Help with large (900liter / 243 gallon) aluminum diesel tank design
Posted: Mon Nov 08, 2021 5:51 am
Hi all,
I'm hoping for some help in repairing a leaking aluminum diesel tank. I'm on a 54 foot GRP sailboat on a circumnavigation and boats being boats, the tank started leaking in the middle of nowhere and needs to be repaired or replaced. The boat is currently in French Polynesia, so while I'm lucky enough to get access to a decent aluminum boat building shop, they are asking that I provide design/specs for the replacement tank. This project would be much easier if I was in Europe or North America, but I'm not.
The tank has poultice corrosion underneath it (see photo), where it was placed on carpet. All was fine for years until we had a small saltwater leak and the carpet got soaked. In it's current state, I cannot access the bottom to repair it and there are 6 baffles internally, so pouring a sealant inside is not a feasible solution.
The existing single tank on the boat is made of 5086-H111, 900liters, in the engine room and was installed prior to the deck being glassed in. See attached drawing of the EXISTING TANK. The tank is 200cm long, 42cm deep and around 110cm high. The low point is on the left side of the drawing, near the diesel shutoff valve, labelled CSA 05 019. The engine room opening is only 85cm, so to remove the leaking tank, I will need to cut the tank.
The tank is original and around 14 years old. The bottom is sloping as the diesel is gravity fed via a stopcock on the lowest point of the tank and being an ocean-crossing boat, the tank needs to fit perfectly without any movement.
OPTION 1 - Repair
Because I am not 100% confident that the local shop can build new tanks that fit perfectly and have the correct geometry to ensure lack of movement and proper diesel flow, I was considering cutting it out in 3 pieces, indicated by the two cuts on the attached drawing. I would have the local shop try to replace the failing bottom panel and then install new side panels where the cuts were made. We would then find a way to secure the 3 repaired tanks.
There appear to be some small pinhole leaks in the vertical welds, so I was considering roughing it up and applying Belzona 1161 to all existing welds.
My desire to repair it is really only to retain the existing tank, which I know fits. Of course, due to the 85cm engine room opening, the one tank will be replaced by 3 interconnected tanks. The repair cost will likely not be much cheaper than replacing them.
OPTION 2 - Replace
The second option would be to cut out the old tanks and have 3 completely new tanks fabricated. The local shop can only source 5083-H111, which I think is fine.
Given that the original tank is 14 years old, should I just bite the bullet and go through the pain of having them build 3 new tanks? My fear is that I fix the original tank and it leaks again in an even more remote location. Again, it won't cost much more, my bigger concern is getting it to fit perfectly.
Common elements and questions
Whether I choose repair or replacement, I will still need to connect the tanks and make a few changes and being that this is the first tank I've designed and well, it needs to work perfectly since we're going to even more remote places soon, I'd appreciate comments and suggestions.
1) For the inter-tank connections, they will weld male 1.5 inch threaded connections to each tank:
(i) Tank 1, on the right most and where diesel is initially poured into the tank system, will have an outlet on the bottom left side of its 1151mm high face, the low point.
(ii) Tank 2, in the center, will have an inlet on bottom right side of its 1151mm high face, the high point and an outlet on the bottom left side of its 1151mm high face, the low point of this tank.
(iii) Tank 3, on the left, will have an inlet on the bottom right side of its 1151mm high face, the high point. The outlet, which feeds the Racor filters, will be the existing one indicated by CSA 05 019.
These male threaded connections will have these 316 stainless shutoff valves:
https://www.kent-marine.com/en/product/ ... 1-1-2.html
with 90 degree outlet elbows, ending in a nipple and connected via fuel hose and hose clamps (see photo with items circled in red). On top of each respective tank, we'll have 16mm T nipples welded onto each tank and have fuel hose connecting all the tanks for venting purposes.
Underneath the tanks, I plan to use 1/4inch Starboard HDPE, bedded to the aluminum using polyurethane sealant to keep the tanks above any stagnant water.
The engine room walls around the tank are covered with sound deadening insulation and was resting up against the tank. I plan to leave a gap of at least 1/4 inch to prevent future poultice corrosion issues. As mentioned, the tank is in the engine room, which, while it has a suction fan, is quite warm due to the presence of the auxiliary Volvo motor and the Onan generator. Originally the boat builder covered the engine room side 1151mm x 2000mm face of the tank with insulating foam. Upon removal of that foam, I could clearly see some poultice corrosion underneath.
My plan is to NOT insulate that face of the tank, but should I be concerned about possible heat issues since the tank is exposed to the engine room? We can motor for days on end and I'm concerned perhaps the tank/diesel will get too hot?
