Wellington has a bug in it. It has the radar Mark III until end date of June 1, 1944. Then nothing. The Wellingtons radar drops off until end of the war after that date. There should be another radar set to upgrade ASVMark something better than MarkIII until wars end of December 31, 1945. At least let change the date to wars end.
Means alot of Wellingtons are flying around after June 1, 1944 to war end with NO radar specified. Unless the Wellington is removed from the game after that date.
[Sensor 3]
NodeName=R01
LinkName=ASVMarkIII
StartDate=19430601
EndDate=19440601
Avengers have NO radar!!
[Sensor 4]
NodeName=R01
LinkName=NULL
StartDate=19380101
EndDate=19451231
So in Jan 1944 only Wellington will have radar ASVMarkIII, while the Avengers NEVER have radar

.
Th ASVMark III default game value is 12 km range at 0.3m MinSurface. I moded that from 0.3m to 1.5m.
The Wellington will spot your snorkel if it is stick more than 1.5 meters high above water surface. At periscope depth, I had to go about 1 meter lower to get the snort below that mark and not get detected. Waves play into this too, hiding you but also shutting off snorkel on/off.
The Avengers have NO radar so they cannot see you using your snorkel except by Visual sensor (Mark I Eyeball

)
If you were stopped in the water, underwater, you should be able to snorkel with no problem.
I am suspecting the surface area defined for the snorkel is too big in its dat file. The Periscope does not get detected so easily.