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.

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.

The mounting would have to be engineered to survive a rough sea and corrosion. More forces at work than gravity.

High Wire said:

The mounting would have to be engineered to survive a rough sea and corrosion. More forces at work than gravity.

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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.

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.

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....

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".

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.

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?

Can RTF insert a vid here of Arthur Defever rolling in his grave?

Greetings,
Mr. c. Sorry, I got nothing...

Scary said:

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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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.

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.

Hull extension?

manyboats said:

Hull extension?

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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.

Marty.....................

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.

Peter,

Your post # 16 made me smile.

It's 100% Peter B.

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.

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.

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.

Daddyo said:

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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Helps but doesn't change my opinion.

A. If the tank is then displacing water, you just put tremendous pressure on it and on your platform. The risk to platform and boat is significant. Typically you'd need to reinforce the means by which the platform attaches to the transom. But also you might put too much stress on the transom itself.
B. A few hundred pounds becomes many hundreds if not thousands when now moved beyond the existing water surface. Think of a see-saw.
C. Don't know where the 340 gallon tanks are now but this may add to the problem rather than reduce. You're now adding weight beyond the stern and removing weight forward some distance. So could be impacting the distribution of weight even more.
D. Not as minor as you think. Changes the entire weight distribution. I've seen batteries moved make from behind the engine to in front of it make significant change. Shifting 4000 pounds from somewhere I'm guessing forward of the engine to outside the existing boat is not minor even to a 60,000 pound boat.
E. Yes they do. Doesn't mean those boats work well with that load. Doesn't mean they aren't dangerous. Depends on the boat and what it was designed to accommodate. As to small fishing boat, I don't know any small fishing boats designed or approved for 1400 hp.
F. Advertises. Key Word. They show two examples. I have no doubt it would increase speed just like trimming tabs way out. As to the flotation chamber of the Armstrong, I believe perhaps if the exact right combination. Also the right mounting and transom strengthening if necessary. They also looked at boats that did not have good attitudes before the install so started with bad situations. Easier to look good.
G. Armstrong's design isn't well known and well proven for a boat like you're talking about putting it on. Actually the lightness of the boat makes putting the extra pressure on the transom much less of an issue. Now you're putting 60,000 pounds of weight on an extended surface not originally part of the design.

I think making such a modification as this without a naval architect checking all the factors would be a serious risk. Could it work? Possibly, of course. Would it work? I think a significant possibility of problems. Fact is we can't know for sure what the results will be.

Some boats work well with extensions. Others don't. They all need an architect working to determine what the new boat will be like.

Some more clarification:
I have not purposed doing this without NA approval (don't know how that got started). This does not in anyway involve the swim platform. The current primary tanks 680g are aft of the engines. The secondary tanks 300+ are less than three feet from the transom in the lazerette. The majority of the existing tankage is above the waterline and mounted all the way outboard. The new tank would be centerline and a good 4+ feet lower then the existing tanks. The new tank would add nearly 3' to the hull length.

Perhaps playing Devil's advocate here a bit. But........

A. I think you are forgetting that as the boat pitches in a sea that the transom goes up in the air and can even be out of the water. Plus it's always been considered a bad thing to put to much extra weight out on either end of a boat.
B. What's that amount of gallons times 7.1?
C. True. But you're removing it from where the original designer intended it to be. That may or may not be a good thing.
D. Maybe. But again an NA may be a better judge of that.
E. - G. I think you might be comparing apples to oranges.

And E. If this is such a good idea why aren't boats designed that way on a regular basis?

I've seen a few "radical" alterations made to production boats in the years we've been our marina. They all made sense to the owners although most other people wondered what the person was thinking.

The question I would ask with regards to removing stock (I assume) internal tanks and replacing them with a tank mounted on the transom is what this might do to the value of the boat? Not to the person having the mod done, but to potential buyers later down the road. Somebody might like the idea, of course, but it's been my observation that people who want to buy a Grand Banks, for example, want to buy a Grand Banks and not some owner's idea of what a Grand Banks should be.

In fact that was a condition that our broker made sure we put as a qualifier on the offer we made on the boat we have now. The boat had to be what it was supposed to be: a stock Grand Banks 36. No major changes to the interior or exterior configuration of the boat. No moving the galley into the rear cabin or whatever, no matter how much the owner thought it was a great idea.

