Inspection, Cleaning, and Maintenance of SHEARWATER’s Water Tanks
By Tom Assenmacher
SHEARWATER 1975 MK-II Yawl (#157)
(We’ve owned SHEARWATER since 1982)


Hit Counter Visits since 6/28/2011

Posted 6/28/2011


SHEARWATER has the typical MK-II aluminum water tanks:  The larger aluminum tank under the V-Berth; and the smaller tank just aft of the mast step.  Years ago we added another large water tank where the original aluminum holding tank was located – but ‘that’s another story’…..


We normally add a small bit of household bleach (chlorine) to the water tanks whenever we fill the tanks, in order to keep the ‘microbes’ at bay.  Unfortunately, chlorine and aluminum don’t ‘get along well’ with each other, cause pitting of the aluminum, with subsequent water leaks and tank failure.  Not wanting to have to eventually replace the water tanks (don’t think the V-Berth tank could be removed from the boat in one piece), about 9 years ago we researched the use of anodes to prevent the pitting.  We found that the use of a magnesium anode will help prevent the corrosion and pitting caused by the use of chlorine in aluminum tanks.  Further research showed that magnesium anodes are available from McMaster-Carr, an industrial supply house. We subsequently installed a magnesium corrosion inhibiting rod (anode) in each of the 2 aluminum water tanks aboard SHEARWATER in 2003.

{Ed. Note: McMaster-Carr has a great web site, offers great service, and is an excellent  source of  ‘marine stuff to make things’, such as 316 SS tubing/plate/fasteners; high density poly-ethylene – HDPE  sheets; etc…. all at ‘non-marine’ prices.  We can order stuff from Mc Master-Carr on Monday and have it delivered by UPS on Wednesday.)


We had not opened and cleaned the water tanks since we installed the anodes in 2003 – we had however, checked the condition of the ‘screw-in’ anodes every year.  This spring, we decided it was time to replace the anodes, as one anode was nearly completely ‘used up’, and the other (in the V-Berth) wasn’t in great shape (as shown in the following photos).  We also wanted to check the interior of both tanks to check on the pitting of the tanks.  When we initially installed the anodes in 2003, we noted no pitting in the small aluminum tank just aft of the mast step, and several minor pits in the V-Berth tank.


The following photos ‘tell the story’ of opening up the tanks (removing the inspection plates), tank cleaning, and replacement of the anodes.


Small Aluminum Water Tank (Aft of Mast Step)
Just Before Opening the Inspection Plate
(Copper Foil attached to the access plate is part of the SSB ground plane – all metal tanks are part of the SSB communications system.)


Interior of Small Tank
Note whitish sediment in tank.


Cleaning Old Gasket Material From Access Plate Area



Cleaning Access Plate Screw Holes With 10-24 NC Tap


Cleaning tank interior using shop vac.



View of inside of cleaned tank.
Very minor pitting – some discoloration.


Old magnesium anode (top) and new magnesium anode (bottom).
(McMaster-Carr Part # 3642K16)


Rubber gasket material (from Ace Hardware) for tank access plate.
(Need 1 sheet per access plate.)


Cleaning tank water pickup tube and access plate– note white oxide residue on tube – used rotary wire brush to remove residue.  Tube was in good condition.

Access Plate Cleaned, New Anode Installed, New Gasket Applied – Ready to Install
(Note Copper Water Pick-Up Tube)


Access Plate Installed On Tank
Note SSB Copper Grounding Foil Attached To Tank


V-Berth Tank Showing Access Plate – Same Procedures Apply As For the Tank Aft Of Mast



Renewed Access Plate/Anode/Pickup Tube Assembly Installed In V-Berth Tank
Note – Used a small amount of Silicone Grease On Both Tank And Access Panel To Facilitate Gasket Sealing (and subsequent removal when required to clean tank).  Anodes are easily replaced without removing access plate – simply unscrew the old anode, measure and cut the new anode to proper length, and replace.


Aft Water Tank Constructed of Wood/Epoxy with Screw-in Access plate