posted 03-24-2003 04:29 PM ET (US)            

Is there another way floor tanks are installed? Every one I have seen is sealed in place with foam. Could an aluminum tank be made safer if it had a coat of some sort of anti-corrosive product painted on it prior to installation? Has anyone here personally used a source for replacing a floor tank with a polyethylene tank? I have that on my list of "things to do" to my Revenge with a "condemned" floor tank. My saddle tanks are aluminum and will get very intense scrutiny prior to each trip.

posted 03-24-2003 07:43 PM ET (US)            

There are many questionable points. For example, select marine grade aluminum alloys basically do not corrode. Certainly, an alloy not intended for marine service will corrode.

Further, two ounces of gasoline does not have the same explosive power as several sticks of dynamite! The article author's is strictly being totally subjective - without any objective information/data at all.

Further, aluminum tanks are normally baffled whereas poly tanks are not baffled. As Where2 attempted to point out above, the sloshing loads are significant. However, the sloshing fuel is dynamic and involves the momentum of the fuel mass acting on the tank.

Further, aluminum tanks are far more expensive than poly tanks whereas the article states that aluminum tanks are cheaper - to explain why manufacturer's are using aluminum tanks.

When I toured the Whaler facility in Edgewater in about '98, I noticed poly tanks being installed on some boats. I questioned the engineer (I had kind of a special tour)and later one of their managers about this practice. I was informed that tanks below 50 gal were poly tanks and tanks over 50 gallons were baffled aluminum tanks. As an engineer with a lot of accident analyses and safety experience, I was satisfied. My only heartburn is - I suspect the smaller boats with the poly tanks might take more of a beating than the larger boats via a higher speed and more motion because of the lighter weight.

But, and as I knew before going to Florida, the tank on my '96 17 OR was aluminum - I would be extremely hesitant to have a poly tank in the boat.

posted 03-25-2003 11:31 AM ET (US)            

You are absolutely, positively wrong about marine aluminum not corroding. It absolutely, positively will corrode & eventually leak, if subject to crevice corrosion due to water being held against the aluminum surface for long periods, as can happen in a foamed in installation. I have seen this happen on several aluminum fuel tanks personally, in factory installed tanks from Mako, Bertram & Aquasport, and in aftermarket tanks made by reputable tank building companies. If you do not believe me, ask anyone in the boat repair business, or in the marine tank business. They will all tell you that all available marine aluminum tanks CAN & WILL eventually corrode & leak if left in constant contact with water for long periods (many years). Perhaps there is some alloy of aluminum that won't experience crevice corrosion when left in long term constant exposure to water (I doubt it though), but if there is, it is not being used to build aluminum fuel tanks by anyone.

As to the proper method for installing aluminum tanks (and a discussion on crevice corrosion on aluminum tanks), check out these links:

yachtsurvey.com/fueltank.htm
continuouswave.com/ubb/Forum5/HTML/000138/html
continuouswave.com/ubb/Forum3/HTML/003271.html

Note that crevice corrosion can eventually corrode even high grade stainless steel...Pascoe has pictures of stainless steel screws eaten up around the screw head by crevice corrosion elsewhere on his site.

As to whether to PAINT an aluminum fuel tank, here are the pro's & con's as I understand them: Most tank builders will tell you NOT to paint the tank...why? Because if you sand, zinc prime & paint the tank with some type of epoxy paint, and then the tank is exposed to constant water exposure such as in a bad foamed-in installation, the water can seep through the paint layer & cause paint blisters filled with water on the tank surface. (These are the same type blisters you'll get on your hull if you painted your hull with topsides epoxy paint & then left it in the water for months...the paint layer seems intact, but small water blisters show up in the paint surface). If this happens, you have created the perfect conditions for crevice corrosion: water held against the tank by the paint layer, but no exposure to air to allow the aluminum surface to create a protective oxidization layer. If the tank isn't exposed to constant contact with water, your painted tank will be fine...but so will an unpainted tank, as air exposure will allow a protective oxidation layer to form, preventing the aluminum from corroding deeply.

