In my search for the ideal boat, I found the FW tended to be a lighter boat than the other manufacturers. Especially when compared to a Crownline, for example.
Keep in mind, those kids are only going to get bigger, and want to do more stuff, faster. Right now it may just be 1 tube, soon 2, then 3. Then ski's and Wakeboarding. You don't need to go with the biggest engine available, but my rule of thumb is about 100 Hp for evey 1000 lbs. Not real scientific, but works well. As I said, I test drove a 21ft Ebbtide with the 5.0 EFI (250 hp I think?). That was quite the pooch with just myself, the mechanic, and 1/2 gas! I couldn't imagine chopping off another 30hp from that!
Now, I think the 6.2/320hp is a little overkill for the 210 (ok, not really). Certainly wouldn't be worth the extra $3k, but at least take a good look at the 350 mag, and maybe settle for the lesser 5.7, or even the 5.0 EFI if that's what the wife decides. Just don't make the mistake of getting way too little and regret it too much later.
Just remember, you don't have to drive WOT all the time. But, it's nice to know you've got the extra umph if you have a full boat, or just by yourself with nothing but glass in front of you.
As far as propping the boat right, try this link.....
http://www.go-fast.com/Prop_Slip_Calculator.htmI hope that helps.
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Walt B
"Debt-Charged"Black '07 H210SS6.2L Bravo One turning a 21P HIFive
Wakeboard Tower with Perfect Pass Stargazer version
Missouri River; Kansas City, Mo.