Waste Tank - Fiberglass Installation

[Clay Greene]

We would like to install a waste tank sensor so we have some advance warning of the need for a pump out.  To install the sensor, we need to run two adhesive strips down the vertical length of the tank.  The only place to do this is on the aft end of the tank.  Unfortunately, this is going to require removal of the wood frame piece that looks to be glassed into the surrounding area.  We should not have any problem cutting the old wood panel free with a Dremel.  I am looking for thoughts on what we would use to reinstall the wood panel.  Would it just be fiberglass mat and West epoxy?  I have not had to do any fiberglass work before so I am looking for some guidance on how to go about this.  Thanks!

[Ken Juul]

Can you just remove a section of the plywood rather than the whole piece?  Then you wouldn't need to worry about replacing it.  Most of the tank monitor strip systems I have seen are about an inch wide, don't need the whole width of the tank.

[Wayne]

I installed a holding tank sensor a few months ago; great upgrade.  I don't recall the brand, but I went with one that utilizes floats.  The sending unit replaces the access cap on the top of the tank and three floats hang down inside the tank.  I know it sounds a bit messy compared to the tape on the outside, but if it ends the need to start cutting into the boat . . .  Just another thought.

[tonywright]

Be careful. I tried the outside tape strip solution, but it gave false readings (take showed full when it was really only half full), and despite all kinds of help from the vendor we could not solve the problem. It seems that having the strips in close proximity to the plywood causes a problem.

I also switched to the float type: no problems with that solution. And you won't need to remove/reinstall the wood bulkhead.

Tony

[Clay Greene]

This particular sensor system got the highest rating in "Practical Sailor" so I have some degree of confidence that it will work; plus, I have already bought it so I am committed to some extent.  The comment about the close proximity to the plywood is concerning but I think I will give this a try and consider other options if this does not work out. 

We looked at the possibility cutting out a section of the wood frame but this raises some additional problems.  First, it would be ideal to have the entire vertical surface open because the tank exterior needs to be cleaned to get good adhesion plus we need to press down on the entire length of the strips for the same reason.  Second, cutting a vertical separation in the frame for the strips would eliminate the structural strength of the frame.  Only the fiberglass matting to port and starboard would be holding it in place.  I don't particularly want the waste tank shifting back and forth in that space.  Third, there is a risk of cutting into the tank in the process of cutting the frame because there is no real separation between the two (and the reason I need to do this in the first place). 

It actually does not look at all difficult to cut the fiberglass matting that holds the aft wood frame in place.  The matting actually forms the joint between the wood and the bulkhead to starboard and another wood frame piece to port.  The only real issue I see is how I go about reinstalling the wood, which is essentially replicating what is there now. 

[Bob K]

I installed the tape type sensor on the forward end of the tank.  Could not install on aft end as you want to because my battery charger is mounted there.   Anyway, I cut an access hole in the plywood wall separating the tank area from the forward storage area under the seat.  Irregular shape hole, I'm guessing aound 8" high, 6" wide.  Before I cut, I was able to confirm there was several inches of clearance between the tank wall and plywood wall.  I cut the hole with a jigsaw with shortened blade, but there was some hand sawing required in the lower corners.    A hole in the middle of the wall does not compromise the strength the way a slot from above will, though you could always add a reinforcement bracket if you are concerned.   I was able to clean the area and install the tape with some effort.   Used the hole cutout to close the hole.  Did so by cutting a slightly larger piece of 1/4" plywood and attaching to cutout piece.  The edge overhang from the larger size 1/4" ply created a lip area to screw it into the wall.   I wanted to have easy access in case the tape peeled, which it hasn't yet (2 years).  After installing the tape, I sprayed over the tape and tank area with some sort of adhesive/sealant - I can't remember what it was, but it was clear, and somewhat thick.  Might have been recommended on this site or mfr site.  Helps to keep the tape from peeling.    If I find it, I'll post.  I've had  no issues with the monitor.  A very good addition.

[Ken Juul]

So we are back to the original question, how do I do it?  

Cut the fiberglass tabbing that is currently holding the plywood in.  Remove plywood, using a sander/grinder remove remaining tabs from plywood (do off the boat if possible).  

Install the sender.  Test the sender to make sure it works.  

If possible, grind the remaining tabs off the hull.  Vacuum entire area, clean hull & plywood with acetone or similiar cleaner.  

Dry fit plywood.  Cut fiberglass cloth into appropriate sized pieces for new tabs.  

I have always been told to use epoxy rather than polyester resin for this type of repair.  Mix enough epoxy to complete the job, using a throwaway bristle brush (foam melts) coat the areas of the hull and plywood with a light coat of epoxy.  Place pre cut tab in wet epoxy, smooth out rinkles, put heavy top coat of epoxy on tab.  The tab should be well saturated. Move to next tab, repeat. Allow to dry.

Make sure boat is well ventilated so the fumes don't get you.  Wear throw away gloves.  Follow the mixing directions, that should give you 5-8 minutes of pot life, so you have to be ready to go before you mix.  Plenty of time to do the job, but can't daudle.  Once the epoxy starts to kick, stop applying, it won't work.  If really unsure, mix enough epoxy for one tab.  Do it for practice.  Then mix a second batch for the rest.  99.9% of the time the repair will be out of sight...don't worry too much that it won't look professional.

[Clay Greene]

Thanks, Ken - much appreciated!  Sounds like a good rainy day project. 

[Rick Johnson]

I made a vertical cut in the plywood using a Makita battery powered 3 3/8" circular saw (I set the depth so I didn't nick the tank).  That allowed me to pull the plywood away from the tank enough to spray the adhesive and mount the foil strips and the sensor.  I sanded both sides smooth so there were no rough edges and then used another piece of plywood to cover the cut.  I used 1/4" plywood and the piece was about 6" wide.  I used ss screws and epoxy to glue the plywood together.  Painted every thing white and I believe I used a wooden paint stir stick to smear some 5200 on the plywood face next to the tank (again to make sure there were no rough edges.  This sounds way more complicated then it was.  It seems plenty strong and the tank monitor works perfectly.  

Cheers,

Rick

[jmnpe]

Thanks Ken for the treatise.

I need to remove and replace my tank ( old/original and very stretched = stinky ) and will require the entire aft panel removal and re-installation. The new tank will include the SensaTank 100 sensors applied to various sides of the tank to complete the triad of tank level sensing available with the SensaTank system ( 2 fresh water tanks + holding tank ).

John