Stabilizer of ship rolling

Abstract

The roll-stabilizer consist of short wings, with aspect ratio r=0.5-0.75, fastened immovably to the ship hull in staggered rows or in lines along the bilges, at port and starboard sides. The height of the wings is such that they do not protrude the maximum breadth and base line of the ship. The transverse distance between two wing lines of the staggered row is 0.5-0.7 of the wing's chord, and the stagger between wings of the two lines is equal to the chord. The wing profiles are parallel to the longitudinal axis of the ship or form a small angle of 2-4° with this axis. At zero forward speed the roll damping moment of the staggered stabilizer is almost equal to that of continuous bilge keels having the same height and area. During motion of the ship ahead the lift forces of the wings produce a large damping moment, considerably reducing the ship rolling.

Description

The invention belongs to sphere of shipbuilding, in particular to stabilizers of ship rolling, which do not contain any parts moving relatively the ship hull. It is known ordinary bilge keels as narrow bars fastened to the ship's plating along the bilge turn, at port and starboard sides. The length of the bilge keels is 0.3-0.6 of the ship length L. Complete area of the bilge keels is about 4% of the product LAB, where B—is the maximum breadth of the ship. The reduction of roll amplitudes, secured by continuous bilge keels, at zero forward speed is 40-50%; however, during advance motion of the ship in irregular waves with a speed of 10-20 knots the reduction of rolling is not more than 15-20%.

It is known also roll stabilizing wings attached in longitudinal lines to boards or bilges of the ship (patent GB 501081, 1936). Two wing lines can be provided on each side, then the wings of one line can be staggered to those in other line. To increase the strength of the wings, they are interconnected by a plate, the ends of which are fastened to the ship hull. The length of the wings, measured athwartships, and intervals between them are more than 2-3 chords of the wing. At such aspect ratio the wings extend beyond the ship cross-section, and at such large intervals the wing's stagger has no effect on the roll damping of a stopped ship. Decrease of intervals between the narrow wings to about one chord is unacceptable since it extremely reduces their lift forces. “A major disadvantage of this system is its vulnerability to damage and a weak quenching effect when the ship is at anchor. Hence, the system is definitely unacceptable”—H. E. Saunders, “Hydrodynamics in Ship Design”, New York, 1957, p. 553. Already 70 years the arrangement is not applied in shipbuilding, and it has no prospects in the future.

The aim of the suggested invention is a considerable decrease of rolling both at stop and at forward motion of the ship in sea-way conditions. This aim is attained by arrangement of short wings (aspect ratio r<0.75) having a sufficient strength and producing large roll-damping moments.

Description of the arrangement is explained by following drawings:

FIG. 1—Transversal section of the ship in region of the short wings roll-stabilizer;

FIG. 2—View on the staggered row of the short wings.

Wings 1 are fastened immovably in a staggered order to the ship 2 along the bilge, at port and starboard sides (FIGS. 1 & 2). The wing profiles are parallel to the longitudinal axis of the ship or form a small angle β with this axis so, that the angle β in the one line of the staggered row is in opposite direction to the angle β in the other line.

To avoid damage of the wings 1, their height h is taken so that the wings will be within clearences determined by maximum breadth and base line of the ship (dotted lines in the FIG. 1). The chord of the wing b exceeds its height, and the aspect ratio is in limits r=0.5-0.75. Relative thickness of the wing profile is from 0.05 to 0.15 and can be varied along the wing's height. The length of the wing row is 25-50% of the ship length L. The transverse distance between two wing lines of the staggered row is d=(0.5-0.7) b. The longitudinal interval between the wings in each line equals to chord b. The stagger of wings in one line relative the wings of other line is also equal to chord b (FIG. 2). Consequently, unlike the above-mentioned stabilizer of GB 501081, having great intervals between the wings, in the suggested stabilizer the adjacent wings are separated one from the other only in transverse direction by a small distance d, i.e. there are only small transverse gaps, and no longitudinal gaps in the staggered row. Therefore, at zero advance speed of the ship the roll damping moment of this stabilizer is almost equal to that of continuous bilge keels having the same height and area.

During motion of the ship with advance speed v and with angular velocity of rolling ω, the water flow relatively the wing has an average velocity V=ν+μ, where μ=ωR; R—distance from the middle point of the wing to the roll axis G (FIG. 1). The angle α between vectors V and ν together with the angle β form the angle of attack of the wing profile (α+β) or (α−β) (FIG. 2). The moment M_i of the wing's lift force Y_i about the roll axis is directed against the angular velocity of the ship ω and thereby decreases the ship rolling.

At forward speed about 20 knots and roll amplitudes 10-15° the maximum angle α=μ/ν is equal 3-5°. Therefore it is useful to have the angle β in limits 2 -4°.

The lift coefficient C_y depends on the angle of attack and on the wing's aspect ratio r=h/b according to the formula C_y=5.8(α±β)r_e/(2+√{square root over (r_e²+2)}), where the effective aspect ratio of the wing, attached by one end to the hull, is equal r_e=2r.

The aspect ratio of the wings r=0.5-0.75 secures enough great values of the lift coefficient C_y, unlike the ordinary bilge keels having, as a rule, r<0.04.

The lift force of the wing Y_i is proportional to the lift coefficient C_y, to square of the speed V and to wing's area S_i=h•b.

The lift forces of a wing line with angles of attack (α+β) will be much grater than the lift forces of the other line, where the angle of attack at this time is (α−β). As a result, the injurious interaction of the two wing lines of the staggered row considerably declines in comparison with the case when β=0.

The total area of all the wings S=Σ S_i is about 0.04 L•B, therefore the stabilizer's drag does not decrease essentially the advance speed of the ship.

The suggested stabilizer provides a summary roll damping moment M=Σ M_i which reduces 2-3 times the rolling amplitudes during motion of the ship with considerable forward speeds in sea-way conditions.

It is useful and convenient to install the staggered rows of short wings on ships with small midship coefficients (C_M<0.9) and rounded bilges.

Ships with large midship coefficients (C_M>0.95) or with angular bilges have a fairly large damping moment of the bare hull at zero forward speed. Therefore, the short wings roll-stabilizer can be installed on these vessels without staggering, i.e. by one wing line on each side of the ship, with the angle β=0 and longitudinal intervals between the wings equal to the wing's chord.

Claims

1. Stabilizer of ship rolling, consisting of wings, immovably fastened to the ship hull in longitudinal lines or in staggered rows, on the port and starboard sides, characterized so, that for the purpose of considerable decreasing the roll amplitudes of the ship in sea-way conditions with a sufficient strength of the wings, the aspect ratio of the wings is 0.5-0.75, the height of these short wings is such that they do not protrude beyond the maximum breadth and base line of the ship, the intervals between the wings in each line equal to wing's chord, and in a staggered row the stagger between wings of two lines is equal to the chord, while the transverse distance between the lines is 0.5-0.7 of the chord.

2. Stabilizer of ship rolling according to claim 1, characterized so, that for the purpose of reducing the injurious interaction of wings in the staggered row, the wings form an angle of 2-4° with the longitudinal axis of the ship, so that the angle in the one line of the staggered row is in opposite direction to the angle in the other line.