if you will maintain such good read rates at speed. As a general rule of thumb, the through hull tansducers lose bottom earlier than an external, in water transducer. You could go the external 1kw job but they are a big unit, and they are normally best with a drop down bracket assembly. Otherwise they can tend to throw big amounts of spray whilst travelling and are more susceptible to damage whilst travelling, due to their overall size. 

If it was me, I would go a through hull 1kw transducer, just make sure you don't install it anywhere behind there is a rib, chine, anything that will disrupt flat water passing over the face of the transducer. 

 I'm no guru mate, but I've got a 1kw B175M (medium chirp) hooked to an S2009. It works at any speed unless i'm trimmed right up.. have pushed it to 380m and it was still picking up clear bottom at 10 knots +. Once you go thru hull, I don't think you will go back. Reduced noise and clean water 

 By thru hull i mean drilling a hole in the hull and having it installed through the bottom of the  hull . Jackal have you got any photos of where yours is installed and how does it read in 20m to 100m at speed 

 If you can and have the coin, I'd put a pocket mount in. You need a flat keel to do it. these end up wet faced and perfectly flush if installed properly. Something like PM265LH Chirp which are higher Qfactor and will work with your unit as far as I know. Then you have selectable frequencies in two ranges and if you want to change to chirp later you can do so.

I think these are possibly the best transducer mount possible, but they are the most work to install and will not work with all hulls. I like the idea of two frequency ranges in the one transducer which is like what to have now in non-chirp.

The other option is B164 or multiple B175 chirps thru hull types.

It depend on whether you want the advantages of chirp transducers and the possiblity to change to a chirp unit later. 

Here is where you need to start looking:

www.airmar.com/productinfo.html

 Sorry I missed the aluminium boat bit of your Post. You will probably need the SS versions and the Pocket mount would be harder but not impossible for a good fabricator.

 Got all the ali gear to weld 

I'd be having a closer look at you transducer placement I can get clear readings well past 100m cruising at 18 knots. In saying that I don't think you can go far wrong with a thru hull can't help with the choice of tranny sorry. 

 I have stuffed around with it heaps and im losing patience . Piss it off and try something else i think

 Did you live in paraburdoo as a kid

Thru hull with a fairing will give you best performance at speed but if your fishing 350m you need 1kw which with a fairing is gonna be too big for 6.8m ally. 

SS264 (same elements as your tm260) pair of flush mounts will give you traditional 50/200khz in 1kw or as above a 175H &175L for 1kw chirp which with the lower Q factor will work well on fixed frequency on the 587.

 I think it is i ran into your mum and dad not long ago in wickham . How you been mate . We used to holiday at coral bay with your family when i was a young whipper snapper .

Been looking at options for an Ali hull 1kw as well and a mate sent thru this picture. Might be what you're after?

https://i.imgur.com/9L2dQYd.png

I would be checking your location of the TM260 first.

It took me a few places to get it right but I had no worries reading bottom at 30knts out to 150mtrs on 200khz. I was running a 588 and TM260. It was at 50khz it would not read at speed but thats the case with anyone I know who has the same set up.  

I got the shits with mine also and swapped over to full Simrad and a TM185M for fishing out to 300mtrs.

If you do want to change it out a pair of SS264 would be the pick but not cheap. Also if you dont want to drill a hole in the boat or cant access under the floor Bluebottle do a transom bracket for the through hull transducers like what bennym_82 said. Can either bolt or weld it onto ally boats.

 Just curious ?? Why would you put a SS transducer on a Ali hull ?? Corrosion would be horrible would it not ? Surely there is a plastic 1kw ? ... as stated above id be getting your current one looked at as it should read at well over 35kt

 Brass to ally is the bad one. The stainless transducers come with a plastic bush so it is fully isolated from the hull.