The FCV-585 is an excellent fishfinder. If fact it is one of the best on the market right now. I would recommend the 527ID-IHD in-hull transducer over the transom mount. The 1kw power is going to out perform that of the 600w transom mount. There is a 1kw transom mount (525TID-TMD) but it is not made to be used while running up on plane it is only recommended use is trolling.
Neil - Scuba Steve's Marine