It will depend on the depth of water you want to fish in.. If your like me and stay shallower than 50m then a standard transducer is fine. Yes the bigger transducer is better but not enough for my $$. I'm running garmin echomap 9" ($1500) and it's brilliant right up to 30kn after that it still works fine but I get a bit of interference on the screen.

Lots of choices out there

i wouldn't put much faith in using sides and to find ground out there. Good for bays and creeks and finding snapper against the rock walls etc

Cats can be a real PIA for transducer fitting. Might be worth looking around to see if you can find someone with the same boat that has worked it out.

As for screen you can get the 12" TZ2 at a good price now and has a 1kw sounder module

I would put in the 1kw transducer if it was me. Not just about power but they are more sensitive too

 Thanks guys, good advice and comments above. Feral, if I can get a good reading at 30 knots I'd be stoked. Tim, great to hear your advise, I know you've done a lot of work in this area. I've emailed Powercat to see if they have a suggestion for what works, will see what they come back with.

 Hi Browndog , I've fitted a HDS 12 touch to a friends 3100 , this cat has pods and outboards fitted so there was a step underneath in the hull where the pods joined that I used to tuck the standard lowrance total scan transducer too. This worked well for a while but he kept getting it caught on his fab dock getting out and I ended up siliconing it to the inside of the hull where the inboards used to be .still works fine but has a bit of interference when your hooking along ( has twin 300's ! ) This guy is not really a fisher just a bit of a weekender / rotto / occasional cray potter , so take the info as you need but the transducer is do- able .
I would suggest going for a through hull type transducer over a transom mount , and I would personally pass on the side scan type units , just a typical sonar type image for me ( just depends on what you learnt how to use I suppose)

hope this helps cheers Carl . 

  I mounted an Airmar 1kw in the big yellow wet box on my 28ft Bruce Harris Sharkcat. Glassed the box onto the outer portion of the bottom of the starboard hull midships. Mitch from Geraldton Marine Electronics recommended putting it there to have pretty well no interference from wave slap from the port side hull. Spend the bucks and go Simrad. Do it once and do it properly. As for getting a reading at 30kts it's doable yet I'm shitting myself!.

 Thanks Fishfish & Silver Fox, good info. I'm pretty much 100% sold on getting the in hull transducer, now just need to figure out brands & model of transducer. From what I can see there isn't much available that will support side scan, not 100% sure I need it, but thought if I'm spending a heap of $$$ it would have been good to have. Thanks for the info on mountiung location Silver Fox, I was planning something similar.

Cheers,

BD.

 Thanks for all the comments and advice guys. At this stage I'm leaning towards a apir of linked 7" displays, one at the helm mainly used as a plotter and one up in the middle of the dash that can be seen from the rear of the boat as a sounder. Definitely in hull transducer, but for now thinking a 600W P79 with option to upgrade down the track to M260 or similar. I like the look of the Simrad NSS7 EVO 3 units, anyone with experience on these?

I've heard several times now about people who have taken a standard transom mount transducer and siliconed it inside the hull, is it really that simple? Can I run two transducer's on the one Simrad unit to get sidescan/Chirp features?

Cheers,

BD.

Airmar website says that installing in hull transducers doesnt affect sensitivity, it just affects depth performance by about 5 to 10%.  I have a mono fibreglass hull that is solid glass where the transducers sit internally.  They won't work with foam sandwich because the air pockets stuff it up.  I have a Lowrance 50/200khz transom transducer at 600W siliconed in,next to an Airmar Medium chirp 1 kw transducer with the supplied wetbox, with a NSS Evo3.  I also have a sidescan total transducer on the transom externally and it is useless above 5 knots.  Both internal transducers I run together, and there is very little interference as the frequency for the medium chirp 1 kw is 80 to 135 Khz from memory, and the Lowrance of course operates at its fixed frequencies.  They both from memory can be run on the Evo 3 as it has 2 plugs at the back for sounders, but I use my Lowrance sounder as a chart plotter and to run the Lowrance transducer.  I get good readings at all speeds even at 30knots, it all depends how rough the ocean is, as the more I bounce around the worse the readings are.  As long as the back of the boat stays in the water the readings are fine.  Depth wise I get good readings so far on both at 130 metres, haven't been deeper.  Quality of the readings comparing the two transducers - the 1kw is probably a bit more sensitive, but the 600W reads really well.  The advantage of the 600W siliconed in (not sikaflex, too hard to move) is that it can be shifted around if its not reading well in that position, just trial and error.  I got it right first go.  Clean the surface really well with some acetone, make a pile of bathroom silicone without air bubbles, push the tranducer in moving it around to remove air bubbles and push down until it is on the hull, but not too firmly. Leave the silicone around the sides on the transducer to help hold it and leave for a couple of days to set properly.  Look up the specs of the medium chirp airmar in hull transducer if interested.  I never use sidescan by the way.

https://www.airmar.com/uploads/InstallGuide/17-573-01.pdf

Ring this mob and see what they would recommend and or put in contact with someone who has a evo 3 in their boat for recommendations .

https://www.leisurecat.com.au/

Ring this mob and see what they would recommend and or put in contact with someone who has a evo 3 in their boat for recommendations .
Had a mate with a Arrowcat, not sure what ducer but was down at Aquarama western finger jetty ,you could try arrowcat also.
https://www.leisurecat.com.au/

is a M135M in hull.  Refer to it on the Airmar website.  I have an EVo3 in my boat.  I have tried siliconing in the Total scan Transducer and it doesn't work.  The 600W transducer have been working in hull for 9 years without any apparent heat issues.