Bulging Fuel Tanks

[Adeline]

Joe, Adeline is still running stock 18 gallon Tempos and like you I'm thinking of upgrading. This old post may help you out. Pete



Joined: 03 Nov 2003
Posts: 48
Location: Vancouver, WA
C-Dory Year: 1989
C-Dory Model: 22 Cruiser
Vessel Name: Adeline
Photos: Adeline
Posted: Sat Jul 31, 2004 11:28 am Post subject:

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Hi all, I talked to Buck @ C-Dory and was told that 25 gallon tanks would be available by October and that the 20s would probably be discontinued. They'll allow room for a single 6 gallon between them. This will be a 40% increase in fuel capacity for Adeline! An upgrade kit should be pretty close to 400.00 and will include the "Starboard" faces and mounting hardware, but no hoses. I was told that there will be no sending units for my fuel gauge. Has anyone installed sending units in their 20s? Tempo offers one with a maximum depth of 24 inches http://www.tempoproducts.com/2004/gauges.html . I know that the new tanks will be opaque so the fuel level can be seen, but I'd prefer not to leave the helm to check. My fuel gauge has always worked well and the holes are already there (gauge and selector) so I'd like to keep them. Pete
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Adeline

[CAVU]

Joe,
I like the plastic tanks and think the problem is not with the tank material but more an engineering problem with securing them. Just like in woodworking(my other hobby)if you know the way your material is going to flex and move then you make allowances for it. The construction of the C-dory bottom with two thin fiberglass skins and end-grain balsa core does not allow for a very secure attachment of flanges or braces. I owned a 17' Whaler for many years and that model had small wood pieces molded into the hull at any place where hardware was to be attached.
I also had 12 gal aluminum tanks in the whaler and never had any problems with them. I would be a little concerned with putting them in this boat unless the bottom could be elevated slightly to insure moisture wouldn't cause corrosion on the bottom. I have read several articles indicating this is a problem in boats with internal tanks in a sealed compartment. The C-dory is open enough that it may be a non-issue?

[Almas Only]

Two photos of the installation and thrust block on Alma's Only, with text, are now posted. As to using aluminum, my only comments are that as long as the distortion of the plastic is not extreme, doesn't significantly affect capacity, and is stable, why bother? Also, I really like being able to look at the sides of the tanks, and see how much fuel is left. We added fuel in 5 gallon increments, and marked the sides so we can get reasonably accurate readings, without having to rely on any gages.

[Adeline]

Hi All, Especially Alma's Only and Cavu, I'm preparing to install my new 25 gallon tanks and have some questions. Alma's Only stated

[CAVU]

Adeline,
I decided to go with 3rd Bytes ideas on my repair. I have epoxied bolts into the floor to hold the starboard strips for the front of the tank and I will use side cleats attached to the splashwell at the top edge of the tanks for lateral support. I am reinstalling my original tanks in their original position so I have no advice about where you should relocate your larger tanks. I would guess that the final shape of the tank would not be evident until it had been through a few hot/cold seasonal cycles?

[Almas Only]

Adeline:

Several possible causes for fuel tank distortion come to mind, and there might be more. All causes must have one thing in common: the application of force to the sides of the tank. Possibilities are (1) gravitational force as the gasoline tries to push out the sides of the tank, (2) thermal expansive forces resulting from temperature differentials between the inside and outside surfaces of the tank, (3) “curing” of the tank over time as volatiles escape and cause the edges to contract, and the enclosed sides to bulge in or out, and (4) “curing” which results in the inner and outer surfaces of the sides being of different lengths, and the side bulging toward the longer of the surfaces. (2) and (4) are similar in that they both result from different lengths of the inside and outside surfaces, much like the distortion in a wall thermostat coil.

You could check for gravitational and thermal causes by filling the tanks with water from your hot water heater. You should look for thermal distortion within a few minutes of filling the tanks, since thermal forces from hot water will diminish as the water cools to ambient room temperature.

Given the rather thick walls of the plastic tanks, and the fact that the sides sometimes distort inward, I’m doubtful that gravitational or thermal forces are at play.

The only way to check for “curing” causes of distortion is to let the tank sit for an extended time, preferably while full of gasoline. On the assumption you want to use your boat, why don’t you just go ahead and install the tanks, keep them full when you’re not cruising, and wait to see if there’s any distortion? If the sides bulge in, the only consequence is a reduction of the amount of gasoline the tank will hold. If the sides bulge out, you might pick up a little capacity, but will also have the possibility of the distortion causing the tank to move and put additional lateral force on the mounts.

