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I was at the marine store and they had some small mercery outboards. The 3.5 hp caught my eye because it has an on-board gas tank. I am going to have to by an outboard soon and had been looking at larger motors (6-9.9 hp) because they have alternators and could charge my batteries (6 amps ) or at least let me run my lights if the battery did go dead. But now I am thinking it would be really nice not to have a gas tank on the floor in the cockpit. The only question is can 3.5 hp push a 22' boat at a reasonable speed in a reasonable amount of current.

No. Won't have enough power to get you out of trouble in moderate wind or current.
This link has some good info: http://www.onlineoutboards.com/article_ ... rd_hp.html

I have a 4hp 2 stroke on my Mac 24. I only use it to get in and out of the channel. It works fine most of the time, but when current is strong it is marginal. My shaft is longer than a typical short shaft. If the 3.5 is a short shaft, you may have trouble with cavitation. Remember, even with a built in gas tank, you still have to carry gas in a gas can for backup. That is unless you like to live on the edge...

No, IMHO, this motor is not adequate for your boat. I'd go with at least a 6hp.

Not enough power.

A displacement hull is going to move at hull speed relative to the water, whether the water is moving or not. If the boat is moving at hull speed of say 6 knots into a current of 3 knots, a larger motor isn't going to make it go faster than 3 knots over ground because that would be greater than hull speed in the water. Wind is a different story. I believe 3-1/2 HP will push your boat to hull speed, however it will be working much harder to do so, and it may not be able to hold that with a strong wind on the nose.

All that said, imagine leaning over the transom in 1-2 foot waves trying to refill that tiny tank.

I use a 3.5 hp two stroke long shaft Nissan as a dinghy motor. I carry it on the starboard side transom of my 26X, on a retractable mount. It works fine as a dinghy motor, and I am able to lower it and power the X as well. I've mainly run it in this configuration as a trolling motor, when fishing a downrigger for salmon or lake trout. It pushes the Mac fine at 2 knot trolling speed, and when opened up to full throttle, can get us up to 4 or 5 knots, if the water is smooth, with no wind or current. However, it can't get the job done in windy or rough conditions. I once had problems with my 50 hp motor, and was relying on the 3.5 to get us around. We were anchored near shore, and a strong wind came up, with gusts over 40 mph. We started dragging and I tried to hold us off the beach with the 3.5 hp kicker. Against that wind, I couldn't even keep it running. As I increased throttle, it simply stalled out.

Pete - check out the "Tender motor research" thread in Performance and Tuning

I cant answer your question about what HP your 22 Venture needs, but there are some tidbits in there that you might find interesting.

One of them is that there isnt any difference between 4-6 HP motors in many lines - other than a carb change. If you can get a 4HP cheap, you might be able to upgrade it to a 6HP