Since most sailing yachts today have auxiliary power, it is important to consider the design of the propeller and the power required under different circumstances. There may be three reasons for having an engine in a sailing yacht. First, yacht harbours are often crowded, and it is difficult to manoeuvre under sail in the limited space available. In some harbours it is not even permitted for safety reasons. Secondly, if sailing conditions are not perfect, many cruising skippers prefer to use the engine, particularly if they are short of time. Thirdly, the engine may be a life-saver under critical conditions in rough weather.
The first case does not put any major demands on the engine-propeller design, since only very limited power is required. It is important, however, that the propeller works reasonably well when going astern. In the second case, speed is an important factor, while in the third case enough thrust should be developed to escape from dangerous situations even against strong winds and heavy seas. These two latter cases put different demands on the propeller, and it is important to find a good compromise to achieve a reasonable performance in both situations. Perhaps the most important requirement is that the propeller allows the engine to work close to its optimum under severe weather conditions.
In the first part of this chapter we will consider the total resistance of the yacht based on our discussion in Chapter 5. This will serve as a basis for the propeller design in calm weather, while for the rough weather case we will also introduce the added resistance in waves, and the windage from the above-water part of the yacht. Having found the resistance under the two conditions we will show how the optimum propeller and the required power may be obtained under each condition. The final choice of the propeller has to be a compromise between the two requirements, and we must also consider what is available from manufacturers, both as to the propeller and the engine. After selecting a suitable combination we will investigate its performance. Finally, we will discuss the added resistance due to the propeller when sailing.
It should be pointed out that the calculations in the present chapter will be more approximate than those of Chapters 5 and 6, in which the fine tuning of the yacht and appendages was discussed. To obtain a suitable propeller/engine combination this accuracy is not needed, and it is also very difficult to obtain, since many of the influencing factors are not known with great accuracy.
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.