Anything larger than 80HP WILL get your boat up on plane, so yes, like Skunkboat said, it will push your boat. NO, it will not push it fast, but it will get you on plane. My guess will be in the low 30's, depending on load, wind, current, tide and prop.
Would I put a 120 on my boat? NO, not for a second. (Well, that's not quite true... if you look at my signature you'll see that I have a 120 Force on my Wellcraft Airslot. It's a good engine, but remember that my Airslot is only 16.5 foot long). Would I hang one on my V21? No, I would consider it way too underpowered. For me, the least I would use is a 150HP. (With the exception of the Suzuki 140 that thinks it's a 150)
Pic is of the 120 Force pushing my Airslot.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God)
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.