Sub-Troll 900 Speed/Temperature Indicator
Author: Capt. Mike Gnatkowski
With more than 20 years of experience as a Great Lakes charter captain I've found that two of the most important variables for consistently making good catches of trout and salmon are speed and temperature. A unit, like Moor Electronics' Sub-Troll 900, that gives you speed and temperature at the cannonball can be worth its weight in gold for the information that it provides. I can't imagine trolling without one.
Temperature often, not always, determines where trout and salmon will congregate. Certain species of fish have certain water temperatures that they prefer. Frequently though, fish will be out of temperature. They have no choice. Baitfish that they feed on may not be in the fish's preferred temperature range. A fish has to eat so they will move out of their ideal temperature range to feed.
There also may not be any water in the lake that is within a fish's preferred temperature range at certain times of the year. During most of the year, the lake is colder than the fish would like. Only during the summer months are fish likely to find ideal water temperatures. Quite often, that preferred temperature will be at some depth. Knowing at what depth the preferred temperature is or the temperature at the depths that you're catching fish, can be critical. Many times you're looking for the warmest water you can find. The difference between 45° water and 48° water is huge and being able to locate and note changes in water temperature on the surface and down below is critical.
Once you've found the fish is where speed comes in. Speed determines how your lures act underwater. Lures have ideal speeds that make them dart, flip, wiggle, snap and spin to mimic something that a fish wants to eat or is going to bite out of anger, curiosity or frustration. If the lure isn't acting right because it's not being pulled through the water at the right speed, the fish aren't going to bite it. Pretty simple. So finding the right temperature will help you find fish, and knowing the right speed will make the lures work so you can catch 'em.
For years I fished without the aid of any electronic device to give me speed and temperature at the cannonball. After spending lots of time on the water you get a feel for how far back your downrigger cables are or the amount of bubbles coming off the cables; how your diver rods are bent with relation to how fast you should be trolling to catch fish. Once you get a strike, you could note how many rpms you were doing, surface speed on your graph or surface speed indicators or speed over ground on the GPS.
Most graphs come with built-in speed indicators or they are available as an option. GPS units and LORAN C devices also give you speed-over-ground data, but your surface speed or speed-over-ground is not always indicative of how fast your lures are actually traveling down below. There are deep currents that affect lure action. Wave action can also influence the speed of your boat and what trailing lures are doing. Length of lead is also a critical factor. The only way to know for sure what speed your lures are traveling at and the temperature where they are is to have electronics that will tell you.
After several years of fighting the urge, I finally decided to purchase a unit that gave me speed and temperature at depth. My initial experience was not a good one. The model I purchased was a digital unit that required a transducer mounted on the transom. After pulling the boat, drilling the holes to install the transducer and mounting the unit, I didn't get the results that I was hoping for. The unit never gave consistent readouts. It turns out that the frequency of the speed/temp unit was encountering interference from other Lowrance electronics on the boat. When the unit did work, the digital readout was inconsistent, jumping wildly from 2.0 to 1.6 to 2.4 and not providing accurate readings. The digital screen was also difficult to see from the back of the boat because of its size, and glare from the sun made it impossible to see.
During the winter, I ran into Duane Simano from Moor Electronics at a sport show. Moor Electronics manufactures speed and temperature indicators, including a unit called the Sub-Troll 900. I related my frustrations with the unit I'd purchased, and Duane assured me that I would not have the same problems with the Sub-Troll 900. For one thing, the Sub-Troll 900 is simple. It only has three interacting parts: the display readout, the antenna and the probe or sending unit. The sending unit attaches to the downrigger cable and transmits a signal to the antenna mounted on the downrigger boom. From the antenna, the signal is sent to the display through a cable. The word "antenna" here is a little misleading. You might think of a larger radio or GPS antenna, but the antenna for the ST-900 is just a small, spiral-wire unit that attaches to the downrigger arm. You hardly know it's there. The ST-900 uses a radio signal to transmit the information to the display unit, so you won't have the problems with interference associated with sonar-type systems. The ST-900 also doesn't require mounting a transducer or grounding rods. You can mount the unit without taking your boat out of the water.
Most speed/temperature units eat batteries. Not the ST-900. The sending unit operates on one 9-volt battery. Over the course of a season, and more than 100 days on the water, I might go through two batteries. The ST-900 underwater sending unit automatically turns off when removed from the water. It also has an illuminated display that makes the readout easy to see during that critical early-morning bite or after dark. The display is easy to read from anywhere on the boat and in direct sunlight.
One problem that every speed/temperature unit manufacturer has had at one time or another is leaking probes or sending units. Moor Electronics was not immune to the problem, but has solved it by pressure testing every unit that leaves the factory, and by using an O-ring seal that is designed to hold out 1,500 psi of pressure. The probe rarely, if ever, leaks.
One of the biggest advantages of the ST-900 is its accuracy and analog gauge. The large display accurately shows data in increments that are easy to see and read. Digital instruments are great for displaying a precise measurement that doesn't change rapidly. Analog gauges are much better for displaying data where not only is the current information important, but also the direction and rate of change. An analog gauge allows you to see pattern and trends, which is important when trolling.
How important an accurate, dependable speed/temperature unit is when trolling was illustrated to me many times over the years. There were too many times to count where my surface-speed indicators were saying I was speeding along at 3.5 mph, too fast to catch trout and salmon on flashers, when the ST-900 said my lures were only doing 1.6 mph. I'd bump the speed up to a smoking 4.0 mph on the surface and the downrigger rods would start popping when the display on the ST-900 said my lures were now going 2.3 mph.
Other times, I would have rods clustered in the magical 54° band above the thermocline. Just for kicks I'd drop a flasher down another 20 feet and the digital readout on the ST-900 would plunge to 42°. Too cold for salmon right? Bang! The 'rigger would go off with a big king salmon screaming out line. Because of the Sub-Troll 900, I found that some of the biggest salmon were consistently in water colder than what you'd expect.
The Sub-Troll 900 proved to be a real eye-opener and a learning tool for what really happens below the surface. It's become one of the most important pieces of electronics on the boat. It will be on yours too.
Click this link to purchase: Sub-Troll 900 Speed/Temperature Indicator