Getting Fuel Tank Tested

ssobol said:
Are you sure that those pads are not going to cause stress points and deform the tank? It appears that large areas of the tank skin will be unsupported. For instance, roto molded water tanks are supposed to be supported at least every 2 inches.
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It's a good question!

Probably the most direct, easy to understand reasoning is that this support spacing is (as Pat points out) no worse than what the tank (and likely all 100 gallon aluminum tanks in 25's) have. That is, a couple of rubber strips spaced about 24 inches apart.

Here, not only is that spacing reduced, we have additional supports at the outboard-most ends of the tank.
The supports are also placed on or near the location of the internal baffles -- that makes the local section of the tank extremely strong.

I'll just point out too that roto-molded tanks may not be the best comparison -- plastic/hdpe has about 1/10 the strength of aluminum depending on what it is you are considering.
 
LuckeCatanias said:
It's a good question!

Probably the most direct, easy to understand reasoning is that this support spacing is (as Pat points out) no worse than what the tank (and likely all 100 gallon aluminum tanks in 25's) have. That is, a couple of rubber strips spaced about 24 inches apart.

Here, not only is that spacing reduced, we have additional supports at the outboard-most ends of the tank.
The supports are also placed on or near the location of the internal baffles -- that makes the local section of the tank extremely strong.

I'll just point out too that roto-molded tanks may not be the best comparison -- plastic/hdpe has about 1/10 the strength of aluminum depending on what it is you are considering.
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It's not where the pads are that I was questioning, it's where they aren't. If it were me, I'd consider adding more pads (another 3 rows). Maybe it's not necessary, but it certainly wouldn't hurt.
 
HDPE rails in.

Since HDPE is difficult to bond to, I’m relying on the mechanical strength of the adhesives.

I shaped a 0.25 wide keystoned groove in the faying surface. This groove took fiber-reinforced 5:1.

Around the periphery, the vertical walls got another 0.125 groove all the way around.

This groove took 5200 as the visible fillet was formed.

Tank going in is next!⛽️


IMG_1800.jpeg
 
UPDATE --

The tank fabricator built me a tank which is slightly different than the one being replaced.

- They added hold-down tabs. Four ells welded to the upper surface of the tank near the four corners. Just surfaces for bolting the tank to structure.
- They shifted the center line of the fill ports inboard by ~1.5".

By adding both of these things they have created interference issues with the decking.

- No issue so far with the aft tabs. They may need to be trimmed slightly to avoid interference with the bottom of the sole.
- At the FWD side of the tank, the deck will need to be substantially modified to remove interference with the fill ports. The FWD tabs will need to be trimmed substantially.

It will be very tight with these additional sole modifications to ensure they are completely covered by the steps when they go back in.
 
We have to remember that the early years C Dory had different fuel fill locations. The early had only one fuel fill in the middle of the cockpit. Next two fills. in the middle of the cockpit and then later were the two fuel fills forward, with the removable cockpit floor (which the early boats did not have.). I'll have to check with my son as to what his "Cruise Ship" has--these are a small group of C Dory 25'ss built in 1995/96. None were produced (I believe) until 2002.

There are often issues, similar to these documented in this thread, and more--which are very difficult to find on the usual Marine Survey. All of these are "fixable"--but some can be on the expensive side if you hire someone to do them. At a minumun I would use a remote camera/borescope to examine the top and sides of the fuel tank--and if possible the aft side of the aft cabin bulkhead. The issue not covered in this post, is where the leaking fuel tank allowed ethanol to disolve the inner layer of laminate resin under the fuel tank.--then water gets into the core under the tank. The only way to diagnose this is to pull the tank and that is not going to be done on the usual survey.

That this boat has these repairs properly done, it should be good for many years, and makes the boat more valuable.
 
Here is how close I’ve trimmed the sole to provide clearance for the fills. Super close. Also had to chamfer the edge of the steps that go over the hole.IMG_1862.jpegIMG_1861.jpeg
 
You might want more margin. The coefficient of expansion of fiberglass is different than the tank. On my CD22 there are some things that can only be removed/installed in the summer when the boat expands.
 
ssobol said:
You might want more margin. The coefficient of expansion of fiberglass is different than the tank. On my CD22 there are some things that can only be removed/installed in the summer when the boat expands.
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Good thought, thanks!

I do think it'll be okay after considering the numbers. I don't have any more margin to give anyway without completely rebuilding the steps that cover this area.

It might not look like it, but I have ~ 0.150 clearance on each fill to the sole.

If you assume the coefficient of thermal expansion for aluminum is 12.8E-6/*F and zero for fiberglass while applying that to a decrease in temperature/shortening of the tank dimension (critical in this case since that would drive the hard ride) of -50*F (conservative) over an 8 foot tank width (conservative) that yields ~0.060 contraction/reduction in clearance.

Certainly more than I would have thought. Glad I ran the numbers!

It does make me want to re-consider how I am attaching the tank to structure - I don't want to induce stresses so I'll want to provide attach points that can slip in a controlled way. I have nylon washers with clearance fit holes. I can probably do something there.
 
I don't know what the coefficient of expansion is for FG, but it is not zero. Depends on the type of FG and how it is structured.
 
Here's a link to when David McKibben had to replace his tank on his 25. He built new steps because of gap issues. For what it's worth...The pic of the new step is on page 4 of the thread.
A

Thread 'Gas Tank leak on my 25'

I had been suspecting I had a problem about a month ago and then decided it wasn't. This afternoon I found about 8 gallons of gas in the bilge. Boat has a full tank, in the garage..I moved it out and have removed the floor panel after much much cutting of the sealant that they used to finish it off after screwing it in place. There is no evidence of any leaks at either of the fill flanges or either of the vent hoses. Nothing at the center where the standpipes or senders are. Only thing I can think is there is a leak in the aluminum tank. I have filled all my neighbors gas tanks and...
 
Fiberglass expands roughly half as much as aluminum and 2–3× less than many plastics,
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this is from Chat GT...
When we cut the transom out of our Cal 46 (about 9' wide--there was not a mm of change between night and day temperatures, so that was one reference point I have. Second, we put in Lexan windows and the co-effiecient of expansion of the Lexan was 7 to 10 times that of the fiberglass of the pilot house windows cut out. We had to float the lexan on a sealant which woud allow this degree of expansion and contraction, and any fasteners had to be "loose" with a plastic washer, and little pressure on the lexan, or the touching the sides of the hole drilled for fasteners.

At 2x the expansion of the aluminum, of the fiberglass, I doubt there will be any issues.
 
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