Replacing One Fuel Tank with Two

[Mr Efficiency]

I would say talk to their technical people about the aluminium, the fella I alluded to earlier I am pretty sure was repairing an alloy tank, but I am not 100% sure

 

[fallguy]

My second sentence was largely intended as a joke. However a boat could, in theory have a tank high enough to be inspected from below.

The rules as I see them are simple. You are required by abyc to have an inspection port which implies the ability to see and fix potential problems with the fill connection at the tank. And afaik, this would apply to diesel as well as gas. At some point, that body made most of the rules the same for gas n diesel. I cannot cite the rules for Europe, but they are typically stricter.

The idea to connect the tanks at the bottom is a bad workaround, okay?

Same as the tank lining idea. The tank lining works best when the tank can be moved. Edit-unless only corroded on bottom

In order to provide access to all those tank holes in your proposed system, you'd need something like 7 access plates.

 

[fallguy]

At least here in the US, you don't get to cut a hole anywhere to stare at the tank. The holes are to service fittings and level gauges, etc.

Also, you are not 'dim', but you have confirmation bias. That is, like many who come here, you want holy water thrown on the idea. But you won't get it.

Kind regards. Many people think boat building and repairs are easy, but when you really go to the details and do the job well, safely, and correct, it is generally not so.

I belong to a cruisers club on Facebook. Some of those folks face serious challenges with massive tanks and engines with no means to install or remove new ones.

Based on that rat's nest of wiring I see, you've got some work to do things right.

 

[fallguy]

Also, as Gonzo, who I believe is a US surveyor or was points out, you also do not show a sender or gauge. Typically, the sender and supply and return lines share an access and the fill and vent another, so you'd have two typically.

 

[fallguy]

I do apologize. An inspection port to inspect the tank internally and access ports for the fittings are two different things.

You may or may not be required to have inspection ports in the tanks, but it is certainly wise. Not sure why they'd be on the side. Another potential for leakage.

You will be required to access the other fittings most likely unless Europe has a different, less restrictive rule.

It seems you are making this effort to avoid cutting into the floor, but done correctly, access theough the floor is required anyhow..

 

[fallguy]

Also..if you have a finished floor, you would be able to carefully cut it woth the proper tools AND make it so it still looks nice. For example, use an oscillating tool on carefully drawn lines and add a cleat for the floor section. Use a 6" hole saw to cut access panels, etc. provide a lip of timber around the cutout and screws with finish washers to reattach, etc..

After removing the floor section, joint the edges straight, sand the remaining side flat n smooth.

This is the right way.

 

[Barry]

DIESEL tanks can be split and bottom connected by way of fuel hose and double clamping. GAS not so. ie with say a 2 inch or 1 1/2 inch pipe at the bottom of the tanks and double clamps.

Each tank needs a vent which can be T'd so long as the vent line does not have any dips in the line. If you are using 2 inch ID filler pipe, use vent ID of 5/8 minimum. ONLY MARINE approved hose, not
automotive. Only Marine hose clamps, not automotive as the automotive clamps often have a zinc or cad plated worm screw while the marine are all stainless

So long as there is NOT a valve to separate/isolate the tanks, you can tie the return fuel line to either tank. MOST IMPORTANT if you decide to INSTALL a valve at the bottom connection this would mean that you will require each tank to have a draw tube/or bottom gravity fuel pick up, and if you turn a valve to pull fuel from either tank then you must switch the return line at the same time or you risk the return line trying to overfill the already full opposing tank.
I would not mess with isolating the tanks as they were not isolated in the first place.


Anderson Brass makes (or did) make a double stack brass valve that with one turn, switched the draw line and the return line at the same time.

The ABYC United States guide for Diesel tanks section is H-33 and if you can find it on line, normally a subscription or outright purchase but another contributor has referenced ABYC info occasionally. The section is maybe 8 - 10 pages long and deals with proper mounting procedures but one sometimes missed is that you can only use aluminum or 300 series fittings in contact with the aluminum tank

 

[fallguy]

DIESEL tanks can be split and bottom connected by way of fuel hose and double clamping. GAS not so. ie with say a 2 inch or 1 1/2 inch pipe at the bottom of the tanks and double clamps.

Each tank needs a vent which can be T'd so long as the vent line does not have any dips in the line. If you are using 2 inch ID filler pipe, use vent ID of 5/8 minimum but I would recommend 3/4 inch. ONLY MARINE approved hose, not
automotive. Only Marine hose clamps, not automotive as the automotive clamps often have a zinc or cad plated worm screw while the marine are all stainless

So long as there is NOT a valve to separate/isolate the tanks, you can tie the return fuel line to either tank. MOST IMPORTANT if you decide to INSTALL a valve at the bottom connection this would mean that you will require each tank to have a draw tube/or bottom gravity fuel pick up, and if you turn a valve to pull fuel from either tank then you must switch the return line at the same time or you risk the return line trying to overfill the already full opposing tank.
I would not mess with isolating the tanks as they were not isolated in the first place.


Anderson Brass makes (or did) make a double stack brass valve that with one turn, switched the draw line and the return line at the same time.

