By: Dr RL Townsin.
The following is an up-dated version of the guidance developed during the 147 surveys reported in Ref. 2 below.
WHAT IS TO BE MEASURED
- Bear in mind that it is the roughness of the intact paint coating that is to be measured.
- Therefore, avoid running the stylus over regions where the surface is uncharacteristically, severely damaged locally, or the substrate is deformed, eg. weld beads. Docking block damage or grounding scrapes, for example, should be noted but avoided by the HRA (Hull Roughness Analyser) in its pass.
- A roughness survey is required to arrive at a measure which can be used to determine the added friction drag due to the roughened, but foul-free, coated surface. This purpose is the principal reason for the following standardized procedure.
- A survey can also be used as a quality assessment of a newly coated surface, together with thickness measurements. Notes should be made by the surveyor, of observed overspray, runs, drips, detachments, grit inclusions and other application faults. Photographic records can be helpful.
- Surveys before and after re-coating can indicate application quality.
HOW TO MEASURE
- The surveyor should aim to take measurements with the HRA at about 100 sites, as evenly spread as possible, over port and starboard sides and the flat of bottom, encompassing all of the loaded wetted surface. Measurement on one side only, is unlikely to be typical of the whole hull surface.
- A note of each site position should be made on a profile and bottom plan or wetted surface drawing, as available.
- The surveyor will need to plan for access in dry-dock, probably by ‘cherry-picker’. Some surveys have been made in-water by trained divers using a waterproof HRA. Whilst this mode is exceptional, it has the advantage of relatively easy access when there is adequate bottom clearance.
- At each site, one pass of the HRA over the intact paint surface should be made over about one arm’s length, say, just under 1m. The direction of the pass should be approximately along the flow line ie. for the sides this will be more or less horizontal.
- Each pass will provide a value of Mean Hull Roughness (MHR) in mm, which must be identified with the site eg. by a site number.
- The average value of the 100 or so readings of MHR gives the Average Hull Roughness (AHR) over the hull.
- The ball-headed stylus of the HRA measures the roughness displacement perpendicular to the local surface, whilst the hand held trolley wheels will measure longitudinal distance. In the case of low surface energy, ‘foul-release’, coatings, the stylus may judder as it passes over the dry, rubbery surface, giving an incorrect reading. This may be avoided by using a hand held water spray to dampen the site. However, the consequently wetted coating tends to be slippery with these coatings, so that the surveyor needs to ensure that there is no wheel slip. Wheel slippage will increase the measured MHR value.
- Good practice will involve the surveyor running the HRA over a sample ‘foul-release’ surface, to become accustomed to the circumstances. It is, in any case, good practice to run the HRA over the calibrated roughness strip provided with the instrument, whatever the coating, before setting out on the survey.
- The final measure of AHR can be used in Townsin’s formula to assess the added friction drag due to the roughness, from which the speed/power penalties resulting from the added roughness, can be calculated.
- It can be helpful, in evaluating the surface roughness, to plot histograms of MHR (frequency of occurrence versus MHR value), especially for comparing surveys before and after re-coating. It is also instructive to compare averages of MHR for port and starboard sides and flat of bottom.
- Poor application will result in increased initial roughness. The surveyor should be aware of potential faults, record those he detects and use histograms of MHR to help describe them in his report.
Some insight into application faults and problems of surveying using an HRA, may be found, inter alia, in a paper read as far back as 1976 :-
Ref. 1. Townsin RL, Wynne JB, Milne A, Hails G.: Hull condition, penalties and palliatives for poor performance.
4th Int. Congress on Marine Corrosion and Fouling. Juan-les-Pins 14 June 1976.
A development of the use of MHR histograms to determine application quality may be found in :-
Ref. 2. Townsin RL, Byrne D, Milne A, Svensen T.: Speed, power and roughness: the economics of outer bottom maintenance. Trans. RINA Sp. Mtg. 1980.
RLT. Sept. 2012.