerged in water

(whilst the user is sat in the kayak). In order to FIX THIS disadvantage, the control

mechanism was then redesigned to be easily accessible to the user whilst in the kayak, this

makes it possible for the user to change the angle of attack according to their speed/desire.

The mounting mechanism for the hydrofoil wing will be similar to an adjustable ratchet

strap clamp with a heavy duty strap. Ideally it would constructed from a solid material

would be used as the mounting mechanism, however because of the emphasis on

retrofitting and the requirement to be compatible with a range of different kayaks each with

unique shapes. Furthermore, it would be impractical to produce multiple solid mounts that

would need to fit a vast number of kayaks. The utilisation of a strap also results in a lower

amount of resistance when travelling at low speeds because of its thickness.

The mount for the hydrofoil will be lined a thin layer of buoyancy foam, to allow the

hydrofoil to float in the water, in the event that the hydrofoil is not attached securely to the

kayak, allowing for easy retrieval. The strap will be rubberised to ensure adequate grip on

the surface of the kayak after being attached, it needs to prevent movement of the strap on

both longitudinal and latitudinal directions.

The material chosen for the construction of the hydrofoil wings and the mount were

composite material. The main reason being it would be easier to manufacture both as a

prototype and for mass production. The use of moulds makes it possible for easy repetition

for high volume production to a high level of quality. The use of materials such as

aluminium, were considered however the higher financial implications would not justify the

superior material characteristics, although it is always desirable, since there would be no

necessity to have a material that could be viewed as extravagant for the application. In the

event that the hydrofoil wings/mount are damaged lightly (chip, scrapes) on the surface, it

would be relatively straight forward and inexpensive to perform a basic repair either by

professionals or the users themselves.

The term composite materials are used to describe materials that are made up of 2 or Fibre-reinforced plastic (FRP) and bonded by resin. In general, laminates are made of various layers of lamina. Although the entire production process seems relatively straightforward, it plays a huge role in influencing the final weight of the vessel, its strength and most importantly the final cost.

Composites materials are used extensively across many industries mainly because of its ability to have specific properties simply by incorporating special fillers. Furthermore, for marine applications the flexibility of being resistant to corrosion, lightweight, fantastic impact habits, and many other advantages over constituent materials.

Therefore Classification Societies have deduced a specific guideline on how particular

combinations of fibres, cores and resins should be arranged in both monolithic and

sandwich construction. Engineers often utilise the building rules provided to ensure that the

laminate schedule would be ideal even during the design phase of the project.

The laminate schedule is essentially the exact instructions to building the FRP vessel, it

explains the types of fibre and fibre weight fraction that should be used for each layer, the

order in which it should be laid. Moreover, the laminate schedule is usually used as a key

document in determining whether a structural failure was caused either by failing to follow

the engineering design or by poor built practices.

Polyester is used heavily in the marine industry. Specifically, Isophthalic polyester resin is

becoming the favoured resin in the marine industry, because of it exceptional water

resistance, due to its chemical structure.

Table 5 shows the laminate schedule for the hydrofoil wings and mount, and the figures that

have been estimated

Layer Fibre Type

Fibre Orientation

Wf Layer Weight g/m2

Layer Weight (g)

Layer Thickness (mm)

Gel Coat - - - 250 173.125 0.217

Back-Up 300 CSM - 2.5:1 1050 727.125 0.784

1 450 CSM - 2:1 1350 934.875 0.978

2.1 300 WR 0/90 1:1 600 415.5 0.386

2.2 300 CSM - 2:1 900 623.25 0.632

2.3 300 WR 0/90 1:1 600 415.5 0.386

3 450 CSM - 2:1 1350 934.875 0.978

4 300 CSM - 2.5:1 1050 727.125 0.784

Flowcoat - - - 250 173.125 0.217

Totals 7400 5124.5 5.362

Table 5

The total weight for a hydrofoil wing and its mounts are estimated at 5.12 KG, (5124.5 g).

Which is an expected figure, as the structure would be required to support the loads of 70

KG in total at the minimum. It would be possible to create an additional product range in

which more advanced materials are incorporated hence lowering the amount of weight,

thus increasing performance.

The materials used include both chopped-strand mat and woven rovings of E-glass, and

polyester resin. The costs of these materials are relatively cheap compared to the more

advanced composite fabrics and resins that are readily available. The use of carbon fibre is

highly desirable for its good weight to strength ratio, good resistance to corrosion, fatigue

and other elements, above all it is known for its aesthetic surface, which more suited to

applications that are not mainly dictated by cost.

