Theoretical Study on the Holding Power of the Anchor

This report deals with a study on the holding power of the anchor from the view-point of soil mechanics. It is considered that the holding power of the anchor is effected by the resistance on the flukes when they shear the sea bed. To simplify the matter, the next conditions are assumed. 1) The pressure exerted by the soil of the sea bed follows the Coulomb's Theory. 2) The side effect of the fluke is not considered, the subject being treated two-dimensionally. 3) The anchor shank lies horizontally on the sea bed. 4) The sea bed is considered to be saturated by water and perfectly loose. The author investigated theoretically the holding power H of the anchor fluke, and have got the following formula. H=f(θ, φ, δ) γ_t h^2 B where, h is the fluke length, B is the fluke breadth, γ_t is the density of the sea bed, θ is the fluke angle, φ is the angle of repose of the sea bed, δ is the frictional angle between the fluke and the sea bed. f(θ, φ, δ) have been computed numerically by an electronic computer, because of complexity. The result is that the holding power increases according to the increase of θ, φ, and δ. Considering the experimental results, it is not reasonable that the holding power increases simply according to the increase of θ. The author thought that it was due to the lifting force acting on the anchor fluke, so he estimated the force with regard to JIS type anchor. In the result, the theoretical value was perfectly consistent with the experimental value: i. e., when the fluke angle exceeds a certain critical angle, the assumed condition 3) becomes not to hold, then the portion of the fluke in the sea bed decreases, and the holding power decreases. Therefore, in order to give the holding power most efficiently to the anchor, it is necessary that the anchor is made in such a way that the weight is minimum, and the fluke angle is maximum as far as the flukes are not lifted up, and the fluke length is as long as possible. The critical fluke angle decreases according to the increase of φ and δ.