The classification societies requirement is that holding down bolts be checked by a surveyor, within each survey circle. this interval of time may be too long and the bolts should preferably be checked at six monthly intervals, unless there is a case history of the bolts going slack more frequently. Checking holding down bolts can be carried out on board the ship itself.
In new vessels, the bolts should be checked within one month of the commencement of the maiden voyage, or earlier if possible. The interval may then be gradually increased if all is found in order. After a vessel has been through bad weather, the bolts should be checked as soon as possible.
A rough method of checking holding bolts is the hammer test. Hold the tip of the thumb on one side of the nut face and strike the nut on the opposite side. If the nut is slack, the nut and stud spring against the thumb and then retract. the movement can be felt against the thumb.
If a holding down bolt is of the fitted type, this test cannot be used, and a hydraulic jack must be used. Due to the presence of bilge water on the tank top at various times, the holding down bolt nuts may rust and seize on the studs.
In this case, the seized condition makes it seam as if the nut is tight. The hammer testing method, however, can be used in finding slack nuts, even when they are seized on a stud.
