Fairing block for transducer

[bromleybysea]

I am about to fit an Airmar echo sounder transducer to my boat, a long keeled deep-draft 60's cruiser racer. The Airmar is a chunky bit of kit compared to the existing transducer, probably an old Seafarer one. There is no surface of the hull (GRP) that is "horizontal" under the waterline so I will have to fit at the turn of the bilge with a fairing block inside and out to get the transducer perpendicular to the waterline. My question(s) is as follows:

What is the best material to use- the Airmar is plastic and the blurb cautions against fitting to a wooden hull because of the risk of cracking it as the wood swells- I have some teak and thought of using that with a reasonable clearance and lots of mastic? (having spent an unfeasibly large sum on the instruments I balk at the prospect of having to buy a new bronze transducer housing)

What's the best way of fixing the fairing blocks to the hull? Epoxy? Sikaflex?? And would I be best advised to through-bolt the inside and outside blocks together?

Thanks in advance chaps- it's been giving me sleepless nights!

 

[whipper_snapper]

At the price of somewhat reduced maximum working depth, you can almost certainly get away with mounting it inside the hull. You then can use a simple bit of plastic drainpipe cut at an angle to sit square and filled with oil, then simply shove the transducer in the top in contact with the oil. As a test to see if this will work for you, you can make a temporary arrangement. Some people have reported simply bedding the transducer to the inside of the hull with bathroom sealer stuff so that it sits vertical. I imagine this causes very significant reduction in range because I imagine the sealer is a very poor transmitter of the signal. You could try that as a quick and simple test - maybe put a plastic bag round the transducer in case the solvent in the sealer damages it.


Having said all that, we have a hardwood fairing block to mount the sender externally. It is epoxied to the hull and the transducer screwed straight in. Nothing has cracked yet!

For us max depth is important if we are to use it for nav information and a warning of nav screwups, because we sail in water with very steep drop-offs from lethally shallow to very deep in the space of a few yards. But in other places the differences between a max of 50m and 100m is not very important.

 

[john_morris_uk]

I agree with whippersnapper. You might get away with putting it in a tube inside the hull, but I prefer to not degrade the instrument by putting the attenuation in the signal path. Use some of your teak to make a fairing block and a suitable wedge for inside the hull and don't squeeze all the mastic out when you tighten it up. This means that there will be a 'gasket' of flexible mastic which will allow the teak to swell a little.

The only other solution would be to cast something in epoxy, but that would be expensive and probably ott for what you need.

 

[michael_w]

I think Airmar make a self adjusting fairing piece just for this problem. FWIW I've found in-hull transducers give poor performance.

 

[jon]

Airmar supply a plastic fairing block that you can bandsaw to the angle.

 

[EastCoastChris]

I am just in the process of doing the same thing, my thoughts have run as follows.

The Airmar block is very expensive (£90 I think for my triducer) and not well suited to cutting for any sort of complex/compound curve, so unless you have a conveniently shaped hull that suits a single bandsaw cut, if you slip up you could end up buying more than one!

My old (too small) fairing block was teak and had not adhered particularly well to the hull. Teak can be problematic with adhesives.

I'm thinking I'll make mine from a good lump of oak which I intend to epoxy coat.

Chris