Almas Only
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Several possible causes for fuel tank distortion come to mind, and there might be more. All causes must have one thing in common: the application of force to the sides of the tank. Possibilities are (1) gravitational force as the gasoline tries to push out the sides of the tank, (2) thermal expansive forces resulting from temperature differentials between the inside and outside surfaces of the tank, (3) “curing” of the tank over time as volatiles escape and cause the edges to contract, and the enclosed sides to bulge in or out, and (4) “curing” which results in the inner and outer surfaces of the sides being of different lengths, and the side bulging toward the longer of the surfaces. (2) and (4) are similar in that they both result from different lengths of the inside and outside surfaces, much like the distortion in a wall thermostat coil.
You could check for gravitational and thermal causes by filling the tanks with water from your hot water heater. You should look for thermal distortion within a few minutes of filling the tanks, since thermal forces from hot water will diminish as the water cools to ambient room temperature.
Given the rather thick walls of the plastic tanks, and the fact that the sides sometimes distort inward, I’m doubtful that gravitational or thermal forces are at play.
The only way to check for “curing” causes of distortion is to let the tank sit for an extended time, preferably while full of gasoline. On the assumption you want to use your boat, why don’t you just go ahead and install the tanks, keep them full when you’re not cruising, and wait to see if there’s any distortion? If the sides bulge in, the only consequence is a reduction of the amount of gasoline the tank will hold. If the sides bulge out, you might pick up a little capacity, but will also have the possibility of the distortion causing the tank to move and put additional lateral force on the mounts.
Since you plan to use angle retainers along the front and inside lower edges (seems like a good idea to me), the only outward bulges which would tend to make the tanks move against the mounts would be those occurring on the rear or outboard sides. You could accommodate this possibility by installing a strip of one-half inch starboard between the angle sections, and the sides of the tank. The starboard could be the same size as the vertical face of the corresponding section of angle. If the tank distorts and tries to push out against the mounts, pull the corresponding starboard strip, shave a little off one side to compensate for the movement of the tank, and put the strip back in place.
Once everything stabilizes, secure the starboard permanently as follows: remove the strips, insert a temporary metal backing plate between the angle and tank, and drill a hole through the angle toward the tank properly sized to clear a number 8 or 10 screw. Repeat at another location on the same angle. Then, remove the metal backing plate, insert the strip, and put screws through the holes in the angle, and into the starboard. The trick here is to be sure to use a backing plate so you don’t drill a hole in the tank, and make sure the screws are short enough so that they don’t go through the starboard, into the tank.
I’d be inclined to secure the starboard strips as indicated right from the start, and cut or grind the tips of the screws as necessary to match any shaving I did to the starboard to compensate for actual expansion. That way, you don’t run the risk of the starboard sliding out of place, and the tanks moving around and knocking the angle mounts loose, while you’re waiting to see if the tanks distort.
I’d also highly recommend a thrust block, as detailed in previous posts on this subject, to help keep the tanks from moving side to side.
Let me know if I’ve confused you, or if I can be of further help.
Alma's Only
Several possible causes for fuel tank distortion come to mind, and there might be more. All causes must have one thing in common: the application of force to the sides of the tank. Possibilities are (1) gravitational force as the gasoline tries to push out the sides of the tank, (2) thermal expansive forces resulting from temperature differentials between the inside and outside surfaces of the tank, (3) “curing” of the tank over time as volatiles escape and cause the edges to contract, and the enclosed sides to bulge in or out, and (4) “curing” which results in the inner and outer surfaces of the sides being of different lengths, and the side bulging toward the longer of the surfaces. (2) and (4) are similar in that they both result from different lengths of the inside and outside surfaces, much like the distortion in a wall thermostat coil.
You could check for gravitational and thermal causes by filling the tanks with water from your hot water heater. You should look for thermal distortion within a few minutes of filling the tanks, since thermal forces from hot water will diminish as the water cools to ambient room temperature.
Given the rather thick walls of the plastic tanks, and the fact that the sides sometimes distort inward, I’m doubtful that gravitational or thermal forces are at play.
The only way to check for “curing” causes of distortion is to let the tank sit for an extended time, preferably while full of gasoline. On the assumption you want to use your boat, why don’t you just go ahead and install the tanks, keep them full when you’re not cruising, and wait to see if there’s any distortion? If the sides bulge in, the only consequence is a reduction of the amount of gasoline the tank will hold. If the sides bulge out, you might pick up a little capacity, but will also have the possibility of the distortion causing the tank to move and put additional lateral force on the mounts.
Since you plan to use angle retainers along the front and inside lower edges (seems like a good idea to me), the only outward bulges which would tend to make the tanks move against the mounts would be those occurring on the rear or outboard sides. You could accommodate this possibility by installing a strip of one-half inch starboard between the angle sections, and the sides of the tank. The starboard could be the same size as the vertical face of the corresponding section of angle. If the tank distorts and tries to push out against the mounts, pull the corresponding starboard strip, shave a little off one side to compensate for the movement of the tank, and put the strip back in place.
Once everything stabilizes, secure the starboard permanently as follows: remove the strips, insert a temporary metal backing plate between the angle and tank, and drill a hole through the angle toward the tank properly sized to clear a number 8 or 10 screw. Repeat at another location on the same angle. Then, remove the metal backing plate, insert the strip, and put screws through the holes in the angle, and into the starboard. The trick here is to be sure to use a backing plate so you don’t drill a hole in the tank, and make sure the screws are short enough so that they don’t go through the starboard, into the tank.
I’d be inclined to secure the starboard strips as indicated right from the start, and cut or grind the tips of the screws as necessary to match any shaving I did to the starboard to compensate for actual expansion. That way, you don’t run the risk of the starboard sliding out of place, and the tanks moving around and knocking the angle mounts loose, while you’re waiting to see if the tanks distort.
I’d also highly recommend a thrust block, as detailed in previous posts on this subject, to help keep the tanks from moving side to side.
Let me know if I’ve confused you, or if I can be of further help.
Alma's Only
