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Thread: How to figure out how much my boat will drop with added weight?

  1. #1
    Senior Member Endurance's Avatar
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    How to figure out how much my boat will drop with added weight?

    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?
    Last edited by Endurance; 11-19-2014 at 10:42 AM.

  2. #2
    Senior Member easttnboater's Avatar
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    Assuming that your math is correct and the drop will be 2.275 in, then the corresponding rise at the bow is going to depend on the actual pivot point of your boat. How bow down are you now in inches?

  3. #3
    Senior Member Endurance's Avatar
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    I am about 3 or 4 inches bow down right now, so I'm guessing my pivot point is going to be just ahead of the center. This would be a lot easier if Ii knew that a boat really did act like a giant seesaw. I am thinking it is kind of like that, but I am hoping to find out more. Some real world knowledge from anyone who has added or subtracted a bunch of weight to the front or rear of their boat will help.

  4. #4
    Senior Member 42gibson's Avatar
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    if I were going to make that big of a addition to the boat I would get in touch with the builder if their still in business for their input.
    44 gibson executive
    on the muskingum river & ohio river
    marietta,ohio

  5. #5
    Senior Member easttnboater's Avatar
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    I added about 1,300 lbs in cap block to make my boat sit even left to right (port to starboard if you want to be nautical). It was all added in the front 1/3rd of the hull on the left side. I was about an inch off. The 1,300 lbs evened it out - so 1/2 in up on the right side and 1/2 down on the left.

    Assuming your hull will handle the stress, it seems like what you are doing will more or less even your boat out front to back.

  6. #6
    Senior Member Bamby's Avatar
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    Before you invest any money into your floating fuel island be very sure you can legally do what you're proposing. If you were proposing a few fuel cans in a proper locker I'm thinking there would be no problem but a 200 to 300 gal. fuel delivery system opens up another ball of worms. I do know there are DOT restrictions and limitations for on the road transportation of gasoline and somewhere I'm figuring there most likely is one for boats also.
    Respect Our Recreational Resources
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    Boating on the Muskingum River
    1972 35' Crest Pontoon Houseboat
    2007 90 hp. Yamaha

  7. #7
    Senior Member easttnboater's Avatar
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    Here is a real world example that might help. A couple of years ago, the guy in the slip beside me in TN had a leak around the transom of his early '70s steel hulled Stardust. It is a 12 x 42 or so boat. He determined that it was at the top of the outdrive and that he needed to lift the back of the boat about four inches. He did not want to pay to have it pulled out. He first tried some airbags under the stern. He put them under the boat and filled them with air. It raised the boat maybe an inch. The next weekend, he had bought a couple of the inflatable swimming pools and put them on his front deck. When he filled them with water, it pushed the bow of the boat down probably eight inches and lifted the stern about the same amount. He did whatever he needed to do and sealed the leak.

    So, your boat should act like a seesaw.

  8. #8
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    I don't think you want a lecture in naval architecture. To keep it simple, one of the parameters naval architects use is weight per inch of immersion, that is the amount of weight it takes to sink the boat 1 inch. This varies depending on the size of the boat. That's what easttnboater did. He determined how much weight it took to sink the boat a certain number of inches.

    Anyway, yes your boat acts like a seesaw. It rotates around the center of buoyancy. The center of buoyancy is the point which all the upward forces (the buoyancy of the water) act. This point has to remain directly below the center of gravity. So if you move weight forward (moving the center of gravity forward) the center of buoyancy moves forward to stay under the CG. So the bow goes down and the stern goes up. How much is determined by the amount of weight moved or added, the distance it is moved and various other factors. The same thing happens if you put weight farther aft. A similar thing happens when you move weight to one side or the other. You are definitely on the right track. The only thing I would point out is that the CB is usually used as the point of rotation. But the math still works if you use an arbitrary point such as the mid length (30 feet) and make all measurements from there. Essentially what you are calculating is the moment arm around this point. I haven't checked your math but at a glance it looks good.

    Your gut feeling is right. One of the most important factors in determining how much the bow or stern goes up or down is the underwater shape of the hull. Hulls are not perfectly asymmetrical unless it is a canoe shape. Usually there is a lot more volume at the stern than at the bow. This means it takes more weight to immerse the stern than the bow, or using the same weight, it has to be moved farther aft to sink the stern the same amount as the bow. But give it a try and measure the results. If you need help with this, call the builder. They may have already done all the necessary calculations. Or you may have to hire a naval architect but they don't come cheap. However a good marine surveyor may be able to do these calculations.
    Last edited by Ike; 11-24-2014 at 09:14 PM.

  9. #9
    Senior Member Endurance's Avatar
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    I though I'd get good advice here and you came through. Thank you for the great info.

  10. #10
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    My Marinette is somewhat leaning to one side as well. It appears that was always the case with this boat. I have found concrete blocks, Steel slabs, and a portion of railroad track in the port side of the hull to even it out.

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