The RPM Table and Search Patterns

Coast Guard Auxiliary surface facilities need to be inspected each year to confirm that all required items are in place.  One of those items is an RPM table.  What is that?  Very simply, it is a table (or graph) that identifies the boat’s speed at various engine RPMs.  For example, here is the RPM table and curve for SEARCH ENGINE:

In today’s modern GPS world, development of the RPM table is easy.  Run your vessel at various RPMs and document your Speed Over Ground (SOG) on your GPS.  Then turn around and do the same thing in the opposite direction to negate the effects of tidal current and wind and average the values.  This is most accurately done when directly in line with the current and wind (such as in the Oakland Middle Harbor in SF Bay).  If you have a fuel totalizer, you can also document your fuel burn at various RPMs.  And if a twin, document it with only one engine running in case of an engine failure.

In every day boating, having knowledge of your boat’s speed and fuel burn at various engine RPMs is critical for proper trip planning.  But when prosecuting a SAR case, it’s even more important and can mean the difference between a successful rescue or the inability to find a survivor.  The type of search pattern used will determine how to manage boatspeed. 

The Coast Guard uses both precision and drifting search patterns.  Precision patterns include Creeping Line (CS), Parallel Search (PS), Track Line Non-Return (TSN), and Track Line Return (TSR).  All precision patterns are based upon the start position and all turn points being a specific latitude/longitude, i.e. with respect to the sea bottom.  In other words, all points can be plotted on a chart and executed while taking into account current set/drift and leeway.  You will actually navigate to that turn point on the chart.  Chartplotters make this easy as they always show Course Over Ground (COG) and SOG, and you can use cross track information as well. If you are in a current, you should adjust your speed on each leg based upon your speed over ground as shown on the chartplotter.

In contrast, drifting patterns such as the Sector Search (VS) and Expanding Square (SS) have an initial Commence Search Point (CSP) which is a geographical position.  But after that, the pattern drifts with the elements and it is critical that the COG and SOG on the chartplotter is NOT used.  Remember, the GPS only knows your course and speed relative to the sea bottom, not through the water.

Here’s an example – imagine you are executing a pattern in an area of high current (such as the five knots that regularly occurs under the Golden Gate Bridge).  If you were using the GPS speed to set a search speed of six knots, you would actually be making 11 knots through the water when traveling with the current, and only one knot when against the current.  Your expanding square would be grossly inaccurate.

Instead, for drifting patterns such as VS and SS you MUST use speed through the water.  Unless your boat has an impeller- or pitot-driven knotmeter, you need to use your RPM table to set your search speed.  This will give you the proper track spacing for the particular search pattern. Similarly, do not use the chartplotter COG for heading – you need to use the magnetic compass for all heading information.

When I do checkrides with new or requalifying coxswains, I regularly see them execute VS and SS patterns while looking at the GPS for speed and heading.  This is totally incorrect – make sure your RPM table is complete and use it for these patterns.  Your PIW will thank you for it!