Thanks in advance for your input and sorry for the long post!
I'm hoping for some help in repairing a leaking aluminum diesel tank. I'm on a 54 foot GRP sailboat on a circumnavigation and boats being boats, the tank started leaking in the middle of nowhere and needs to be repaired or replaced. The boat is currently in French Polynesia, so while I'm lucky enough to get access to a decent aluminum boat building shop, they are asking that I provide design/specs for the replacement tank. This project would be much easier if I was in Europe or North America, but I'm not.
The tank has poultice corrosion underneath it (see photo), where it was placed on carpet. All was fine for years until we had a small saltwater leak and the carpet got soaked. In it's current state, I cannot access the bottom to repair it and there are 6 baffles internally, so pouring a sealant inside is not a feasible solution.
The existing single tank on the boat is made of 5086-H111, 900liters, in the engine room and was installed prior to the deck being glassed in. See attached drawing of the EXISTING TANK. The tank is 200cm long, 42cm deep and around 110cm high. The low point is on the left side of the drawing, near the diesel shutoff valve, labelled CSA 05 019. The engine room opening is only 85cm, so to remove the leaking tank, I will need to cut the tank.
The tank is original and around 14 years old. The bottom is sloping as the diesel is gravity fed via a stopcock on the lowest point of the tank and being an ocean-crossing boat, the tank needs to fit perfectly without any movement.
OPTION 1 - Repair
Because I am not 100% confident that the local shop can build new tanks that fit perfectly and have the correct geometry to ensure lack of movement and proper diesel flow, I was considering cutting it out in 3 pieces, indicated by the two cuts on the attached drawing. I would have the local shop try to replace the failing bottom panel and then install new side panels where the cuts were made. We would then find a way to secure the 3 repaired tanks.
There appear to be some small pinhole leaks in the vertical welds, so I was considering roughing it up and applying Belzona 1161 to all existing welds.
My desire to repair it is really only to retain the existing tank, which I know fits. Of course, due to the 85cm engine room opening, the one tank will be replaced by 3 interconnected tanks. The repair cost will likely not be much cheaper than replacing them.
OPTION 2 - Replace
The second option would be to cut out the old tanks and have 3 completely new tanks fabricated. The local shop can only source 5083-H111, which I think is fine.
Given that the original tank is 14 years old, should I just bite the bullet and go through the pain of having them build 3 new tanks? My fear is that I fix the original tank and it leaks again in an even more remote location. Again, it won't cost much more, my bigger concern is getting it to fit perfectly.
Common elements and questions
Whether I choose repair or replacement, I will still need to connect the tanks and make a few changes and being that this is the first tank I've designed and well, it needs to work perfectly since we're going to even more remote places soon, I'd appreciate comments and suggestions.
1) For the inter-tank connections, they will weld male 1.5 inch threaded connections to each tank:
(i) Tank 1, on the right most and where diesel is initially poured into the tank system, will have an outlet on the bottom left side of its 1151mm high face, the low point.
(ii) Tank 2, in the center, will have an inlet on bottom right side of its 1151mm high face, the high point and an outlet on the bottom left side of its 1151mm high face, the low point of this tank.
(iii) Tank 3, on the left, will have an inlet on the bottom right side of its 1151mm high face, the high point. The outlet, which feeds the Racor filters, will be the existing one indicated by CSA 05 019.
These male threaded connections will have these 316 stainless shutoff valves:
https://www.kent-marine.com/en/product/ ... 1-1-2.html
with 90 degree outlet elbows, ending in a nipple and connected via fuel hose and hose clamps (see photo with items circled in red). On top of each respective tank, we'll have 16mm T nipples welded onto each tank and have fuel hose connecting all the tanks for venting purposes.
Underneath the tanks, I plan to use 1/4inch Starboard HDPE, bedded to the aluminum using polyurethane sealant to keep the tanks above any stagnant water.
The engine room walls around the tank are covered with sound deadening insulation and was resting up against the tank. I plan to leave a gap of at least 1/4 inch to prevent future poultice corrosion issues. As mentioned, the tank is in the engine room, which, while it has a suction fan, is quite warm due to the presence of the auxiliary Volvo motor and the Onan generator. Originally the boat builder covered the engine room side 1151mm x 2000mm face of the tank with insulating foam. Upon removal of that foam, I could clearly see some poultice corrosion underneath.
My plan is to NOT insulate that face of the tank, but should I be concerned about possible heat issues since the tank is exposed to the engine room? We can motor for days on end and I'm concerned perhaps the tank/diesel will get too hot?
Thanks in advance for your input and sorry for the long post!