Mike — part of the reason to use a backing block is to make sure you have a flat spot in the rounded hull to mount the transducer; it’s not so much for reinforcement. It also gives you more of the shaft of the transducer to seal with whatever goop you use as a sealant; I use LifeSeal when I install transducers. Goop up the flange and bottom several threads of the transducer, and HAND-tighten the nut inside on top of the rubber gasket; then come back a day later and tighten the nut a bit more, if it will easily turn.
If your location is really flat, go ahead and mount without a block; but I would recommend using the gasket from the manufacturer. Then if your boat sinks and you can prove you followed Airmar’s instructions, you can sue them… :^) — Fred Fred Street -- Minneapolis S/V Oceanis (1979 C&C Landfall 38) -- on the hard in Bayfield, WI :^( > On Apr 1, 2017, at 4:45 PM, Michael Crombie via CnC-List > <cnc-list@cnc-list.com> wrote: > > Hi gang, > > Last fall I wrote about removing my depth sounder as water was leaking in. > It was a traditional external depth sounder with a mushroom head. It was > held in place with a locking nut that screwed down against a wood backing > block. > > I dug out the rotten wood and ground down all the glass that had been used to > hold it in place. The installation was really overkill. I wasn't able to > remove the transducer without stripping the threads, so I bought a new one. > > I now have a smooth, pretty flat surface around the transducer hole. I bought > the same transducer (Raymarine, i.e. Airmar,) > > So here are my questions: > > 1. I have made up a fibreglass backing block, but do I really need a backing > block given that there really aren't any stresses on the transducer? It's > not like a through-hull where you are opening and closing a valve. > > 2. The new transducer came with a rubber washer that sits on the inside and > the locking nut is supposed to be hand tightened down onto it. Is this really > a good idea? Won't the rubber shrink a bit with age? > > 3. For my sealant, is there any difference between 3M 4200 and Sikaflex 291 > sealant/adhesive? I'm not sure if the Sikaflex is suitable for underwater > applications. > > My current thought is to use the backing plate given that I've made it up > already and skip rubber washer. > > Thanks, > > Mike > Atacama. 33mkii > Toronto
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