The ease and capability of a yacht in a sea way are not wholly dependent upon length and breadth, but are largely dependent upon her depth under water and her height out of water.
With regard to the height of freeboard a great difference of opinion would appear to exist, inasmuch as yachts of 150 tons exist with only 2ft. 9in. of side above water at the lowest point of the deck, and yachts of 40 tons are frequently given as much side as this. One rule for determining freeboard is based on the assumption that a yacht should have so much side that when she is heeled none of her deck is immersed until a certain angle of heel is reached. This angle is generally put at 30° for small yachts of 5 tons, and diminished to 24° for yachts of larger tonnage. This rule is a trustworthy one so long as the yachts are of from four times to four and a half times their beam in length ; but when the length much exceeds these proportions it is inapplicable. For instance, the Jullanar is nearly six times her beam in length, or 16.88ft. to 100ft., and if her freeboard had been apportioned by the rule that her deck was to reach the water at 24° inclination, she would only have been given 3ft. 2in. of side at the lowest point. This would have bees a sufficient proportion for a yacht, say 72ft. long and 16ft. broad, equal to 77 tons ; but the Jullanar is 27ft. longer, and has the propelling force in the way of sails of a yacht of nearly double tonnage ; she therefore requires increased side to assist her stability so that she may carry her canvas effectively, and at the same time keep her as dry in a seaway as the shorter and slower yacht (see page 46). It is thus quite plain that the freeboard of a yacht should not be decided upon by her beam alone, irrespective of her length; neither would it be satisfactory to decide upon freeboard by length without reference to beam, and a rule is required that will include both length and breadth in its terms. It is found that the freeboard* of well-proportioned yachts of four and half beams to length varies as their half-breadths multiplied by the sine of 24° ; also that their freeboard varies pretty regularly as the square root of one-fifth their length on the load water-line ; and the mean of these two quantities will give an approximate height of freeboard for yachts of any proportion of beam.
The sine of 24° is 0.4; half of 16.88 is 8.44ft., and 8.44ft. x •4 = 3.37ft. Next one-fifth the length of 100ft. line is 20, and the square root of 20 is 4.47. Next 4.47 added to 3.37 and divided by 2 make 3.9, and the exact freeboard of Jullanar at the lowest point is 3.9ft. For Freda, 5 tons, 04x es) + 030.5) the freeboard by the rule would be 2 5 = 1.9ft., which is • ( 2 04 X equal to her freeboard. Applied to an Itchen boat 5 = 1.9ft., which is the exact freeboard of the 23ft. Itchen boat described further on; in short, the rale, if applied to yachts of any extreme proportions, will give a suitable height of freeboard for them.
In designing merchant ships it is usual to consider freeboard in relation to depth as well as length and breadth, but as a yacht's free board may be regarded as an unalterable quantity after she is once ballasted, depth under water need not enter into the calculation, as the quantity of freeboard given by the rule is sufficient for any depth a yacht is likely to reach, and is at the same time appropriate for shallow yachts ; indeed, the freeboard might be reduced for very deep yachts without unfavourably influencing their performances ; but in shallow yachts freeboard becomes an element in insuring safety, as it largely assists in lengthening out the range of stability.
Great depth of body under water, coupled with good freeboard, will insure a great range of stability ; and, coupled with suitable length, ease, power, and dryness in a sea way ; indeed so valuable has depth proved for these qualities that in reducing length and breadth a corresponding decrease in depth is seldom resorted to ; thus whilst in large yachts of 300 tons we find the depth only equal to about half the beam, in small yachts of 5 tons the depth is nearly equal to the whole beam. [Depth as here used means the distance from the load water-line amidships to the top side of the keel at the centre of length of the vessel.] How ever, although depth does not vary directly as the beam varies, we find that it does vary in a ratio with the cube root of the beam, and a suitable depth for a yacht of any given beam can be found from the formula.