20" shaft length should be fine, but why carry the weight if you're not going to use it for trolling? If you really want something that is a back-up, you're going to need about 20-25HP - for trolling 8 or 9.9 would probably be fine, and it will still get you home - albeit slowly... The problem is weight on the stern of a 20-25 outboard when you will (hopefully) never use it. And if you do - and the weather is bad - even a 25 is not going to make for a pleasant return trip. :( My advice - have a main engine that you are completely confident with - keep it maintained and it won't fail you. Your 225 Merc can be as dependable as dirt if well maintained - failures almost always are predictable. Don't ignore any problem, watch the poppits and thermostats, change the lower unit oil each year, look under the hood regularly for any corrosion or loose fittings (like oil injection tubes or water lines), and you should be fine. Have a dependable 25 watt marine radio with an 8 foot antenna and a back-up 5 watt handheld so you can call for help if you need it. Here's an interesting statistic from the aviation side: years ago, someone did a study of engine failure in private aircraft. The results showed that multi-engine aircraft had a higher rate of failure than single engine, even when you took into account 2 engines versus 1. The study concluded that owners with 2 engines were not as finicky about maintenance compared to owners with single engine planes. The owners with only 1 engine know there's no backup, so they were less willing to defer maintenance. The owners with 2 engines would sometimes push the envelope a bit more because they had a "backup." For those of who are pilots with some time on the logbook, we know losing one engine on a twin-engine plane in some conditions can lead to catastrophe. Yes, the plane will fly in normal conditions with one engine, but Murphy says the critical engine will fail at the worst time, and that may not be an easy time to recover. The bottom line - one well-maintained, dependable engine is better than two with deferred maintenance and latent problems...
