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  • Writer's picturePhoenix: Reimagined

Boat Project: Black Tank Replacement

Updated: May 5, 2023

Do you have a smell in your boat that you just can't seem to get rid of? Does it seem like you're having to discharge or pump out your black tanks too often? Is your tank showing full even after you've discharged/pumped out? These are all symptoms we were dealing with prior to tackling this boat project!

We have a lot of lessons learned to share after completing the black tank replacement on our cruising catamaran. We even re-engineered the original tank design by deleting the inlet dip tubes to hopefully mitigate future calcium buildup problems! Let us tell you the tale of the day the stink died.

black tank project
Removing the starboard black tank

During the pre-purchase survey of our 2008 Leopard 46 (4 cabin/4 head version) it was noted that our aluminum black tanks (starboard and port) were worn. SV Imagine has saltwater flush heads (toilets) and as you know, saltwater and metal aren't friends. It was only a matter of time before tiny pinholes in the sides of the aluminum tanks started to appear which caused a whole host of problems (not limited to) contributing to a stinky situation once the tanks were filled to a certain level. Another issue we were having is getting enough suction to pump out the tanks while at a marina or "on the hard"in a boat yard. It turns out that even though we were paying for pump outs, the tanks were not being emptied because of a lack of suction. As the tanks were aluminum, the only way we could verify the amount of liquified matter inside was to open the inspection port in the top (pictured above with the black cover).

I'll spare you the gruesome details and get straight to the point: sometimes the realities of boat life stink!

Step #1 - Ensure you have the correct (old) tanks installed in your boat

This sounds like a no brainer but when we went to replace these tanks we found that at least our port-side tank had already been replaced once, but not with the correct port-holed tank! It was replaced with a starboard-holed tank which has the holes lined up differently. When we replaced the port-side tank, we went ahead and replaced it with a starboard-holed (as had already been done) tank so we wouldn't have to do another repair to the side deck fiberglass.

There are plastics companies out there that have the original drawings for the Leopard 46 tanks. If we had not confirmed the tank design we actually needed (two starboard-holed tanks) beforehand, we would have purchased a starboard and port tank by mistake which would have cost us another deck repair.

Here's what I mean:

boat project replacing black tank
Starboard Waste Deck Fitting on SV Imagine

black tank deck fitting
Port Waste Deck Fitting on SV Imagine - You can see where the fitting had been moved and a deck repair had been performed previously

black tank
Shown: Bottom of both tanks removed from SV Imagine. From left to right of each tank: toilet inlet, discharge to ocean, toilet inlet. As you can see, these two tanks are IDENTICAL and they should not be!

Step #2 - Reach out to us! We may be able to help!

After working with Ronco Plastics on the modification of our 2008 Leopard 46 (4 cabin/4 head version) black tanks, we decided to partner with them to provide our modifications to YOU! Ronco Plastics has been ultra responsive and the tanks we received were top notch. Shipping was fast and pricing was very reasonable! By ordering through us, we will provide an additional layer of support to your project with no additional cost to you! The tanks will be shipped directly to the address you provide.

Don't have a Leopard 46 but still want to replace your tanks? If you'd like additional assistance, reach out to us. We can help you modify your boat's aluminum tanks to replace them with plastic.

Phoenix Reimagined, LLC

Reason and Nicole Phoenix

Subject Line: Black Tanks

Step #3 - We will walk you through verifying the drawings and making any modifications

There are plastic tank manufacturers that already have the original Leopard 46 tank drawings but you may not get the specific support you need. Once you contact us, if you don't want to do any modifications, the verification process is simple. Just verify that all the holes line up with the drawing on each tank. Some preliminary measurements should do the trick, however, we have heard several horror stories about calcium build up in the toilet inlet dip tubes caused by saltwater reacting to urine causing rock-like mineral deposits in the tubes. When we removed our tanks, we noticed that the toilet inlet dip tubes were severely restricted. There was no way of knowing this without removing the tubing and looking into the inlet holes (gross!).

Once the dip tubes become clogged with build up, there is no way to clear them without using harsh chemicals (again, speeding up the need to replace the aluminum tanks!) so we re-engineered the tanks and removed the inlet dip tubes. One symptom of the inlet dip tubes being clogged would be undue pressure on the joker valves which could cause them to fail early and allow back flow of dirty water into the toilet after flushing (again, gross!).

Photos of SV Imagine's clogged toilet inlet dip tubes in the old aluminum tanks.

What are dip tubes?

On the original Leopard 46 tanks, the inlets from the toilets are in the bottom of the tank! The inlet is attached to a small diameter tube that runs vertically nearly the entire length of the tank. Liquified matter from the toilet comes into the inlet (in the bottom of the tank) and is pumped through the dip tube to fall like a waterfall into the tank.

See diagram of the ORIGINAL tank design below:

ORIGINAL Leopard 46 Black Tank Drawing Modified by Phoenix Reimagined, LLC

We modified the tanks for SV Imagine to DELETE the inlet dip tubes (discharge dip tube is still there for pump out) and instead have the inlets in the TOP of the tanks. The liquified matter from the toilet is pumped directly to the top of the tank with a check valve in the tubing about halfway from the toilet to the tank to prevent back flow and reduce head pressure on the joker valve. There is no chance of the tank itself being clogged.

