Hi Neil and Ashley,
yes the petrol engines originally used did proove quite reliable, but I think the sails were there because it was the transitional stage from rowing/sailing to motor/sailing so gave confidence to the crew as a back up should they have mechanical failure, and also this was the war period, so any fuel shortages that might possibly have stopped a long deployment could be assisted by auxillary sail power. If you look at modern lifeboats you'll still see gear hark back to the ol'days – our Mersey still has two boat hooks on deck, one white and one blue, the old port and starboard colours for the oars!!
I know how change can often be seen as a backward step, and if you have always used sails then being without them could cause major worries. The young accept change much more readily, but they would have been called up for war, so the crew would have been the "old guard."
As Neil states, once the engines were swapped to more powerful diesels, the sails were most probably left in the sail locker at the boathouse. As with thames barges converting to diesel engines, they very soon found that having lots of mast superstructure actually added to the fuel bills, so masts were shortened or removed, engine power prevailed, the sailing barge was no longer required!
Still, the photo shows the sails, so the modeller wants them on. Facts is facts!
Thanks for your comments, always welcome, Kimosubby.