My PT 1/4 boat didn't come with one either and got different stories from different sailors as to if I should need one or not.
Looking @ the marina I'm at, I saw everyone else had one! (hmm, if they ALL have one and I don't...)
Then I got Don Guillette's "The Sail Trim Users Guide" and read it over til understood it, (my second year sailing, also my first year with a REAL boat. so I've no clue what I'm doing at times).
The Boom Vang helps controls a few things, draft and draft position of the sail, sail twist, (the wind is stronger as you go up the sail from water level. Twist OPENS or CLOSES the top of the sail to "dump" air out, this equals PSI through out the entire sail). Draft is where the "belly" of the sail is.
As well as other controls on your main sail.
After reading it and looking close at my main sailing,I ordered a boom vang. It just arrived from West Marine.
You can use the main sail sheet as a vang to keep the boom "down" and contrl the sail twist, but after reading this book, I THINK you really need one to get the most outta your main sail.
You can (like I was last year and this year) just go out and have fun with your boat and set the sails as best you can, but if you wanna really set up your boat to work it's best, I THINK you need one! (I figure it'll keep me outta trouble is a strong poof hits me- again).
Go to "U Tube" and type in Boom Vang,see the vidoes on it and the whys and you'll see if you need one, (U Tube is a great source of info on ANY sailing topic)!
Joe
This is just my 2 cents worth, any advice I spew out of my pie hole should be taken with a few beers!
