Do I need a blowoff tube?

I have a 6 gal carboy which leaves more room at the top for a 5 gal batch. Would I still need a blow off tube to avoid a blow-out or would I be safe using my airlock?

I would use a blowoff tube still. Chances are you'd be fine, but you never know. I have a 6.5 gal glass for my primary and my 5.25 gallon batch of ESB using WYeast 1098 slowly pushed foam out of the tubing for like 10 hours. You can take it out and put an airlock in after the intense fermenting is complete.
EDIT: I use a blowoff tube with every batch, just in case and remove after the crazy churn is over and put in the airlock/stopper.

I fermented 5.5 gallons of a wheat in a 6.5 gallon carboy and had a fairly violent blowout. I had the same amount of a nut brown in the same container and had no issues.

Additionally, the higher the amount of fermentables in the wort (higher OG) the more vigorous the fermentation will likely be increasing your chances of blowout. The IPA I just bottled overflowed my airlock but didn't blow it out and it was a moderately high OG (1.05).

Why risk it when it only costs $3 or $4 bucks for a tube. Put the other end in a milk jug partially filled with sanitizer solution.



By the way, the picture above is my Extra Light Ale that I just brewed on Saturday. Hopefully it will be like BMC with taste added. I have no way to do lagers yet, so I had to make an ale equivalent.

The short answer is... yes. Well it depends on so many things. To be safe if you are concerned about it use one. Never hurts and you will save yourself a mess possibly.