Transducer Replacement

[idearat]

I'm replacing 35-year-old transducers (depth and speed) with new ones (Airmar, w/ Raymarine ST40 bidata). I have some questions (and jitters); any help appreciated!

- The new through-hulls are about 1/8" smaller in diameter than the old. Am I okay to just us sealant / caulking, or am I going to need to build it out with Epoxy? (Solid glass hull.)

- Any hints on removing the old through-hulls? Both the old transducers are removable, so I'm hoping I can just carefully cut through the tube radially in a couple of places and break them inward.

Thanks...
Perry.

[Mark1977]

I don't think sealant or caulking will cut it. I would use epoxy.

[mrm]

Mark, you are probably right, but I am looking at Airmar DST800 installation manual
http://www.airmartechnology.com/uplo.../17-355-01.pdf and the drawing (Fig. 3) at the bottom of page 2 caught my attention. In one of the configurations they show a hull hole of a noticeably bigger diameter than thru hull and use of isolation rings, where the bottom one also acts as a centering bushing.

Maybe in this case it would be easier and cheaper to use this approach?

Marius

PS: In the same literature Airmar instructs to

Originally Posted by Airmar

[sensor]Surfaces exposed to salt water
must be coated with anti-fouling paint. Use water-based anti-fouling paint only.

Without the intention to go off topic I would like to ask for a recommended product for this application. I think this will be of use to OP too.

[Ziggy]

The isolation bushing is meant for use with stainless steel transducer housing and a metal hull. I don't know if it would work well with a plastic housing (if that's what you're buying). You might send an email to Airmar tech support and ask their advice.

I use MDR water-based transducer paint: Transducer Antifouling Paint

[mrm]

Ziggy, thank you for hint about paint.

Marius

[idearat]

Originally Posted by Mark1977

I don't think sealant or caulking will cut it. I would use epoxy.

I was afraid you'd say that... guess I was hoping 1/8" difference in diameter (i.e. 1/16 all around) was small enough for the caulk to fill.

How would you go about the epoxy thing? I'm thinking some kind of exactly 2" tube with wax or something, tape across the hole on the outside of the hull, push thinnish putty down the (1/16") gap between the tube and hull? Not sure how to centre the tube in the hole.

Re transducer antifouling, nothing much available locally (and not enough time to order on the web). Seems most locals just leave it bare... I expressed surprise when the guy at the chandlery told me that, and he said "Just take a walk around the boatyard and see what you find"... and sure enough, the ones that aren't just painted over with the regular bottom paint are just bare. He did say you could spread some lanolin on the transducers and the prop to good effect.

Thanks for your help!
Perry.

[Ziggy]

Originally Posted by idearat

... guess I was hoping 1/8" difference in diameter (i.e. 1/16 all around) was small enough for the caulk to fill.

If it's really only a 1/16" gap, it may be fine to fill it with caulking. However, the old transducer may not have been a tight fit to begin with, so the gap may be closer to 1/8" all around, which would begin to be more problematic.

If you want to make the hole smaller, clean out all the old caulking and send the hole well. Then put a sheet of release fabric (or saran wrap) backed up by a small scrap of plywood or some other semi-rigid material tight against the bottom (you can hold it in place using a wooden batten). Then mix some epoxy with colloidal silica and high density filler to peanut butter consistency and fill the hole from the inside. When the epoxy cures, drill the new hole. Since the difference in diameters is pretty small, you don't need to worry about feathering the hole and laying up multiple layers of glass.

[idearat]

Thanks for the clear instructions, Ziggy. Guess I'll see what it's like when I get the old one out.

[capngeo]

The issue is not so much one of sealing, but mechanical strength, with that in mind, I'd be leery of installing a through hull of any kind in a hole much more than 0.125" diameter larger. If you have the thread thickness on the TH, I would have a machinist turn out a flanged bushings for the outside, and a large washer for the inside. Nylon, Noryl, Delrin, or even high density polypropylene.

[Ziggy]

By the way, to get the old transducer housing out, get two flat pieces of wood and drill a hole through each to accept a long bolt. Put one of the pieces inside the boat acrross the top of the housing, with the bolt through the piece and extending through the housing through the hull. Then under the boat put the second piece of wood on the protruding end of the bolt and secure it with a nut. Put two blocks of wood as spacers between the hull and the flat piece on the bolt, so the flat piece straddles the thruhull. Now start tightening the nut, so the bolt and the piece of wood inside the boat pushes the housing out.

This method probably won't work with 3M 5200 but I used it to remove plastic thruhulls that were bedded with polysulfide with no damage to the hull or the TH.

[lancelot9898]

Originally Posted by Ziggy

Put one of the pieces inside the boat acrross the top of the housing, with the bolt through the piece and extending through the housing through the hull.

I have a bronze TH transducer which is solid such that a hole needs to be drilled in the transducer itself to allow the bolt to pass thru. If I can pass a bolt thru that hole then your method seems to be a good idea. I orginally planned to put a pipe wrench on the thru hull and twist it after removing the retaining nut to break it loose for the caulking. There is really little overhead clearance where it is installed but the bolt idea will possibly work provided I can drill a center hole(from the outside) in the transducer and have enough clearance to get the bolt through the hole and/or get a wrench on the upper nut/bolt head to keep it from moving when tightening the other nut from the outside. Thanks for the suggestion!

The old impulse transducer(25 year old) instructions called for drilling a 2 1/16 hole and when I go the airmar site most transducers say a 2 inch hole is needed. I asked airmar about using the orginal hole, but no response. The yard installed the old transducer 25 years ago so I'm just assuming a 2 1/16 hole is there so will not know exactly until I get the old transducer removed.

[capngeo]

if you in fact do only have 1/32 extra,, just use 52 goo (3M 5200)