I get this question a lot.  And, there are a lot of misperceptions/beliefs about the differences between the two types of lines.  I often hear “A DT line casts better on a bamboo fly rod”, or “I use a DT 4 or a WF 5 on my bamboo fly rods”, or “I just don’t like DT lines so I always use WF lines.  So, let’s look at line construction for a bit.  Below are the line profiles for the popular Corland 444 Peach fly line taken from the Cortland WEB site:

Cortland 444 Peach Line Profiles

You’ll notice that the Level Tip and the Front Taper are the same on both kinds of lines.  It’s only until you get to the end of the body on the WF line that things change and you get into the back taper and running line.  So, if you’re casting either line and do not have more than 34 ft. of line (the length of the Level Tip + the Front Taper + the Body on the WF line) past the rod tip, the two lines will cast the same.  In this instance, both lines have the same profile and weigh the same…so they’ll cast the same.  If you have more than 34 ft. of line past the rod tip, then the WF line will start to have problems because the small diameter running line will not transmit energy through the fly line effectively.  The DT line will continue to cast well.  That said, with any longer casts, the WF line will shoot easier because the smaller diameter will move through the guides on your rod better.

I’ve looked at a lot of similar profiles of lines from a number of different line manufactures.  Those manufactures that make specific lines in both WF and DT, the front profiles are the same.  However, we’re now seeing some manufactures coming out with special DT lines (not available in WF) with front head profiles specifically designed for delicate presentations, and these lines might be desirable for bamboo fly rods.  And, DT lines have the added advantage of being able to turn around on your reel and use both ends, allowing you to get longer life out of your fly line.

Most of us don’t have good enough casting strokes to carry a huge amount of line past the rod tip at still get great presentations…I certainly don’t.  And, I find that most of my fishing is done from 10 ft. to 30 ft. in front of me.  If you consider 34 ft. of fly line + a 9 ft. leader + the length of your fly rod, that covers most fishing conditions where we actually fish.  Bottom line…use the line that you like on your bamboo fly rods.  There aren’t huge differences between DT and WF lines for most fishing conditions.

If you want to know more, noted casting expert, Bruce Richards, has written some great articles on the subject of fly lines.  Check out this article at:   http://www.sexyloops.com/articles/wfvsdt.shtml.