I have installed a humminbird 595c per the instructions  on my 1992 Stratos F/S.  I the transducer mounted exactly where I am supposed to, the far right of the step up the underside of the hull.  The unit works great except when I am under engine power.  Once I start to go across the lake, I lose my depth reading and the bottom view.  This bothers me since I would like to see what the depth is doing as I am going along.  I have not found any good fixes for this in my research and I am hoping someone has the answer.  Thanks very much.

Move the transducer.  you are getting either air bubbles or too much turbulence from the motor

If you have it mounted on the step up which is above the drain, it is not even wet when underway. If you must have a transom mounted one, it has to be on the lowest part of the hull next to or on the pad.

another note: it also has to be positioned so it won't hit the trailer bunks. If you don't have it mounted so that is a possibility, you have it on the wrong part of the hull.

I moved the transducer today.  It is now just to the starboard side of the drain and is directly in line with a bunk.  The bunk is about an inch in front of the transducer when tightened to the trailler.  I have the bottom of the transducer even with the bottom of the hull.  It is level though, not tilted to match the angle of the hull.  I believe this is how it should be according to the instructions in order to create the most fluid flow of water.  I'll have to try it out to see if this fixes it.  Thanks for the advise.

The lengthwise line at the middle of the transducer is supposed to be even with the bottom of the hull according to the installation instructions. That would make 1/2 of the bulk of the transducer below the hull. Most setups like that require the rear end to be tucked down a couple of degrees. If you mounted the bracket correctly, you will have the vertical adjustment needed to get the proper water flow across the transducer.

After you get frustrated with trying to get it to work properly, you can just epoxy it on the inside of the hull.   Good luck.

Like WP said, drop it down so the center of the Xducer is even with the bottom of the hull.  Also make sure it pointed straight to the front and as mentioned, drop the back to add a one or two degree tilt.  It should then give you depth readings at just about any speed your boat will run.  

When you mounted it way out on the hull, you put it over one of those hooks Stratos built into your boat to help it get on plan and keep it from porposing, no way it would have worked out there.  Probably also had it lifted out of the water way out there also.

I mounted it the way it was described so I have plenty of room to drop the bracket down a little if I need to.  Ill try it where it is as it is close to the center or the transducer now but may be a little higher than half.  If I need to, I can drop the whole thing down and then the transducer will have its centerline in line with the bottom of the hull.  The instructions said the transducer should be level so I am surprised to hear the back should be a little lower than the front.  I guess those that have working depth readings would know.  Thanks for the help so far.  Any other suggestions would be great in case this doesn't work either.

In the event I do have to epoxy it, which I would love to avoid, I would want to do that in the area in front of the drain inside the hull as close to the center as possible...correct?  I have heard horror stories about sensitivity loss etc.  I hope it doesn't come to this.

Had the same problem on mine.  Some transducers are hinged at the bracket.  If not locked in, they will be forced up when underway and signal loss is imminent.