Diesel Keel Tank Replacement - SailNet Community

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  #1  
Old 02-03-2013
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Diesel Keel Tank Replacement

I am contemplating the purchase of a 40 year-old center cockpit 42' fibreglass ketch project boat. The diesel engine and diesel generator both need to be rebuilt as the boat dragged onto the marsh during a hurricane and then took-on some water at high tide. Since both have to come out of the engine room, now would be the time to address the center in-keel aluminum diesel tank. I know that it is fairly common for sailboats of this age to develop leaks in the keel mounted tank - a number of this boat's sisters have either abandoned center tanks or have replacements.

The boat originally came with three tanks with a total capacity of 210 gal. My guess is the center tank is 75-80 gal, leaving a capacity of approx 130 gal between the port and starboard tanks.

I am retired and single so I plan to live-aboard while working part-time as an Adjunct Professor, probably in SE Florida. I would then plan to sail over to the Bahamas during semester breaks. For the summer I will head north probably to at least Chesapeake Bay but might want to go as far as LI Sound up to Nantucket. I don't plan a circumnavigation at 65 years old, but should I not have to work in the next few years I might like to go further south than the Bahamas.

Based on my intended use of the boat do you think that a 130 gal diesel capacity is sufficient? If I decide to abandon the center tank will I be excluding a large number of potential buyers when I go to resell? Any comments from experience in restoring a swamped boat would also be appreciated.
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Old 02-04-2013
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Re: Diesel Keel Tank Replacement

130 gallon tank on a 40ish foot boat should give you a motoring range of ~700 miles at 6kn. That's a pretty good chunk of motoring for a boat this size. Just look at 40 foot comparables, my Beneteau 381 has 40 gallons onboard. What you are really loosing is some time on the hook with a generator going.

Frankly given what you will have left I am not surprised that many sister ships have taken out the tanks, I would think you would be looking at a lot of time and money for a negligible gain. In actual operation that tank is probably going to be empty 95% of the time anyway, just to keep from storing bad fuel.
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Old 02-04-2013
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Re: Diesel Keel Tank Replacement

The boat's designer(s) put an ~80 gal tank in the keel for a reason: they expect that much ballast to be there.
Can you remove the lowest tank, replace it with a reserve water tank and live with the remaining 130 or so in fuel tankage? Probably.
Could you install a tank that could be used for either diesel or water? Probably.

So you are looking at a h'cane Sandy boat? Good for you for wanting to rescue one.
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Old 02-04-2013
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Re: Diesel Keel Tank Replacement

I frankly don't see a problem. If you can get to the tank you might even want to rip it up and make it storage space of move the batteries to there.
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Old 02-04-2013
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Re: Diesel Keel Tank Replacement

Well.... seems to me, from the little your telling us, it's a project boat. It may be years to get her ready for the high seas again. Tanks; I'm guessing are a small part of the big picture. rebuild both engine$$$$$??? eek. what about the keel bolts, stringers, soft decks, bent masts...rigging.....

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Re: Diesel Keel Tank Replacement

Thanks for the good advice Greg & CalebD!

I think my range will be a bit less - the boat has a Ford Lehman 80hp engine.

I hadn't thought about the ballast effect of a keel tank. I agree the extra capacity will likely be empty 95% of the time. Assuming that's the case we're looking at moving up to 572 lbs (7.15 lbs/gal x 80 gals when full) from about 10" above the bilge to say 12" below the bilge if I leave the other two tanks empty and use the bilge tank as my primary tank. Now if I use the keel cavity as an integral tank, I'll gain probably up to 20 gals capacity from the 4" of foam under the current aluminum tank, say 1/2" around it and say 4" above. That's another 143 lbs for a total of 715 lbs vs. 8,000 lbs lead ballast, about 9%. That could make a significant difference in a bad storm.

So, now that you've resolved that question for me, what do you guys think of removing the old aluminum keel tank and converting the cavity to an integral tank? I've read a bunch of blog posts that warn of leaks from integral tanks due to hull flexing, and I think I read that integral tanks can't be forward of the mid-point of the boat, but I expect this applies more to a power boat. The lay-up schedule for this boat built in 1975 called for a 1" thick solid glass bottom. I would expect the keel is even thicker. Do you think there can be any flexing there?
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Re: Diesel Keel Tank Replacement

ChucklesR - I don't think the keel cavity would be a good place for batteries as it will be the lowest point of the bilge - batteries & water don't mix well.

