Hello Salvor and welcome to the forum!
First you need to lower the side range in your settings and slow down while scanning.
The side range should be around 2-3 times the water depth your in. This is the best compromise concerning details on the screen versus area covered.
You get good side images if your boat speed is not faster than 6 mph.
Your graph speed must be in relation to your boat speed to avoid "stretched" or "shrunken" pictures. I wrote something about this here:
http://forums.sideimagingsoft.com/index.php?topic=242.0Concerning your set up:
Your hull is quite a challenge. I would suggest either a dual SI transducer set up or a telescope mount with a single Si transducer. Here's how I built my telescope:
http://forums.sideimagingsoft.com/index.php?topic=158.0I think you can avoid the prop wash if you would go deep enough beyond your propeller.
Hope this helps
Harry