The instrument to measuring hull roughness is the BSRA Roughness Gauge. The instrument measures the Average Hull Roughness (AHR), which is defined as the average of minimum 100 measurements of Mean Hull Roughness (MHR). MHR is the average of highest peak to lowest trough in 50 mm measuring length. MHR is comparable to “Average Rmax” or Rtm.
AHR on newbuildings typically is in the range of 130-160 micron (full A/C and A/F system). A well prepared new steel plate after blasting and shop primer typically has an AHR of 55-60 micron.
Measurements of hull roughness have nowadays been abandoned by most owners, but had much attendance when the self-polishing antifoulings were introduced during the mid-1970’s to mid-1980’s.
It should be noted that is not possible directly to compare AHR with other definitions for roughness, as the AHR is inevitably linked to the BSRA AHR gauge and stylus.
Surface roughness is closely linked to the paint specification. The specification for the necessary anchor pattern covers at least the roughness (sometimes also the profile). Surface roughness is given as “roughness numbers” according to ISO 1302 with reference to the roughness comparison specimen, Rugotest No. 3, which accords to ISO 2632/IT.
Roughness numbers are related to Ra, the arithmetical term for the deviation of the profile. If a numerical nominal roughness is needed, for instance Ra = 12.5 micron, the specification will be “Rugotest No. 3, N10”.