13 CHAPTER 4 Design and material selection

Design and Fabrication of a Hovercraft Chapter 4: Design and Material Selection

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Tunku Abdul Rahman College

CHAPTER 4

DESIGN AND MATERIAL SELECTION

4.1 Introduction

In this chapter, the author will discuss the design and criteria of selecting the material of

hovercraft. First of all, it is important to understand the principle of hovercraft and

know how it’s work. Below are simple’s principle using in hovercraft:

There are two main principles:

1) Lift

2) Propulsion

A skirt is required to make the hovercraft float.

The shape body of hovercraft will affect their stability.

No contact with ground hence friction is eliminated.

Main parts of hovercraft:

1) Lifting fan (leaf blower)

2) Thrust propeller

3) Skirt


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Figure 4.1: Principle of hovercraft

Theoretically, hovercraft is a simple machines but a plethora of problems exist to make

a functioning hovercraft. Firstly, the plans as well as the design must be flawless. On

top of that, to build a hovercraft one must be well aware of the demands of construction.

Only then can one design a hovercraft. (CJ Samson 2010).

4.2 Design

Figure 4.2: Final design of hovercraft

Figure 4.2 above shows the final assembly of hovercraft. The hovercraft was designed

by separating the craft into four separating sections, namely Thrust System, Lift System,

Skirt and Control System.


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4.2.1 Thrust System Design

Figure 4.3: Thrust system assembly

Thrust system is a system responsible for lateral propulsion of the hovercraft. In order to

keep the gravity as low as possible, the engine was mounted directly onto the plywood.

To maximize the overall stability of the hovercraft, the center of gravity was largely

being taken into account. In order to drive the propeller, the drive shaft of the engine

will connect to a pulley which will be attached to the shaft of propeller via V belt.

Hence, the shaft is attached directly to the propeller using a key and bolt connection.

Besides, two bearings are used to withstand the shaft and also the radial and thrust loads

imposed by the propeller. To withstand the forces created by the propeller dynamics,

angular bearing was chosen to be appropriate to handle the thrust load. The final design

for the thrust system can be seen in Figure 4.2. For the control system which is not

inside the figure, author will discuss later for more details.


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4.2.1.1 Engine’s Frame

Figure 4.4: Engine’s frame assembly

Figure 4.4 above shows the design of engine’s frame. In this design, teak wood was

used as a main material to build the frame because it’s must stronger than others wood

such as maple wood and oak wood. However, one of the disadvantage is teak wood is

heavy as compared to others wood. Two pieces of plywood were used to build the top

and bottom. 22 L brackets were used in this design to fix the engine’s frame.


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4.2.1.2 Damper

Figure 4.5: Damper

In this hovercraft design, four (4) dampers were used to reduce the vibration from

engine to hovercraft.

The parts list as listed in Table 4.1.

Table 4.1: Main parts list for thrust system

Item Description Quantity

Engine 5.0hp Robin EY20-3 horizontal shaft engine 1

Propeller 22" diameter fiberglass propeller 1

Dampers Rubber dampers 4

V belt length of belt 2

propeller shaft shaft 3/4'' custom shaft 25'' length 1

Angular Bearing 3/4'' bearing 2

Teak wood build the frame 3

L bracket fix the frame 22


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4.2.2 Lift System Design

Figure 4.6: Lift system assembly

Lift system is a very important aspect of the hovercraft since it is the power generation

to raise the hovercraft on its bed of air. This (combined with skirt design) will give the

hovercraft its ability to float and traverse the various specified terrain. The main

function of lift system is to providing the necessary pressure to underneath of hovercraft

so that the hovercraft can lift and weight off of the ground. In lift system, the pressure is

created by using a leaf blower. The function of leaf blower is to forces the air into the

bag skirt through an L bow pipe, and then the air will inflates the skirt and creates

pressure under the hovercraft. The overall final design of the lift system is shown in

Figure 4.6 and a list of the main parts can be found in Table 4.2.

Table 4.2: Main parts list for lift system


Leaf blower Stihl SR420 1

Pipe elbow pipe / straight pipe 1

Leaf blower


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4.2.3 Skirt Design

Figure 4.7: Bag skirt assembly

Skirt is most important aspect of design to a hovercraft. This design is indispensable as

it separates the lift air flow and creates high pressured area beneath the craft which lifts

the hovercraft. Normally skirt are divided by three types, which is wall skirt, bag skirt

and figure skirt (refer chapter 2 for more details). For this hovercraft design, the skirt

design will be a bag skirt, shown in figure 4.7. The reasons for choosing bag skirt are

because that it’s easy to build and less air pressure needed to lift the craft. Besides, bag

skirt is more stable than the wall skirt and it’s more suitable for this hovercraft design.

This design is readily used in industry as one of the more reliable constructions. In this

hovercraft design, a plastic plate is place in between bag skirt to prevent any friction

across in skirt and also prevent the skirt from damage, shown in figure 4 below. In bag

skirt, the skirt is inflates once the air is forces into the skirt. After that, the bag skirt

allows the air exit under the craft through specified holes in the skirt. This air flow

under the hovercraft will creates a high pressure which then lifts the hovercraft.


