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Why would a mate want a boom vang in the first palce? Here's why. The boom vang is the primary mainsail sail trim control for TWIST. Without either a soft vang, boom kicker or rigid vang, twist adjustment becomes a lot harder and without some type of vang you're only other twist adjuster is the mainsheet. The problem with using only the mainsheet to adjust twist is that while your adjusting twist you're also adjusting the ANGLE OF ATTACK and DRAFT POSITION neither of which you may not want to be adjusting at the time.
I've never been on a boat that had a boomkicker. One advantage it seems to have is ease of installation - it appears to be simplier to install than the Garhauer Rigid Vang.
My PERSONAL preference, for what it is worth, is that on boats under 27' I would consider the boom kicker as vangs like the Garhauer rigid vang and others might be overkill but for boats over 27' I prefer a rigid vang. In fact, the first piece of hardware I installed on my Catalina 30 was the Garhauer rigid vang. When I lived in So Ca I sold and installed them. I could easily install one in under 2 hours.
After I installed the rigid vang the first piece of hardware I eliminated was the topping lift. I have nothing against the poor old topping lift but I hate a loose line flapping in wind against the sail - it messes up my karma!!
So, the bottom line is, if I had a Hunter 31, I'd go for the rigid vang but there are probably a lot of sailors that would install the boom kicker - that's difference of opinion is what makes the sailing world go round and round.