Depth Transducers

I am hoping to improve the reliability of my depth transducer system at the next haul out. My current system is a cheap bullet transducer in a wet well mounted in the bilge on the inside of the hull. The wet well is made from a plastic sandwich box with the bottom cut off, and I periodically fill it with water and/or radiator coolant. It works most of the time, but not enough of the time to be reassuring.

Previously, I had thought of two options
1 Through-hull transducer mounted in a hole-in-the-hull. The transducer before my current bullet is like this, which means I have a 30mm hole already in the hull. I've kept the transducer in place for obvious reasons (if I remove the old transducer, I have a 30mm hole in the hull below the water line. I have a plug, but don't want to risk the changeover until the boat is out of the water)
2 In-hull or external bullet. Most cheap fish-finders are designed to be mounted externally on the transom of a tinny. They can be mounted inside the hull using a wet well, like my sandwich box, which can be improved by filling with resin or goo. This approach will not work for sandwich-hull construction or timber, but seems to be OK for solid GRP hulls, like mine. I had previously bedded the bullet in with strong goo, but slightly damaged the housing when I tried to check it and relocate it. Next time I will use a soft or weak goo.

There is also a third option, demonstrated in the re-fit of Lyra by Tim Lackey lackeysailing.com/lyra-60/. This uses a cut-out and moulding in front of the keel for the transducer. I emailed Tim, who replied that the transducer was provided by the boat's owner, and is a side-scanning sonar like a Garmin GT30-TH. buy.garmin.com/en-AU/AU/p/537083 (the photos show that it is not the exact model). Tim advises that "Any regular transducer (i.e. not side-scanning) will shoot through a solid fibreglass hull from the inside, if installed properly (usually in an air-free mass of silicone or epoxy against the hull). Or invest in a good, metal (bronze) through-hull transducer (which I admit is still my preference)." I didn't ask if Lyra's transducer will be painted with anti-fouling.

So, I'm thinking of the following plan
* Get a new good-quality through-hull to replace the old through-hull in front of the keel (My chart plotter is on the blink as well, so I might go for an integrated system, if funds allow)
* Relocate the cheap bullet elsewhere in the bilge and fill the well or bed it in with soft goo. This is a backup, and makes use of something I already have.

Finally, the mass-market is oriented to fish-finding, which is not what I am looking for. I want to reliably know the depth in less than 5m of water to prevent grounding and to estimate the length of anchor-chain needed. I am not interested at finding fish at a depth of 200m. The solution seems to be the higher frequency transducers, which have a shorter range but greater accuracy than lower frequency transducers.

I had a very out dated depth sounder and a very outdated plotter and updated to a ( now old but not outdated sidescan fish finder plotter) integrated Humminbird which is accurate to three feet in position and allows me to side scan for isolated bommys or coral out crops in a potential anchorage by doing a small loop in proposed anchorage, and I don't think I will ever regret it or go back.

Sorry to say the cut a hole in the hull option imho should never be done.

In a past yacht (Compass 750) I had an old basic Piranha 1x depth sounder and wanted an in-hull mounted inside of the hull laminate scanning through the hull skin no holes needed (single skin grp as you say - no good for sandwich grp or timber) transducer for it - I emailed bla and they immediately came back with;

As long as your Piranha is a multi-pin Transducer it will fit. Part no you will need is - 103307 - Transducer Puck 200/83khz SGL/Dual Beam - Retail $ 189.00 Yes it is available You can purchase via any Marine Dealer nearest to with a BLA account You can find these via our Store locator on either - www.bla.com.au or www.humminbird.com.au

I bought this and installed it as per the instructions and it worked perfectly - depths were only in feet though. Maybe your readout has the metres reading but even so to avoid a hole in the hull I would be happy with feet again and am looking to repeat the above with a used readout and new transducer in current single grp skin yacht, no holes. On the slipping of this yacht in August I will fill in and cover over the old through hull transducer hole and speedo transducer hole - for both epoxy 9mm ply doubler pad probably 100-150mm diameter inside, tapered periphery and epoxy resin and glass over it, high strength epoxy filler to fill it, epoxy glass faired in on edges outside to finish off.

Revisiting this thread because I still do not have a working solution.

My attempts at mounting an external puck in an internal wet well have not succeeded. I am not getting reliable depth readings at shallow depths.

I also have a donated Airmar B19 through-hull, which I have not fitted yet. I emailed Airmar a photo of the plug (see below), asking what I might need to plug it into a Garmin head, and Airmar responded with "That picture is not a connector that we are familiar with and has not been used on B 19 transducers."

Laying out the cards on the table ...

* Looking for something that gives a reliable reading of depth at shallow depths, at low speeds and on muddy bottoms. Not interested in finding fish at 200m. Not interested in something that "performs well at high speeds" (a phrase often-repeated in the on-line blurbs, obviously appealing to the fishing market).

* Most reliable option appears to be a through-hull in a metal housing.

* Airmar transducers are used by most (all?) of the instrument-makers (Garmin, Lowrance, B&G, etc). You just need the right connector, or an adaptor, to plug one into the other (Airmar's confusing email to me notwithstanding)

* I don't want to spend less than I have to, but also don't want to spend much more than I have to, for features that I don't need (e.g. I don't need a speed wheel). Do I need CHIRP?

However, looking at the range of options on-line gives me a price range from about $220 (Airmar B169) to $810 (Airmar B75H), which is too broad to work with. Please help narrow this down.

Achernar, just wondering how your sender dilemma went ? I was watching closely as I had to get a simple and reliable sounder for my work boat, in the end I went with raymarine i40 display and b17 transducer.

@woko I might have a solution. I used the weakest filler I could find , Parfix, to glue the bullet inside the hull. It appears to provide readings that are reliable enough to avoid the shoals. Trying to keep silicone away from the boat.

So starting to line up all the bits & pieces I need for jobs I want to do on my new old wood hull, looking over the installation information for the depth transducer ( airmar they all are ) says to use waterbased anti foul, I've been rolling the like of jotun sea Guardian over a very similar unit on my other boat for 8 years. What does everyone else do ?