Depth sounder in hull transponder fluid

[Lahara]

I'm replacing my old B&G depth sounder which has a through-hull transponder with a new Furuno model having an in-hull transponder. The new transponder is fitted into an Airmar (P79) base attached to the inside of the hull and is set down into a fluid bath contained in the base.

For the fluid, Airmar recommend using propylene glycol or mineral oil, with the former seeming to be their preferred choice. I understand propylene glycol is, or is the major consituent of, antifreeze fluid for use in radiators of water-cooled engines. Though I could probably buy antifreeze fluid locally in the city, I'd prefer not to start using it as all my cruising is done in Brisbane and points north - i.e. in the tropics. It seems to me that if I ever needed to top up the reservoir of fluid, say at Thursday Island (at the northern tip of Australia), antifreeze may be difficult to obtain from shops set in amongst the palm trees.

I appreciate the purpose of the fluid bath is to provide a constant acoustic path between the transponder and the hull by excluding air and I can also see that use of antifreeze fluid for boats operating in climates much colder than Australia makes a lot of sense. Having gleaned from diverse sources the use of olive oil, baby oil, castor oil, and even plain water, as the fluid in transponders installed by others, I was wondering what were the views of the wider YBW forum. Leaving aside its antifreeze characteristic, does propylene glycol have a particular property that makes it ideal acoustically? Is there a 'mineral oil' that has similar properties? Or to put this another way, if I'm not intending to use antifreeze, what is the best mineral oil to use as transponder fluid, and why?

 

[ccscott49]

Castor oil is best IMHO.

 

[Lahara]

Thanks for reply Englander.

Castor oil, olive oil and peanut oil have all been used at some stage, according to my google hits. I have to say I haven't been inclined towards using a vegetable oil, even though there is considerable advantage in their being readily available from food or chemist's shops just about anywhere.

I have not yet had any direct experience of the use of a vegetable oil as a transponder fluid but have plenty of knowledge of how aggressive tropical insects can be when there is a food source available, particularly in some dark and infrequently visited place like an in-hull transponder. Admittedly, in my case, the transponder fluid is sealed in its base so this denies a direct path to a food source; still, I'd prefer to avoid use of them unless a particular vegetable oil - you suggest castor oil - has a technical characteristic that enhances transponder operation. Is there a particular reason why you favour castor oil?

 

[tjbrace]

Similar to my previous boat where we used Castor Oil wiyhout any problems

 

[VicS]

I think you will find that almost any fluid that does not attack the GRP or the transducer is not going to freeze, is not volatile, and is chemically stable will be quite satisfactory.

As Englander said castor oil was the a popular choice and was what was recommended by Seafarer when I installed mine.

Propylene glycol is used as an antifreeze but by far the most common for vehicle engines is ethylene glycol. The latter is toxic whereas the former is not and is therefore used in situations where toxicity is relevant. I would not use ethylene glycol without establishing that it was compatible with GRP.

You seem to be in favour of using a mineral oil. Any mineral oil will do but you would not want to choose one with smelly additives like an EP oil I guess.

Silicone sealant or Sikaflex sealant are other possibilities rather than the oil filled tube. Some transducers were not supplied in a shape suitable for mounting in a liquid filled tube and were attached to the hull with Araldite I remember. The important thing with non liquid mountings is to avoid trapping air bubbles.

I have seen it suggested that a blob of chewing gum is a good temporary method to check on a chosen location. Important that the are no voids in the GRP where it is mounted! Although I think it has been reported that Blue tack is not so good.

 

[Lahara]

Thanks TJB and VicS for your comments.

Yes, I do favour, for reasons I've already mentioned, the use of a mineral rather than vegetable oil. However, it is useful to have confirmed that castor oil works, particularly as castor oil, if desperately needed, should be available even in out of the way places. I suspect that in the end all that matters is the exclusion of air from the face of the transponder and, unless I am informed otherwise, will probably elect to use baby oil, which is mineral based and unattractive to insects, and which is likely to be as readily obtainable as castor oil in out of the way places.

Still, though only a short time since my message has been posted has elapsed, I am uneasy that I've not heard from any YBW correspondents that use mineral oil. Given your UK-base, peraps this is a climate related matter.

 

[ccscott49]

You can also use silicone grease. but make sure no bubbles.

 

[whipper_snapper]

To hijack this thread - does anyone know if a 'normal' transducer will work through ply/epoxy maybe 6mm thick ?


Thanks

 

[pampas]

Another way of saying engine oil. /forums/images/graemlins/laugh.gif

 

[moody1]

Generally manufacturers recc'd Castor Oil. Easily obtained from chemists and cook shops.
Other variants people use are Cooking Oil, water, mineral oil - probably based on what they had at hand at the time.
Reason that oils such as natural ie Castor Oil is that they are heavier density than water to try and keep density of transmission medium near constant with hull, they do not aerate easily providing a good pathway. water does work but may depending on hull thickness degrade signal.
We have in past smeared large amount of fast set-epoxy on a transducer and bonded to the hull without tube. Works fine. Even relatively easy to remove transducer as it doesn't bond totally permanent.

 

[gjgm]

do you need to bother with tubes and fluid? Just a smear of silicone and stick it on. You can use epoxy, but its a bit permanent if the location isnt perfect!

 

[moody1]

Tube and fluid. It's the way near all manufacturers advise to fit them, of course they like to sell you the kits to do it. Plumbers pipe and car filler does just as well.
Epoxy is a good way and actually can be broken away from transducer head easier than imagined..
Silicon works as well.
I think it all depends on location in the hull, is it to be absolutely permanent.
I wonder if tube / oil combination has anything to do with vibration and damping ?

 

[Lahara]

Hi Moody1,

Yes, I've long thought that simply sticking the transponder down with lots of silicon sealant was the best option. Seems to me one gets the desired air free contact and a fitting robust and permanent enough to handle vibration yet it is not that permanent that it can't be easily removed. But I chickened out when it came to the crunch and bought the stick-on (with silicone sealant, no less) base to fill with oil. It's extra work but almost the same, though I guess I have the option, if it doesn't work, of being able to have a legimate whinge to the supplier if it doesn't work, given I've followed the official instructions.

 

[yoda]

I've used sea water for over 20 years without a problem. It is after all consistent with the fluid upon which the boat floats, cheap (free) and always available. You also don't have a load of oil that can end up inside your boat.

Yoda