Here is an example of the modified drawings, note that there are now (3) 1" ports on the top of the tank. One for the vent (not modified) and two for the toilet inlets (modified). The inspection port was also moved to allow access for the three 1" ports. The inlet hoses were kept at 1" diameter the entire distance to reduce head pressure on the joker valve, allowing it longer life.

We've been using these black tanks successfully for almost a year. The proof is in the...pudding. (Sorry, I had to!)

Modified drawing
Modified Leopard 46 Black Tank Drawing by Phoenix Reimagined, LLC

Step #4 - Remove the old tanks

Pro Tip: EMPTY the old tanks as much as you can! DO NOT assume that just because you got a pump out that your tanks are empty! If you are in the water fill your tanks with fresh water then head offshore to release your tanks using the discharge. If you are on the hard, I hate to say it but a 5 gallon bucket outside and a person on the inside releasing the discharge seacock is your friend. If you do not ensure your tanks AND the discharge hose are empty, you will have a disaster in your boat. It's a shitty situation (pun intended) that is avoidable.

Pro Tip: DO NOT cut the hose that goes from the waste deck fitting to the tank! It is too short of length and too large of a diameter to be flexible so the desire is to cut it. We made the mistake of cutting the hose only to find that the sanitation hose is an irregular size that we were unable to source in the USA. Instead, remove the waste deck fitting (this will give you a chance to check for leaks in the deck anyway) and the hose together from the tank.

When we removed the port waste deck fitting, we found that there had been a leak around the fitting that we hadn't known about. This allowed us to dig out all of the affected Balsa core and make an epoxy repair before reinstalling the waste deck fitting after the new tank was installed. If we hadn't removed the fitting, the damaged area of Balsa could have turned into a major repair.

Pro Tip: CHANGE out the discharge seacock and hoses. A heat gun will be your best friend while removing and reinstalling hoses. Not to get too gruesome but while we're on the subject, the best time to replace the discharge seacock (not the through-hull, just the valve) is when the tank is 100% empty. When we purchased SV Imagine, we did not know that our port discharge valve had a minuscule leak. Normally this wouldn't be an issue as long as you are exercising the discharge seacock regularly but SV Imagine had been left for over a year with this small drip. As a result, all of the liquid accidentally leaked out and only solids remained. The solids solidified into cement in the discharge hose making it impossible to discharge our port tank. Of course, we had no idea until we went offshore for the first time and that full black tank got shaken up while at sea! PHEW!

Take this opportunity to do some preventative maintenance. Change out your hoses (all of them you can (except for the deck fitting hose)) and the discharge seacock. Cut the new hose long so you can fit it properly once the new tank is in place.

Check the macerator, change the joker valve etc. This isn't something you want to have to revisit right away when you are visiting some beautiful location!

Note: If you are changing out the tanks while in the water (we do NOT advise this!), you can use a bung (inserted from the OUTSIDE) as a temporary stopper to allow you to change out the discharge seacock. We had to do this while in the Bahamas. It helps to have a diver/hookah and will require two people (one inside, one outside) with predetermined sound signals (knocking on the hull). Be careful here and USE CAUTION! You are opening a 2" hole in your boat! AGAIN, we DO NOT advise this!

Pro Tip: Take a lighthearted approach. Make the most of this shitty situation. Jokes will keep morale high!

Step #5 - Install your new tanks!

Your shiny bright new tanks have arrived! Marvel at how amazing they look!

Pro Tip: Use 3M 4200 Marine Sealant (Not 5200) for all deck/tube/hose fittings. Avoid using 3M 5200 because you want flexibility and the ability to disassemble in the future. In addition to 3M 4200, also use stainless steel hose clamps.

All of the hose fittings should have come with the tanks from the manufacturer.

Pro Tip: Install your tank level indicator before placing the tank in the hull. The hole for the sensor was not pre-drilled by the manufacturer. We drilled the holes ourselves and used an aftermarket sensor on Amazon. I hesitate to post the sensor here because we did have to modify it to reverse the light so it was turned on when the tank was full instead of being on all the time and only turning off when the tank was full. I may do a separate tank sensor indicator modification post.

Pro Tip: Shorten any of your overly long hoses. Our port discharge hose was so long that it had a "low" spot in the middle in the bilge. This means that unless the tank was pretty full, there wasn't enough pressure to empty the hose. Unbeknownst to us, this also contributed to the cement situation we faced. We were able to cut off over two feet of extra hose to allow for a more streamlined flow.

Pro Tip: The last hose you should install is the Waste Deck Fitting hose. Install this hose by dropping it in place through the waste deck fitting hole in the deck. Wait to install it until you are ready to install the deck fitting. Save this for last so you can still move the tank around as you need to while installing the inlet and discharge hoses.

Pro Tip: Verify no hose leaks by flushing with FRESH water before you start using the heads. The last thing you want is "waste" water in the bilge after you've done all this work! Also, take this time to check the shower supply lines. We found that our shower supply lines were very corroded and had started to leak so we replaced them.

Find this helpful? Need more help for your specific project?

We are offering limited consulting spots for marine electrical, 12v battery systems, plumbing, and air conditioning, as well as project management assistance. Contact us via if you'd like to start a dialogue.

We promise that if we don't feel like we can provide value, we'll tell you right away. If we can't help, we'll point you in the right direction.


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