Ships – Anchor – Fluke type
Patent
1994-02-07
1996-04-30
Basinger, Sherman
Ships
Anchor
Fluke type
114310, B63B 2132
Patent
active
055115063
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to marine anchors.
2. Description of the Prior Art
The fundamental requirement of a marine anchor is an ability to dig into a mooring bed when pulled forwardly, and to remain stable in the dug-in attitude in the bed when pulled further, it is also well established that for high holding power the anchor should be relatively deeply buried during anchor setting. The nature of mooring beds varies enormously, for example, from hard soils of granular noncohesive dense gravels and sands or cohesive stiff clays to soft soils of cohesive muds. The mooring bed may also be rocky whereupon the anchors must be able to hook satisfactorily onto a rock for mooring. Satisfactory operation of an anchor in a particular mooring bed has necessitated the anchor to have a particular geometry including a fluke angle compatible with the mooring bed soil. The fluke angle is the angle formed between the fluke and a line in a fore-and-aft plane of symmetry of the anchor extending between the rear of the fluke and an anchor line attachment point in the forward end of the shank. At present, it is known, (see, for example, The Quarterly Transactions of the Institute of Naval Architects, Vol. 92, No. 4 Oct. , 1950, pps. 341-343) that for operation in a sand bed a low fluke angle in the range 23.degree. to 32.degree. provides peak holding power in the deepest burying anchors. Fluke angles of 25.degree. to 32.degree. for medium dense to loose sands generally provide satisfactory performance. For a relatively soft mud bed, the fluke angle for peak performance is larger and is in the region of 50.degree. to 55.degree.. In sand with fluke angles over 32.degree., the moment about the anchor line attachment point of the resultant of soil normal pressure and friction forces on an anchor fluke is insufficient to counterbalance the sum of the moments about the same point of soil edge resistance force on the fluke and soil resistance force on the shank during initial penetration. The anchor is, in consequence, longitudinally unstable during pulling, and rotates about the attachment point into a nose-down attitude wherein it fails to bury below the surface of the mooring bed or even breaks out of the soil altogether. A fluke angle of 32.degree. or less has thus generally been adopted for the deepest burying anchors to permit effective use in both hard and soft soils. The resulting disadvantage in soft soils is usually mitigated by maximally increasing fluke area at the cost of reduced structural strength for hooking on rocks. However, even with increased fluke area, such anchors typically provide a soft mud performance less than 15 per cent of their sand performance. This illustrates the problem involved in providing an anchor with a single compromise fluke angle capable of producing high holding capacity in both hard sand and soft mud.
The applicant's European Patent No. 0180609 describes a marine anchor which, by the provision of a barrier plane aligned with transverse non-cohesive soil flow at the rear of the fluke and with a restriction passage between the barrier plate and the fluke, causes a stalled wedge of mud to accumulate on the fluke during burial in a soft mud bed. This mud wedge shears between the leading edges of the fluke and the upper edges of the barrier at an angle of 20.degree. to the fluke (which is set at a fluke angle of 30.degree. for sand) so that an effective fluke angle of 50.degree. is established at the incident-mud/stalled-mud-wedge interface. This large effective fluke angle at the surface of the stalled wedge enables the anchor to operate satisfactorily in soft mud. In a sand bed, the restriction passage, although too small to permit a significant through-flow of cohesive soil (mud), allows escape of non-cohesive soil (sand) aft from over the fluke whereby shearing occurs at the fluke surface so permitting effective operation of the anchor in sand at the actual fluke angle of 30.degree.. However, although this arrangement does provide
REFERENCES:
patent: 2743695 (1956-05-01), Bowman
patent: 3685479 (1972-08-01), Bruce
patent: 4523539 (1985-06-01), Granger
K. P. Farrell, "Improvements in Mooring Anchors,"; Quarterly Transactions of the Institution of Naval Architects, vol. 92, No. 4, pp. 335-350 (Oct. 1950).
Basinger Sherman
BruPat Limited
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