The sheetbend knot is very similar to the lap knot (castwell knot).
Does anyone know if makes any difference in the effectiveness of the knot for securing a looped leader to a fly line?
This page shows both the sheet bend and the lap knot
http://www.realknots.com/knots/sheetb.htm#Xschootsteek
I ask because the Sheet bend seems to be a knot still in my memory from my boy scout days a few decades ago. My mine wants to tie it instead of the lap knot.
Quoting “The Ultimate Encyclopedia of Knots and Ropework”
Sheet Bend
“This knot is neither strong nor secure. It reduces the strength of lines by 55 per cent, and can spill if subject to spasmodic jerking.”
I don’t know about spasmodic, but I generally hope to get some jerking on my fly line and leader. To be fair, the “Ultimate Encyclopedia” probably doesn’t carry the same authority as The Ashley Book of Knots, but even Ashley rates the sheet bend as not particularly secure. Of course, neither mentions the lap knot so there is no way to compare them directly.
James Castwell said his knot has never failed him in over fifty years. Why mess with a sheet bend? The Castwell knot is just as easy to tie.
“spasmodic jerking.”
I think this is very important…IMO it is one of the reasons the Chinese grip type of loops tend to let loose if extra measures aren’t taken.
Sorry if this is a little highjacking but IMHO CM probably nailed it 
Both knots are so much a like that I would think the strength of both are about the same. For a fishing knot joining the fly line and the leader it does not need to be that strong of a knot it just needs to be stronger than the tippet.
My gut feel is that either knot would be also be about a secure as the other. from the comments of people that have used the Castwell knot it is obvious that it must be adequate for the purpose.
I was hoping someone might have experience rather than just opinion. My opinion on the matter is no more accurate than anyone elses opinion.
Guess I will have to consider doing my own experiments.
It has been a long while since I have had a spasmotic jerks on the end of my line other that the jerk at the butt end of my rod.
Let us know how your experiments work out.