How many HP motor for raft?

2hp will keep you going.
Rafts aren't built for speed, so more hp won't really do you much good.
5hp would be MORE than enough.

Don't know about the Missouri River Breaks, but wind can be miserable on any river
I have a 9.8 HP Nissan, not fast on my Aire 18' cataraft, can certainly buck head winds and easily push our crew's rafts

A five should keep you going in most typical rafts. Don't expect speed, but unless you are piled super high and create a major frontal wind load, you should be ok.

Agreed, 5 hp will keep you moving. I have a Honda 5 hp and when the winds pick up (20- 30+ mph) I get about 3 mph, but this also depends on your load and the river flow. My boat fully loaded is about 1000-1300 lbs (3 humans, 3 dogs, and all of the other "essentials"- or so I'm told.) Don't forget the extra gas tank(s)!

yeah, 5hp seems a good balance, imo. easy for one person to handle and move around at ~60lbs but I can get up abit over 7mph in a fully loaded out multi day raft with three adults on board in dead current or a lake, as well.

A thing over half throttle is wasted fuel anyway. Be content to putter along and be glad you're not fighting the wind with your back and arms.

We did a few trips up there and now is a good time!!! If you go to low AND the wind blows (which it will) you need a 9000hp motor (it’s a engine not a motor damnit!) bad news is it’s like 10 inches deep sometimes lol. Anyway we did a all natural trip no engine and I tied on buckets and made it out but it was terrible. I bet a big raft could hold a ten horse before it started to squish or wheelie? Mines 6 and would rip up that no problem. Pretty float and great hunting up there. Make sure and side hike a canyon or two. We found a giant ram skull and a bunch of rattle snakes. There’s a placard too where Lewis and Clark camped that pretty cool to imagine. In their diary they said they killed 40 elk or something stupid lol. Anyway 6-10 unless it’s a zodiac then 50? Lol have fun. Go soon for the water flows!

For a oar raft, after about 9hp you're really just wasting horsepower. I get down stream at about 7mph with a Yama 8 on the back of a 16' Maravia self-bailer.

As important as the motor is the transom it's strapped to. A transom that joins the frame will work much much better than one that sits on the rubber with straps.

It may be instructive to know that the Westwater rangers used five horsepower Hondas for years.

And on a real windy day down below, I had been known to lash onto as many as three or four worn out and struggling rowers, to help them get to Cisco. You don't move fast, but you do move...

Oh and check the regs your only allowed an engine on the white cliffs stretch certain calendar days!!

I had a similar question and found outboards between ~3-6HP had the same size and weight, just a different carb to gain more/less power. I went with a 6hp since I was carrying the same engine it might as well carry the most power.

Not sure if this is true across all outboards, but was the case with my Yamaha.

Pretty sure I found that information here on a previous post and it really spoke to me.

4 to 5 hp is plenty. Like stated above... you'll just keep moving in wind, not looking for speed.