I have a new to me Pathfinder 2200XL which as a 997 on it. My previous PF 2200V had an 898 si on it. I was able to hold bottom up to about 40MPH with that unit. Anything over 10-15mph I loose my reading on the new boat. Some one said they were able to purchase a shoot through the hull transducer that worked with there transom mounted side imaging transducer. I checked the HB web site and I don't see anything that says it is compatable.
There claim was the shoot through started working as soon as they exceeded the speed that the SI transducer would work on.
However, you will see some reduction in 2D sensitivity vs the element in your transom mounted transducer. The degradation in performance you will get will be determined by your hull type and mounting location. You should be able to track depth at speed without a problem. Only your fish finding and bottom discrimination should be affected.
I would try to tweak your transom mount transducer before adding a shoot through the hull. Every installation is a challenge. I've put Humminbird transducers on several boats and moved them between boats. But I've never had an installation work best the first time.
If you aren't tracking bottom at speed, the first thing to try is to lower the rear end of the transducer. Just a slight tilt will sometimes let you track at speed. Next, try lowering the transducer a bit. I like to put the inside edge about 1/8" below the V of the hull. That puts the outside edge quit a bit lower still. This will sometimes give you a rooster tail when you run at speed. If this is unacceptable or you still can't track bottom, try installing a Sternmate transducer mount:
I've had good luck with these. They set the transducer back a few inches, sort of like a jack plate, which lets it run in cleaner water than right of the transom.
I put a through hull Airmar B60 on my Grady to allow me to track bottom at speed (as well as providing better bottom discrimination). But since adding the sternmate, I find that the stock Humminbird side imaging transducer that came with my 997 actually tracks bottom up to my top speed of 46 mph. That's better than I can do with the through hull unit. Before adding the sternmate, I would loose bottom with the Humminbird transducer once the boat got on plane.
Thanks Doc! That is some good information. I think I might try to add a 3/4" piece of Starboard to try and accomplish what the Sternmate does. I already have holes in my transom. I will also check my angle. I am not sure that it isn't tilted down rather than up.