Determine Direction of Troll
Written by Capt. Gregory Houtteman   
Monday, 30 April 2007 18:11

Downspeed, the speed of your boat measured at depth, is a very important component to your success in trolling on the big lake.  Downspeed gives you an accurate read of the conditions and speed affecting your baits at a specified depth of water.

First though, let’s understand speeds and the different measurements and terminology.  Boats measure speed in a multitude of different ways.  The most basic is the paddle wheel that provides feedback into a fish finder, chart plotter, or other display device; simply measuring the boat speed by the speed of rotation of the paddle wheel as it moves through the water.  Probably the most common now is the GPS Speed Over Ground (SOG) measurement which actually measures how fast the GPS unit is moving.

ImageSOG is a combination of the boats speed and the surface current, generally the tide in ocean applications, but not applicable for Great Lakes since our tidal movement is barely perceptible.  If your boat is going directly into a surface current, the speed over ground will be the boat speed less the surface current speed.  So if you’re trolling and your boat is going 3 MPH and the current is coming towards you a 0.5 MPH then your SOG is 2.5 MPH.   If that same 0.5 MPH current is going with you then your SOG would be 3.5 MPH.

Now if you have a paddle wheel for your speed on your boat and SOG GPS you will notice a wide differential in speed readings.  The way a paddle wheel works is rotation caused by water moving over it – so the boat moving forward and the current moving past it will actually register a faster speed even though you are going slower then it reads.

Measuring downspeed can be accomplished using any number of devices including the:

  • Cannon Speed-N-Temp
  • FishHawk
  • SubTroll
  • Depth Raider
  • Walker Speed and Temp
  • others…


These devices are typically connected to a downrigger above the weight and then are dropped to a specific depth for fishing.  The downspeed units have an integrated speed measurement device (probe) that is usually a simple paddle wheel and transmits the speed data through a wireless connection, coated cable, or a regular cable.   Think of downspeed as the equivalent of a surface paddle wheel in that it’s a combination of boat speed through the water and the current at the depth of the probe.  So if the boat speed is constant the fastest downspeed reading will indicate the boat moving against or into the current.

Keep in mind that the SOG and the downspeed have little relation other then the boat is moving through the water.  The current at depth could be completely different, and usually is, then the surface current.  Surface current is typically driven by the wind, except in the case of rivers, and down current is typically driven by bottom structure or other natural occurrences.

ImageThe easiest way to determine the direction of the down current is to make one or more slow circles without changing speed and recording the compass heading and downspeed to determine the fastest speed.  The fastest recorded speed will indicate the boat moving into the current, and similarly the current will be moving 180 degrees from the fastest boat heading.

Once you have determined your downspeed and current direction, then it’s up to you to determine what the best course of action is.  It’s a personal preference as to what direction you will troll and sometimes the weather will help make the decision for you.  What I have learned over the last couple of years is that the preferred direction to troll is directly into the current, wave state permitting.  Trolling into the current is most productive for downriggers and divers but cores work also.  The least desirable direction is to troll with the current.  In this case the best approach is to use more cores as they produce the best in this situation.  Cutting the current can be very productive but you generally want to do it related to going with the current.  This will often produce something we have all seen from time to time – one side of the boat doing well while the other side stinks – this is generally indicative of a cross current situation.

Regardless of which direction you choose to troll, it is important to know the direction of the down current and your speed at “the ball”.  Speed is important, not just from a direction determining standpoint, but also because it allows you to know how your baits are running.  If you find yourself in any trolling situation you want to make sure that your baits are performing as you would expect.  You don’t want your spoons just hanging there because your down speed is only 0.5 MPH even though your SOG is 2.5 MPH.  And you also don’t want to be ripping your three fly rig along at 5 MPH while you are only going 2.5 MPH SOG.

Downspeed and speed maintenance is an important part of your trolling tool kit – use it!

 
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