Yelkenli teknelerde direnç deneyleri

Abstract

Yelkenli tekne konusunda çok fazla yayınlanan bilimsel çalışma yoktur. Ticari çalışmalarında saklı olması nedeniyle, yelkenli tekneler hakkında faydalanılabilecek çok fazla veri bulunmamaktadır. Türkçe kaynak azlığı nedeni ile yelkenli teknelere etkiyen kuvvetlerin dengesi, yelkenli tekne performansı ve stabilitesi ile yelkenlerin aerodinamik direnci hakkında geniş bilgiye bu çalışmada yer verilmiştir. Yelkenli teknelerin direnç deneylerinin yapılması gemilere nazaran farklı ve biraz daha zordur. Yelkenli tekneler hiçbir zaman doğrusal durumda seyretmezler. Bu nedenle yelkenlilerin dirençlerinin yanında, yan kuvvetleri de ölçülmelidir. Eğer direnç ve yan kuvvet değerlerini ölçebilirsek teknelerin performans hesabını yapabiliriz. Performans hesabı özellikle yarış tekneleri için çok önemlidir, fakat Türkiye'de bu konuda yapılmış bir bilimsel çalışma henüz yoktur. Bu çalışma Ata Nutku Gemi Model Deney Laboratuarında, yelkenli teknelerin hidrodinamik karakteristiklerinin saptanabilmesi için bir deney sisteminin dizaynını ve test edilmesini kapsamaktadır. Çalışmada değişik hızlarda, meyil açılarında ve sürüklenme açılarında tekne üzerine etkiyen direnç, yan kuvvet meyil momenti, savrulma momenti, dinamik batma miktarı dinamik trim açısını ölçebilen 6 bileşenli dinamometre üzerine kurulmuş bir aparat geliştirilmiştir. Aparat detaylı bir şekilde kalibre edilmiş, direnç değerleri için diğer direnç dinamometresine göre test edilmiştir. Dinamometre ile kullanılmak üzere 20 m boyunda, tek direkli bir yelkenli tekne dizayn edilmiş, laboratuar model atölyesinde 1/8 ölçekli modeli imal edilip, deneylerde kullanılmıştır. Model takıntısız halde ve 5 ayrı salma ve bir dümen modele monte edilerek seçilen hızlar, meyil açıları ve sürüklenme açıları için denenmiştir. Yan oranları birbirinden farklı olan ilk üç salmanın sonuçları ve deneyler süresince yapılan gözlemler çalışmada verilmiştir. Salma tiplerinin performans üzerine etkileri incelenmiştir Çalışmanın son bölümünde, elde edilen sonuçların yorumları ve gelecekte yapılabilecek çalışmalar için öneriler yer almıştır.

