I am getting a Helix 9 DI/GPS unit and will be mounting the transducer on the jackplate. Since this will not allow me to get DI or sonar readings while on plane, I thought I could connect my thru hull transducer with a "Y" cable. I couldn't find such a cable on line so I called Humminbird and was told that there was no such cable because the Helix units wouldn't process dual beam signals. They said I could mount the Helix transducer in the hull to obtain readings while on plane. This would be a real pain and I have already bought the hardware from Transducer Shield and Saver to mount on the jackplate.

Any suggestions?

Not trying to argue with Hummingbird and I'm sure Wayne will be here soon to correct. The transducer shows to be the xnt 9 di t and is showing to be compatable with the 800, 900, and 1100 series which would make you believe the plug pin out is the same. If this is the case, the Y cable used for the above series should connect a in hull transducer and this transducer to the unit. The website also shows the Helix 9 to be a dual beam unit (83/200/455/800 or 50/200/455/800) so it shouldn't have a problem processing an 83/200 or 50/200 transducer. The Y cable just splits the wiring from the transducer.
John

"it ain't that simple"

There is a Sonar, a DI, and a SI version of the Helix 9, 10, and 12.

A DI unit uses 200 and 455 kHz for 2D sonar. The included XNT 9 DI T is tuned to 200/455 for 2D and 455/800 for DI and a 200/455 crystal is smaller than a 83/200 kHz crystal. That is why the specs for 200/455 kHz is 28/16 degrees instead of 20/60 degrees like with 83/200 kHz.

The AS SIDB Y cable is to split off the 83/200 kHz crystal from a Side Imaging transducer.
There is no Y cable for a DI transducer and no shoot-thru 200/455 kHz transducer.

While the Helix 9, 10, and 12 DI units have a setting to use 83/200 kHz for 2D sonar when using the trolling motor puck XTM 9 WIDE DI 20 T, I have not seen the application for a dual beam shoot-thru and Y cable with a DI unit.

BUT, a DI transducer can be used with a SI unit to provide sonar data for one side of Side Imaging to avoid the blended SI version of Down Imaging and produce a sharper DI image on a temporary basics.
That is discussed on the Humminbird website and some forums using the TS3 switch to select either the SI or DI transducer for the DI view.

Using the AS SIDB Y cable MAY work for your application when using the 83/200 setting for 2D sonar. It will cost you the price of the Y cable to find out.

One suggestion. Use a holeshot plat on the bottom of your jack plate and mount the SI transducer in the middle. If you mount it on the side of the jack plate you could pick up a shadow from the other side of the jack plate. That's how I have mine set up and it works fine. The SI doesn't work above about six MPH, but when I'm up and running I have the chart with the depth markings on the screen to keep me out of trouble. It's going to be pretty difficult to see fish on SI when you are on plane anyway. I first mounted mine like this because I dry stacked my boat and I didn't want them tearing off transducers with the fork lift, but even now that I trailer, it works well enough for me that I just left it there. YMMV

The OP will have a Down Imaging unit. No side clearance issues.

Thank you all for the great information and suggestions.
I have my current 798C SI unit connected to a dual beam transducer via a "Y" cable so it will be easy to switch it over to the Helix unit to see if it works. Not expecting to see SI images on plane, but sonar would be nice.

I am not familiar with a holeshot plate but will check into that solution as well.
Once I get rigged up, I will report back to let you know how it went.

Thanks again for taking the time to respond.