If you only fish in shallow water you probably won't need the higher output power. Shooting through the hull does attenuate the signal some, but it usually isn't a problem. I think I saw somewhere that shooting through the hull cuts performance by 1/3 to 1/2. I would think that in rivers, estuaries and lakes a lower power sonar shoud be able to get you a clear bottom picture. If you plan on fishing deeper water it would probably be wise to get a unit with a higher power output. I have a couple of Humminbird Matrix 500W fishfinders that are rated for 1500ft (457m). Even if I had 50% signal loss it would still work for me because the offshore areas that I fish are usually less than 200ft (61m). The only time I actually take advantage of the power of my sonar is when I am over the canyon in the local fishing grounds. I can regularly meter the bottom in 900ft of water.
Check out this site for fishfinder comparisons. I'm not sure if all these brands are available in Australia, but it should give you a good idea of what the current state of technology is.
I've had Rhynobars on all three of my kayaks. I like using the Rhynobar, but there are pros and cons. I like having my fishfinder mounted higher up on my kayak. The Rhynobar with the Humminbird mounting plate raises the finder by about 10cm. The Rhynobar gives you great flexibility in mounting equipment. It keeps things off the deck and in front of you. I've used mine for mounting radios, GPS, fishfinders, lights, electrical swithbox, tool holders, external FF transducers. It also makes a convenient attachment point for my paddle leash and other bits and pieces that I take with me. The Rhynobar is also very sturdy and can take more of a load than something that is just attached to the hull with rivets. I have used aluminum backer plates and fender washers when mounting my Rhynobars and I think that the kayak would be destroyed before anything could pull it off. The downside to the Rhynobar is that you have a large metal object permanently mounted above the cockpit rail. This arrangement makes it difficult to stack multiple kayaks or to store your yak upside down. You would probably have to use saddles when transporting your yak rather than just laying it upside down on a roof rack. Depending on the design of the kayak and placement of the Rhynobar, you might also have difficulty getting to the forward hatch. I didn't have this problem on either my Ocean Kayak Drifter or Prowler 15 because the hatch is closer to the cockpit. Having the Rhynobar on my Malibu X-Factor makes it almost impossible to move far enough forward to release the front hatch straps. Weigh the options before you buy. I would also suggest contacting Jeff "Rhyno" Krieger (mailto:[email protected]
) for specific information on price and shipping. He is a great guy and a living legend of the Southern California kayak fishing scene.