SJ23 Tech Tip C18, (Updated 2021-09-29) Bob Schimmel


Rigid Water Tank & Pump.
Construction, Poly Tanks, Water pump, Water Quality, Winter storage, Sailing Perffomrnace.

Shortly after I bought Panache I moved the water tank from under the cockpit to the empty void under the forward berth because I needed the storage space under the cockpit for light bulky stuff. It seemed like the most logical place to put the tank.  Later I discovered that the extra weight in the bow helps to offset the crew weight in the cockpit which makes for faster sailing, more responsive handling, higher pointing and less hunting at anchor.  SJ23s sail at their optimum while on their water line.  Leave it to me to find a couple of technical reason for a nice practical solution!  See Note 1

The rigid 10 gallon rectangular tank is made of white polyethylene.  It is translucent so in daylight it is fairly easy to see how full the tank is.  While it just fits through the access hole in the forward berth, it was a hassle to pull out for end of season service as I always scraped my knuckles.  Problem is, it is difficult to support the tank properly over the V shape of the hull and also keep it in place while heeled at 250 in pounding waves.  A full 10 gallon tank weighs about 100 pounds and when not supported it generally slid to an odd angle, jamming between the hull and the bottom of the berth.  I doubt this was any good for the tank, the fittings, hoses, let alone the hull!  The bow location has operational advantages, so I made a proper shelf to support the tank and sloped it aft so water could drain for winter storage.  This has worked so well that the tank has never been removed since it was installed here in 2000.

Fig 1 shows the tank installed.  What is not shown are the long wedges of dense neoprene foam that I jamb between the top of the tank and the bottom of the berth to securely hold the assembly down, vibration free. 

fig 1 - The water tank nicely tucked under the forward berth, leaving lots of storage space aft of it.


fig 2 - The shelf that supports the tank showing scribe marks on the aft leg to align it to the hull for gluing with Sikaflex.


fig 3 - The small 12V demand pump that fills my coffee pot quicker than asking, "May I have a refill?"


fig 4 - The fill hose is just visible inside the left side of the anchor locker.


fig 5 - Tank fill hose & deck fitting. The vent thru hull fitting is just visible to right of the fill hose. 


fig 6 - Deck view of the black filler cap on the bow.  It seals off the filler hose shown in fig 5.


CONSTRUCTION - To support the tank over the V shaped hull I fashioned a plywood shelf with two form fitted feet glued to the hull with Sikaflex.  The shelf has never moved.

  1. SHELF - Cut a section of 3/8" thick plywood (fig 2) to fit under the tank, plus a 1" wide fillet at the perimeter.  Draw an outline of the tank on the center of the plywood.  Using stainless screws from underneath, attach a (1x1)" fillet of solid wood to fit snug around the tank.  I used old hockey stick handles as they are a renewable resource of excellent quality wood in Canada.  The fillet keeps the tank from sliding off the shelf.  Drill a 3/8" diameter drain hole through each corner of the shelf.  Sand all corners smooth and coat with your favourite stuff to protect the wood.

  2. SHELF FEET - (This step requires a bit of juggling and art work).  Cut two 2x4s the full width of the shelf.  Determine where they will rest on the hull and then draw an outline of each 2x4 on the hull.  Use cardboard to make a template for shaping the bottoms of the 2x4s to match the shape of the hull.  Since the shape will be slightly different from one side of the 2x4 than the other side, make a forward and aft template.  Place both templates against the same 2x4 and draw the outlines.  Then shape the bottom of each 2x4 to conform to the hull shape.  Make the aft 2x4 slightly lower than the forward one to ensure the tank has an aft slope, towards the drain.  This way all the water and any debris can drain out, which is important if you want to pump out the last drop or totally drain the tank dry for winter freeze up.  Draining dry is preferable to using RV antifreeze which must be flushed out in Spring.  I don't like the after taste of that stuff.  The final shape of the 2x4s should match the curve of the hull to within 1/8", without hard spots. 

  3. Assemble the shelf and feet for a dry fit.  Now glue the feet to the hull using plenty of Sikkens Sikaflex under the 2x4s.  Ensure the tops of the feet are parallel to support the shelf.  Create a fillet for additional holding power.  This is definitely NOT a job for bathroom silicon sealant as a full 10 gallon tank weighs 100 pounds.  When you slam the hull into a wave you need all the holding power you can get.  The advantage of using Sikaflex is that is has some give that helps to absorb shock. 
    NOTE - If the shelf is screwed to the feet (no glue) it can be removed for future access.  So glue the 2x4 feet to the hull and screw the shelf to the 2x4s.  Its a great way to recoat the shelf.

