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Hydrodynamic Characterisation and Structural Design Analyses of an Airboat

Ibelema Faango Keribo*, Daniel Tamunodukobipi

Department of Marine Engineering, Rivers State University, Port Harcourt, Nigeria

Abstract: This paper presents the structural design and hydromechanics performance characterization of a prototype airboat of length= 2.42m, beam= 1.11m, draught= 0.32m, and powering =30 kW. Modified Savitsky’s model and test-data are utilized for the analysis. The results show that the hull-trim increases from static trim to a peak value of 5.2° at Fn =2.50 in the non-planing speed regime; and then decreases to a plateau in the planing-speed regime. This phenomenon is explained by the aftward drift of the point of action of the resultant lift force towards the centre of gravity. For higher Froude number, bottom-velocity ratio increases towards unity: i.e. a reduced wake effect. Resistance curve for airboat is “Ogive” shape, rather than parabolic; whereas the effective power is parabolic. Correlation of analysis and test data shows a good agreement, except at transition speed. Therefore, the analysis is valid for characterizing airboats parameters.

Keywords: hydrodynamic design of airboat, stability analysis of planing hull, resistance and speed characteristics, design and structural analysis of airboat

Citation: Ibelema Faango Keribo, Daniel Tamunodukobipi, Hydrodynamic Characterisation and Structural Design Analyses of an Airboat. American Journal of Mechanical Engineering, 5 (5). 199-204. January 2017. doi:10.12691/ajme-5-5-2

1. Introduction

The difficult operational terrains for oil/gas explorations and exploitations in the Niger Delta Area of Nigeria have placed significant emphasis on the use of low draught, high-speed surface-planing crafts (HSCs), for deployment of personnel and delivery of logistics between companies’ shore-base and operational fields. By ABS standard: a vessel with Froude number of is a high-speed craft [1]. Conventional marine vessels with submerged propellers are largely incapacitated by discontinuous water bodies, craggy shallow waterways, semi-submerged obstacles (water hyacinths), mudflats and swamps [2,3]. These challenges have necessitated the design, development and increasing usage of all-terrain pliable, high-speed airboats. Less than a decade ago, the use of aircrafts as a palliative measure was popular. However, the associated huge expenses of hiring and the non-availability of safe landing platforms in the mangrove-forest fields have coerced oil firms to resort to cheaper but more reliable recourse means of transport. Airboat and hovercrafts invariably emerged as the most suited for their operations.

1.1. Definition of Problems

Hydrodynamic phenomena affecting HSC hulls are in various ways more complex than those of displacement crafts [2]. Designers of airboat planing hulls encounter problems resulting from widely varying friction drag (aero-hydrodynamic effects), hull trim and draught. Also, the trade-off between buoyancy and hydrodynamic lifts is difficult to optimize. Savitsky [4] established that repeated random dynamic impacts and porpoising during heavy weather are crucial considerations for hull form, stiffeners and material selection. However, a light hull structure for better hydrodynamic performance is readily achieved at the expense of structural strength for safety. Most authors present airboat designs without adequate theoretical analysis for optimal performance. Sverre [5] argued that an optimal design must entail theoretical modeling, detailed experimentation and results validation. Thus, predicting performance characteristics of planing boat is necessary but rigorous because of the myriad of widely varying operating conditions which combine in various proportions, depending on the terrain, to influence a vessel hydro-mechanics [6]. Invariably, this heightens the need for a comprehensive virtual tool for modeling and analyzing airboat parameters for reliability, good sea-keeping and energy efficiency under any conditions [6].

1.2. Methodology

Therefore, this paper presents a detailed structural and hydromechanics design, and performance characterization of airboat. It utilizes a modified Savitsky’s model [4] with the inclusion of thrust-induced-moment trim change for the hydrodynamic characterization. Several relevant technical data sheets for hull-form and performance evaluation are used [7]. Formula-based stability analysis and buoyancy lift predict its sea-keeping behavior [8]. Structural integrity analysis is based on shear/bending stress calculations and mechanical properties of materials. Model resistance test is performed in a towing tank, and the results extrapolated to the full-scale airboat. Correlation of theoretical and model test results is conducted to validate the analysis. Nonetheless, the mathematical model and design procedure advanced in this paper may be useful as a good framework for preliminary design and performance analysis of airboats.

1.3. Description of Airboat

Airboat is a hard-chine, structurally reinforced fiberglass or aluminum, ultra-light HSC which is commonly powered by a diesel engine. Its hull is flat-bottom with no protrusion below the waterline. Some hulls may have gentle dead-rise from the centre-line which diminishes progressively towards the aft. Such feature facilitates easy maneuvering in water, but at the expense of its stability performance on ice or grassland. The bow is modified into a 45° - 60° extended rake as shown in Figure 1. This prevents water ingress into the craft from wave sprays or during descent from land into water. Teflon layer on the bottom provides protection against abrasive wear and minimizes Coulomb friction on land. The side-chimes serve as stabilizers and preclude sprays from flowing into the side of the craft.