The fibres have been stacked in the specific order to ensure the maximum adhesion and

bonding, chopped-strand mat for Layer 2.2 because it prevents the woven roving from being

directly on each other, as this would cause it to slide because of its fibre type (texture).The

process is known as Lamination Notation.

Equations

Lift

Coefficient Of Lift

Concluding sections

In the future, if the project were to be repeated. It would be ideal to use specialized

software that is capable of the analysis of 3-Dimensional Wing Design. In addition, it would

also be able to accurately calculate values for the drag, moment coefficients, longitudinal

stability and lift for hydrofoil sections. Functions such as screen for cavitation, ability to

analyse cross sectional shapes would be useful in order to further improve the efficiency of

the hydrofoil. The software can also produce outputs in the form of interactive graphs,

tables and other formats, which would make it easier to comprehend the correlation of

different values. Further in depth analysis is also possible with features such as vortex ring

layouts and other aspects if required.

It essentially makes it easier to make adjustments to the values for a hydrofoil and be able

to see how it changes visually, as opposed to having to perform multiple calculations before

being able to produce a 3D model. This also makes it much easier to perform accurate

estimations for potential alterations/improvements, by providing quantitative data which

would support

Another useful process would be to build a full sized prototype of the hydrofoils and the

mounts, as opposed to just carrying out theoretical estimations and calculations. The

process of perfecting the ergonomics would be much more realistic in comparison to using a

CAD model of a mannequin. It would also be possible to measure the performance in terms

of the lift generated by the hydrofoils against the values that were estimated.

The construction of working scale models of the hydrofoils would most definitely be

advantageous in exploring the different ways in which mechanical solutions could be

applied/adapted to the hydrofoils. Seeing that, scaled models are easier to fabricate and the

actual time/effort required to perform alteration/modifications would be considerably less

compared to full sized models and possibly even for models in CAD programs in certain

situations. Thus being able to explore and analyse more potential solutions in a shorter

period of time.

Another element that would be considered/analysed in greater detail is the method of

mounting the hydrofoils. Although the current solution does fulfil its functions, it would be

further improved. It would be interesting to dedicate more resources and time to

potentially create a solution/mechanism that is build of solid material yet still offers the

amount of flexibility that the straps do.

The build quality and strength would be another element to be considered, the best

approach for this, would involve constructing test pieces of composites using the laminate

schedules that was been calculated. The use of other mixtures of fibre types and resins

would provide a realistic degree of improvement that would be possible if building the

hydrofoil from the respective combination. Destructive testing would then be carried out

repeatedly on several test pieces to get a range of results. Destructive testing would then be

carried out repeatedly on several test pieces to get a range of results.

In addition to attempting to improve the efficiency and strength of the existing design,

developing on the basic idea and make the product even more advanced, such as having a

dual use for the hydrofoils when they are not required.

Getting a wider range of opinions and inputs from more people that are interested/work in

the watersports industry would be ideal. Although there have been many in depth

conversations and discussions with a small group of people, it still seems to lack any

suggestions/thoughts that targets the issues from another perspective of thinking other

than an engineering approach.

As a future advancement of this project, it would be ideal to investigate the incorporating

the use of pedal powered kayaks and hydrofoils in the search for increasing the speeds of

human powered kayaks.

Acknowledgements

It is probably one of the most clichés when compiling a project report to express thanks to

your supervisor. I truly have to express my utmost gratitude towards Alex Whatley. I would

like to say thanks for his supervision and patience throughout this project, but also for his

support in my previous projects.

Special acknowledgement to Mash Derick whose lecturing in Computer Aided Design greatly

assisted me in the designing process of the project.

I would also like to thank my fellow colleagues for their invaluable inputs throughout the

entire project.

Appendices

Fig 1 The surface area required for the 3 different sized hydrofoils

Fig 2 The rotation movement of the hydrofoil wings

Fig 3 The circular support that acts as a pivot allowing hydrofoil wings to move

Small (0.2345m2)

Medium (0.2862m2)

Large (0.3384m2)

Small (0.2345m2)

Medium (0.2862m2)

Large (0.3384m2)

Fig 4 The hydrofoil wing with an increased angle of attack when moved

Fig 5 The hydrofoil wing at idle position

Fig 6 The hydrofoils and mount attached to a kayak

References

Ray, V (2009). Hydrofoils: Design, Build, Fly . United Kingdom: Peacock Hill Publishing. P15-

33.

Stephen M. Duray Ph.D.1,Stacie S. Martel M.S., D.C.2. (2006). Age-Related Changes Of

Arachnoid Foveae. A Quantitative Method for Estimation of Volume Changes in Arachnoid

Foveaewith Age. 51 (2)