sloshing forces using unbaffled plastic fuel tanks

I just installed two 99 gallon Moeller plastic fuel tanks in my 34 Silverton. I thought they had some baffles built in when I ordered them, but I don't think they do. Now I'm concerned about the forces caused by sloshing fuel on the control of the boat in rough seas. Are there any suggestions on how to calculate the forces, and what effect they may have on my boat?

The tanks are each 69" fore to aft, 24 inch wide, 17 inches high, slightly tapered sides. They are installed in the bilge, with the rear of the tanks about 10 ft. forward of the transom.

The 1978 34 Silverton Sedan is about 12,000 lbs. I have removed the inboard engines and am installing a 30" bracket and powering with two 225hp Mercury Optimax engines.

 

Tanks that size should have at least two rigid baffles.. Have they inspection hatches of any kind to get it checked? In plastic tanks it's quite difficult to have baffles installed afterwards..
There might be some possibility to install a special of foam to that kind of use thou I don't recall the brand etc..

 

Yep, you can get foam to put in the tanks. I don't recall mfr. names either, but a google search should get you some leads. I know they make foam blocks or triangles even for motorcycle tanks to reduce sloshing. You need the right kind of foam, though...

 

Foam fill is an effective method to mitigate the slosh problem. The correct foam is common in the auto racing industry. Consult your local shop that caters to that kind of sport. You will need enough foam to fill the entire tank. Yes it takes up some space but does not seriously reduce the tank capacity. Subject foam does have a useful life span. The race car guys usually change the foam every season. Fortunately it is not prohibitively expensive.

If you do not find what you want locally, try Summitracing.com. That outfit is one of the larger national suppliers of racing stuff and they do a huge mail order business.

 

The tanks do not appear to have any baffles in them - although the drawings for the tanks show four "towers" that are 3" x 1" x 2" tall - in the bottom of the tank. Would these be enough to prevent sloshing problems?

 

No. To be effective, baffles must reach or nearly reach the surface of the liquid in the tank. You are trying to prevent the liquid from moving around in the tank. The easy, although expensive fix is to buy enough fuel tank foam to nearly fill the tanks, cut it into pieces that will fit through whatever holes you have in the tanks, and stuff it all in. Be SURE the foam will stand prolonged immersion in your fuel of choice. The other fix is to find out from the manufacturer if the plastic is weldable. If so, they should be able to recommend a procedure to install baffles.

 

With no baffles the tank end loads from sloshing will be high.

I would at least box the tank ends , although we prefer to box every plastic tank on 5 sides.

FF

 

I have the tanks strapped down with two 2.5" wide polyester straps, U-bolted to a 3/4" plywood deck that is bolted and epoxied to the stringers and bulkheads. I can add some 2x4s to the ends to block it in, but the tank manufacturer, Moeller, says that the tank will swell with gasoline about 2%, so I will wait until I fill it to block it in.

The tanks do not have any cleanouts on them - they have a 1.5" hose fitting and a fuel level sensor. If I fill the tanks with foam, a float level gauge wouldn't work - I guess I can get another type. I'll check out the foam - anyone else out there have any experience with the foam?

If sloshing really will be a problem, I would have thought that Ocean Link (Moeller distributor) or Moeller would have given me some cautions about this. Neither gave any indication that sloshing force would be a concern.

 

In checking their marketing, Moeller claims their tanks are CG certified. Among other things that means they passed a slosh test without failure of the tank. The test is that they partly fill the tank and rock it back and forth 15 degrees each way for 500 times. I would think the failure mode would more likely be when 300+ pounds of fuel fetches up against one of the ends. The most common failure is that the tank shifts enough to break loose a fitting. They also chafe through the tank sometimes. This shifting of the tank is caused by the forces generated by sloshing liquid. I guess what I'm driving at is that the problem with sloshing is not so much with the tank as it is with the way the tank is secured against movement. In needs to be much stronger than 700 pounds of lead or concrete would require. Because your tanks are so long, I would pay close attention to Moeller's instructions and exceed them if possible, taking care not to put anything in contact with the tank that is harder than the tank. It is a lot easier to replace Neoprene padding than it is to fix the tank. I think I would block the aft end of the tank solidly and leave the swell allowance at the front.

 

"Because your tanks are so long, I would pay close attention to Moeller's instructions and exceed them if possible, taking care not to put anything in contact with the tank that is harder than the tank. It is a lot easier to replace Neoprene padding than it is to fix the tank. I think I would block the aft end of the tank solidly and leave the swell allowance at the front."


The other way is to allow the tanks to swell, ans squirt in stiff foam spray to fill any space at the ends.

FF

 

Moeller emphasizes not to foam the tanks in place. But my big problem is not securing the tanks into the boat, it is that the forces resulting from sloshing fuel will cause handling problems in the boat.

I bought the new tanks - $1200, mounted them in place (16 man-hours), now I realize they have no baffles. To put foam in the tanks, I will have to cut inspection plates in both tanks ($?) and buy and install $1200 worth of foam from Fuel Safe Systems. So my $1200 tanks are going to cost me close to 3 grand... or do I scrap the two plastic tanks, and go buy an aluminum one?

 

Call these people, they make lots of approved tanks. Getting another opinion
is free.http://www.incaproducts.com/site/why_plastic.aspx

 

The handling problem is a concern, of course, but maybe not as bad as you may think. Race cars and motorcycles have tanks mounted high and are constantly moving back and forth. Long narrow tanks low in the boat will not be such a problem. Tanks like that have been put in boats for a long time. When you accelerate, fuel will move to the back of the tank and stay there until you reach steady speed, when it will level out. Opposite when you throttle back. A 6' long tank mounted low won't have much effect o a 34' boat. The center of gravity in the tanks won't move more than a couple of feet or so. The fuller the tank, the more weight, but it moves less. Here are some websites.

USCG spec requiring baffles in large vessel metal auxiliary fuel tanks
http://ecfr.gpoaccess.gov/cgi/t/tex...n=div6&view=text&node=46:2.0.1.2.15.9&idno=46

USCG spec for boat fuel tanks- no requirement for materials or baffles, only that they pass the tests.
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&tpl=/ecfrbrowse/Title33/33cfr183_main_02.tpl

Canadian law-again, baffles in metal tanks.
http://www.canlii.org/ca/regu/crc1486/whole.html

As long as your tank says
"This tank has been tested under 33 CFR 183.580" , you're good to go.
http://newboatbuilders.com/pages/fuel.html

some info on PE tanks here, and other good stuff
http://www.brokeboats.com/badtanks.html

USCG advice for backyard boatbuilders mostly on installation
http://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf

In case you can find someone who has bought a copy of the books of standards, these are the ones you want to look at.
ABYC H24 for us
and
ISO 21487:2006 for everyone else.
These books are expensive so usually they are bought by designers, manufacturers and builders.

 

Naturewaterboy,

To answer your question:

Fuel weighs close to 60 pounds per cubic foot.

I would calculate the weight of half a tank and

then what acceleration it would be subjected to

through what distance in the tank under the worst

pitching, rolling, heaving circumstances you can

imagine, and see how you sleep that night.

I would remove the tanks and install several

smaller ones in the same space. That's my opinion,

it is you who has to "live" with it. In my home built boat,

I installed five, ten gallon tanks instead of one 50 gallon

in the same space.

Sleep well, Tom.