  4. Finally, use a couple of poly straps or ropes around the tank and plywood shelf  to hold the tank within the fiddles shown. (fig 1 above)

  5. FILL HOSE & FITTING - The water filler hose connects from the filler cap (fig 6), down through the anchor locker (fig 5) & (fig 4) to the top forward corner of the tank.  The hose is made from an opaque vinyl to inhibit bacteria growth.  The filler cap is also opaque.  A 1/2" vinyl vent line parallels the fill line and is connected to an OMC gas tank vent installed through the hull at the upper side of the anchor locker.  See Note 2.  Both lines are supported to eliminate a sag that can trap water or grunge.  The vent line is equipped with a screen to keep bugs out.

  6. My portable water jug for carrying water to the boat can lie on the deck while it drains into the deck fitting (fig 6).  

  7. TANK DRAIN - The tank drain is located at the aft right corner (fig 3) with a 900 barbed fitting that points down to direct water to a small 12VDC demand pump.   The centrifugal pump is below the tank and gravity feed primes it.  Occasionally I have to prime the pump by sucking water through the tap.  Yes it gets cleaned!  The water line runs from the pump, under the port settee to the faucet in galley.  The pump is controlled by a switch located next to the faucet.  Push the switch you get water, push the switch again and the water stops! 

  8. Shown at right is the completed shelf glued to the hull on shaped feet.  Note the scribe marks on the shelf and hull are still in alignment after 30 years of sailing. 

ABOUT POLYETHYLENE TANKS & FITTINGS: Most polyethylene tanks come without hose fittings unless it is a purpose made tank.  So if you purchase one, fit it in the boat and determine where you want the fittings to be.  Mark the spot with a felt pen and take the tank to the dealer or RV repair shop where they can "weld" the fitting to the tank.  If you have a leaky fitting or want another one installed, they can do that as well.  These shops have a tool that resembles an electric drill that holds the fitting while it is spun against the body of the tank.  The friction created between the fitting flange and the tank melts the polyethylene on both sides.  When the two surfaces become soft enough, the operator simply stops the spinning the fitting and holds the tool steady against the tank till the conglomerate mess cools and solidifies.  The operator knows from experience when to stop turning.  Leak fixed!  One sure way to screw up a fused fitting like this is to apply side strain on it from a hose.  So support the hose to eliminate all strain on the fitting. 

  • FILL FITTING - This fitting should be located on top of the tank or at the top of the sidewall.  The reason for this is that if the hose comes off you keep your water.  There is also less force on the fitting from water slopping in the tank. 

  • DRAIN FITTING - Ideally the drain fitting should be installed through the top of the tank and be equipped with a pickup tube that extends to the bottom of the tank.  This is to retain the water if the hose comes off.   Unfortunately the majority of RV tanks are equipped with an external bottom fitting for gravity feed.  But this also means you don't require a self priming pump.

  • VENT FITTING - This fitting must be attached to the top of the tank to pass air freely. 

  • BAFFLES - Ideally the tank should be equipped with baffles to prevent water from sloshing around too much, straining the tank.  This is hardly necessary on a SJ23 because the tank is not big enough to be concerned about it. 

  • The tank rests on high density foam to provide abrasion protection for the bottom. 

SELECTING a WATER PUMP for the SINK:  It is most desirable to have a dependable pump so you can draw drinking water from the tank at any time.  After all, drinking water is sort of important to sustain life on this planet.  The irony is that you may be floating on the stuff but can't drink it!  Having said that, the pump should also operate quietly so you don't disturb the sleeping crew.  This is why many people choose a manual pump.  It's usually very dependable and operates without electrical power.  The down side is that you have to operate it by hand or foot depending on the style.  This is OK as you will use less water.  I prefer a foot operated pump as they free your hands to hold a glass under the tap.  For an electric pump I prefer a demand pump because they are quieter and consume less power than a pressure regulated demand wobble pump you find on a holiday trailer.  Some boats have two pumps, a manual foot pump for sea water and an electric pump for drinking water.  This saves the drinking water when all you need to do is rinse. Salt water is a natural antiseptic. 

A foot pump installed through the settee wall close to the sink is relatively easy to install since it requires such a small hole.  Make the corners of the hole slightly rounded to prevent cracks from developing.  If done right, the pump's mounting plate should restore the strength of the settee wall hole.  Just make sure you install it where you are unlikely to trip on it.  Also make sure you can access the pump reasonably well for maintenance.  By the way, you can judge the complexity of a boat by the number of pumps on board! 

