how big of an outboard could I use?

I just bought a late 40's early 50's meyers aluminum boat. It's a 15footer with a 67" beam. Currently it has an Evinrude 18hp Fastwin on it. I am 6'1" 300lbs and my buddy about the same, it moves us ok but this boat is wide, and any chop or turning really bogs it down. Theres no data plate, dont look like there ever was one, so not sure what size motor i could go up to safely.

There is a coast guard formula to figure this out if you have the dimensions of the boat and the transom width. My guess would be 25hp would serve you just fine without pushing the limits of a boat of that age. Remember the Evinrude 18 was prior to OMC prop rating there motors so it was likely closer to 15hp at the prop. So a post 1985 25hp (prop rated) motor should push that boat fairly well compared to your old 18hp.

Check this out....
https://www.tinboats.net/forum/viewtopic.php?f=4&t=12144

thanks for the info, doing that calculation i came up with 31, which would be 30HP. Now a guy on another forum responded to my question and said he was looking at a new meyers with same beam width and it was rated for 35, so i should be ok in the 30 to 35 range i would think right? also i will only be looking for vintage 2strokers, no newer 4 stroke types

If the boat and transom are solid then you will have no problem with a 30hp. My boat is rated for a 25hp and I run a 30hp without any issues at all. My motor is badged as a 25 though :mrgreen:. Also my boat through coast guard regs comes up as 31hp max also but is tagged for 25 and I believe this is the boat makers way of being on the cautious side.

If you buy a 2 stroke OMC (Mercury also I believe) prior to 1985 then get a 35hp because they were not putting out this much HP at the prop, if you buy a motor from 1985 to present then go with the 30hp because they were prop rated in 85'. That is why the 35 was re-badged the 30 in 1985. Most 2 stroke 30-35's in the 70's, 80's and 90's were about 110lbs to 130lbs depending on what model, make and accessories like electric start or longshaft versions. So your transom should have no problems holding these light 2 strokes.

You should be just fine with no worries....... and a 30-35hp will make you much happier than that 18hp is. Seeing speeds of 25-30mph is highly likely if propped correctly. I can touch the 30mph mark with my 30hp in my 14' tin with around 800-850lbs total weight (boat, motor, gear, my 6yr old son, battery, me (210lbs).....etc)