There has been recently some interest in sailing yacht racing in Turkey. Such boats are normally acquired by importing from overseas. Even though it is possible to build such a boat in Turkey, know-how is the design of this boats. Scientific methods are required for designing a sailing yacht with a good performance. Although the scientific research on sailing yacht hydrodynamics and aerodynamic was initiated by 1750' s, there is limited information and data due to commercial confidence of such information. The available literature in Turkish is even more scarce. The methods for predicting yacht performance have been developed in two alternative ways. Numerical prediction methods such as velocity prediction programs appeared in the mid 1970' s. A few years later van Oossanen presented a method specially designed for 12 m yachts. The real breakthrough appeared as a result of research at Massachusetts Institute of Technology resulting a well known IOR rule rating program of various sail -hull -keel configurations. The second method for performance prediction is based on the model experiments both in wind tunnel and towing tanks. Wind tunnel experiments have suffered from the difficulties of modelling sail shape and fabric properties in the scaled models. Research at Southampton University, has pioneered development on sails in 1960's. The other side of experimentation by towing tanks had been established the fundamental principles since Davidson's pioneering work in the Stevens Institute. However there has been widespread opinion among designers that the optimum design had been reached. Furthermore due to some notable failure in the early 1970' s confidence in yacht research and tank testing in particular was very low. By the victory of scientifically designed yacht "Australia II" in 1983, the interest was boosted not only in the races but also yacht research and development. Before going further it is better to summarise the simple theory behind the calculation of a yacht performance. A simple aerofoil produces lift and drag and a yacht making leeway at a given speed through the water employs a very similar flow mechanism as shown in Fig. 1. SIDE FORCE Fig. 1. - Flow mechanism The resistance of even an ordinary ship form cannot as yet be calculated easily and in the case of a yacht hull the situation is rendered more complicated because the magnitude of side force and its relationship to the resistance is determined by the effective aspect ratio of the immersed part of the hull. Also the effect of heel and the consequent asymmetry of the hull are difficult to deal with analytically. APPARENT WIND DIRECTION SAIL LIFT HORIZONTAL COMPONENT OF SAIL FORCE HORIZONTAL COMPONENT OF KEEL FORCE WEICHT Fio. 2. - Balance of forces XI The sail is even more readily replaced in our minds with a simple aerofoil and the forces associated with it may be superimposed on Fig. 1 to give Fig. 2, which represents the situation when the yacht is sailing to windward. Since the resultant of the aero and hydrodynamic forces (shown dashed) are balanced, the yacht will continue through the water at the speed associated with the particular values of side force and resistance shown. The balance of forces in the heeling plane is also simply illustrated in Fig. 2 The capsizing couple due to the sail force and hull sideforce has to be resisted by the natural stability of the hull which results from hydrostatic forces and the weight acting through the CG. The amount of sail that can be carried without suffering a capsize is largely determined by the hull stability. It will be appreciated that not all the aerodynamic forces in an actual practical case are due to the sail and also there will be aerodynamic interference between the hull and sails. Thus the balance of forces implicit in Fig. 2 can be simply written down as an equation as follows. Hull hydrodynamics + Hull aerodynamics + Interference effects of sail on hull aerodynamics + Sail aerodynamics + Interference effects of hull on sail aerodynamics = Complete yacht performance The apparent wind direction and velocity is not the true wind direction and velocity. The true values are modified by the ship speed in a manner illustrated by the vector diagram shown in Fig. 3. SPEED MADE GOOD TO WINDWARD Vfa Fig. 3. - Velocity vector diagram The value of speed made good to windward (Vmg) for a given yacht will depend in the main on the angle between the ship speed and wind vectors and also on the strength of the wind. XI i The progress on yacht research in Turkey initiated by Prof. Ata Nutku in 1950' s. He has given an original diagram of elements and factors affecting a yacht performance. HULL SPEED MADE GOOD 14 V* BOAT R SAIL Wind 13 K SPBBDrr-r^^CûUa&E \Beoo/br^ m Cxi ?Â.nq/e TRUE WINO- couess. Scolei F F> UULL Dimensions Form 1 Dyn. WIND SESISTAMCE OF BOAT nmaMm."v.. ~^ow AppuK&/,Budc/er FORCE. I FORCE H£ELM II P*g%{:îtv£ SA 1 SA'L\ AfVGLE AREA OF ATTACK L/D Yaw xAng/c (Jee/XAngJe L J LAteoaI I STABILITY polar ami SAIL L AREA. OP LATERAL PLANE I 1 L İk PROFİLİ. Aspect ren Camber nC/o+f> rorasif-y 2+C..A etc. HULL Asp.RoM vo Fig. 4. - Elements and factors affecting a yacht performance Xlli However there has been difficulties due to look of appropriate measuring systems for drag side force and moments. This work is aimed to develop such a measurement system. A six component electronic dynamometer was bought in 1974 for cavitation tunnel use. This dynamometer has been utilised by addition of two piece of equipment which are designed and constructed wihtin the project, first enables the model being free to heave and the second being free to pitch. Hence the model is tuned free to heave and pitch but fixed to roll, surge, yaw. Electronic signals of drag, sideforce, heel moment and yaw moment are outputs of dynamometer. A linear vertical displacement transducer and a rotary potentiometer are used to record the heave and trim of the model respectively. As a matter of interest, during each run at pre selected speed, displacement, heel and vertical sail force following variables have to be recorded: Sideforce. Yawing moment. Angle of trim. Waterline length. Angle of leeway. Temperature. Resistance. In order to test the dynamometer and analysis of the system, a sailing yacht in 20 meter in length was designed and 1/8 scale model was built. The model was tested with current dynamometer and a mechanical dynamometer in upright position. The difference between two measurements is less then 5% over the full speed range. A set of keels consist of 3 keels with varying aspect ratio and 2 with varying planform are built and tested with the model. The model was tested in three heel angles (5°, 10°, 20°) and four leeway angles (0°, 4°, 8°, 12°) for four speeds corresponding to ship speed of 4, 6, 8, 10 knots in addition to upright position tested in speeds of 4 to 12 knots. The results of the project can be summarised as follows : 1- A suitable rig for yacht measurements is designed, built and tested. Sufficient degree of accuracy was achieved. 2- The results of the system have been proved by testing one model both by current dynamometer and by mechanical dynamometer. The results are satisfactory across the speed range. 3- Five keel types are tested and the results are given. xiv The hull used in the test, performed satisfactorily. The bow form could be changed due to bow lines along the bow form. The flow lines at the aft is well shaped until 8, 8.5 knots. 8.5 knots is the optimum speed for the hull. Fn - CT diagrams have a hump about 7 knots and a hallow about 8. 5 knots. 7- The increase of resistance by the increase of heel angle is observed from resistance diagrams. 8- When the leeway angle is higher than 8°, the resistance augment comparing to upright is considerably higher than the resistance augment between 0° and 4° degrees of leeway. 9- Out of three keels tested, the keel with highest aspect ratio has shown the best performance. 10- The wave resistance appears above 4 knots. At Froude number of 0.33, the wave resistance is about 30 % of the total drag. About Froude number 0.5 the wave resistance accounts to 90 % of the total drag. 11- Stall related to leeway angle has not been observed across the leeway angels tested. 12- At 0° leeway angle, the hull without keel and rudder shows the least resistance. The keel results about between 40 % and 20 % resistance increment. 13- At 9 knots yacht speed the aft sinks into the water, as a results of the sinkage it is not optimum to sail at this speed. On this study we could say that this work is a basic stage on sailing yacht model test and measuring hull forces and moments. We hope that this experimental set up will be used in both scientific and commercial research in the future.