Here's what I have done with good success on a number of boats.

Since big differences in image quality can vary with a few degrees, I use a digital level or more economically my iPhone with a $3 app called "iHandy Level". Very accurate after you calibrate the app.

First: Put the boat in the water loaded with an typical load then cruise at the speed you would normally use while scanning. (This is usually 4-6 MPH). Using the level record the angle that the boat makes by reading at a flat place on the boat. On newer 21ft bass boats, this usually falls between 3.5 and 5.0 degrees. Your boat and your load may be different.

Second: Put the boat back on the trailer and do one of two things. To make it simple, chock the trailer wheels and un-couple from the hitch. Then tilt the boat with the trailer jack until you position it at the exact angle read while scanning on the water. At that point it�s a simple matter of adjusting the transducer to be exactly level.

If your good at math, simply read the angle of the boat as it sits on the trailer and adjust the transducer to give the transducer angle required.

I haven't tried the scum line method but I question if a typical scum line is the same as the angle obtained while scanning at 4-6 MPH. The nose of most boats rise significantly at typical scanning speeds.

USMarine

From your statement above, I would think that your transducer needs to be raised on the aft end instead of lowered.

Interesting approaches. When I installed my HDS8 a few months ago I got starting position using the scum line as Dayne suggested then the next time I got the boat on the water I used a spirit level like Bobby suggested to see how the boat sat while scanning. Then after getting the boat back home I jacked up the trailer to get the same position on the spirit level, then set that big Gen2 transducer to "level". This seems to have worked pretty well re image quality.

Useful to compare approaches with other folks!