depends mate on a number of things, what do you want it to do ?

 Umm, show ground, fish etc. That sounds like I'm taking the piss but I'm not. Trying to upload some photos but not working for me.

all dependant on sea conditions

if it's glass , mine'll read bottom at 30 + knots

if it's crapola , you need to be going really slow off the plane  ,  and even then , it can be bad

(transom mounted)

first heck your tranny is mounted correctly, u can get diagrams off the net pretty easy, if correct, could be sensitivity, if your sounder is set to auto, change it to manual, then as u go faster u will need to increase sensitivity to pick up more detail, if that doesnt work, try increase colourline too. take note of settings b4 adjusting lol as it takes a bit to get used to .

not all sounders work the best on auto sensitivity, eg my lowrance hds10 has different sounder modes eg deep, shallow, trolling,etc so u can choose the right 1 and adjust it to perfection, dont be afraid to muck around with it, thats why its got all the buttons lol best of luck.

 Yep, I have the Lowrance HDS10 as well. I normally keep it in auto mode but it doesn't read much at all if I`m travelling. I don't think increasing the sensitivity will work as it`s just a mess. Does it change from day to night?

turn surface clutter on and try again, im pretty sure thats wot i did :)

i can read the bottom eg 40m deep at 20knots no dramas, of coarse the detail is less but it doesnt lose sight of the bottom unless the gain/sensitivity is turned down, and my ping speed turned down

santhmore, turn your ping speed up to max and try medium or high surface clutter filter and then adjust your sensitvity and colourline up until u start to see the amount of detail u want, i dont want to see jelly fish but i can see flare ups and higher or lower ground doing 20knots, futher investigation on those spots at a slower speed shows more detail but i at least saw it and marked it so i could look at it on the way back in! i think i used x2 scroll speed too but ill check that for ya :)

 Thanks mate, I'll give that a go. There is defineately something amiss with mine. 

On my boat I found the most important thing to getting a reading at speed was how the tranny was mounted. Once i put it deeper rather than shallower and adjusted the angle it worked much better. You still need the right settings (e.g. gain, ping/scroll rate) but if the transducer is popping out of the water while on the plane or has any bubbles under it at all then no settings are going to help you. While a mate is driving, look at the transducer and see what's happening. Try it lower and see if it helps. Can also try in the river where chop isn't a problem. If you can't pick up a solid red line with no clutter while at speed then your transducers not set right.

Thanks for the advise guys. I'm in Brazil for the next month but will give it a go when I get home.

Yeah Harro I never wrote that original post well, the sounder doesn't seem to work when going above 4knots. The screen just fills up with spag. I've got a few things to try when I get home.