Hull roughness, surface roughness & Propeller roughness – some definitions. As “roughness” is not just roughness, definitions of some relevant types of roughness might come in handy.
 
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 steelplate after blasting and shopprimer 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 it 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”.
 
Note also that different comparators may be based on different roughness parameters. E.g., the Keane-Tator Surface Profile comparator uses Average Maximum Peak Rz - the ten-point height of irregularities.

Propeller roughness

There are three main choices in technique for measurement of propeller roughness.

1. Stylus instruments in situ.
2. Stylus instruments following replication.
3. Comparators.

Stylus instruments are used in situ for the finishing of new propellers in the foundry. Compact instruments are available with digital readout, but the instrument is considered too delicate for routine dry-dock use.....

For more info download attached file

Infomarine On-Line Technical Library

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Propeller_hull_roughness.pdf	Hull roughness, surface roughness & Propeller roughness – some definitions	43 Kb