Hey I just put a new 788ci HD DI combo on my boat when I get to about 8mph I am loosing signal on fish finder, transducer is mounted on transom this place work find with old hummimbird up to about 25mph what give I have move transducer to inside the hull hoping this will help is DI that sennesitive but I loose water depth to, give me some input

Without knowing what type boat and hull material is involved, it is hard to recommend any solution. One size does not fit all.

Post a picture of your transom.

I got and 18 ft. skeeter fiberglass hull

I got and 18 ft. skeeter fiberglass hull

That means you have a stepped transom and getting satisfactory on-plane performance with a transom mounted transducer is marginal especially the Humminbird transom transducers. They are not as hydrodynamic as the Lowrance torpedo transom 2D transducer.

The DI transducer can be used as a shoot-thru, BUT you have to install it correctly by testing for the best location BEFORE attaching it to the hull and using the proper SLOW CURE epoxy.

Yes, you may loose some sensitivity with a shoot-thru mounting, but that application is very common for your hull style.

If you had correctly installed your former transducer as a shoot-thru, you would have been able to get depth readings at the boat's maximum speed.