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How to (not) install a Thru Hull for transducer

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bjoye
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USA
105 Posts

Initially Posted - 01/25/2017 : 10:15:51

How to (not) install a Thru Hull for transducer

1. Read Instructions
2. Install plastic thru hull housing which came with transducer
3. Stay up all night wondering if it will leak
4. Remove said thru hull, yep, the boat would have sunk
5. Read Maine Sail articles on thru hulls
6. Install bronze thru hull correctly with backing plate
7. Sleep well

We finally have successfully installed the bronze thru hull for the retractable instrument transducer. The boat originally had 2 old non-functioning transducers under the V-berth. Both literally fell out when removed. We glassed in one hole with West Systems Epoxy (that is another story). Installed the plastic housing that came with the transducer as directed. Ripped it back out, convinced that we did not know what we were doing.

Several issues:

First, the plastic housing that came with the transducer. While the mushroom part seemed to be robust, the threaded vertical cylinder is very fragile. Either while installing or when I knocked it with my knee while working in the V-berth, the threaded cylinder cracked at the vertical seam. A bronze replacement is $100 and well worth the peace of mind.

Any thru hull in the V-berth must have a backing plate, the interior hull slope is too much. Even though I sanded the interior surface smooth, there is too much curvature and the locking nut makes minimal contact with the compression washer, introducing stress on the housing (see above).

We read the various articles by Maine Sail http://www.pbase.com/mainecruising/boat_projects on thru hulls and backing plates, along with other excellent articles here.

First we made a backing plate. We mixed West Systems Epoxy and 7 layers of glass (4 woven, 3 mat) and cut a 3 1/2" circle using a hole saw (for 2� housing hole).

Important: we cut the outer hole, but not inner hole, that comes last.

Next, we wrapped the 2� hole saw in plastic tape (to act as a release agent) and mounted the backing plate to the top of the hole saw, using the existing pilot hole and thin bolt.

Now we mixed epoxy with 406 Colloidal Silica to the consistency of peanut butter, slathered the back of backing plate and dropped the plate/hole saw into the hole from the inside. The hole saw held the plate perpendicular to the angle of the hole and parallel with the outer hull surface.

Next day, we removed the bolt, and popped loose the 2� hole saw from the outside. Then we use the hole saw to drill the inner hole from the outside. The existing hole guided the saw to a perfect centered cut.

Finally, we installed the bronze thru hull housing using 3M 4200

Because the backing plate is flat and parallel to the exterior hole surface, the locking nut firmly engages the compression washer. One note: I did not run the sealant up the threads to the point of the locking nut as directed by the instructions. I felt that this step is a crutch for poorly installed housings. Now with a proper backing plate, the locking nut and compression washer should function as designed. We will see how it holds up.

"Frayed Knot" 1989 C-25 WK/SR #5878

islander
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3992 Posts

Response Posted - 01/25/2017 : 10:47:42

quote:
Any thru hull in the V-berth must have a backing plate, the interior hull slope is too much. Even though I sanded the interior surface smooth, there is too much curvature and the locking nut makes minimal contact with the compression washer, introducing stress on the housing

This is why I and Dave suggest to install the transducer under the quarter berth where the hull is close to flat and the transducer points down, Not off to the side.No need for a backing plate to make things level. Anyway looks like you did a good workaround. Nice job!

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Edited by - islander on 01/25/2017 10:52:09

SKS
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161 Posts

Response Posted - 01/26/2017 : 12:41:36

My personal preference would be to use an inside the hull transducer located just aft of the keel where the hull flattens out.
I use my GPS for both depth and speed.
Yes, I get over the land speed, not necessarily through the water speed, but I'm OK with that.

"Lady E" 1986 Catalina 25: Fin Keel, Standard Rig, Inboard M12 Diesel, Sail No. 5339
Sailing out of Norwalk Cove Marina, Connecticut

Edited by - SKS on 01/26/2017 12:42:09

islander
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3992 Posts

Response Posted - 01/26/2017 : 13:21:31

Water speed doesn't make much sense to me. Doing 5n into a 5n current = 0 forward progress. If I'm going somewhere it's SOG that I'm interested in and the GPS will give you that and tell you how long till you get there,

Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound

Edited by - islander on 01/26/2017 13:24:57

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