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I get a lot of questions and see a lot of comments from this site on depth finder problems, especially at speed. When someone contacts me the first thing I ask is "Does the depth finder work when your sitting still in the water?" If the answer is yes, then the problem is almost certainly an improper installation. Air and bubbles are a transducers worse enemies.

Here's a good article I found on Garmins website dealing with transducer installations.

Question: What is the best way to mount my transducer?
Answer:
Transom Mounting:
Choose a location where the water flow is smoothest within a minimum turbulence and bubbles especially at high speed. The transducer needs to be continuously immersed in water and the sounder beam unobstructed by the keel or propeller shaft. Make sure that there is adequate headroom and there is a minimum dead rise angle. NEVER mount the transducer near water intake or discharge openings or behind strakes, struts, fittings, or hull irregularities. Also, water near the keel can be quite turbulent which will negatively affect performance. For more information about preferred mounting locations, contact the hull manufacturer.
A disadvantage of a transom mount is air bubbles will affect the sonar.
In Hull :
Oil bath is the best mounting method for most boats. It works well on single skin G.R.P hulls but is not suitable for sandwich construction, steel, aluminium, Ferro Cement or wooden hulls.
The ultrasonic signal produced by the transducer passes easily through liquids and solids but is greatly attenuated in air. It therefore follows that air inclusions in the fibreglass or a stream of air bubbles on the outside of the hull will both affect performance. The signal will pass through up to 0.75`` thickness so avoid sitting the transducer over spray rails or reinforcements.
The signal from the transducer is in the shape of a cone (approx. 22 degrees). If it is mounted too close to the keel it can pick up an echo from the keel, so ensure adequate clearance. The optimum installation site can be found by temporarily mounting the transducer in a blob of grease, or placing it in a polythene bag of water (wet on the outside too) and holding the bag against the hull.
The most vital point to remember with the Oil Bath Method is that the oil will need regularly topping up. The most common cause of Depth Sounder problems is no oil around the transducer. Any oil will do.
 
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