Measure the transom. 15" transoms are for shorts. My 34lb electric works fine for trolling on speed 2-3 without any wind and great for sneaking around bass fishing. Upload a picture of your boat. I had a 12' valco rated for 7.5 and I started with 6. it would plane me but 2 guys was slow and 3 stupid slow. got a 9.9 really quick solo but not quite fast enough with 2 and 3 people again too slow. so I doubled the rating and got a 15 and bam. need a bit of bow weight to run solo or she bounces a bit but to guys is quick and with 3 its still quick enough. and you can't get in trouble for going above the rating as long as you do it in reason and 8, 9.9, and 15 horse motors come in the same package and are light. especially 2 stokes so there is know stern weight issue which could be a problem. Like I said take a pic and I'll let you know what I think would be a good motor for your 12'er. I am on my 3rd 12' boat and I wouldn't put my 15 on my first boat.