How smooth does a sounder bottom need to be?

[9D280]

Busy fityting my new (to me) toy, which came with a transom mount depth transducer.

Said transducer had been fitted internally to the previous owner's boat, so I have spent a couuple of evenings carefully chiselling off the epoxy that held it in. It's now back to plastic but with one tiny groove on the bottom face caused by the chisel when I probably should have stopped chipping & gone to bed 10 mins before.....

So, the question - knowing it has to work in as smooth water flow as possible, should I be sanding this "deep scratch" out the plastic, or are they robust enough to handle that kind of damage? I assume some must have worse from being grounded? (scratch is about 4mm long and about 0.25mm deep)

Also can anyone tell me how smooth the surface is on a "fresh" one? (or do I need to polish it - or will a careful circular sanding with an old rag & 1200 grade wet'n'dry be sufficient?

[Chris]

I would use some epoxy to fill the scratch then sand it smooth.

[jezza2011]

yep Fill in the scratches with epoxy and epoxy it to the outer hull to shoot through.
I have had on in my sump for years.
Had to change the transducer a few times and never had any issues.
make sure the epoxy is fairly bubble free and your away.

But geneerally its not too important. as the sound wave should not be too much effected by the state of the transducer. the odd scratch should be fine.

[Erin]

I really doubt a scratch/groove will cause a problem. If you consider that they are often mounted in hull firing though fibreglass and antifoul I'm sure you'll be fine.

[Poly]

Its detecting changes in "density" of the material, on the back of the boat I wouldn't expect a problem its going from plastic to water and it won't have the resolution to "see" a 4x0.25mm feature - unless it doesn't wet properly - because the groove is too thin - then it will effectively be trapping an air bubble and might not be so happy.

Likewise if you decide to mount it in the sump and silicone/epoxy it in place you want to be sure you aren't getting a bubble in your "ding".

[9D280]

Thanks guys.

Sump - I wish! This is going to have to be transom mounted.

Pol has hit the nail on the head - the air / vcaccum bubble it might create at 20+ odd knots was my concern. I guess if it tells me obvious rubbish at speed it's a wide angle transducer so I could probably angle it ever so slightly down at the rear to try & up the fluid pressure on the bottom face a bit & reduce the likelihood of that happening.

In the meantime I'll sand it a bit smoother & see what happens... as I motor off into the sunset thinking Jaws is hot on my tail