1. The 15 inches from the prop required by HB is to protect your motor cooling water inlet from turbulence cause by the large dollar bill sized transducer. Nothing to do with your picture.
2. Wow U do have a very deep V much deeper that J.White. If U put a yardstick on bottom of J.Whites ducer it appears U would be well below his boat where as on Yours its looks like U would be level with those water intakes-drains, sure U may be slightly behind them but the turbulence from water flow under boat & then hitting them seems to be major cause of low speed problem. Yes moving ducer back a inch or 2 & tilting motor may really help. Also trying a tad deeper may help & all this can be done without putting one new hole in U boat. Fact is u can move ducer any where u want even on run(slow)
3 I love that deep v must be a great ride Especially for old farts like me with a wired/screwed back. Because of that deep v its more turbulence then sidescan ducer can live with U will have to go with a 2nd ducer with Y wire as I think J.White mentioned, however need to spend only about $100or so for this dual beam not side scan ducer(this would offset the $40.oo ducer bracket). This could give U side scan at slower speed & dual beam at hi-speed. And U will know if sidescan works before buying 2nd ducer.