Most species of fish like certain temperature ranges and that’s where they tend to be found.  Find the right temp…find the fish.  And presenting your baits to them at the right speed is essential…to make sure the lures are running correctly, and to maximize the chances of triggering a strike. 

Underwater currents, structure, wind and wave action and surface currents can all affect your “down speed” or speed “at the ball”.  And knowing the speed differential between surface speed and down speed can help you properly orient your boat to the underwater currents.  Not all currents occur over a large area or just near the surface.  Some water currents occur in narrow areas far beneath the boat.  By looking at the speed differential between up speed and down speed, you can determine the strength and direction of the currents.  For example, if your surface speed is 2.0 mph and you are heading into a down current of 1.5 mph, your down speed is only .5 mph. This speed is way too slow if you are running a lure that was designed to work at 2-3 mph.  On the other hand, if you are running at 2.0 mph, in the same direction as a down current of 1.5 mph, your down speed is 3.5 mph…this might be too fast for the species of fish you are targeting.

Some of the most well known speed and temp units are the Fish Hawk 840, the Moor Electronics Sub-Troll 900, the Depth Raider from Kell Laboratories and the Cannon Speed-n-Temp.

Fish Hawk 840
The Fish Hawk 840 consists of 3 pieces…a helm-mounted display unit, a transom-mounted transducer, and a probe that attaches to your downrigger cable.  The Fish Hawk is the only unit which simultaneously displays both the surface and down speed and the surface and down temperature.  The surface information comes from the transom-mounted transducer, while the depth information comes from the probe which is fastened on the downrigger cable above the cannonball.

The Fish Hawk is a wireless device, in that there is no direct connection between the probe and the transducer (there is a cable running between the transducer and the helm display unit).  There is no special coated cable required…it will work with all downrigger brands or cables.   The downrigger cable is not used for data transmission…data is transmitted through the water like a sonar system.  In my experience, the Fish Hawk is only effective to a little over 100 feet, although the manufacturer says it will work as deep as 200 feet.

Because it is wireless, the installation is quite easy…no modifications are necessary for it to work on any downrigger.  However, because of the transom-mounted transducer, the Fish Hawk system does need to be installed on dry land.

The special sensors for the temperature and speed at the surface and the downrigger, continually relay data to the microprocessor in the helm unit.  All information is displayed at the same time on a large digital readout on the helm unit.  Temperature data can be displayed in degrees Fahrenheit or Centigrade and speed can be displayed in MPH or Knots. The 840 also features a calibration capability for fine adjustment.  The probe has good water sealing with O-rings on the battery cap, and uses a standard clip-on 9V battery connection.  I find the battery connection to be a weak link, since we replace our battery frequently to make sure that we are getting the best performance possible.  During tournaments, we will even put a fresh 9V Duracell battery in each day.

Sub-Troll 900
Moor Electronics makes the Sub-Troll 900 (ST-900). The ST-900 gives you both speed and temperature information at the lure. Moor has engineered their system to install easily on any downrigger, even while the boat is in the water.

The ST-900 system consists of three parts. The display readout, the antenna, and the sending unit. The ST-900 is wireless in a similar fashion to the Fish Hawk, although it does not have a transducer mounted on the transom of the boat. The sending unit (probe) attaches to the downrigger cable and transmits a signal to the antenna which is mounted on the downrigger boom.  From the antenna, the signal is sent to the display through a cable. There  is no physical contact between the sending unit and the display readout. The readout may  be installed up to 25 feet from the downrigger using the supplied cable.  Because there is no transducer, the ST-900 can be installed even when the boat is already floating.

Because the ST-900 uses a radio signal for the transmission of information, you eliminate some of the problems associated  with sonar type systems. The 9-volt battery will last up to a full season and, will not interfere with sonar or other electronics on the boat.  This can be a problem with the Fish Hawk and certain brands of fishfinders.  Because there is no transducer, you will get a constant reading on the display, even in choppy conditions.

The ST-900 underwater sending unit automatically turns its transmitter off when removed from the water. This insures that the 9-volt battery will remain ready for your next fishing trip.  While trolling after dark, the ST-900 Illuminated display makes it easy to see speed and temperature data.

Cannon Speed-n-Temp
The Speed-n-Temp Monitor from Cannon is designed for use with any downrigger on the market.  The sealed underwater transmitter attaches to the cable of the downrigger just above the weight. The Sensors on the Speed-n-Temp Monitor register the absolute depth of the lure and the lure's true speed as well as light intensity (water clarity) and water temperature at the lure's trolling location. These readings are then electronically transmitted up through the downrigger cable and digitally displayed on the Speed-n-Temp Monitor.  Absolute depth of lure eliminates blow-back estimations that anglers normally must guesstimate.

Using a standard downrigger cable, the Speed-n-Temp Monitor operates up to a maximum depth of 100 feet.  Using a coated downrigger cable increases effective operational depth to up to 300 feet.  The Speed-n-Temp unit activates when the speed wheel enters the water and automatically shuts off after 15 minutes of inactivity. A replaceable 9-volt battery will power the transmitter for up to 200 hours of operation. Battery voltage is also digitally displayed on monitor screen so that you know when a battery change is necessary. 

The Cannon's Speed-n-Temp Monitor has a 7 inch high-resolution screen, which provides an easy-to-read display of all the transmitted information. The monitor can be easily mounted at any location on the boat. The Cannon Speed-n-Temp Sensor attaches to the downrigger cable just above the weight, and provides true underwater conditions and lure depth.

In addition, the Cannon Speed-n-Temp probe can be integrated with the Humminbird 900, 700 & Matrix series Fishing Systems.  In that case, the Speed-N-Temp unit data is displayed on the Humminbird fishfinder screen along with the rest of the fishfinder data, giving the angler a truly integrated multifunction display.

Depth Raider
The Depth Raider from Kell Labs is a compact unit that displays speed in MPH & temp in degrees Fahrenheit on the display readout.

The Depth Raider system measures and displays this information with digital precision. The Depth Raider monitor tells you the exact speed and the exact temperature of your lure at it's running depth.

The Depth Raider requires the use of a coated downrigger cable.  When placed into the water, the underwater transmitter probe unit continuously measures speed and temperature, then periodically transmits a digitally encoded RF (radio) signal via the coated downrigger cable.  Like the SubTroll ST-900, the signal is received by a receiving antenna mounted on the downrigger boom. This signal is then routed to the receiver/display unit via a coaxial cable.

The display unit continuously monitors transmissions from the underwater probe unit. Once a transmission is received, the RF signal is decoded and the data is converted into speed and temperature info which is then displayed on the LCD display.

Coated downrigger cable is necessary to prevent the water from absorbing (and diminishing) the signal transmitted from the probe. The coating on the cable provides electrical isolation, since water is a very good conductor of electricity. Without the coated cable, as the depth of the probe continues to increase, the signal level at the antenna would diminish until it is no longer able to be deciphered by the display unit. 

While the Depth Raider system will operate without the coated cable; installation of a coated cable is highly recommended to provide proper operation to 200 feet.
 
Conclusion
These units all have their pros and cons, but they will all do a good job of informing you of conditions far below your boat.   Think about your installation requirements (coated cable or not…transom transducer or not…integrated fishing system or standalone unit),  and of course, your budget, when making your decision, and then go out and get one of these devices.  As they say, knowledge is power…and these Speed and Temp units will give you a lot of new knowledge about what is happening down where the fish are.  They are an indispensable tool that will help you put more fish in the boat.