The ABYC United States guide for Diesel tanks section is H-33 and if you can find it on line, normally a subscription or outright purchase but another contributor has referenced ABYC info occasionally. The section is maybe 8 - 10 pages long and deals with proper mounting procedures but one sometimes missed is that you can only use aluminum or 300 series fittings in contact with the aluminum tank

access plates to fills are required and that is an issue here, but you don't mention access

since access is an issue now; it seems strange to make it worse

 

[Barry]

access plates to fills are required and that is an issue here, but you don't mention access

since access is an issue now; it seems strange to make it worse

I did not mention access ports as the OP has said that he has access to the side of the tank in post 7. I assumed that the original tank had access to the "tank connections and fittings shall be readily accessible, or accessible through an access panel, port or hatch H33.10.4" . Thus if he has easy access to the side of the tank, then with proper fitting management, his original fuel fill, draw, vent and return locations can be used. His access issues are the 4 hose clamps on the bottom of the tanks ( and a method to determine fuel level, either a fuel sender or sight glass), an extra hold down strap for each tank (assuming there are 2 straps on the one tank) and another vent line connection access.

I have seen 3 of this type of retro fit, in the US, boats we were looking at to buy and the owners of two of them provided a recent survey and the split diesel tank was not mentioned. One of the boats had 3 such daisy chained tanks.

 

[Wideocean7]

At least here in the US, you don't get to cut a hole anywhere to stare at the tank. The holes are to service fittings and level gauges, etc.

Also, you are not 'dim', but you have confirmation bias. That is, like many who come here, you want holy water thrown on the idea. But you won't get it.

Kind regards. Many people think boat building and repairs are easy, but when you really go to the details and do the job well, safely, and correct, it is generally not so.

I belong to a cruisers club on Facebook. Some of those folks face serious challenges with massive tanks and engines with no means to install or remove new ones.

Based on that rat's nest of wiring I see, you've got some work to do things right.

I think you have me wrong Fallguy. I'm not wedded to this "solution" at all but it was the one that seemed to have the most prospects of working. That I am learning from your and others comments is why I posted and thanks very much for that. For me it's an iterative process which I hope will lead me to a sensible solution money, effort, disruption, reliability etc wise. This idea stood out because I can get those sized tanks through the floor and companionway without losing too much capacity, which I need for where I intend voyaging.

 

[Wideocean7]

As someone who made a lot of money cutting out rotten aluminum and repairing old alloy boats, I find it hard to believe that the tank rotted without damage to stringers or plate. Assuming the the tank is a tight access area? Is there any chance of some pics to see what were up against when giving advice?

Apologies Comfisherman. I didn't reply. There's no other corrosion, poultice or otherwise. There was up to 1" of gunk in the bottom of the tank (inside obviously) that I hazard a guess caused the deep pitting due to oxygen starvation.

 

[Wideocean7]

The tanks should not be cross-connected at the bottom. Diesel engines have a fuel return, which makes multiple tanks more complex. The easiest way is to have a switch valve to use one tank or the other. The valve needs to be double, to switch the feed and return simultaneously. Also, each tank needs a fuel level gage and fill inlet.

Hi Gonzo. Are you saying no no bottom cross-connection because of the potential for leaking fuel? I'll try to get hold of the ABYC codes as they sound like a useful source of info. But I guess they don't apply here in Australia or in my native UK/Canada?

 

[Wideocean7]

your slave tank is filled on the bottom and that fill is not 'accessible' per most rulemaking bodies

of course, I could be wrong if you can put inspection ports under the tanks

I guess having an inspection port - which I'd want anyway - negates the use of poly tanks which I'd heard can't have them structurally?

 

[Wideocean7]

I did not mention access ports as the OP has said that he has access to the side of the tank in post 7. I assumed that the original tank had access to the "tank connections and fittings shall be readily accessible, or accessible through an access panel, port or hatch H33.10.4" . Thus if he has easy access to the side of the tank, then with proper fitting management, his original fuel fill, draw, vent and return locations can be used. His access issues are the 4 hose clamps on the bottom of the tanks ( and a method to determine fuel level, either a fuel sender or sight glass), an extra hold down strap for each tank (assuming there are 2 straps on the one tank) and another vent line connection access.

I have seen 3 of this type of retro fit, in the US, boats we were looking at to buy and the owners of two of them provided a recent survey and the split diesel tank was not mentioned. One of the boats had 3 such daisy chained tanks.

Hi Barry. Yes I do have pretty good access to the one side of the tank beneath the floor of the raised deck-saloon. I'm not thinking of hose clamps for the fuel interconnect line. I'll post a variation of another option next.

 

[Wideocean7]

Sorry everyone for bombarding you with all my posts. It was easier replying that way. I appreciate and am taking on board your views. I said in an earlier post that I'm not committed at all to the solution I've described here. I am happy to be corrected on the errors of my thinking and logic and will move on to another idea if need be. But I don't think I've run this one to ground just yet. Why is it that I like this idea? Because I don't need to remove floors and fit-out and it gives me the least reduction in capacity -still down from 300L to 200L (that's a big loss but I'll live with it)

I've amended the design to include a dip-stick/sender port. I've placed this, plus the fill and 2 vents where there will be access from above. The fill and return lines are now from the side with the aim of them being accessible via the inspection port. Also, I've changed it so that the fuel draw and fuel return lines are to/from the same Master tank. Unless I'm mistaken, the tanks will need to be metal as all the sites I've followed say that poly tanks can't handle a large-ish inspection port. Both tanks will be tabbed and bolted to the frame. Then I'd have a cross-brace to tie the 2 tanks together.

I get it that the big downside is the risk of leaks from the equalisation line between the Master and Slave tanks. I haven't downloaded the ABYC standards yet. If I went down this track, it wouldn't be using hose clips, but something more substantial along the lines of that shown in the photo of the existing port-stbd equalisation line. Still, there's always the risk of leaks, but we all have to contend with that already with our thru-hull fittings.

Thank you again for spending the time on this.