Figure 1. Airboat (a) principal features and (b) aerial propeller in steel cage

Unlike canonical marine vessels, airboat is propelled by aerial propeller and steered by a pair of vertical aerial rudders which direct a stream of forced air towards starboard or portside as required for maneuvering. Both propeller and rudders are mounted in a protective cage to prevent damage resulting from flying objects or

Overhanging tree branches, (Chensiyaun, 2010). Airboat carriage capacity depends on buoyancy, mission, and nature of route. Some airboats have buoyancy sufficient for carrying more than 10 passengers and may be unsinkable because of built-in floaters. In the absence of floaters, a flooded airboat sinks very quickly (8-15 seconds). In some, passengers are sheltered from the harsh environment using a fixed or retractable canopy. Airboat can readily climb in and out of water having a bank inclined as much as 45°. Typically, airboats do not have brakes and reverse motion. Stopping and reversing are dependent on the operator’s maneuvering competence (Ed Fitzgrald, 2009). Their characteristic Teflon-coated flat-bottom in conjunction with the absence of protrusions below the waterline enables them to safely glide over delicate vegetation, marauding animals, craggy river canals, grassland, and frozen lakes (Howcherg, 2001).

2. Structural Design and Model Construction

The model is constructed with a 2mm-thick aluminum sheet and finished using a 300 grit wet-and-dry paper. Aluminum is chosen over fiberglass and steel for airboat hulls because of its strength-to-weight ratio, toughness, workability, corrosion resistance and durability [10]. On the hull, 10 stations and 5 waterlines are marked out and their respective displacements calculated using Simpson’s rule as shown in Table 1. Figure 2 presents the design loading diagrams for the airboat floating at a draught d = 0.158m in calm water. Based on the loading, the maximum bending moment is calculated to occur at 1.2m forward of the transom. Table 2 presents specifications for the scantlings.

Table 1. Table of Offsets for the hull design of the model

Figure 2. Loading diagram for prototype airboat including: gravity loads and up-thrust

Table 2. Scantlings

Using the scantlings and the bending moment of each section, the structural analysis in Table 2 is summarized as follows:

Section modulus to the bottom ZB = 6.468 x 10⁻⁴m³

Section modulus to the deck ZD = 2.480 x 10-4

Bending stress to the bottom σB = 0.12183 mpa

Bending stress to the deck σBD = 0.317742 mpa

Stability behavior of the airboat in calm water is investigated to ascertain its seakeeping performance and maneuverability. The analysis considers static and dynamic stability. The associated metacentric heights from bottom (KMT and KML) and from CG (GMT and GML) for transverse and longitudinal are calculated, respectively, from Eqs (1) to (4).

[picture] | (1)

[picture] | (2)

[picture] | (3)

[picture] | (4)

While the righting lever (GZ) in metres and angle of heel (θ) in radians are obtained by:

[picture] | (5)

[picture] | (6)

Transverse stability is more crucial for airboat performance to avert capsize during maneuvering since the length-to-beam ratio is always greater than unity. Thus, the calculated transverse stability parameters are: KMT =0.7289 m, KG = 0.3798 m, and .The area under GZ curve = 0.05 rad.m, using Simpson’s rules.

3. Hydrodynamic Design and Performance Analysis

A proper hydrodynamic design of airboat is imperative for swift motion, good maneuvering, dynamic stable and energy efficiency. Principal parameters affecting hydrodynamics of planing crafts are examined.

Mean wetted length-beam ratio is given by:

[picture] | (7)

[picture] | (8)

[picture] | (9)

By Savitsky [4], the Reynolds number (Rn) and coefficient of lift (CLb) are expressed as:

[picture] | (10)

[picture] | (11)

From Eq. (11) the trim can be written as given in Eq. (12). Recall that airboat thrust is not along the axis of CG, but has a bow-dipping moment arm, . This tends to reduce Savitsky’s planing boat trim by a magnitude given by Eq. (13)

[picture] | (12)

[picture] | (13)

Hence, Savitsky’s trim value is corrected by deducting the value of due to thrust induced moment. Neglecting air drag, the resultant resistance for airboat is given as;

[picture] | (14)

Note that the wetted surface and bottom velocity ratio can be estimated, respectively, as;

[picture] | (15)

[picture] | (16)

4. Analytical Results and Discussion

In Figure 3, the trim obtained from analysis rises sharply from 4.2° at Fn =1.75 to a peak value of 5.2° at Fn =2.50, before descending gently to 2.8° at Fn =5.75. The boat, between Fn =1.75 and 2.50, exhibits the characteristics of a typical semi-displacement craft; while at the peak, it transits to a surface planing craft. This is because the point of action of the resultant lift forces drifts aftward until it coincides with the CG at full planing. Then a dynamic equilibrium is maintained with marginal changes in trim as Fn →∞.

