Re: Trolling motors for saltwater boats
Around here (PacNW) most people would have a 6 to 8hp outboard on a boat that size for trolling and emergency backup. Most 6hp and below are single-cylinder motors, and 8 and above are two-bangers. The later tend to be a lot smoother running.
You will probably need to install a "kicker bracket" on your transom. The kicker (trolling) motor clamps to that, and the bracket itself can move up and down.
Electric motors tend to be for smaller boats (e.g. skiffs) and most are not salt-water rated. At any given time Seattle Clist will have dozens of kicker motors in the 3-10hp range listed so there is usually lots of choices.
If you are really serious, each of the manufacturers even make a motor specifically designed for pushing a large boat slowly (but with only a 17 I don't think it is necessary to have one of these motors). For example I have a Mercury 9.9 "Bigfoot Pro Kicker". Lower gearing, bigger and flatter prop, and (supposedly) a stronger transfer case than their other small motors.
-dm