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Old 17-07-2008, 03:15   #1
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Fuel Tank Baffles

Is there a rule of thumb for tank baffles.
I am building a tank 800mm wide 500mm long and 750mm high for about 300 liters.
As I am unlikely to have this full I am tending towards putting a longitudinal divider and turn it into two tanks 400 wide by 500 long and 750 high.
Would these need baffles of any sort, maybe a half hieght cross baffle to limit the for and aft surge. At half hieght I would only need one 6" access port per tank.

Mike
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Old 17-07-2008, 04:56   #2
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Quote:
As I am unlikely to have this full I am tending towards putting a longitudinal divider and turn it into two tanks 400 wide by 500 long and 750 high.
The purpose of the baffle is to act as internal bulkheads to support the span of the long side and reduce the wall thickness of the tank. A full tank of fluid on a moving boat can make some serious forces.

With a volume of 300 liters that would mean the volume if filled with water would run about 300 kg. You can see as you get bigger the baffles are meant as structural not just to reduce the sloshing. Even if you don't plan on filling it you need to make sure you could. Overfilling the tank would then be a time bomb waiting to happen.

My water tank is stainless steel The baffles are solid across but there are notches in the baffle where it is attached to the sides and the bottom to allow the tank to be level between them. That gives a good attachment to hold the long sides from pushing outward as well as maintain the rectangular shape of the tank under dynamic loads.

So the rule of thumb is you need to minimize the deflection of the faces of the tank based on the tank volume and the material of the tank walls. Long expanses tend to push out and worse yet the tank can tend to "rack" and break the seams at the edges. Reducing the sloshing also reduces fluid motion and noise that also reduces the dynamic moment when pounding through surf. 300 kg bouncing around is a lot of forces. The seams actually stress and release and can fatigue.

You also want to secure each edge of the tank in two directions and at least two or more places so the hull will support the tank motion. The idea is the tank should be strong enough to freely support itself when full but also have no place to go as you fly through the water. Repeated pounding will fatigue the metal and probably worst at seams.

It would not hurt to actually have the tank engineered and avoid just a rule of thumb. It's just a box but it has to handle a lot of dynamic load.
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Old 17-07-2008, 06:06   #3
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I probably should have provided a little more info.
The tanks will be constructed out of cored balsa panels of which most are existing structural panels which would have been used to anchor a loose tank. These will have 2 additional layers of glass on the inner side,450g/sqmeter, for a total of 1500grams and an extra layer on the outer which is more than many hulls. The base is the bridge deck itself and has external longitudinal stiffeners. The aft vertical side will in addition have signifcant horizontal bracing at about the mid level of the tank. I doubt strength will be an issue and the tank cant really get loose. Compared to what some local "Pro" boat builders do for tanks these are serious overkill.
I am more worried about the fuel sloshing more from the perspctive of noise and airation. The base of each tank would be only 500mm for/aft and 400 wide. I suppose a half hieght cross baffle would spread the stress some more so probably a good idea.

Mike
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Old 17-07-2008, 15:11   #4
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Hi Mike
You state that you are using balsa cored panels for the baffles in your tank. I am very sceptical about any glass construction using balsa. Especially for fuel. I know that many boats have been built using this coreing and have been very sucessfull.
However...I have had to fix a good number of balsa core defects. In my opinion it is only used because its cheap..most important..it absorbes resin well and therefore the adhesion to the matting is good..it is light..important too. But if it fails...and it does you can have serious problems down the road a way...I am always banging on about the next ten years. Every boat has a new owner sometime and a fix is a legacy which eventually can come back to haunt the new owner.
If I might suggest Airex foam. It is a closed cell foam which means it will not transport fluid in the event of skin failure due to collision, compression etc..and these events do happen. You might not even know its happened. It has all the adhesion qualities needed..is light..you can bend it..it never rots..Ok so it costs more..Not an issue as far as I am concerned.
Do you know that the existing panels you speak of are sound..A moisture meter might be a good idea if you are set on using them.
I'm not trying to be a negative voice here, but be sure that you have all the bases covered..Cheers..Jerry
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Old 17-07-2008, 18:37   #5
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Jerry
The boat is currently under construction so is dry. The panels are
"Duflex" which are prelaminated balsa and due to the method of manufacture prevent the spread of moisture further than a couple of inches beyond the area of damage. All edges are decored and filled then glassed over, with a couple of sealing coats it all becomes very impervious.
From all my research foams are not the panacea that they are purported to be so I am staying with balsa. Thats a topic that gets debated adnauseum and not really for here.

Mike
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Old 17-07-2008, 21:49   #6
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The other consideration might be the resin used to seal the core material. I have cut open diesel tanks I built several decades ago of plywood sealed with WEST System epoxy. No penetration of diesel, and the wood still smelled like it did when I built the tanks.
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Old 17-07-2008, 22:28   #7
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Doing a quick conversion in my head I think your tank is around 80 gallons. Dividing it in two gives you about 40 gallons. The rule of thumb I have seen is to put baffles in any tank over 30 gallons in size.
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Old 17-07-2008, 23:11   #8
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Roy
The resin I am using is west. With 3 layers of glass plus sealing coats penetration won't be an issue.
Steve
Thanks. As one tank it would be nearly 80 US gallons so breaking it into 2 and then a half hight baffle in each sounds right for your rule of thumb.

Mike
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Old 17-07-2008, 23:49   #9
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Just another 2 cents worth: IMO err on the side of caution if one are not 100% sure of the "thumb rule". It is easy to add a baffle or two at this stage and another inspection port if necessary.
The sea always seem to hunt the weak link but then again I am guilty of over-engineering sometimes .
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Old 18-07-2008, 08:02   #10
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Originally Posted by Wotname View Post
Just another 2 cents worth: IMO err on the side of caution if one are not 100% sure of the "thumb rule". It is easy to add a baffle or two at this stage and another inspection port if necessary.
The sea always seem to hunt the weak link but then again I am guilty of over-engineering sometimes .
To err on the side of caution is always a good idea. Dividing it up into 4 compartments is about as far as I can go I think. Have changed it slightly to 800x450 by 850 deep with the cross baffles 550 deep. This makes the footprint of each tank 400x450 or each compartment 400x225. Seems extreme to put in more than one inspection port per tank as each is 200mm diameter with a 150mm opening, thats why the cross baffle is short. 4 ports would be almost all of the top .
I think the weakest panel will be the front so it will now get some cross reinforcement about mid level.
Compared to the roto molded tanks that are just strapped down I think this will be far stronger.

Mike
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Old 18-07-2008, 20:20   #11
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Sounds good;
FWIW, I remember a tank maker in Brisbane in the 80's whose prime business was making replacement tanks for "production" yachts that had made their first off shore trip from the southern states on their way to the reef.
Their original tanks would burst somewhere between Sydney and Brisbane and he had a good business replacing them with stronger tanks. Guess I always remembered the lesson.
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