I have a friend at work asking me about his transducer for a HB 899. It is a transom mounted transducer and he says that when he gets up to speed, the depth goes to 2', ever though he knows it is deeper. My guess was that when he gets on plane, his transducer loses contact with bottom, but he says he still gets sonar siginal. Has anyone else experienced this, and if so what was it and how do you correct it?

IF the transducer is installed so it can give the depth when on plane, it will.

Post a picture of the transducer installation for critique.

I will see about getting a picture from him.

He needs to add a second transducer and splitter cable to get depth at high speeds. See the diagram below.

The second transducer needs to be mounted inside the hull also

I thought I posted this earlier, but dot see it:

I was looking at the thru hull transducers on HB website, they all say for single hull fiberglass hulls. Is there a thru hull transducer for aluminum hulls? He has his 899 on a Lund aluminum hull boat. He is suppose to be taking me a picture of his transducer mounting. He did have it "professionally" installed. I think he had the people at BPS install it.

You have your transducer terms incorrect.

A thru-hull transducer is installed by drilling a 1"+ hole through the hull bottom to put the mount stem through and the transducer is on the exterior.
Look at the picture of the XPTH 9 HDSI 180 T on the Humminbird web site.

A shoot-thru is what is used inside single layer fiberglass hulls. There is no shoot-thru Side Imaging transducer.
Side Imaging is done with an external mounted transducer.

There is only one shoot-thru 2D transducer that is made for metal hulls---Alumaducer.

You will need two transducers connected with a Y connector. The 2D will shoot through the hull and the external side imaging transducer handles side/down imaging duties only. I have this setup.

Is that true only with the newer HB 899? I had the older 898 with SI/DI and 2D and only one transducer and got all the readings I needed when running on plane. This is the first I heard of the need for two transducers...

No, that applies to all models of SI units. The installation and transducers used is based on the hull type and its performance capabilities. You customize the installation for the best performance.

The SI transducer does perform all the functions, but those functions may work better in different places.