Another undated postcard from Kevin Blair, this one shows tugs moored at North Shields.
HENDON is easy the spot but I think the other two are Crosthwaite tugs.
Looks like JOSEPH CROSTHWAITE next to HENDON: http://www.tynetugs.co.uk/huskisson1896.html
Also LEWIS CROSTHWAITE behind: http://www.tynetugs.co.uk/colonelthys1912.html
I think.

Didn't they both do well to have such long careers, down to being built with good materials and good maintenance bearing in mind that most of their working lives things like Boiler water treatment and the more modern filtration in the Hotwell were not available, the filtration in the Hotwell would be something like Coir or Coke ( not the Colombian marching powder ) along with regular Salinity Testing of the Boiler(s) which was carried out by taking a sample of Boiler Water from a Sample Cock (rather than a Valve) at the back of the Boiler(s) into a tall metal jug type Container dropping the Stainless Steel tester in which looked like a Fishing Float and taking the reading, all this was done whilst wearing thick Asbestos Gloves and the concern particularly on the older vessels that if the Sample Cock on the Boiler snaps off you are in deep doo-da as you are stood in the limited space between Boiler and usually a Bulkhead depending on the Ship design, although admittedly not as bad as crawling into an hot empty Firebox whilst the Boiler is on reduced pressure to plug off a leaking Firetube No wonder I was thin in the days of Steam.

Were the North Shields Crosthwaites related to the Tees Crosthwaites?

No idea NE although it would seem a bit of a coincidence bearing in mind they were both in the Towing business if they were not related. In later life I did meet up with "Dickie" Crosthwaite on a regular basis to carry out some "community business" where inbetween doing the "business" we talked about Tugs mainly the unique "Acklam Cross" and if her propulsion system was worth the extra expense ( around 50% more than a conventional Steam powered Tug at the time she was built) along with his flying antics.
Certainly "things" took a strange turn of events after his flying accident.

John Proud in his seminal book on Tyne Tugs says in the chapter on Crosthwaite Tugs "It is worth pointing out that this Crosthwaite family, contrary to popular beief, is not connected with the Crosthwaite of Middlesbrough who managed Tees towing Co Ltd."

I have of course heard of John Proud and his excellent work then that is the answer published by Whickham, certainly a bit of a coincidence In my conversations with "Dickie" he of course mentioned his Papa in relation to "Acklam Cross" but that's about as far as it went as I was not aware of Crosthwaite Tugs other than those operating on the Tees.
One for Ancestry.com