15 hp will push a 16' boat, just not fast and not up on plane with a couple people and gear....
I would not want anything less than a 40 hp on a 16' boat, especially a heavier built one like a Hewes.
Like I mentioned earlier, I have a fairly light 14' Sylvan that will do 20+ mph with two people in it but that is without much in the line of gear. I tried going camping once with my ex and the dog on Chelan and it could not get up on plane with the extra weight, of course the ex was getting kind of large by then...
Also, when pulling downriggers with it, the shorter boat does not turn well since the downriggers produce a fair amount of tail drag.
As mentioned, I agree a 16' boat sounds ideal for what you are looking to do. The 17' Alumweld would be a good one if the price were right on it. They are a little lighter than the Hewes boats I think.
