Peter B. You may be the armani of tshirts but you are the gomer pile of props.
You definitely do not have 12"x15" props on your 23" Most likely you have 14x17'or 19's. I am sure because I had 13x19's when I had twin yami 130's.
To answer this thread I run 22" ss props on my twin 115 Suzuki 4 strokes and I get 28mph and 10.5gph at 4k rpm's for about 2.7mpg and that is with a full tank and 3 people. Max speed 38kts at 6300 rpm's. I may go to 24" props to drop the rpm's down to 5900 or so.