If there are paints available that absolutely, positively won't blister or allow moisture to seep through them after years of constant water exposure, then I would imagine that painting a tank with such a paint would be a good thing. But I don't know if such paints exist. Using the same logic, you CAN cover an aluminum fuel tank with fiberglass & resin, thereby creating a (relatively)waterproof barier around the tank. This is actually a viable aluminum tank repair method, as well, although I'd personally feel safer replacing a leaky tank as opposed to trying to repair it. But a safer bet would probably be to install the tank corrctly in the first place so that crevice corrosion doesn't become an issue. This can be done as David Pascoe describes, or by using metal tank straps to hold the tank down (a more common method).

I thought that there were baffled polyethelene tanks available for under-deck installations, but perhaps I'm wrong. Aluminum had the abvious advantage of being custom buildable to whatever size is required. Otherwise, I'd rather have a plastic tank, personally.

posted 03-25-2003 04:46 PM ET (US)            

I didn't discuss the tank restraints during the tour. Also, the restraints did not enter into the discussion with a manger following the tour. I recall seeing a tank embedded in foam - but unfortunately, don't recall any restraint details.

John W. -- The article you reference by Pascoe is interesting - in fact, I saved the article and will spend more time in it. However, I question how you can have oxidation without oxygen being present. Who is this Pascoe? What are his credentials? A journalist? A chemist? A metallurgist?

I will also talk with a chem engineer and a metallurgist and see what they have to say regarding the article and also the corrosion aspects of select aluminum alloys.

But, everyone should be aware that there are many alloys of aluminum and stainless steel available. Some intended for marine service, some not. Now, some of these materials are coming from off-shore. I am not sure of the alloying conformance - however, I know of counterfeit bolts being used in this country - compliments of non-American companies. That issue involves boron being alloyed into high-strength (series 8) bolts. Boron makes the material easier to machine - but also decreases the strength. Kinda got off the target subject - sorry.

posted 03-26-2003 11:53 AM ET (US)            

Pascoe is a marine surveyor, not a chemist or a metalurgist. I don't agree with everything he writes, and in any case, I doubt he claims to be the ultimate expert on metals. But his article on corrosion makes perfect sense regarding crevice corrosion & how it works. His description of crevice corrosion explains how aluminum open to the water (such as an aluminum boat hull) won't corrode significantly, but aluminum (or the stainless screws he displays) will corrode when trapped against a small amount of water...that small amount of water becomes acidic & contributes to the corrosion, versus the open oceam which does not become acidic. Anyone interested in this should read the whole article, or at least the part on Crevice Corrosion midway down, and also his discussion of aluminum in marine applications at the bottom.

I have seen first hand this process on aluminum fuel tanks...as I wrote in one of the attached links, my father & I recently pulled the tank on an Aquasport he is restoring. The tank sat on two stringers, had open air in the bilge underneath & above it, but the sides were in direct contact with (closed cell) foam. When the tank was removed the top & bottom looked almost new after cleaning...no corrosion at all on these faces. But the sides in contact with the foam were pitted badly, on one side to the point of leaking. The corrosion was from water trapped between the foam & the tanks...the water was not in the foam itself. This will occur from the expansion/contraction of the tank & condensation on the tank sides. We took the tank to Mattson, a tank building shop in Tampa FL with a very good reputation. They told us this is common on foamed in tanks, and encoouraged us to mount the new tank they built for us in a manner which won't trap water against the tank (like foaming in place will).

posted 03-26-2003 03:27 PM ET (US)            

Since I've already been too wordy & monopolized this post, I see no sense in stopping now, so I'll offer 2 cents more:

"Should we all pull the decks of our revenes & outrages before it's too late?"

I own a 1989 Mako with a foamed in tank, and despite all my talk on the subject I've done nothing about it. The job on my boat will be difficult and/or expensive, as all console wiring would have to be removed. The outboard mechanic I use also runs a boat repair shop, and has pulled numerous tanks from Mako's & similar. He maintains that around the 20 year mark is when a foamed in tank is likely to start corroding through. But this is hardly scientific evidence. If your tank is very old & foamed in, then pulling it, replacing it or re-installing it might be a worthwhile project. If you own the boat long enough it will eventually need doing anyway. My father's 1973 Aquasport I mentioned above now has it's THIRD aluminum fuel tank since new...the one we recently replaced was not the original tank.

