isljam

Hey folks.

Ok, maybe a dumb question, but how do you level the transducer horizontally, i.e. starboard to port (not vertically, I get how to do that) so it is perfectly parallel with the bottom. Obviously, I’m doing the install on dry land with boat on trailer.

I know the driveway is not level. And even it was, I can’t assume the boat is perfectly level sitting on the trailer.

I decide to use a spirit and wheel jack to get the boat level. But what surfaces can I assume are level on the boat.

Not sure the transom is level or the carpeted floor. Do I just lay a spirit level from starboard beam to port and assume that is a level surface?

Man It seems whenever I undertake a DIY, seemingly simple steps become overwhelmingly complicated. But I’m determined to get this done.

I'd just aim to draw a line on the transom from each corner where the bottom of the hull meets each of the sides.
In reality unless you have perfect weight distribution in the boat at all times, the horizontal level will always be out anyway.
Get is as close to level and spend more time fishing.
 

If its a traditional transducer without side scan, then it's shooting out a cone shaped signal, and the size of the cone depends on the frequency that you're running. As you can see in the image below, the depth displayed is never the actual depth below the boat, but the shortest distance between the transducer and the first return signal.
You will never get the transducer installed exactly plumb as the pitch of the hull varies depending on the boat speed, and roll changes depending on how the boat is loaded on the day. You're over thinking it. Just install the transducer pointing down when the boat is on the trailer. As long as its not out by 5 degrees or more, you are going to get decent readings. 

Hey folks.

Ok, maybe a dumb question, but how do you level the transducer horizontally, i.e. starboard to port (not vertically, I get how to do that) so it is perfectly parallel with the bottom. Obviously, I’m doing the install on dry land with boat on trailer.

I know the driveway is not level. And even it was, I can’t assume the boat is perfectly level sitting on the trailer.

I decide to use a spirit and wheel jack to get the boat level. But what surfaces can I assume are level on the boat.

Not sure the transom is level or the carpeted floor. Do I just lay a spirit level from starboard beam to port and assume that is a level surface?

Man It seems whenever I undertake a DIY, seemingly simple steps become overwhelmingly complicated. But I’m determined to get this done.

Do not file / grind the weld unless you want to cause future problems.
As @Corry mentioned a Transducer mount plate is definitely preferred as not doubt you will need to play around with or adjust the height of the transducer via trial and error.
Install it and see how it's reading at low / high speed. If you then find issues with the transducer reading make small adjustments. (Plenty of info on the internet)
The tip is is you value high speed reading then the angle of the transducer is more forward facing. Slow speed then stick to the install recommendation.
Consider one of these to help with adjustment (if you don't get a plate welded on)
https://www.whitworths.com.au/aluminium-flat-adj-centered-transducer-bracket
 

Have you got a transducer mounting plate welded to the back of the transom? If not, I would suggest to go see an alloy welder and have a plate welded on, that way you can bolt the transducer to the plate without having to drill holes in the transom. At least then if you upgrade or change it later, your not left with holes below the water line. Be careful drilling the holes in the mounting plate that you don't drill through the transom, slide a piece of steel in between to help. 
As for mounting, don't worry about the transom weld, most boats have it, definitely don't go filing or grinding it away. As far as height to mount it, there is plenty of info out there in the google world, but majority of the mounting brackets allow vertical movement, I've found that I've needed to mount it lower than 'recommended' to keep good picture at higher speed, you might have to 'play' with it over time to see what works. They say centre of transducer should be level with bottom of hull, and mounted between any rakes or chines, half way between outboard and side of boat. 
Hope this helps somewhat.
Cheers, Corry

Hey folks. I am doing my first transducer install on my 4.5m Aluminium Brooker runabout and need a little advice please. I’m aware I need to choose a flat, unobstructed point on the transom to mount. Avoiding chines, welds and hardware etc, to ensure unobstructed water across the transducer. The concern is that the weld along the bottom of the transom creates a small ridge. So, running along the hull it is smooth until it meets the transom. There, the water would have to travel over the small ridge created by the weld. The manual for the unit recommends mounting 10mm below bottom of hull. Is this going to create a disturbance in the operation or the transducer. Of course, there is no way to avoid it as the weld runs the entire length along the base of the transom. Should I attempt to file / sand this weld line flat or could this jeopardize the integrity of the weld? Thanks