Is a 5hp motor much more powerful than a 3hp?

I have a 14' jon boat and use a 3hp merc to push it. It does a decent job on local lakes. The river we live on here moves pretty swiftly and the 3 hp is a bit too small to use. I have a chance to buy a 5hp motor and wonder if it would be any difference. Do forum folks have an idea if the 5hp is that much more powerful than a 3 hp? Thanks

Depends, top speed probably only 3-4 more mph, but if loaded the boat down a lot or for you with swifter river current, the 5 hp will push it much better than a 3 hp. I'd jump to at least a 6 hp if you can find one used in good shape. I find the 9.5-10 hp engines are too heavy and cumbersome if not left on the john boat all the time. When I traded in my 9.5 hp 1970ish Johnson since way too heavy (like a 20 hp in weight almost) to take on and off my 14' V-hull with no trailer (throw in back of truck), I wanted a 5 hp, but ended up with a 4 hp found a good deal on. Only think I don't like which I really wanted was reverse and neutral gears instead of just forward gear since when start it, it takes off forward right off the bat and not really ready for it. My 4hp doesn't push my 17' alumacraft lunker boat in swifter current hardly at all going against the current, so I would opt for a 6 hp just to be safe if using in river currents a lot. I'd say if the 5hp is a good deal and you can unload the 3 hp for a decent price, go for it though like I did wanting a 5 hp and settled for a 4hp. Swift river current still sucks though with only my 4hp like I said, so a 6 hp would be best without going too heavy with a 9.9 hp.

You'd be a lot happier with a 10hp or 15 hp on it than a 5hp.

Depends on the age of each. The newer motors are much stronger than the older ones. . 5 should be 40 % stronger than a 3.