Denise - I hear you! Would you like to barter some help in restoring her in exchange for some future winter sailing time in the Bahamas? Seriously, it isn't as bad as your post makes it sound - if I can believe the broker. She sat on her side in the marsh when the tide went out and then took on "some" water when it came back in. I'm told 2-3" above the sole. Was also told the interior was completely cleaned after the water was pumped out and there was no damage to hull, keel, rudder or anything else other than water damage. I haven't seen the boat yet as it is an 8+ hour drive away. I'm in the process of trying to do my due diligence so I don't waist my time and money on a three day trip!
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Re: Diesel Keel Tank Replacement

Sailor,

The ballast value of the tank is going to be pretty minimal. No designer worth discussing is going to rely on the ballast value of a consumables tank for anything. It MIGHT make the ride a bit more comfortable, but it could just as easily give the boat a snap roll when filled. Frankly given the location the difference is going to be roughly the same as one person sitting on the centerline versus moving 3' towards the high side. There just isn't enough lever arm to matter.

As far as what do do with the space... I would need to see it. I could imagine batteries would be fine if you could build a box for them, or bulk long term storage (canned food, booze, rice&beans). Or perhaps just give yourself a deeper bilge. It really depends on the arrangement.


Whatever you do, do not turn it into an integral tank. This does work with metal boats, but there are numerous reports of diesel eating away at the resin in fiberglass tanks, let alone fiberglass hulls. Water is almost as bad, think blisters on the inside of the hull. Not to mention the issues of dealing with an integral tank can be massive (how do you install a drain in it?).

Really I think you are over thinking this, your existing fuel storage can get you a good ways across the Atlantic, I wouldn't worry about it. Figure you will get about 6kn at 1gallon/hr fuel burn. Likely much better than this, but being conservative. My Beneteau 381 is more like .5gallon/hr @6.5, and my old Irwin 54 was just under 1gall/hr at 7.5kn. I doubt you will get worse fuel economy than the Irwin, and will almost defiantly fall in between the two.

Trying to eek out the last but of fuel range just isn't worth it.
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Re: Diesel Keel Tank Replacement

Stumble - Thanks for the good advice. You're right I don't foresee needing a range more than 780 kn. But regarding your comments on fiberglass resin and diesel, from what I've read that doesn't seem to be a problem with epoxy resins. And it's the ethanol in gasoline that attacks the resin correct? No ethanol in diesel. In any event, what others have done with this boat is to add glass using epoxy resin to the cavity and then paint several layers with an epoxy paint. As for draining the tank, how is an integral tank any different than the aluminum tank that is permanently foamed-into the keel cavity in that regard?

Jim McGee- SailNet let me compose an answer to you PM but because I'm new and haven't posted 15 times yet, SailNet won't let me send the response to you as a PM. The answer to your question is neither. Warn your friend though to check with his insurance broker - I just spent most of the evening composing a response to my potential insurance broker's questions. If the boat's been declared a total loss by its insurer it may not be insurable ever again.
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Re: Diesel Keel Tank Replacement

Foamed in aluminium tanks are about the surest way to guarantee the tank will fail prematurely. Poltice corrosion is a sure bet. Having had to clean tanks before, I wouldn't own one I couldn't drain and wipe down. It isn't something that needs to be done very often, but when it does buried tanks are a major problem.

And yes, epoxy and diesel are compatable. But the underlying resin may not be. If you really believe you can make a perfect tank, well I wish you luck, but I know commercial builders that every now and then make mistakes and have a bad mix. If we are talking about water intrusion, well it isn't that big of a deal, if an integral tank fails, now you have a major problem, because the diesel can disolve other resins. So you have just introduced a solven that can do major structual damage to the bilge, right around the keel bolts...

It isn't a likely problem, heck I would even be willing to accept that it is a pretty rare problem, but the consequences of a mistake are huge. And so to me the risk/reward analysis just doesn't work.
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