There is lots of talk about this on the Simrad/Lowrance page on Facebook and The Hull Truth.

• Check that there are no chines/pickups/ports forward of the transducer that could be disrubting the waterflow before it hits the transducer.
• Try adding the 4.5deg wedge to angle the rear of the transducer down.
• That height should be good but maybe try raising it 3-4mm to the LHS edge of the transducer is exactly level with the hull.

 It's all trail and error, i.m cuirrently going through it myself.

 It looks pretty good, as Darren has said maybe try the wedge to get the back edge down a little more 

One way to tell if you need to raise or lower your transducer is to monitor the display as you make a steady turn left or right while maintaining a constant speed.

Assuming transducer is mounted on starboard side:

- If the image improves while turning to port you need to raise the transducer. 

- If the image improves while turning to starboard you need to lower the transducer.

 My moneys on it needing to go lower going from the photo . A side on photo would be better to tell that plus angle

 looking at the pic i would say go a bit lower. i think the right hand side should be level with the hull. placement looks ok tho

 Interesting responses - really appreciate it. After doing more searching and research there are alot of forums that suggest running the Airmar P66 about 10mm lower than recommendations. 

I've taken and added some side on shots with a straight edge placed along the hull line. It does look like it's a little bit high. Will try to drop it 10mm. The wedge that comes with the P66 seems to create a large angle so I am reluctant to put that in yet. So maybe I will lower it and try again, then I'll add the wedge if still no luck.

I think I know the answer to this one, but keen to hear what others would do - obviously I squirted marina sika in the holes and on the screw threads when I installed this. Would it be better to completely remove each screw, remove as much of the dry sika as possible and re-apply sika ? Or if I only wind them back enough to lower the height, then tighten them back up will the sika still form a seal ?

10mm lower should improve it. Try it first before adding any more wedge. 

Best bet is to just change one thing at a time so you are sure what improved or didnt

 Drop it 15mm mate going off your photo angle looks good . I have a p66 on the opposite side of a tm260 and it sounds really well up to 20 knots or more

 Yep I agree with dropping but try 3-4mm or so at a time. I found my sweet spot that way. 

Hey mate, like you I've recently installed one myself (FCV 628 and a P66) and I’m getting solid bottom in over 20 knots (very fair conditions)

My P66 is definitely installed a lot lower than yours, try moving it as low as possible without drilling new holes. You may be lucky and this will do the trick.

The only time I have lost bottom is when I smash through/over solid wind chop but I am putting that down to air moving under the hull. (or when i lift the prop )

 Excellent - I will start dropping it and give it a good test tomorrow ! Will post an updated photo of the eventual location that improves the reading (if it does indeed improve the reading).

thanks 

 Put that bracket as low as it will go and you will be laughing.

 as the other said drop another 10 mm.  If you have on going issues I would also make up plate from rear of  transom out to transducer  so to eliminate any gaps where water and air especially can form between to two surfaces.  

 I have seen it discussed at length on other forums, and that is the actual placement relvant to prop rotation. For example the OP has it mounted on the downsweep side of the ( assuming RH rotation) prop. Conventional wisdom in other places states it should be on the upsweep ( ie, in this case ,left) side of the prop. Let me stress i have no opinion on this subject, just throwing it out there for comment.

When it come to the perfect set up with p66 they need to be buried passed disturbed water. Usually on the side closed to the keel needs to be 10-15mm lower then the Hull. The next thing is angle, working off your Hull isn’t a true indicator. You need get on the plane with a level and work out how your boat sits then level your tranny from there. Most smaller boats quinnys, savages etc ride nose down on the plane. I had to give mine some angle and looking at it looks wrong but mine reads 29knots flat out even in crap conditions. One way to work out if the angle is wrong is if your loosing bottom while going into the plane as the nose drops if so more angle. Just play around with it mate once it’s set mark around it with a permanent marker and you will always have the spot to mount it

When it come to the perfect set up with p66 they need to be buried passed disturbed water. Usually on the side closed to the keel needs to be 10-15mm lower then the Hull. The next thing is angle, working off your Hull isn’t a true indicator. You need get on the plane with a level and work out how your boat sits then level your tranny from there. Most smaller boats quinnys, savages etc ride nose down on the plane. I had to give mine some angle and looking at it looks wrong but mine reads 29knots flat out even in crap conditions. One way to work out if the angle is wrong is if your loosing bottom while going into the plane as the nose drops if so more angle. Just play around with it mate once it’s set mark around it with a permanent marker and you will always have the spot to mount it

 Cheers guys, I dropped it today and tested, then dropped more (as low as I could go with the screw locations) and tested again. I think I got marginal improvement but not much. Otherwise I was super happy with the reading I was getting (even if the fish I appeared to be seeing didn’t seem to want to bite!)

Will try to insert the wedge and see if that makes any difference, otherwise I might need to patch up the holes and try again.

Interesting point re: being installed on the downsweep side. I’ll have to revisit this and read into it as I thought I put it on the correct side.

 Just a follow up to close this out and help anyone else out - I ended up having no luck with the original mount location OR using the shim/wedge when over 10knt, so I moved the transducer over a bit closer to the keel and removed the shim. I added a new photo and you can see the 3 previous holes  (need to finish the patch job). I didn't have to move it over too far and I can still go lower if needed but I am getting a solid reading at 20knts now ! Very happy. 

It does start to break up over 20kn, but if I turn to starboard, the transducer goes deeper and the image clears up. I don't think I will be bother lowering it further tho because I can't be bothered playing around with silicone again and I rarely exceed 20knt unless im done for the day.

I also changed the Transducer setting from "600W" to "525STID-PWD". Apparently this is the part number for the P66. Not sure if that actually helped or not but I'm super happy with the result now.

Hope this helps someone else.

Cheers