CHARACTERISTICS OF MANEUVERING MOTIONS OF PHILIPPINE OUTRIGGER CRAFT IN WIND
Ritsuo SHIGEHIRO (Faculty of Fisheries, Kagoshima University, Japan)
Glenn D. AGUILAR (University of Philippines in the Visayas, Philippines)
Takako KURODA (Faculty of Fisheries, Hokkaido University, Japan)
Harald E. KAGARUKI (Graduate School of Fisheries, Kagoshima University, Japan)
Abstract: Most fishing craft of the Philippines are constructed in the double outrigger form with a main hull and floats at both sides. Generally, a float and its beam are made of bamboo. The craft has met with a sea accident such as a collision or aground in a strong seasonal wind. A great deal effort has been made on the issue, little is known about maneuverability under the wind forces for the craft.
In this paper, a prediction method of the maneuverability for the craft is proposed to investigate the maneuvering motions under the wind forces. The point about the development of this method is divided into different parts of the hull and the outriggers. Furthermore, the model of hydrodynamics forces on outriggers is described as a simple mathematical formula. The accuracy of the method is checked by model tests. From these simulation studies, a safer and more efficient craft could be designed.
When considering the maneuvering motions of outrigger craft under the wind forces, field survey provides a starting-point. Aguilar (1997)i)2) has conducted field surveys to measure and record the technical characteristics of the craft and to build a database that can be used for analyses. Furthermore, Aguilar and Shigehiro (2002) 3)4) have conducted the sea trials of an outrigger craft to investigate the ship's speed and turning ability in the Philippines. However, there is not enough data to apply the other outrigger craft in generally. From these remarks, it is clear that the simulation study is useful method to improve the performance for the craft. As the craft is constructed in the double outrigger form, it is important to take account into the effect of hydrodynamics forces on outriggers.
The purposes of the studies are to develop a prediction method to estimate the maneuvering motions and to clarify the effect of outriggers and wind forces on maneuvering motions for the craft. In this prediction method, the mathematical model of hydrodynamics forces is divided into different parts of the hull and the outriggers. Firstly, the accuracy of the method is checked by comparing between the results of numerical simulation and turning motions of model tests in a calm condition. Secondly, the simulation studies are applied to investigate the maneuvering motions for the craft under the wind forces. From these simulation studies, the effects of outriggers and the wind forces for the craft are able to discuss quantitatively.
2. PREDICTION METHOD
The prediction method of maneuvering motions for the craft is based on numerical simulation. The mathematical model for the simulation is the typical modular type that is called MMGmodel5) 6) in Japan, and it is described as follows:
where m : mass of the craft, Izz : moment of inertia at the center of gravity of the craft, u,v : velocity of x-direction and y-direction and r: angular velocity of yawing. The notation of X, Y and N represent the hydrodynamics forces and moments acting on the center of gravity of the craft.
Fig.1 Co-ordinate systems.
The subscripts of H,P, R and E refer to the hull, the propeller, the rudder and the external forces, respectively. The co-ordinate systems are shown in Fig.1.
2.1 Mathematical Model of Forces and Moments Acting on the Hull
XH,YH and NH include the effect of outriggers and are expressed by the following polynomials of u',v'and r'.
where ρ : density of water, L : length between perpendiculars, d : the mean draft and U : resultant velocity of u and v. v' = v/U ≡-β,r'=r(L/U).
A thing worth of note is that the derivatives of the craft are largely different from those of a mono-hull ship. Detailed account of the influence of outriggers is given in the next section (3. INFLUENCE OF OUTRIGGERS).
Xp,Yp and Np are expressed by the following formulas:
and where t : coefficient of thrust reduction, n : speed of rotation per seconds, Dp : diameter of the propeller, KT : coefficient of thrust,xp : location of the propeller (x'P=xP/L)and wo : hull wake fraction in straight motion.
XR,YR and NR are expressed by the following formulas:
where δ is the rudder angles, xR represents the location of the rudder and tR, aH are interactive coefficents among the hull, the propeller and the rudder. FN is the rudder normal force and is expressed by the following forms:
where δo: neutral angles of the rudder, rR : flow straightening factor around the rudder, ∧ : aspect ratio of the rudder, AR : movable area of the rudder and lR,ε,κ are interactive coefficients among the hull, the propeller and the rudder.