The size of the new Echo Voyager is in the middle of the German U-Boat lengths which were 40-60 meters [previously incorrect feet units in article] long depending upon design. the 51 foot long Boeing drone is about 40% of the length of the short U-boats.
U-boats dominated the seas for several years due to the fact that thousands were produced.
A militarized Echo Voyager could have a similar role if they were mass produced at low cost in the event of a conflict.
Boeing unveils the water drone of the future in Huntington Beach https://t.co/ygGuDItXkg pic.twitter.com/cLVHF4H7hp— ABC7 Eyewitness News (@ABC7) March 11, 2016
The Echo Voyage has a dive depth of about 11000 feet or nearly 2 miles.
The smaller Echo Seeker can go to a depth of 20,000 feet (3.7 miles)
The Type II German U-boats of World War 2 were about 40 feet long.
The Type VII German U-Boats of World War 2 were about 60 feet long.
SOURCES - Youtube, Boeing, Wired, RT, Wikipedia