SP KED BOAT DKSIGN
II. STABILITY AND CONTROL BY
R. R. POOLE, B.Sc.
HAVING dealt in the last article with the general design of the hull from the point of view of resistance to motion, we may pass on to consider those factors governing its stability both when running on a straight course and when cornering. Instability manifests itself in three principal directions : the boat may tip over, it may be unable to maintain a straight course if left to itself, and it may pitch to such an extent as to be almost unmanageable.
Next to being able to float the most obvious essential of a boat is that it shall float right way up, and to ensure this we must eliminate all those factors which might invert it, and further, must construct it so that the natural tendency is towards the more comfortable position. This appears rather obvious, but it seems to have been overlooked in some designs, if one may judge from some recent races. Secondly, the boat must be able to hold to a straight course without constant correction for small deviations caused by wind or rough water, and it must be able to corner neatly without undue rolling or sideslipping. Provided that the hull is reasonably wide and low as to its centre of gravity, there is no reason why it should invert itself when travelling straight, even in rough water. Most spills occur when rounding a buoy, and they can generally be attributed to bad rudder design. Our th…oretical ideal boat, consisting simply of an inclined plate skimming on the surface of the water, could presumably corner perfectly since the whole mass is concentrated at the waterline, but a practical skimmer hull necessarily has most of its mass considerably above the water, so that any change of direction produces an outward force tending to overturn the boat. Now this can be counteracted if we can produce a force in the same direction but below the waterline such as is provided by a stern rudder. Bow rudders, since they operate the other way, will only accentuate the outward roll, and their use should be limited to very wide, flat bottomed boats, or as small auxiliaries to a deep rudder at the stern. Reference to Figure 1 will make this clear. A turn in the direction indicated can be effected equally well by either the bow or stern rudder. In each case the thrust on the rudder blade can be divided into its two components. 111. in the direction of motion, and constituting merely an extra head resistance, and P. transversely, the latter component producing the turn. Since these forces necessarily act well below the waterline, the one will oppose and the other assist the overturning moment. The turning effect of a rudder depends on its distance from the centre of pressure on the whole hull. In a well balanced single step hull, the C.P. will be roughly midway between the centres of pressure of the two immersed areas, which would make it some two feet behind the step in a 12ft. skimmer. Further, the turning effect is independent of the depth of the blade, except in so far as turbulence and the “following zone” are concerned, so that it is possible so to arrange the stern rudder as to compensate exactly for the overturning moment at all speeds. For if “Ii “is the height of the centre of gravity of the hull with the engine and pilot in position, ” M ” lbs. the total mass, ” d” the depth of the C.F. of the rudder blade, ” v” the velocity and ” r ” the radius of the curve taken, then And since PT is proportional to v”, it follows that the compensation is the ‘same regardless of the speed. The actual calculation of PT is rather difficult, as the effect of eddies, as shown in Figure 2, is difficult to predict. The common treatment of the inclined immersed plate, as expounded in books on hydrodynamics, generally leaves so much to empirical constants based on fifty-year old experiments, that little value can be attached to it, and experience is the best guide. The usual size of the rudder on small boats is about 15 to 25 square inches, as larger plates oiler too high a frictional resistance when trailing. The value of this resistance can be found from the ordinary formula, counting both sides when reckoning the area. For such small areas the figure given by the formula is usually on the low side, but the coefficient f may be increased to allow for this, and the relation
P = .005 Av 2 lbs.
is fairly accurate. Although a bow rudder is a potential source of danger, a small fixed fill, well forward on the bow section, is undoubtedly a help when ruddering sharply, as it reduces the tendency of the bows to sideslip on the turn, and as the after section moves in response to the rudder, it will tend to bring the boat round more quickly. The overturning moment is still present, of course, but its effect is small if a shallow fin is used and placed well
forward to secure the maximum advantage. A bow fin alsol helps to keep the boat on a straight course in a gusty side wind, and there is very little else in the bow section to offer much resistance to lateral motion when planing.
Two parallel fins are sometimes used, particularly with flat bottomed hulls, but this is not good practice, since the water is generally not moving in a straight line except near the mid line, and unless the fin is accurately parallel to the streamlines in its vicinity it will give rise to eddying and will have a considerably increased head resistance.
If the rudder is constructed so as to turn about an axis not far forward of its centre of pressure, the thrust on the forward part will tend to counteract that on the afterpart, and a light control will result. Much caution must be exercised in the design however, since, although the C.P. is not far from the geometrical centre when the rudder is trailing, it moves forward rapidly as the angle of incidence is increased, owing to the creation of a partial • vacuum behind the forward part, as shown in Figure 2. This may cause the rudder to overbalance on reaching a certain critical angle, unless the forward part is fairly small, say a quarter of the whole surface.
The rudder spindle should be faired for minimum resistance, and a good plan is to have this fairing in the form of a fixed strut with the spindle down the middle. The fairing can then be carried neatly into the hull bottom with a minimum of hydraulic interference between any of the parts. The rudder thrust bearings are then carried at the bottom of the strut. See Fig. 3. In most outboards the rudder is made in one piece with the driving unit, though in larger inboard boats it is sometimes advantageous to use two coupled rudders, one on either side of the screw, to avoid interference from the screw turbulence.
If a single rudder and screw are used, the usual practice of combining the propeller strut and rudder post is perhaps the simplest. The most common failing of small boats, particularly outboards, is the tendency to lift in the bows when planing. We have already seen that the most efficient
angle of incidence is not more than 50 and it is evident that a boat travelling tilted at 15° or so, is both inefficient and unstable, since side winds and small waves can the more easily cause the bows to deviate.
Non stepped boats are the worst offenders in this respect, as they generally run with not more than two feet of the hull in contact with the water, and a simple experiment with a saucer in a bath will convince anyone that a short blunt object is very difficult to steer straight when propelled from behind. However, very few people now build any but single stepped hulls.
With the weight of an outboard engine at the stern the pilot should sit rather forward of the centre of gravity in order to maintain the correct turn when planing. The impression of great speed, caused by flying spray and tilted bows when one sits well aft is quite false and the pilot should adjust his position so as to keep the boat practically horizontal, the correct angles of incidence of the bottom planes being arranged for in the construction of the hull.
If the hull is trimmed so that the two parts share the weight equally very little ” slapping” or deviation from the set course should occur. If most of the weight is concentrated on the after section the bows will be bounced by mere ripples, and while off the water can exert no control over the direction. The use of horizontal fins or hydrofoils under the bow section to act as” dampers” to the pitching movement is a fallacy, since once the movement has been started the fin will be tilted so as to accentuate it,
while if placed under the stern section, its function is already performed on a much larger scale by the bottom of the after body. One or two outboard engines have a horizontal fin on the strut, but it is difficult to see any use for it whatever, as there is no particular point in increasing the already large resistance of the driving assembly, and it can have practically no stabilizing effect on the boat, nor any control over the steering.
To summarize then : A deep stern rudder counteracts the tendency to roll on corners, while a shallow bow fin assists cornering and reduces deviation when running at speed. Pitching or slapping is reduced mainly by care in trimming the boat, and good design of the bottom.