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Figure 4.8: Bag skirt design

Figure 4.8 above shows the design of bag skirt. The skirt was design in 6 inch height

since it’s more stable as compare to skirt in 4 inches and 7 inches height (from

research). Besides, bag skirt can jolt on very smooth surfaces and it’s easy to make with

low cost material needed. The parts list as listed in Table 4.3:

Table 4.3: Main parts list for skirt design


Skirt Black-8 linear yards 1

plastic plate Reduce friction and protect the skirt 2’x2’ 3


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4.2.4 Control System Design

Figure 4.9: Control system assembly

The main function of control system is use to control the moving direction of hovercraft.

In the control system design, two plate of plywood were used to make its rudder.

Besides, two ropes were used to control its direction left and right.

4.2.4.1 Steering

Figure 4.10: Steering assembly

The figure 4.10 above shows that the design of steering was used in hovercraft. The

function of steering is to control the direction of two rudders on the back with moving

left and right. Two ropes were connected between steering and rudders to control its

direction.


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4.2.4.2 Pulley

Figure 4.11: Small pulley

In this hovercraft design, four (4) pulleys were used. Basically, it is used as the roller to

scroll the rope which connects the steering and rudder.


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4.3 Material selection

4.3.1 Leaf blower

Figure 4.12: STIHL SR420 mist blower

STIHL SR420 mist blower is chosen for the lift system since it’s already available at

author’s home. It’s a powerful mist blower for spreading fertilizer but it also can

convert to a leaf blower. Below shown the specification of left blower:

Standard features:

Anti-vibration system

Carrying system

Multi-function control handle

Technical data:

Displacement 56.5 cm³

Weight: 11.1 kg

Max. Air flow rate: 1260 m³/h

Vibration levels left/right: 2.3 m/s²

Sound pressure level: 101.0 dB (A)

Sound power level: 113.0 dB (A)


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4.3.2 Thrust System’s Engine

Figure 4.13: Robin engine

In order for the hovercraft to make turns, it was determined that the thrust fan, which is

located at the back of hovercraft, would have to rotate. So, 5.0hp Robin EY20-3

horizontal shaft engine is used for thrust system. The speed of this engine is 3600rpm

and maximum can up to 4000rpm, so it’s suitable for using in thrust system as it need

high rotate speed. The cost for this robin engine is RM560. The specification of this

engine is shown in table 4.4 below:

Table 4.4: Specification of robin engine

Model FSH20-A

Type air-cooled, 4-cycle, single clinder, side valve

Broke*stroke(mm) 67x52

Displacement(cm3) 183

Rated output KW(HP)/rpm 2.65(3.5)/3600

Max output KW(HP)/rpm 3.73(5.0)/4000

Fuel Automobile Gasoline unleaded

Fuel tank capacity(L) 3.8

Lubricating oil capacity(L) 0.6

Spark plug E6TC

Starting system Recoil starter


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4.3.3 Hovercraft Body

Figure 4.14: Plywood

After compare to those materials like plastic board, steel and fiberglass, plywood is the

most suitable material for built a hovercraft since it is easy to cut and also lighter than

the fiberglass. Plywood has a better “strength to weight” than steel and it’s much lighter

than fiberglass. Besides that, plywood hovercraft can be built with basic hand and wood

working tools. It’s more durable and low in maintenance as compare to other materials

like plastic board and fiberglass. In this hovercraft design, two different thickness of

plywood with the size of 4’ x 8’ were used, which are 6mm and 12mm. The plywood

with the thickness of 12mm is covered by the canvas and it is placed at the bottom of

hovercraft. On the other hands, the plywood with the thickness of 6mm is placed on the

top of 12mm’s plywood.


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4.3.3.1 Teak Wood

Figure 4.15: Teak wood

Two sticks of teak wood (2) were used to prevent bending of hovercraft. Its helps to

balance the weight acting on hovercraft so the hovercraft can work better. (Size:

2.5x4.3x299 cm.

4.3.4 Skirt

Figure 4.16: Canvas

Skirt is one of the important materials in hovercraft design. Basically, it is used to cover

the plywood used at the bottom of hovercraft as the supporting board. The selection of

skirt is very important as it will affect the stability of hovercraft and also the efficiency

of air cushion. Throughout sourcing this material, the author faced the difficulty

whereby the size of skirt required, which is 4x8 feet2’ was unable to be found.

According to one of the salesperson at shop of “Bathroom Accessories”, the standard

size of skirt sell in Malaysia is 6’ x 6’ and thus it is not big enough to build a 4x8 feet2’s

hovercraft. Due of this limitation, the author decided to use the canvas as per picture

shown above as the alternative material.


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4.3.5 Bearing

Figure 4.17: Bearing

Two bearing (2) were used to support the blower shaft in thrust system. This is a 3/4''

angular bearing which manufacture by Japan. It’s necessary in thrust system to rotate

the propeller fan. Each of this bearing cost RM30.

4.3.6 A Belt

Figure 4.18: A belt

Two A belt (2) were used in thrust system to pull the blower shaft by connecting two

small pulleys. The belt must be fit and tight with pulley so that the propeller can

perform in high power.

13 CHAPTER 4 Design and material selection

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