Endurance

11-19-2014, 10:35 AM

I have my boat on dry land for some pontoon modifications including the addition of a gas tank to fuel ski boats from the back of my houseboat. I am thinking that this so-called toy tank should hold in the range of 200 to 300 gallons of gas. My only concern is the effect on flotation from dropping that much weight on the back of my boat. My stern now sits a little high so adding the fuel tank will improve things, but I still don't want to go too crazy and make my stern too low.

My fuel tank will sit entirely above water so the fuel, the tank, and the structure that holds it will all be dead weight and pretty much has to sit near the back of my boat so that I can fill and drain the tank conveniently.

In perfect hindsight, what I should have done was invited about 2000 pounds of my closest friends to stand on the back of my boat last summer to measure the effect of weight.

Since I didn't do the smart thing, I am now stuck with what I can do with common sense and a calculator. Here is the math I have done so far: I have figured that to float 2000 pounds, I need to displace about 241 gallons of water. (Water weighs 8.3 pounds per gallon). (2000 pounds / 8.3 pounds per gallon = 241 gallons) Each of those gallons of water will cause me to lose 231 cubic inches of flotation since that's the volume in one gallon. That means that to displace 241 gallons of water will cause my boat to drop enough to lose 55,671 cubic inches of flotation. (241 gallons x 231 cubic inches/gallon = 55,671 cubic inches.

I have two pontoons that are 34 inches wide each, so a cross section of my boat will have 68 inches of flotation side to side. Cubic inches are, of course, L x W x H, and we're solving for height. That means we need length. Length is where things get tricky. One thought is to assume my 60 foot boat is like a giant seesaw and that the back of the boat will be going down and that front of the boat will be going up. If I make that assumption, the math looks like this: 55,671 cubic inches/68 inches wide = 819 square inches. Assuming only half of my boat is going to go down, my length will be 30 feet or 360 inches. 819 square inches/360 inches of length = 2.275 inches, which is what my calculator guesses my stern will drop. If we take the seesaw theory to its logical conclusion, my bow also has to rise 2.275 inches. That means that my stern will appear to drop just over 4.5 inches in relation to the bow. (2.275 inches x 2 = 4.55 inches).

While this all sounds fine on paper, my gut tells me that the bow will not rise exactly as much as the stern drops. The whole boat has to sit lower in the water after all.

I know there are great minds on this board. Does anyone have any thoughts to share that might help me figure this out?

My fuel tank will sit entirely above water so the fuel, the tank, and the structure that holds it will all be dead weight and pretty much has to sit near the back of my boat so that I can fill and drain the tank conveniently.

In perfect hindsight, what I should have done was invited about 2000 pounds of my closest friends to stand on the back of my boat last summer to measure the effect of weight.

Since I didn't do the smart thing, I am now stuck with what I can do with common sense and a calculator. Here is the math I have done so far: I have figured that to float 2000 pounds, I need to displace about 241 gallons of water. (Water weighs 8.3 pounds per gallon). (2000 pounds / 8.3 pounds per gallon = 241 gallons) Each of those gallons of water will cause me to lose 231 cubic inches of flotation since that's the volume in one gallon. That means that to displace 241 gallons of water will cause my boat to drop enough to lose 55,671 cubic inches of flotation. (241 gallons x 231 cubic inches/gallon = 55,671 cubic inches.

I have two pontoons that are 34 inches wide each, so a cross section of my boat will have 68 inches of flotation side to side. Cubic inches are, of course, L x W x H, and we're solving for height. That means we need length. Length is where things get tricky. One thought is to assume my 60 foot boat is like a giant seesaw and that the back of the boat will be going down and that front of the boat will be going up. If I make that assumption, the math looks like this: 55,671 cubic inches/68 inches wide = 819 square inches. Assuming only half of my boat is going to go down, my length will be 30 feet or 360 inches. 819 square inches/360 inches of length = 2.275 inches, which is what my calculator guesses my stern will drop. If we take the seesaw theory to its logical conclusion, my bow also has to rise 2.275 inches. That means that my stern will appear to drop just over 4.5 inches in relation to the bow. (2.275 inches x 2 = 4.55 inches).

While this all sounds fine on paper, my gut tells me that the bow will not rise exactly as much as the stern drops. The whole boat has to sit lower in the water after all.

I know there are great minds on this board. Does anyone have any thoughts to share that might help me figure this out?