Speed paddle-wheel skin fitting failure

little_roundtop

little_roundtop

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I've just been reading an old PBO (No 537 Summer 2011) and there's an article in there (page 50) about the catastrophic failure of the plastic skin fitting of a speed/log paddle-wheel transducer. Apparently it fractured at the junction between the outer flange and the threaded tube. The author recommends encapsulating the nut and lower skin fitting tube in GRP, apparently his installation instructions called for this to be done although it wasn't.

My speed/log is a similar plastic unit (Simrad) which has performed perfectly since it was installed 7 years ago. My installation instructions make no mention of glassing it in they just call for lots of Skiaflex and not over-tightening the plastic nut.

A search of the forum has turned up at least one other skin fitting missing the outer flange (http://www.ybw.com/forums/showthread.php?t=273661) so it's not a complete one-off. There must be thousands of these out there, so has anyone else had one fail? Are yours glassed-in as the article suggests or not?

Should I be worried?

Tony C.
 
No, I wouldn't worry. It sounds like the failed unit may have been over-tightened, leaving the flange under stress, or it may have been hit by something. Like you say, there are millions of them out there and failures are not common.

Rob.
 
There was a discussion about this some while ago.
I had a NASA log unit come away in my hand while removing the paddle wheel, that concentrates the mind rapidly! It broke exactly as you say, between the flange and threaded barrel, the plastic is VERY thin, just a few millimetres thick.
Afterwards I went back and read the instructions and it does clearly state that the fitting should be encased in resin on the inside. Just regard the plastic back nut as a temporary locating device while applying the resin.
 
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DanTribe said:
There was a discussion about this some while ago.
I had a ASA log unit come away in my hand while removing the paddle wheel, that concentrates the mind rapidly! It broke exactly as you say, between the flange and threaded barrel, the plastic is VERY thin, just a few millimetres thick.
Afterwards I went back and read the instructions and it does clearly state that the fitting should be encased in resin on the inside. Just regard the plastic back nut as a temporary locating device while applying the resin.
Click to expand...

The instructions for mine make no mention of anything other than tightening the nut. They specifically do not mention encapsulating it. Perhaps there are different designs to these things, some require encapsulation whilst others don't??
 
parsifal said:
Seems to me that it's a pretty poor skin fitting if it needs to be encased in resin in case it breaks up!
Click to expand...
You are probably correct, but there are lots of NASA units fitted and I suspect that , like me, quite a few are not fitted in accordance with the manufacturer's instructions. Not a problem until they snap off, and there is no one there to discuss the poor design with at that time.:)
 
My NASA unit is encapsulated as per the installation instructions. I wonder what the insurers might say if your boat sank due to the fitting shearing off, and not being installed in accordance with the manufacturer's instructions? My own opinion is, it's not worth the risk for a few minute's work with a bit of resin.
 
DanTribe said:
You are probably correct, but there are lots of NASA units fitted and I suspect that , like me, quite a few are not fitted in accordance with the manufacturer's instructions. Not a problem until they snap off, and there is no one there to discuss the poor design with at that time.:)
Click to expand...

Mine is encapsulated per NASA instructions but I still don't like it and I only tolerate it because I can't buy a bronze fitting.

However, I reassure myself with the thought that there must be thousands of these plastic skin fittings in use and it's rare to hear of a boat sinking because one has failed.
 
"The author recommends encapsulating the nut and lower skin fitting tube in GRP"

And what does he recommend for a non GRP boat.

Mine says only to use bronze on wooden boats.
 
The installation instructions for my transducer tube ( Airmar) simply say to bed it in mastic and to hand tighten. It says you must not use a wrench ( American for spanner) to tighten - just hand tight.

The steady load on the fitting is about 2kg so a decent sealant should be able to hold that without the flange.
 
I suspect this relates to incidents (10 years ago??) where using the 'wrong' sealant affected the plastic used in NASA fittings.

I think the original discussion has been lost down the back of the sofa, but there are some references here:

http://www.ybw.com/forums/showthread.php?t=35934

http://www.ybw.com/forums/showthread.php?t=55461

More come up with a search.

Unless you have an older NASA sounder, then I don't expect you have any special reason to be worried (over the normal paranoia about random failure of underwater fittings).

Andy
 
I too read the article last year, and checked the NASA instructions for a new impeller unit I acquired. Mine hadn't been glassed in by whoever fitted it originally, so it's now done - only a few minutes work in my case, but more peace of mind now. I wonder if the instructions have changed recently.

As a postscript, I've put a new rotor in my original fitting, and acquired a second fitting. I've cut the co-ax cable, and put a 4-pin mini-DIN plug on a short co-ax from each unit, and a mini-DIN socket on the instrument lead. It works! I can now swap a unit in and out for cleaning and/or repair at leisure - at least, that's the plan....
 
Choice of sealant appears critical

When I fitted a NASA log about ten years ago the instructions were clear - use silicone sealant and nothing else otherwise NASA would not be responsible! I also glassed mine in place using epoxy resin. I still have the boat and it still floats!
 
On my previous boat, I machined out a bronze skin fitting to replace the plastic one.
A friend of mine has a steel boat where the paddle wheel skin fitting is in a sealed locker, such that the worst that could happen is you flood that locker, the top of which is well above waterline level. Access is not great, I have to say.
 
NASA generally make acceptable cheap end of the market kit for boats but the poor thickness and structure of these impeller housings to be fitted in a critical location reflects pretty badly on them as a company.
 
catalac08 said:
NASA generally make acceptable cheap end of the market kit for boats but the poor thickness and structure of these impeller housings to be fitted in a critical location reflects pretty badly on them as a company.
Click to expand...

The one I had on my first boat seemed adequately strong and well made. That was 10 years ago, have they changed much?
 
''I suspect this relates to incidents (10 years ago??) where using the 'wrong' sealant affected the plastic used in NASA fittings.''
It happened to me and it was not a nice situation to face.Luckily i was lying right above it as it started getting worse and i heard water flowing in and bilge pump engaging... apart from being scared as it gave way when i tried to tighten the nut by hand, nothing else happened as i keep those wooden bangs right next to it.Apparently Sikaflex is the wrong material to use and yes i got rid of it.It only cost me as i had to haul the boat out next day to glass the hole it left.One less hole to worry about...
 
This was a big saga specifically related to the NASA skin fitting about 5 years ago with several reported cases of the outer flange becoming detached. I have one myself dating from that time and never fitted. I have NO specific expertise relating to such mouldings, but it certainly doesn't look satisfactory to me. Of course, glassing in on the inside should avoid a catastrophic failure, but that shouldn't be necessary!. I have always been suspicious about claims that the failure was 'usually' related to the sealant used.

It IS possible to substitute the plastic NASA fitting with a standard brass/bronze/DZR through-hull fitting with a little modification. (remove any internal tabs with dremel; shorten length; replace 'O' rings with slightly different size if necessary).

Out of interest, my NAVICO plastic log through-hull has been in place for 27 years (no glassing-in) with no problems at all. It is a much heftier moulding though, but unfortunately a completely different size to any presently available log transducer.

Vic
 
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