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Old 08-15-2014, 02:55 AM   #1
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external transom mounted fuel tanks

So my thought is this. We are in the middle of removing old/leaking/unwanted fuel tanks. When pondering the new tanks location, shape, capacity and material it occurred to me why not mount the new tank to the transom under the swim platform? Heavy gauge aluminum with perhaps a thick "bumper welded around it. The advantages would include: easier installation, increased waterline for speed, cleans up engine room, etc.... The diesel would be lighter then the water displaced.
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Old 08-15-2014, 05:05 AM   #2
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Wow Mark, deep thought for 2:45 in the morning! I know on my boat that weight on the stern would change the balance of the boat significantly but it wouldn't likely make so much of a difference with your overall weight displacement. My other concern would be with a single wall tank if you have a puncture or leak for any reason you would have no means of containment. At least currently if it goes in the bilge it is contained. Not sure that would even be legal in Canada? A double wall tank may be a thought. It would as stated increase your waterline and give some efficiency. I will be interested to see what other folks have to say.
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Old 08-15-2014, 05:48 AM   #3
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The mounting would have to be engineered to survive a rough sea and corrosion. More forces at work than gravity.
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Old 08-15-2014, 07:39 AM   #4
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The mounting would have to be engineered to survive a rough sea and corrosion. More forces at work than gravity.
My thoughts exactly...works for me as far as space allocation...but the size you probably want I'd be nervous to backyard engineer that myself without NA input.

I'd build and install...but only with a NA's suggested attachment strategy.
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Old 08-15-2014, 08:25 AM   #5
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I think you're better off with the original engineering unless you want to run this by a naval architect.

I also think this would greatly increase the risk of dumping diesel fuel into the water in case of an accident. The EPA doesn't take that lightly.
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Old 08-15-2014, 08:38 AM   #6
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I am still pondering if This came to Mark in a dream, woke him up and motivated the posting for interpretation by wise men. Seriously, could work, the real question is would it be a plus after the negatives are factored in. I would also reflect on why we haven't ever seen it on any production boats. I have a buddy that did this on a one off 53' trimaran 70 Bigfoot outboard power. Works fine.
Good luck on finding the wise men....
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Old 08-15-2014, 10:13 AM   #7
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Too bad your not in Miami as I bet you could find guys in Hialeah who could whip that up quite well, quite fast, and properly install it. You can get anything done in Hialeah. As a long time Southern shade tree mechanic, I'm in total awe of these guys creative abilities. When you come from a country where there are no parts or sophisticated (as in electricity) tools available, it's stunning what they can do. "Cabron, of course I can do that".
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Old 08-15-2014, 10:49 AM   #8
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One consideration, well maybe three, would be

Effectively moving your running gear forward with the added length of your waterline could, would change handling, How would you you attach the tank with the amount of buoyancy of the added tank without ripping the tank off. Maybe the correct way would be to add a glass hull extension using the extension as fiberglass tank. Then you would not be looking at a mechanical connection that could fail. I can't even imagine trying to sell a boat with a aluminum tank bolted on the transom below the water line.
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Old 08-15-2014, 10:50 AM   #9
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I would'nt do it because of the variable CG factor. Hard to imagine you would'nt be running out of trim w SST (swim step tanks).

I just completed an auxiliary fuel tank system but it's only 6 gallons and in my salon/pilothouse.

What is the trim of your boat as it is? Perfect or which end down? Quite a few trawlers I see in pics are noticeably bow down and they would probably benefit from your SST CG wise. But if you're talk'in external tanks it does sound scary. And the CG issue is only about ten percent (or less) likely to be positive.

I see from the thread title the SST is external. Now is this tank permanent? In the future would you keep it empty? Seems like a lot of work for a temporary system.