I don't doubt that if enough money is thrown at the proposed transom fuel tank, and as long as whatever USCG, EPA, etc. requirements are met, if there are any, that the project could be completed in a safe, strong manner. Whether it negatively affects the handling and behavior of the boat itself is anyone's guess at this point. And whether or not a potential buyer down the road will think it's a good idea remains to be seen.

I am talking about approximately 300 gallons, so this would be about the same weight when not in the water of two large outboards even though, again when submerged this is added boyancy not weight. This is also a significant lowering of the center of gravity.

I think the issues are there...most of us backyard type guys are just "guessing" with just enough knowledge to be "borderline dangerous".

I'm with you Mark in that I don't think it's as bad as many make it out to be but still not something to mess up as major holes/cracks developed in a seaway would be interesting.

The transom on my trawler isn't as thick as the one on my dingy because it doesn't have to be...so even a bolt on bracket for an outboard designed boat will need some transom work on your boat probably beyond even what a 30 foot center console might.

If the mod is going to make your boat $10,000 dollars more usable...then I'd say explore the possibility at least. By the time you pay an NA to do his thing, buy/build a bracket and mod the transom area....I'm guessing $10-$12,000 but a few phone calls might tweak that number to your liking.

What problem does the current arrangement present? If you want more storage enclose the area over the aft cabin.

The curved transom of the 48 which adds immensely to sea keeping of the vessel would be a bugger to attach your planned bustle. And then when the tanks are emptying they would float the boat in a nose down attitude or just break right off.

On our DF 48 the laz tanks are about 100 gallons each and protrude no further aft than the aft bunk "insert" leaving lots of room for storage or equipment such as diesel heat.

If you want to make the 48 into a houseboat look on Gibsons for sale and travel to Louisville to pick one up, they are a pretty nice vessel ideally suited for your needs.

And no, Art would not be rolling in his grave. He was buried at sea. For sure though he'd be l

psneeld said:

I think the issues are there...most of us backyard type guys are just "guessing" with just enough knowledge to be "borderline dangerous".

I'm with you Mark in that I don't think it's as bad as many make it out to be but still not something to mess up as major holes/cracks developed in a seaway would be interesting.

The transom on my trawler isn't as thick as the one on my dingy because it doesn't have to be...so even a bolt on bracket for an outboard designed boat will need some transom work on your boat probably beyond even what a 30 foot center console might.

If the mod is going to make your boat $10,000 dollars more usable...then I'd say explore the possibility at least. By the time you pay an NA to do his thing, buy/build a bracket and mod the transom area....I'm guessing $10-$12,000 but a few phone calls might tweak that number to your liking.

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None of us know what the issues would be. We're just expressing some that might exist. There are probably others we don't even know about. Only a good naval architect can make a good judgement and they may not be right.

Mark's saying now he wouldn't do this without utilizing one. But he did ask us the question and seems to not like the concerns any of us are raising. Sounds like he's pretty determined to do it. I'm guessing by the time you pay an NA to review the ideas and offer a feasibility opinion, then to do an actually design and spec, pay for modifying and strengthening the transom, pay for special tanks of some sort to meet all requirements the $10-$12,000 is on the low side. This isn't a bolt on to an outboard boat. The outboard boat was always designed to accommodate outboard motors, the modification just moves them a bit.

Weight and size

300 gallons of fuel plus the tank weight would be around 3000lb. If the boats is a 60,000 lb boat that's about 5%. A 300 gallon tank could be 14' by 5' by 1' and span the width of the transom above the water line and boarding platform it would have little effect on boat handling. Probably work fine. But as Marin states could you sell it to some one else. It would turn me off in a heart beat.

BandB said:

None of us know what the issues would be. We're just expressing some that might exist. There are probably others we don't even know about. Only a good naval architect can make a good judgement and they may not be right.

Mark's saying now he wouldn't do this without utilizing one. But he did ask us the question and seems to not like the concerns any of us are raising. Sounds like he's pretty determined to do it.

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I appreciate the comments and have agreed or disagreed with any of them. I have only added details and clarifications. I am not determined at this point at all only as always trying to think outside the box and looking to add value to the mundane like tanks.