Consider first a class of monohull ships moving steadily ahead on a flat calm infinitely-deep sea. Fix the displacement, draft, speed, and hull shape. The length is then essentially the only variable allowed. At any given length, adjust the beam by uniform scaling of all offsets, so as to achieve the prescribed displacement; longer ships are thinner. Now vary the length until the total (viscous plus wave) drag Rt=Rv+Rw is minimised.
The above simplified ship optimisation problem, with length as the only variable, usually possesses a non-trivial solution, i.e. a finite optimum length, for the following reason. Viscous drag Rv is predominantly skin friction, which is proportional to surface area, and as a body of a given volume gets longer and thinner, its surface area increases. Hence viscous drag increases with length at fixed displacement.
On the other hand, for conventional ships at conventional speeds, wave resistance Rw generally decreases as the shiplength increases. Since we are holding the speed U fixed, as we increase the length L, we are decreasing the length-based Froude number F=U/sqrt(gL). At fixed displacement, and at most relatively low Froude numbers, wave resistance is a (rapidly) increasing function of Froude number, and therefore decreases with increasing length. This wave-reducing advantage of long ships is very much part of the naval architectural art.
Since there are opposite trends with length in the two constituents of the total drag, there must be a minimum for their sum, at some non-trivial intermediate length. That length is generally somewhat larger than for conventional ships, but not so for rowing shells and other competition boats.
In fact, sometimes there is more than one local minimum in the graph of total drag versus length, and this phenomenon is discussed in more detail in the following section. There is often a delicate interplay between local and global optima, which makes for an optimisation process that is quite difficult to analyse. In order to deal with this problem, we use here a powerful general purpose technique, described later, called "genetic algorithms".
In the present paper, we perform an exhaustive treatment of this optimisation problem for a family of monohull vessels, covering a large range of speeds . We hold the hull shape, displacement and speed fixed, and allow the draft as well as the length to vary until the minimum total drag is achieved. We treat viscous drag as the sum of skin friction (estimated by the 1957 ITTC line) and a generally small but sometimes crucial form drag contribution which is estimated by an empirical formula. We use Michell's integral for the wave resistance, which is only accurate for thin ships. However, this is a more than usually good assumption for the class of extremely fine hulls that arise from this optimisation process.
The main purpose of the present study is to provide a benchmark, from which extended studies can follow. One class of such extensions obviously involves allowing the shape of the hull to vary. We present here results for a very fine type of hull, appropriate for high-speed and sporting-type vessels. When there are length (or other) restrictions, and hence (for shorter-than-optimal ships) a greater contribution of wave resistance to the total drag, multihulls can have less total drag than monohulls of the same length, because of the potential for favourable hull-hull cancellation of wave resistance. Work on both of these extended studies is nearly complete and will be reported elsewhere.
However, perhaps of greater importance is inclusion of further constraints, such as constraints on maximum length or minimum beam, which arise inevitably from commercial, structural, safety, seakeeping, or sporting requirements. When these constraints are imposed, the ship proportions will return to the more conventional range, but at a price in terms of increased total drag. It is of value to know just how much of a price is being paid.
|Fig. 1.1: Comparison of total resistance for two one tonne monohulls.|
Figure 1.1 shows two typical examples of graphs of total drag versus length (in metres) at a fixed speed, for such a vessel. For the present purpose, it is not essential how the drag is determined or scaled, but we should note that it does include an allowance for form drag, discussed later. The dashed curve is at a fixed speed of 5.56 knots and the solid curve at only a very slightly higher speed of 5.59 knots. In both cases, there are two prominent minima, i.e. two distinct (and remarkably different) lengths are locally favourable, and define "best" and "second-best" boats. At the lower speed, the longer boat (13.2 metres length) is better than the shorter boat (9.8m), whereas at the higher speed, the shorter boat (9.6m) is superior to the longer boat (12.3m). Thus, as we vary the speed and other parameters, there may occur an interchange between two local optima, so that the optimum length may appear to change discontinuously. These changes can occur over a remarkably narrow range of speeds.
|Fig. 1.2: Optimum length for a one tonne monohull|
This type of discontinuity in the optimum length is shown in Figure 1.2, again taken from the family of one-tonne monohulls. This figure gives the optimum length in metres as a function of the speed in knots. The discontinuities indicated above occur only at relatively low speeds, notably at about 5.6 knots (where the change between the two curves of Figure 1.1 occurs) and 4.3 knots, with smaller discontinuities at even lower speeds.
At speeds between the discontinuities, the Froude number based on boatlength remains essentially constant, and examination of the variation of wave resistance with Froude number indicates that this constant value corresponds to a local minimum of wave resistance. What is happening as we increase the speed is that, in attempting to design for minimum total drag, we simultaneously increase the boatlength, in order to stay at that local minimum. This continues as long as possible while we increase the speed, and when it is no longer possible, the optimum boat suddenly decreases its length, so that the Froude number suddenly jumps to the next higher local minimum, avoiding the local maximum in between. This process is intuitively like changing gears!
The length variation in the example of Figure 1.2 is continuous for all speeds above 5.6 knots. However, as is discussed later, if form drag is neglected, there can also be an apparent high-speed discontinuity. It is important to note that, as indicated by Figure 1.1, there is no discontinuity in the actual total drag at these speeds, merely an interchange in the roles of "best" and "second-best" boats. At the speeds where the optimum length changes discontinuously, the residual total drag tends to reach a local maximum, where its rate of change with respect to speed changes discontinuously.
Although these discontinuities are of interest in their own right, they are not necessarily the most important feature of Figure 1.2. They depend on the fact that the wave resistance possesses minima, and these minima are to a certain extent magnified by the theoretical procedure (here Michell's integral) used to compute wave resistance. If more empirical means are used to estimate wave resistance, with the effect of smoothing out the humps and hollows in the wave resistance variation, there will be a consequent reduction in the size of the discontinuities. However, so long as there are at least two minima in the wave resistance curve, a discontinuity is inevitable, no matter what method is used to estimate wave resistance.
Above 5.6 knots, the optimum length of a one-tonne vessel varies smoothly, and it is unlikely that the optimum length is sensitive to the procedure for wave resistance computation. In fact, the range of speeds above that where discontinuous length changes occur is the one of greatest interest in practice; for example, it is the competitive speed range for rowing shells. In that range, the results are relatively robust, and show no surprising features.