If your trim is now bow down and you build the tanks really skookum it should be ok ... Can't say fine as any leakage would be fuel overboard. I consider this a radical idea and usually like radical ideas but I'd consider other options inside the boat. What if you got rear ended? Is it even legal?
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Old 08-15-2014, 10:51 AM   #10
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Can RTF insert a vid here of Arthur Defever rolling in his grave?
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Old 08-15-2014, 11:02 AM   #11
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Greetings,
Mr. c. Sorry, I got nothing...
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Old 08-15-2014, 11:17 AM   #12
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Effectively moving your running gear forward with the added length of your waterline could, would change handling, How would you you attach the tank with the amount of buoyancy of the added tank without ripping the tank off. Maybe the correct way would be to add a glass hull extension using the extension as fiberglass tank. Then you would not be looking at a mechanical connection that could fail. I can't even imagine trying to sell a boat with a aluminum tank bolted on the transom below the water line.
Kind of like the old Ocean 40+2 with the two extra feet under the swim platform. An integral fiberglass extension of the hull from the swim platform down, properly baffeled and sealed could be worth consideration if you really need the several hundred gal of extra fuel.
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Old 08-15-2014, 12:53 PM   #13
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Mark-maybe some numbers can put your idea in perspective. Say you construct a 500 gallon tank in some configuration. There are roughly 7.4 gallons in a cubic foot. So you need a tank with an interior volume of approximately 68 cubic feet. 500 gallons of diesel weighs about 3,600 lbs, wild guess a tank might weigh 300?, so a total weight of about 4,000 lbs hung on the stern. Now, given the cubic foot needs, you have an infinite range of configurations for the tank shape. I would think that what you would want is the widest possible, tallest possible, shortest possible, to keep the gross weight and the CG of the tank as close to the transom as possible. So a tank close to the beam at the transom, say 10' long, 2' available under the swim platform so 2' high, that leave about 3.5 wide.
I think that the construction cost of an add-on to meet all contingencies mentioned in other posts would be prohibitive. I like the idea of an integral hull extension under the swim platform (creating an integral swim platform too!), but that also seems cost prohibitive.
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Old 08-15-2014, 01:03 PM   #14
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Hull extension?
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Old 08-15-2014, 04:41 PM   #15
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Quote:
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Hull extension?
Yep, I think a hull extension is what it would take, like what these guys do. I have seen other yards in the PNW who will do hull extensions like that as well. The ones I have seen have basically added a big lazarette but it could just as easily be fuel tankage. I thing a hull extension would actually increase the buoyancy so it would off-set the fuel weight. I dunno, I am not a naval architect so you would have to talk to one of the yard that actually do this kind of stuff.

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Old 08-15-2014, 09:59 PM   #16
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Mark, for all the above reasons…don't do it. I strongly urge you to replace your tanks with smaller, more easily replaced, fuel grade plastic tanks, interconnected so you can mothball extra capacity for when really needed. Lighter, non-corroding, and more efficient, and minimal effect on trim - better still weight nearer the centre where it is always best to keep it, not wagging the tail as it were. I could never sleep well with all my fuel stuck on my bum.
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Old 08-15-2014, 10:23 PM   #17
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Peter,

Your post # 16 made me smile.

It's 100% Peter B.
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Old 08-15-2014, 10:36 PM   #18
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I've seen people just move their batteries and change their performance and handling significantly. You start talking about tanks full of fuel and not just at the rear of the boat, beyond. There are so many issues and the cost of getting an architect to even figure out if it was possible is high. Also, I've seen the problems people just have with exceptionally low swim platforms and with davits and dinghies on some of them. This is many times those issues.

When it comes to computers and boats and many other things, I'm very Vanilla. As near to the original design as I can keep things I believe the fewer problems I'll have. You want to see an extreme case of what might happen, try the Northern Marine 90. That all started from a basic 85 but then included adding length, adding height, changing this, changing that. It took forever to get ready to even launch and then.....

I don't profess to be a Naval Architect so not going to play one by altering the balance or center of gravity or any other factors of my boat that were actually designed by one....at least hopefully were.
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Old 08-16-2014, 12:51 AM   #19
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Hull extension is exactly what I thought as well. It's done all the time but I would get an NA involved in the process. The boat yard across the river from my store is well know for stretching Bertrams.

Instead of just screwing aluminum tanks on the transom, you might consider a fiberglass hull extension and install the tanks in that.

Think of all the interior room you would gain if you do put the tanks aft!

PK, not quite Hialeah but close.
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Old 08-16-2014, 12:54 AM   #20
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Perhaps a bit more info would have helped.
A. There would be no additional weight aft as the fuel weighs less then the water it displaces, ie: the tank full would float if detached.
B. The tank would be only a hundred or so more gallons then the lazerette tanks I am eliminating
C. The two 340 gallon main tanks would be eliminated thereby greatly reducing the total weight of the vessel
D. You have to take into consideration the overall minor weight/issue as we are talking about a 60,000 boat
E. Armstrong makes brackets for small 5-7,000+ fishing boats that carry two, three and sometimes 4 350hp outboards
F. Armstrong advertises the added buoyancy and increase in speed and reduced fuel burn when using their brackets
G. Copying someone like Armstrongs attachment design would be using a well known and proven design.
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