Investigation of wake characteristics of a fishing boat suitable for Turkish waters

Türkiye sularına uygun bir balıkçı gemisinin iz karakteristiklerinin incelenmesi

Abstract

The wake field of a ship is determined with wake measurement tests. It is so important to know the distribution of wake coefficient on the propeller disk plane in point of cavitation, vibration and noise. Wake field measurements on the propeller disk plane by means of Pitot tubes or electronic pressure transducers are to investigate wake quality and hull efficiency. The Pitot tube (named after Henri Pitot in 1732) measures a fluid velocity by converting the kinetic energy of the flow into potential energy. The method of Pitot tubes or the method of wake comb is used widely. In this method, by using a wake comb consists of Pitot tubes, the values of total pressure pT and static pressure pS are measured at the certain points of propeller disk plane. Later, the values of dynamic pressure are obtained for these points. By using the values of dynamic pressure in the Bernoulli equation, the axial velocities of water flow are calculated at all the points.In this study, a computer aided analysis of the wake measurement tests for the model 148/1C of the parent fishing boat 148/1 was performed. All the tests had been carried out at Ata Nutku Ship Model Testing Laboratory operating within the Faculty of Naval Architecture and Ocean Engineering at Technical University of Istanbul (Soygur and Taseli, 1981). In the tests, a model 148/1C of the parent fishing boat 148/1 had been used. The model 148/1C is made of wood and its scale value is 3/25. This model had been towed in the loaded condition for the situations of no appendages, no inclined and no trim. The wake measurement tests had been carried out at the model speeds corresponding three different ship velocities in 8, 10 and 12 knots. Thus, the distribution of wake coefficient according to ship velocity was studied. In the system of manometer range and wake comb, Pitot tubes connected to a shaft with a hydrofoil. This part is called “Wake Comb”. The pressures on Pitot tubes are evaluated with a manometer range. The wake comb is connected to the manometer range with the elastic and transparent pipes. The levels of the manometer range are immobilized with the electromagnetic valves in the testing region. Thus, it is possible to have enough time for measurement. Air in the system is emptied with a vacuum container and a small pump and also the manometer range is calibrated at zero situations. In the tests, two different types of the wake comb had been used. Each wake comb consists of eight tips. Total pressure is measured with four tips. Static pressure is measured with other tips too. Thus, by carrying out the measurements at the 456 points, the values of total pressure and static pressure had been obtained. Then, the values of dynamic pressure were calculated as follows: $P_D(r,theta)=P_r(r,theta)-P_S(r,theta)$r: Radial coordinate, $theta$: Angular coordinate By using the values of dynamic pressure in Bernoulli’s equation, the axial velocities of water flow were also calculated as follows:$nu_A(r,theta)=sqrt{2gP_D(r,theta)}$ g: Acceleration of gravity Later, the wake coefficients were obtained as follows:$W(r,theta)=frac{nu_m-nu_A(r,theta)}{nu_m}$ vm: Model speed, $nu_A$: Axial velocities of water flow on the propeller disk plane. The manometer range had been fixed with an angle of $50^circ$ in order to read the values of pressures in enough sensitivity. Then, the values for vertical situation had been calculated by multiplying by sin ($50^circ$) (0.76604). The obtained results are as follows: 1. The values of wake coefficient in a big region of the propeller disk plane are less than 0.1 for each velocity. 2. The values of wake coefficient increase suddenly, in other words the water velocities decrease suddenly, in a region of the propeller disk plane near the aft body for each velocity. 3. The region in which the values of wake coefficient increase suddenly becomes narrow if velocity increases. 4. The nominal value of wake coefficient decreases if velocity increases. 5. Some solutions must be obtained to accelerate water flow in the region defined with $theta$ = [$150^circ$ – $210^circ$]. The optimum one from these solutions must be selected. 6. The wake measurement tests according to block coefficient, the ratio of length to beam and the ratio of beam to drought must be carried out systematically to determine the wake characteristics of the Fishing Boats Series of ITU.

Özet

Pervane düzlemindeki iz katsayısı dağılımının tespiti; kavitasyon olayı, titreşim ve gürültü bakımından önemlidir. Bir geminin pervane düzlemindeki iz katsayısı dağılımı, model iz deneyleri yapılarak belirlenebilir. İz deneylerinde, gemi model deney laboratuvarlarının sahip olduğu teknolojiye göre Kempf halkaları, Pitot tarağı, "Hot Wire (sıcak tel)-Hot Film (sıcak film)" ve lazer anemometrelerinin kullanıldığı yöntemler uygulanmaktadır. Bu yöntemlerden en çok Pitot tarağı (iz tarağı) yöntemi kullanılmaktadır. Bu yöntemde, Pitot tüplerinden oluşan bir iz tarağı kullanılarak, pervane düzleminin belirli noktaları için pT toplam basınç ve pS statik basınç değerleri ölçülmektedir. Sonra, ölçüm yapılan her bir nokta için pD dinamik basınç değerleri elde edilmektedir. Dinamik basınç değerlerinden ise Bernoulli denklemi kullanılarak eksenel su hızları he-saplanmaktadır. Bu çalışmada, Türkiye sularına uygun balıkçı gemilerinden 148/1 kodlu ana balıkçı gemisinin 148/1C kodlu modelinin; yüklü (1.00xT) durumda, üç farklı model hızında gerçekleştirilmiş olan iz deneylerinin bilgisayar destekli analizleri yapılmıştır. Söz konusu model, 3/25 ölçek değerinde ve ahşap malzemeden inşa edilmiştir. Deneyler, ana geminin 8, 10 ve 12 knot hızlarına karşılık gelen model hızlarında ve model; takıntısız, meyilsiz ve trimsiz olarak gerçekleştirilmiştir. Model ile gemi arasındaki Reynolds benzerliğinin sağlanabilmesi için, modelin 9.5'inci postasına 2 mm çapında bir tel türbülans yapıcı olarak yerleştirilmiştir. Yapılan analizler sonucunda, üç farklı hız için iz karakteristikleri belirlenmiştir. Ayrıca, nominal iz katsayısının hıza göre değişimi elde edilmiştir. Son olarak, elde edilen bulgular maddeler halinde sıralanmıştır.