Flashback 2006: Replacing fuel/water tanks

Tanks Installed

Recently, Mark wrote up a description for Manta Owners in need, on a job he did waaaay back in 2006 after a trickle of a fuel leak was discovered on Reach.  The issue cropped up due to foamed-in tanks in earlier generations of Mantas where a pin-hole leak developed in our aluminum tank.  Foam for this use is usually not desirable as it can disassociate and promote condensation leading to corrosion.  On the upside as you will see, the foam actually helped in containing the leak from being a lot worse at the time.  

In his usual fashion, Mark contemplated, researched and lost sleep over a solution all summer, waiting until a CT winter to tackle the job.  The fuel and water tanks are under the molded-in cockpit and would need to be cut-out; we replaced both while we had access.  He came up with a way to keep the non-skid surfaces intact, glass it back in on both sides (!) and re-gelcoat without detecting a fix.  

Most people visiting on Reach are amazed to learn that we’ve had the cockpit cut out!  It was traumatic at the time; however, I came to appreciate how miraculous fiberglass repair is, which helped mentally when we tackled the stern extension project years later… 


Cutting out the cockpit and removing the tanks:


Remove the foam board ceiling trim from the cockpit areas of both staterooms.  Remove the drawers from the starboard stateroom.  Try to remove the cabinets by the bed from the port staterooms – ours were glued on and I would have had to wreck them to get them out.  I was able to remove the backs from them, which allowed access to the cockpit glass, but I had to work through the cabinets.


You will see where the cockpit is tabbed down on interior bulkheads in several places.  This tabbing needs to be cut.  There may also be a bit of tabbing along the cockpit footwell that can be reached through the stove area – although this may not have been tabbed there on all boats.  A multitool makes cutting the tabbing easy.

Interior tabbing 1

Interior tabbing 2

Best is to cut along the cockpit sides and along the footwell side – you will need to make a small cut across the cockpit floor at both ends of the footwell to transition.  The reason for this is that you won’t cut through any core material this way, all the cuts will be wide flat surfaces, and all the cuts will be reachable from inside and outside.  This will allow you to grind back proper bevels for glassing, be able to glass both sides of the cut, and make fairing and finishing easier (you can heavily glass the inside of the cut for most strength and glass the outside for finishing ease).

Where to cut 1

Where to cut 2

The big “Gotcha” is that Manta inserted a 1″ pine board in the layup where the seat post bolts into the propane locker area.  This makes removing the cockpit impossible with a single cut because it is too thick to bend forward and out.  So here you will need to remove a 6-10″ section.  Best to just remove that section starting from the port rear corner of the cockpit wall all the way across the seat post mounting area.  In the pictures, you will see I cut it out in two or three pieces.  That was only because I needed to keep cutting out sections until I could remove it.  That pine board really came as a surprise!

Cockpit pine board

Cockpit out small

Once the cuts are made, you can insert screwdrivers in them and lever the cockpit sides forward and lift the cockpit out.

Removing the tanks from the foam can be difficult because that foam really glues them in.  I used a hand saw to cut around the sides and across the bottom as much as I could.  Manta placed structural foam strips across the curved bridgedeck to level it and the tanks are sitting on these with the foam filling underneath them.  So if you feel resistance while cutting under them, just push and pound through them as best you can (they are dense and tough).  I ended up needing to use a hydraulic wedge to finally break the first tank free.  Once one is out, the other is easy because you have much more room to work around it.

leaking fuel

Cockpit and Tank – Prep and Install:


Once you have the cockpit out, the tanks removed and the foam cleaned out, then you need to decide how to mount the new tanks.  You will get many different advices on this, and I will present how I decided.

Many people will try to convince you to install the tanks on rubber mats or similar.  I think this is a very bad idea.  Any carbon rubber will cause galvanic issues with aluminum.  Any mounting choice that allows contact with the aluminum can cause poltice corrosion.

My idea was to glue fiberglass strips to the bottom of the tank so that the aluminum didn’t have any contact with surfaces, and no water could ever get to it.  I used epoxy and glass coated plywood to level the curved bridgedeck and glued the tank strips to this.

Bridgedeck mounting 1

Pictures tell the story easier.

I glued epoxy coated furring strips along the tank well port and starboard bulkheads so that they and the center high point of the bridgedeck were at the same height.

Bridgedeck mounting

I then laid three epoxy coated and fiberglass wrapped 1/4″ plywood sheets across the bridge deck   

The plywood was glued with 5200 to the middle of the bridgedeck, and glued with thickened epoxy and screws to the furing strips.

For the tank, I bought 1/4″ x 2″ fiberglass strips from McMaster-Carr and cut them to be just short of the width of the tanks.  I put 6 strips on each tank bottom and spaced the strips so that two strips fell on each of the three plywood sheets.


The fiberglass strips were liberally buttered with 5200 and stuck on the tanks.  All of the strip edges were filletted with 5200 so they were now permanently glued and sealed to the tanks.  No water can get between them and the tanks.  The 1/4″ thickness makes the tanks stand proud of the surface and makes sure that the aluminum tank has no contact with any surface and any water or condensation will run off without touching the tanks.

Tank Bottom Prep

To mount the tanks, I put heavy beads of 5200 on the bottom of the fiberglass strips and sat the tanks down on the plywood floors.  Be sure you test fit everything because this will be a permanent mount when it cures.


The 5200 will be enough to hold those tanks down permanently, but I also used pieces of nidacore to chock the corners and between the tanks to make sure there is no movement or shifting.

Tanks Installed

Again, you will have to decide the best way for you.  There may be flaws in my thinking that I haven’t found and you may have a better way.


There will be stuff to do before putting the cockpit back on, but it is obvious stuff like putting the little subfloor over the tank fills in and hooking up hoses, etc.


Then put the cockpit back in.  One hint here is that the cockpit will sit lower than it did originally because you have cut the tabbing it sits on in the interior bulkheads.  So drill a couple of holes on both sides of the cuts along the cockpit walls and use zip ties to pull the cockpit up so the cut sides come flush together.  Now you can fillet the bulkhead spaces and retab the cockpit to them.  Heavily glass the inside of the cuts and pull the zip ties out.  Now glass the outside of the cuts and fair them to finish level.

Reglass 1

Reglass 2

Reglass 3

The best part about cutting the cockpit out this way is that you can gelcoat it without worrying about the color match becoming different in later years.  Just gelcoat from the floor nonskid to the seat nonskid – any color change in later years won’t be noticeable because the nonskid breaks the eye and the two slightly different colors will be separated by nonskid and unnoticeable.  There will be two areas where the old and new gelcoat will abut – at both sides of the cockpit well.  But these will be small areas and are out of the normal line of sight.  Of course, match the color as best as you can – but it will slowly change over the years.

Masked for Gelcoat

new gelcoat

© M&M 2019