Instructions to Database
Guide to using the Skin Friction Database:
When opening the database interface (optimized for Google Chrome) the html page shown below will show up.
It consists of 4 distinct vertical areas which is (from top to bottom):
1. Data necessary for extrapolation
2. Interactive plotting of measurement and extrapolation data
3. Data Selection
4. Fuel consumption estimation tool
1. Data necessary for extrapolation
Two parameters are needed for full scale extrapolation, ship (or plate) length and ship speed. The parameter Hscale is optional and corresponds to the efficiency parameter C described in the “extrapolation method” chapter. The data will not be updated before the button “Calculate Friction” is pushed. There is a couple of seconds calculation time before the data is updated.
Setting Hscale to any other value than 1.0 will modify all extrapolated curves and delta Cf estimate except hydraulically smooth surface. Please read the section “Extrapolation method” before changing this value. Changing the efficiency parameter is only intended for minor adjustments, therefore C values outside 0.75-1.4 is not recommended. A result outside this range will still be produced by the database.
2. Interactive plotting of measurement and extrapolation data
In this section the data is plotted based on which surfaces is selected and for full scale what the length and speed is set to by the user. The data can shown in Model Scale or Full scale Reynolds number. The y-axis is CF and the x-axis is the Reynolds number.
Following the extrapolated values in the velocity dimension it is important to use the correct length to calculate the Reynolds number
, i.e. for model scale the plate length must be used (can be found in the surfaces data sheets) and for full scale the length defined by the user. It is possible to show or remove plotting of extrapolated values by clicking the box “Show Extrapolated Values”
3. Data Selection
The measurement filter, list and surface selection is in this section. Three filters are available on the left. The first is the device type. For now only flat plate measurements are included in the database, but if other device type measurements become available more type will be added automatically. Next filter is surface type, where each can selected/deselected reflected immediately in the list to the right, in the graph and in the fuel consumption tool. The same applies for the filter for selecting which labs results should be included.
The list in the bottom right reflects which surfaces have been chosen using the filter. The colored boxed in this list can be selected/deselected adding or removing that surface from the graph and fuel consumption tool. Clicking the text symbol will add a more comprehensive description of the surface, and clicking the “data sheet” link will produce a popup window with the full information of the surface, including photos. More information about tested surfaces are also available in “Rough surface description”
4. Fuel consumption estimation tool
The final section is related to the fuel increase estimation tool. Power (PD ) from full scale prediction (or whatever reference point the user will use), number of propellers, and Specific Fuel Oil Consumption (SFOC) must be supplied by the user. In addition the parameters in the first section must be supplied. Changing the numbers in this section does not require a recomputation, however if parameters in the first section is changed the button “Calculate Skin Friction” must be pushed for the data to be updated.
Output is given in estimated power, estimated fuel consumption and percentage increase of fuel consumption for each of the surfaces selected. All percentage increase is related hydraulically smooth surface.
For an explanation of the method used to estimate the fuel consumption increase please see the section “Fuel consumption tool”
Which rough surface to use
A commercial vessels surface is never hydraulically smooth. Therefore, if the user want to compare the hulls current condition to the newly build condition, a comparison between AHR (Average hull roughness by BMT roughness gauge) 65mikron antifouling to the hulls current surface would be more correct. If the AHR for the newly build vessel is known the roughness can be scaled within reason.
Say that the vessel have now been in operation for some years and the coating roughness have been increased by aging, flaking and/or recoating and is measured to be 110mikron, then the expected increase in CF, power and fuel consumption, will be the relative difference between those surfaces increase from the tool.
Vessel surface roughness are normally not uniform. Sunlight on the vetted surface flat of side, mechanical damage from quay compared to the flat of bottom for instance always creates different surface roughness over the years.
It is possible for SSPA to perform CFD analysis with varying roughness looking at the vessel as a whole, but this is not an automatic procedure. Instead, using a weighted average of different surfaces can be employed. Say, as a simple example that flat of side have mechanical damage on 40% of the wetted surface and that the remaining surface have a antifouling (AF) coating structure with a measured AHR of 100micron then delta CF could be estimated as
meaning that the reference is the 65mikron AF referenced to 40% flaked surface and 60% rougher AF scaled down to 100mikron.
One important advice using the tool is to decide by inspection and preferably hull roughness measurement the state of the vessels condition. Then using the database try and get as close as possible to the measured surfaces and scale them if needed and only if AHR measurements are available. Once the result is obtained consider that the result. If the outcome is not what is expected it makes little point to tweak the values to get the expected result.