"Are metal straps sufficient...?"

The large fuel tanks in large yachts are almost never foamed in place, and are instead held in place with metal straps. So yes, they are more than sufficient if the job is done properly. Pascoe's method of 5200 & starboard strips might be easier to implement on a whaler, although I'd hate to think of the difficulty of pulling up the tank again if you ever needed to. I doubt that foam around the tank serves in any structural way other than to hold the tank in place.

"Why do whaler & other boat builders continue to use this construction method?"

I have no earthly idea. I wish they'd stop. But given that foaming in a tank is an easy & foolproof way to secure it, and that the problems of crevice corrosion are ten or more years down the road for a new boat, the builders may not care. Or perhaps they don't understand the problem of crevice corrosion.

"Is cathodic protection a viable method for preventing or reducing tank corroson?"

No, not for dealing with crevice corrosion.

posted 03-26-2003 03:28 PM ET (US)            

You ask some very intelligent questions. I have often wondered the same things myself.

I do not think there us any question about whether aluminum corrodes. It does. Trapping water against it will accelerate this process. Mother Nature cares not a bit about Federal law.

The internal tanks in the bellies of Classic Whalers are securely strapped down with very specific sets of shims, padding and aluminum straps that are bolted to the hull with stainless steel 1/4"-20 cap screws. I imagine the foam offers supplemental support but I am unclear if it is acceptable to not have it. This would be a good question it ask Whaler.

There was a very good article published by the Coast Guard in 1997 about these very issues. It can be viewed here:

http://www.uscgboating.org/recalls/pdfs/BSC79.pdf

posted 03-26-2003 10:47 PM ET (US)            

I recommend that everyone interested in the aluminum fuel tank issue read this article.

And Tom, from my interpretation of 33CFR183, a strapped in tank without foam is acceptable. The procedure further states that foam shall not be the only restraint. As such, the foam is not providing a significant structural benefit, but rather simply adding more flotation.

I did note their recommendation for plastic tanks and the absence of their addressing loading of the tank via fuel sloshing - which surprises me.

posted 03-26-2003 10:58 PM ET (US)            

The U.S.C.G. article that Tom referred to us carries an amazing statistic - some 23% of owners of gasoline powered boats continue to operate their boats after knowing they have a fuel leak problem. WOW !!!

Most obviously, those individuals are treading on very thin ice, sitting on a powder keg, and any other metaphor which is appropriate.

posted 03-27-2003 08:10 AM ET (US)            

When you read the ads for plastic tanks, my understanding is that some are considered portable or above deck(red), and others are considered permanent or below deck(black or translucent). I would think that below deck or permanent installation tanks are manufactured to more stringent requirements than portable or above deck. My question is, "Can you use one of the black or translucent plastic tanks permanently mounted to the deck of a Montauk under the helm seat?" There seems to be a better selection of shapes and sizes in the permanet tanks.

posted 03-27-2003 05:01 PM ET (US)            

It is quite refreshing to read an intelligent post to this forum that actually relates to Boston Whaler boats and which actually contributes some useful information. Thank you for sharing that information.

It's interesting to note that the Florida tank is made with the .125" thickness, type 5052 aluminum which is precisely what the Coast Guard article cited above suggests as the desirable and more durable material to construct a tank from.

It is a little disconcerting to hear that your original tank was not well secured.

My own 1983 Outrage 18 also had a Florida Marine Tank. I know some 18’ and larger Whalers in the early 1980's were fitted with plastic tanks but this did not prove to be satisfactory and they discontinued that practice in pretty short order.

I guess my thinking now is that if you do replace a tank, you shouldn't use foam to hold it in, but rather allow it to drain to as great an extent as possible. I also like the idea of fitting a drain to the tank cavity.