Principles of Yacht Design

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Fig 6.6 Influence of aspect reality a more interesting question is how much drag is produced for a ratio on lift and drag given side force. As pointed out in Chapter 5 the task of the keel is to balance the given side force from the sails at the expense of the smallest possible drag. With this in mind the lift diagram could be interpreted in a different way. For a given side force the leeway for the two-dimensional keel would be less than one third, and for the AR, = 3 keel

less than half that of the ARc, = 1 keel. Quite different drags would then be obtained as the righthand drag diagram suggests. Note that CD in Fig 6.6 represents the total drag, ie also the viscous components, as presented in Fig 5.4. This is why CD is not zero at zero leeway angle.

The differences between traditional long keels and fin-keels are now obvious. While the long keels have an effective aspect ratio considerably smaller than one, the modern fin-keel ARes are usually larger than three. Large performance differences are therefore to be expcctcd. However, there are also disadvantages to the fin-keel. One of these was discussed in Chapter 3 in connection with roll damping, and it was shown that a long keel is considerably more effective in this respect. Another disadvantage occurs at low speeds. As appears from the lift equation of Fig 6.5, the lift is proportional to the lift coefficient, the speed squared and the keel area. Since fin-keels have a smaller area they operate at higher lift coefficients (which are easily obtained since they are more effective). However, the maximum CL is about the same for all aspect ratios and it is reached much faster for a fin-keel yacht when the speed drops, if the side force is still required. This may happen when berthing, or at the start of a race when there may be a considerable side force from the sails, but the speed is low. The keel then stalls and the yacht starts moving sideways. The difference between long lcccls and fin-keels is quite significant, and many owners of modern yachts have experienced problems when manoeuvring in harbours.

To obtain the advantageous elliptical distribution of the side force the keel may be designed with an elliptical plan form. This means that the chord length must vary elliptically from tip to root. Two geometries that would satisfy these requirements arc the half ellipse and the quarter ellipse (see Fig 6.7). but in both cases the important quarter chord line would be bent, so the force distribution would not be exactly elliptical. In the third alternative the design has started from a straight quarter chord line and the chord lengths have been distributed elliptically in the vertical direction, always keeping the 25% point on each chord on the line.

Trapazoid Fin Design

The elliptical plan form has certain disadvantages, not least from a practical point of view, so trapezoidal keels are much more common. It is, in fact, possible to obtain a force distribution which is very nearly elliptic for this kind of keel also, provided the taper ratio is chosen to fit the sweep angle according to Fig 6.8. As can be seen in the figure, a small taper ratio requires a large sweep back and vice versa. At zero angle the taper ratio should be around 0.45, and for large ratios the keel should actually point forwards, since the angle is negative. Most keels have a sweep angle of 20-30 degrees, which should call for a taper ratio of about 0.1. This is not practical, however, since the centre of gravity would then be too high up, and the stability poor. There is another disadvantage of small taper ratios. If the keel is unswept, and either elliptic or has a taper ratio of 0.45, the area distribution in the vertical direction corresponds to the force distribution. If smaller chords near the tip are compensated by sweepback to get large enough forces in the area, this part will be more highly loaded than the rest of the

Fig 6.8 Optimum relation between sweep angle and taper ratio

Taper Ration

Fig 6.9 Increase in induced drag due to non-optimum taper ratio keel. The local lift coefficient will be higher and this part will stall earlier. In practice, lower taper ratios than 0.2 are not recommended. As a matter of fact, most designers use much larger ratios, 0.4-0.6, for stability reasons. Note that, if a given thickness ratio is used, the cross-sectional area of the keel increases as chord squared, which means that

Toper ratio


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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

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.

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  • elvio ferrari
    How to calculate lift coefficient for a keel principles of yacht design?
    7 years ago

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