Hi Steve
Good choices there. I've looked in vain for info on both of these tugs for several years now.
YY was launched on 3 Mar, 1961 as hull no. 8; YS was launched on 4 Sept 1962 as hull no.15.
Both had a single Ruston engine driving a fixed pitch, open propeller.
The two are not quite "sister ships". They both have the same hull dimensions, and I'm sure their deck arrangements would be similar, if not identical - but YY had a slightly more powerful engine - 874 kW vs 809 kW for YS. There must have been some internal differences also, as YS was rated 149 grt, whereas YY was rated 166 grt.
Last info I found was that YS is registered in Cook Islands, but working in NZ. YY had a name change in 1997 to SUMBER POWER IV, registered in Cambodia. I don't know if this info is still current.
I've managed to glean some photos of both tugs off the web - none are particularly good shots, and you may already have them. If you don't, then send me a pm and I'll flick them to you. I only managed to find these photos on grosstonnage.com (which seems to have gone dead), but only after I had found their respective IMO numbers - 5396090 for YS, and 5396416 for YY.
Des.