As the guys above correctly pointed out, lots of factors come in to play as to whether you can (or cannot) track your gear on the screen of your sonar while fishing.
Strength of frequencies in the transducer and beam angle of those frequencies are probably the two most important factors as to whether you can or cannot track gear. A 1 Kw transducer is going to be firing way more power down into the water then a 600 W transducer will and the strength of the return signals will be that more visible on your fish finder/sonar screen. Add to your 1 Kw transducer a 25 degree beam (the newer ones have that) and you'll have that much larger a viewing "window" beneath your boat
Next--- the downrigger line you're using---are you lined with cable or braid, and if braid, what diameter is the braid? ---the bigger the diameter of your braid, the more "blow back" you'll be getting---the more blow-back, the bigger the chance your gear will NOT be visible in the beam angle of your transducer. The old style downrigger cable, although less convenient to use, tended to promote LESS blow back and consequently, your gear was probably a bit easier to track via sonar
Last---tide action and weight of your cannon ball.
At high or low slack tide, even using 250 lb test Scotty braid, your gear will be pretty much directly under your boat, so even with a 10 lb cannonball chances are it'll show up on a 600 Khz transducer at 200 feet of depth (unless you're ripping along at 3 knots). As soon as the tide starts moving, so does your gear beneath the boat resulting in gear being pulled outside of the sonar viewing window
But add a 15 lb cannonball, it'll tend to show even at speed on your fish finder if you have everything dialed in (proper gain, clutter etc). Some guys get creative and attach glass balls to their downrigger (Christmas tree bulbs, etc)---You're basically mimicking the swimbladder of a fish when you do that, creating an easier "target" and thus, a more identifiable return
As pointed out above, the new CHIRP technology is pretty stunning for target definition. You can fine tune which frequencies the transducer is using based on the depths you're fishing at or the fish species you'll be chasing. It offers a full "palette" of frequencies rather then just the 50/200 used by the older transducers.
Here are a couple of screen shots of what gear looks like with the newer transducers on the market:
In all these shots, the lower line is just a naked 8 lb cannonball; the upper line is a spinning, cut-plug herring
The next three screenshots are all the results of a thru-hull 1 kw straight 50 Khz transducer--- that explains the "blobby" target definition--- lower frequency--good for depth but not so good for target definition---
This shot is "bombing" the face of a cliff with a plug-cut herring--- the numerals disappeared off the screen because there was huge wave action (and bubbles) across the face of the transducer, but because it was a 1Kw thru-hull, it got a pretty good picture of not only the spinning herring but the cannonball perturbing balls of bait as it bounced down the cliff
This shot was using a 1 Kw 200 Khz transducer ---you can see that with the increased frequency, there is much sharper target definition
With the higher frequency you can just about count the individual rolls of the plug-cut
This last shot show target separation with the new CHIRP technology--- even though this was taken in shallow water, this transducer has the same crisp target definition in deeper water . With a 25 degree beam, I fully expect it to track even an 8 lb cannonball in 200 feet of water and provide evidence as to whether my plug-cut herring is still spinning like it should
Equipment--- the rule of thumb in sonar---spend your money on the transducer---the head unit (sonar/fish finder) is of secondary importance in target resolution although having a bigger screen is a good thing
The technology is increasing so fast that for a modest sum of money, lets say $ 400 - 800, you could get a good Lowrance or Garmin head unit that only a few years ago would have cost $ 1,000 +
Add to that a good quality 600 W transducer (Airmar!!!) that's PROPERLY INSTALLED on your transom (not getting dirty water from prop-wash or through-hull fittings etc) and you should be able to track your gear down to 200 feet of water depending on the gear/cannonball/tide action etc and how nimble you are with MANUAL manipulation of the settings (gain/clutter/signal strength etc)
Bump up the transducer to a 1 Kw---that'll make your modest-priced sonar head unit light up like a Christmas tree!
