Installing aluminum fuel tanks, need help!

I posted this in "Open Discussion" and was encouraged to try asking my question here.

I recently removed a 140 gallon tank from my 28' Skipjack. I had to cut the tank out, so as to not damage any of the stringers/bulkheads/deck; therefore, the new tank must be installed in two pieces. I have determined I can install two 82.5 gallon tanks, if I go with belly tanks ... adds a little insurance with more fuel.

The issue I've come across is that the old tank was foamed in and flat on the bottom (secured by foam). Through researching the proper way to install the new tanks, I've come across some people that think the install may have problems. Here is what I intended on doing:

To the V'd sides of the new tank (area that would be in contact with the hull), I would apply strips of 1/4"-1/2"plastic, which would be attached to the tank using 5200. With the empty tanks resting on the hull, I would mark the sides of the tanks, so that the builder can weld on brackets to secure the tanks to the top of the stringers on either side of the tanks. These brackets would ensure the tanks would not move.

The concern that has been shown is the following:

The brackets may flex and cause damage to the tank.

The boat was not designed to support belly tanks.

3/8" may not be enough room between the stringers and the tank for clearance (I'd probably place some 1/4" plastic strips on the side of the tanks, too, to avoid wear).

Does anyone have experience with this type of installation that can advise me how to proceed? The tanks have not yet been built, so now's the time to make changes.

 

aluminum fuel tanks

I'm replacing my Diesel tanks currently with two 100 gal rectangular oem aluminum tanks. They are much easier than yours, but I have some similar challenges. I think you're on track with the concern about bolting them to the stringer - as much for the stress on the tank as the stringer. There may be a lot of flexing along the hull where you're placing the tanks - which will be quite rigid in the shape you're specifying. Could you lay the tanks in with the 1/4" to 1/2" plastic insulators against the hull and side of the stringer - then strap the tanks down using two straps (either nylon or stainless) between the top of the stringer and blocks glued to the inside of the hull? With blocks fore and aft (also UHMW plastic) to keep the tanks from sliding, the straps would keep the tanks from doing anything crazy and the stringers and hull would be free to flex. For the tank's sake - keep wood the hell away from any surface.

 

I spoke with a boat builder who is familiar with my boat. He feels that the hull is definitely strong enough to support the tanks and the brackets should be fine as long as the under side of the tank is snug against the hull. He told me to have the brackets installed slightly higher than the stringers and shim up to them. I think I might loosely secure the tank with the brackets/tabs to stop the tank from sliding fore and aft, but use a strap to secure the tank firmly to the hull ... what do you think?

 

do you have any sole bearers? and are they grunty
this is how we do it in alloy boats, your size

 

Sole bearers? What is that? Are you referring to the stringers?

I can't overlap the aluminum top of the tank, like you have drawn, wish I could.

 

the sole is the (floor) like floor of a house, the bearers would be, like the joists in lubberly terms, in other words the sole bearers are the supports for the sole(floor) of your cockpit

 

Thanks for the clarification, but I don't understand your sketch.

The cockpit floor is supported by crossmember that run athwartship. Those crossmembers appear to be 2x4 material, fiberglassed into the "sole." There are approximately four 2x4 that run vertically between the top of the stringers to the underside of the sole for support. Here is a shot of the area I am talking about:



This is not my boat, but the is the same model. The area you see is currently full of foam. I have removed the foam to allow for the belly tanks. You can see in the rear of where the tank was mounted, the bulkhead that seperates the bilge from the cabin. Above the tank is the aft birth. The measurement between the stringers is 31.75" and from the cut bulkhead (which I have completely removed to allow for the tanks), in the foreground, to the cabin bulkhead is 60". The stringers stand 9" off the hull. The tanks will be two 82.5 gallon tanks that will measure 31" in width, 16" in height at the stringers (19.75" at the keel) and 33" in length.

 

1. getting rid of the foam was good. One of the quickest ways to corrode and aluminum tank is encpsulate it in foam. Whatever you do, don't foam the tanks in again. Leave space around the tanks so that moisture will not collect on any surface of the tank.

2. Brackets and a strap are good. But I think you want more clearance from the bottom of the compartment. If you get any bilge water collecting it will result in corrosion. Aluminum has a natural oxide coating that is dissolved by water. Keep the tank dry and it will last for 10-20 years or more.

3. The boatbuilder seems to have had some experience with this. His sounds like good advice, except the clearance issue.

4. Make sure the tanks ar made out of 5000 series aluminum. Tanks should be 5052, 5083 or 5086. The reason, without getting into chemistry and metallurgy, is that these aluminum series are very corrosion resistant and least affected by salt water and other corrosives found in the marine environment.

 

The tanks are being built by the same marine tank company that built the original, 28 years ago. They are still building marine tanks for Skipjack, so I'm not too concerned that they are using the correct material. To be honest, the only reason I removed the existing tank was because of its age. After pulling it, I discovered it was in like new condition. It was the best time to do this, since I am repowering. Too bad I had to cut it out or I might have just reinstalled it.

As for the clearance issue; there is 3/8" on either side and I'll space the V'd bottom approximately 1/2" from the hull. In the center of the tank, where it is flat across the keel, there will be approximately 3/4"-1" of space between the tank and the keel; however, from the tank to the actual V of the hull will be approximately 3", so I think it will be plenty clearance. The tank is located deep under the deck, the majority of which is enclosed under the aft berth, in an area that very rarely, if ever, sees water intrusion.

 

It all sounds good. I think you have well thought out this installation.

 

Okay, Ike, this is where I am at.

Here's a pic of the area I am working with (Ignore the water, that is from water that was behind the bulkhead from an accident. I drilled a couple of holes to drain it ... I'm addressing that issue, too.)


Here are the tanks:


This is my dilemma; where should I mount the spacers on the tank?

If they're mounted on the edge, closest to the stringer (like the following photo), I think that would have the most support.


If I mount them this way, the tank may not be EXACTLY the profile of the hull and cause a hot spot, where more pressure is applied to a smaller area of the tank.


This next photo shows the best of both worlds; however it is moving the spacer midway between the keel and the stringer, where, I believe, there would be less support.


Any feedback would be most welcome!

 

I like the middle photo option. Just off the top of my head it appears to spread the load over more of the bottom of the tank and I don't think it will create the hard spots for stress. Plus it will expose a little more of the bottom of the tank to air.

Some of you other engineers chime in here. WHat do you think?

 

Well, you're too late.

I ended up doing this:


I secured the pieces of King Starboard in place with 5200.


The reason I decided to do this, is that I feel that most of the strength is going to be along the hull, next to the stringer. The side of the tank is less than an inch from the stringer. I can always put something lower, closer to the keel, if someone thinks it is necessary. Any thoughts?

Oops! I didn't notice the emblem on the bottom right side of the photos; I hope that isn't a problem, is it?

 

I think it will work out just fine.

 

Well, unfortunately, it will not work out fine.

5200 does not adhere starboard to aluminum ... especially in 96 degree weather, like we've been having over the last couple of days. The blocks of starboard just slid right off the tank, leaving a nice shiny aluminum surface.

It looks like I'll have to figure out something else! Any suggestions?