Since you plan to use angle retainers along the front and inside lower edges (seems like a good idea to me), the only outward bulges which would tend to make the tanks move against the mounts would be those occurring on the rear or outboard sides. You could accommodate this possibility by installing a strip of one-half inch starboard between the angle sections, and the sides of the tank. The starboard could be the same size as the vertical face of the corresponding section of angle. If the tank distorts and tries to push out against the mounts, pull the corresponding starboard strip, shave a little off one side to compensate for the movement of the tank, and put the strip back in place.

Once everything stabilizes, secure the starboard permanently as follows: remove the strips, insert a temporary metal backing plate between the angle and tank, and drill a hole through the angle toward the tank properly sized to clear a number 8 or 10 screw. Repeat at another location on the same angle. Then, remove the metal backing plate, insert the strip, and put screws through the holes in the angle, and into the starboard. The trick here is to be sure to use a backing plate so you don’t drill a hole in the tank, and make sure the screws are short enough so that they don’t go through the starboard, into the tank.

I’d be inclined to secure the starboard strips as indicated right from the start, and cut or grind the tips of the screws as necessary to match any shaving I did to the starboard to compensate for actual expansion. That way, you don’t run the risk of the starboard sliding out of place, and the tanks moving around and knocking the angle mounts loose, while you’re waiting to see if the tanks distort.

I’d also highly recommend a thrust block, as detailed in previous posts on this subject, to help keep the tanks from moving side to side.

Let me know if I’ve confused you, or if I can be of further help.

Alma's Only

[Adeline]

Thanks for that reply A.O., In other words, Starboard Shims. Sorta like the padded insoles in my shoes. Pete

[TyBoo]

[Adeline]

I'm actually a little concerned about installing them when it's 40 or 50 degrees and knowing that this summer it'll be 90-100. Picture those tanks all filled up at those temperatures. They're going to be at maximum bulge. It's difficult to plan for such a change in dimensions.

[Larry H]

Adeline,

How about an experiment?

Measure across the bottom of the tanks front to back and side to side. Mark the tank where you measured.

Warm up the tanks to summer temp and measure again.

Not too scientific, but its free and might tell you something.

[Bearh]

I'm just curious--
How does one go about warming a couple of 25-gallon tanks up to summer temperatures? The last time I shoehorned something that size into our oven, my wife told me it was usual practice to peel the cow before cooking it. Who knew? Bruce Holly
P.S. My 20 gallon tanks did the bulge thing after I installed them last year about this time of year (see the previous thread(s) about my vale of tears on this project) but not so seriously that I couldn't readjust things by loosening and then tightening the straps again. "Course, I come out of the "if it runs with duct tape, it's a good fix" school, and God loves fools, apparently, because He seems to love me.

[Larry H]

Bearth,

I would put them under the kitchen table,(or a sheet of ply on two saw horses) drape a tarp over the table and heat with a small electric heater.

Be careful not to start a fire!!

[rogerbum]

Am I mssing something here or wouldn't it be possible to simply put the bolts in the floor point upwards and the put slots in the retaining brackets so they can be moved forward and backward to accomodate any expansion? With a lock washer under each nut that holds the retaining bracket down, "unplanned" movement should be zero...

Roger in the SeaDNA

[CAVU]

Roger,
That is the route I went, although I didn't think about slotted holes in the cleat to allow for adjustment. I like it! I posted a couple of pix in CAVU's album of the bolts. They seem very strong and I think they will do the job.

[Sea Wolf]

Mike-

Quote:

"It is kind of strange that those heavy walled tanks would bulge as much as they do. But every container of liquid wants to be round by nature. A circle provides the largest area of any shape with a like perimeter dimension. (Oh no - if I got this physics statement wrong, Sea Wolf will be all over it!) Otherwise we would have square pipes, and millwrights could be plumbers."

Grade = A. CONGRATULATIONS!

Right you are!!! Pressure in a liquid flows out equally in a liquid or gas and results in rounded shapes: circles in two dimensions and spheres in three.

This also explains why our brains are encased in spherical skulls, namely, so that more nerve cells can be packed in a smaller space, making for the miracle of human intelligence!

It also explains, after a few years of time passing, why some people seem to have lost a good part of their original God given cranial content, since liquids like to flow downward under the influence of gravity, and hence the term "Brain Drain".

Over time, this phenomenon in turn accounts for the many "Senilor Moments" as we wander down that golden road to geezerhood, and thus why we all wind up as old gnarly farts joyously fiddling around in boats and passing time until called upon by our maker to take the final journey.

Thus in the end, it really makes no difference whether we were carpenters, plumbers, millwrights, teachers, or whatever, as we seem to finally wind up in basically the same boat! The important part is that we enjoy what time we have here on Earth, and doing your thing in the right boat can make all the difference! Enjoy your C-Dory!