WATER QUALITY - Always pour clean tap water into the tank.  Then add a cap of Clorox (chlorine) or bleach to a full tank (10 gallons) of water to inhibit bacteria growth.  The vent line is equipped with a screen to keep bugs out.  The end of season tank drain ensures no sediment in the tank.  For these reasons I have not seen it necessary to filter the water to the galley faucet.  But if you find it necessary to treat your drinking water it is recommended to go through the following three-step treatment;

  1. Filter all water that goes into the tank down to 1 micron.

  2. Chlorinate or maintain a chlorinated tank.

  3. Filter the water that leaves the tank to the faucet water to 0.5 micron using a carbon block filter.

DECONTAMINATE A TAINTED TANK - There is a standard sanitizing procedure for recreational vehicles (ANSI A119.2 section 10.8) that works just as well for boats.  We’ve added a few details, but the bones of it come straight from the code and have been reviewed and accepted by the U.S. Public Health Service.

  • Remove any carbon canisters or micron rated filters.  Remove any faucet aerator screens.  Wire mesh pump protection strainers should stay in place.  The plumbing will very likely slough off a layer of bacteria during later flushing steps.
  • Clean and remove the vent screen and flush the vent hose.
  • Use either following methods to determine the amount of common household bleach needed to sanitize the tank.
    - Multiply gallons of tank capacity by 0.13; the result is the ounces of bleach needed to sanitize the tank.  This is 1/8 cup of plain bleach (no fragrance) per 10 gallons.
    - Multiply litres of tank capacity by 1.0; the result is the millilitres of bleach needed to sanitize the tank.
  • Mix the proper amount of bleach within a 1 gallon container of water.  This will provide better mixing and reduce spot corrosion of an aluminum tank.
  • Pour the solution (water/bleach) into the tank and fill the tank with potable water.
  • If possible, allow some solution to escape though the vent.  (If the vent is exterior, prevent any spillage into local waters.)  This will sanitize the vent line.
  • Open all faucets (hot and cold) allowing the water to run until all air is purged and the distinct odour of chlorine is detected.  Leave the pressure pump on.
  • The standard solution must have four hours of contact time to disinfect completely.  Doubling the solution concentration reduces the contact time to one hour.
  • When the contact time is completed, drain the tank.  Refill with potable water and purge the plumbing of all sanitizing solution.  Repeat until bleach is no longer detectable.
  • If the smell of bleach persists after two refill and drain cycles, add a teaspoon of hydrogen peroxide per 20 gallons and mix.  The peroxide will oxidize the hypochlorite to chloride (salt) and oxygen, neutralizing the bleach.  Any excess peroxide will be harmless to drink and will have no taste.  Peroxides are common ingredients in commercially available water freshening preparations.  Don’t use vinegar, which can ferment, undoing all of your hard work.
  • Replace all filters and the vent screen.
  • Note for aluminum tanks: Some sailors are afraid of using bleach in aluminum tanks for fear of rapid corrosion.  This shouldn’t be a concern for infrequent cleaning when the recommended dosage and time is observed.  As an alternative, PuriClean is an effective sanitizer, and it is non-corrosive to aluminum.

WINTER STORAGE - Since there are no sags in either the fill or vent lines to trap water I pump the tank dry while on the way to the haul out dock.  The tank will be almost empty when I pull Panache up the ramp.  Then at her winter parking spot I remove the drain hose and collect the last bit of water out of the tank.  Lastly I blow out the remaining water through the pump, into the sink for winter freeze up.  Then to protect the pump I shake the water droplets out of it and give it a quick spray of cooking oil to prevent internal corrosion.  Sure a lot simpler than my previous method of removing the tank every Fall to drain it. 

SAILING PERFORMANCE - What came as a total surprise was the improved sailing performance with the bow weighted down, sailing the boat on its lines.  Later I realized the combination of the rigid shelf (acting as a grid) glued to the hull added stiffness.  With the added weight of the water plus the foam jammed between the top of the tank and the V-berth, it really helps to deaden the sound of punching through bigger waves. 

NOTE 1 - While much has been said about adding weight to the bow to level the boat for fast sail performance, an easy weight would be to tie a five gallon jug of water in the front of the V-berth.  This will add about forty pounds to the bow and you can vary the amount of water as you need.  There never seems to be enough drinking water when you are cruising anyway.  Five gallons should last about four days if restricted to drinking.  Fifty gallons of water should last about 5 to 7 days if the electric pump is switched off and the hand pump is used. 

NOTE: 2 - A vent line is required to let air out of the tank while filling with water or into the tank while draining.  This permits the fastest flow by preventing air bubbles coming up the fill line, blocking water.  It could also handle an overflow but this action should be discouraged to minimize the extra column pressure on the tank and fittings.  The column pressure increases a disproportionate amount for the slight extra volume stored in the filler tube and is a strain on seals that are NOT designed for it.  The last thing you want is a bilge full of drinking water just because you wanted a measly extra cup of water. 

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