The wetted surface decreases from 3.15 m² at Fn =1.75 to 2.0 m2 at Fn =4.50, before describing a plateau. This reduction is due to increasing hydrodynamic lift which raises the boat out of water until a dynamic equilibrium is achieved. During this period, there is a positive rise in CG. However, between Fn=5.25 and 5.75, the wetted surface tends to increase. This phenomenon is due to the effect of spray. In general, at full planing speed, the changes in trim and wetted surface are marginal and oscillate about their mean values.

Figure 3. Characteristic behaviors of trim and wetted surface as Froude number varies

Similarly, in Figure 4, the wetted length-beam ratio and the wetted surface slope from left to right as speed increases. This is consistent with the change in draught due to hydrodynamic lift. In contrast, Figure 5 presents a slight but progressive increase in bottom velocity ratio. This implies that a fully planing airboat has very little wake effect.

Figure 4. Characteristic decrease of wetted surface and length-beam ratio with increasing Fn

Figure 5. Increasing bottom velocity ratio with reduction in wetted surface

Figure 6 displaces the Reynolds number which increases with higher valises of Froude number, in spite of the progressive reduction in wetted length. It is because the rise in velocity is two order of magnitude higher than the reduction in wetted length. As consequence, the friction coefficient CF, being the inverse of log(Rn), decreases accordingly. The resistance and effective velocity of the model are shown in Figure 7. Resistance curve is steeper, especially in the regime of rising trim (1.5≤Fn≤2.5). For higher values of Fn, the resistance becomes more linear. Eventually, the entire curve assumes an “Ogive shape.” In contrast, the effective power is rather more of a parabolic curve before planing speed, and linear after transition into the full hydrodynamic surface planing regime (Fn≥2.5).

Figure 6. Curves of friction coefficient and Reynolds number

Figure 7. Resistance and effective power curves for airboat model

5. Experimental Results and Validation

To characterize the resistance of a vessel, geometrical, kinematical and dynamical similitude must be ensured between the test model and analysis. This condition is necessary for reliable comparison, reproducibility and validation of results. The model resistance test is conducted in the towing tank at Rivers State University, and the procedure for the conduct of the resistance test is in Ref. [5]. Figure 8 compares the resistance results of the test and analysis. Generally, the prediction before transition is in good agreement with test data. However, at transition, the resistance value drops sharply before climbing gradually at full planing regime. Nevertheless, the disparity is slim, but subject to further investigation.

Figure 8. Comparison of test data and analytical results of resistance

6. Conclusion

Hydrodynamic design and structural analyses are performed for an all-terrain pliable airboat to ensure swift motion, good maneuverability, and safety. Modified Savitsky’s model with the inclusion of moment-induced trim change is implemented for the hydrodynamic characterization. From the results, it is established that stability and structural integrity for safety should not be compromised for light hull in favour of better hydrodynamic performance. This is because capsize in waves and wreckage during dynamic loading could be imminent. Also, it is found that the hull trim increases from static trim to a peak value of 5.2° at Fn =2.50 in the non-planing speed regime; before descending gently as it transits into the planing speed regime. Such characteristic behavior is explained by the aft ward drift of the point of.

References

[1]   | Kramer, R.H. (2005). “US Navy High Speed Craft – Comparison of ABS and DNV Structural Requirements,” ASME Journal, No. D26-2005.
[2]   | Thien, P. Q., Hieu, N. K., and Vuong, P. M. (2015). “Numerical simulation of floating airboat: Estimation of hydrodynamic forces”, Int’l J. of Mech. Engg and Applications.
[3]   | Lewis, E.V. (1988). “Principles of Naval Architecture: Resistance, propulsion and Vibration,” 2nd Ed., SNAME, 601 Pavania Avenue, Jersey City, NJ.
[4]   | Savitsky, D., (1964). Hydrodynamic design of planing hulls. Marine Technol, 32(3): 78-88.
[5]   | Sverre S. (2014). “Experimental Methods in Marine Hydrodynamics,” Lecture Notes, NTNU Trondheim, Norwegian University of Science and Technology, Norway.
[6]   | Wood H.K. and Stapersma D. (2003) Design of Propulsion and Electric Power Generation ‘Systems. IMarEST. London.
[7]   | Blount, D.L. and Clement, E.P. (1963). Resistance tests of a systematic series of planing hull forms. SNAME Transactions, 491-579
[8]   | Derrett, D.R. (1997). “Ship Stability for Masters and Mates,” 4th Ed. Butterworth Heinemann, London. PP 15-45
[9]   | Tamunodukobipi, D.T.; Ogbonnaya, E.A.; Koumako, K.E.E. (2009). Characteristic Behavior of High Speed Craft at transition from Bow-wetting to full planing. Journal of Engineering and Applied Sciences 4(3) 189-196
[10]   | Kohansal, A.R., Ghassemi, H., and Ghaisi, M., (2010). “Hydrodynamic characteristics of high speed hulls, including trim effects,” Turkish J. Eng. Env. Sci. 34, pp. 155-170.