Lee, it has been a long time but in many instances of choice between AL and Fe the Al was thicker and nearly the same weight. At most 10% weight reduction. Like Matt stated, need to know the thickness. I would use the original weight and be happy with a reduction. The difference will probably be less than the weight of 2 gallons of fuel.
Looking at beer kegs there may be 20# difference so make that 3 gallons.
According to these drawings, the wall thickness is 3/16".
Just actually studied that drawing and have a suggestion ---- If in the future your customer decides to repower with a fuel injected gasoline motor done right with a tank return, OR a nice modern diesel that definitely needs a tank return, another 3/4 NPT port in the top of the tank would really come in handy and would be nice & cheap now as long as the tanks are on paper and not in the boat without a needed extra port :-)
please see the attached excel sheet. I took the dimensions and created a volume calculation.
Aluminum weights .0975 lbs/ in^3,
steel weights .28333 lbs/in^3
So the tanks as specified will come in at aprox 125lbs.
To use the chart just change the values and the weights will automatically update. I added a steel tank and mage the wall .06 thick, It would weigh about 115 lbs.
With bungs and weld it will be heavier.
Edit: looking at the drawing once more, the baffle size is not noted and is not considered in this calculation. oops....
Well that’s no fair - Dick got drawings??
steel is approx. 3 times heavier than aluminum (168lbs vs 490 libs per cubic inch). Do you know the weight of the old tanks?
I see online that a rectangular 150 gallon steel tank weighs about 190 pounds, so I am guessing an aluminum tank would weigh about 65 pounds, or to be safe round up to 80 pounds.
to confirm, a 160 gallon tank would be about 48"x27"x27". if the wall is 0.125, that means it is about 830 cubic inches, or about .5 cubic feet of aluminum, so the tank would weigh around 80+ pounds (168x.5). this changes a bit depending on the shape of the tank, with a cube weighing less than a long flat tank.
Are there baffles? Original tanks were baffled and the new should, baffles need not be same guage as the wall.
LEE........ post pictures of the Charleston project............B