View Full Version : How to figure out how much my boat will drop with added weight?

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?

11-19-2014, 11:43 AM
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?

11-19-2014, 11:53 AM
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.

11-19-2014, 01:51 PM
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.

11-20-2014, 06:12 AM
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.

11-20-2014, 08:34 AM
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.

11-21-2014, 06:03 AM
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.

11-24-2014, 09:09 PM
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.

11-25-2014, 09:49 AM
I though I'd get good advice here and you came through. Thank you for the great info.

11-25-2014, 10:21 AM
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.

11-25-2014, 01:33 PM
Glad I could be of service.

IN my many years with the Coast Guard I saw a lot of boats that had a list (leans to one side) at rest. It was a simple matter that the builder didn't do their homework, what designers call weight and moment calculations. For a boat to sit level you have to have the same amount of weight on both sides. This means actually weighing the stuff you are planning to put on the boat (or getting the weight from the maker) and then adding it all up and seeing if it all comes out even. If it doesn't you have to move things around until it does. Some builders simply don't do this so at launch the boat leans. This may not be dangerous (it can be) and if it's a planing hull the boat may level out at speed if the heel is minor like 1/2 or 1 degree. Adding weight may correct the heel but it causes all kinds of other problems. Number 1. The boat weighs more, therefore displaces more, takes more power to go the same speed and uses more fuel. 2. even more problematic, it changes the stability of the boat. A boat that may have been very stable may now not be stable at large angles of heel, like when you take a huge wave from the side and lean so far over your passenger start falling out of their seats. Adding weight is not a good solution. removing weight can help but it might make the boat more tender. The best solution is to move stuff around until the boat sits level. This can be very hard to do becasue it means moving equipment, sometimes structure, and literally requires redesigning the boat. So the builder takes the asy way out and adds weight.

Of course there may be another problem. When the boat was launched it may not have sat "on it's lines" meaning it didn't float at the designed waterline and actually needed extra weight to bring it down to the waterline. Or it could have been down at the bow or stern and need weight to even it fore and at. All of these are indicators that the designer didn't do the math.

If it truly bothers you and you don't feel it is safe, then take out the weight and start moving heavy stuff around, Batteries, generators, appliances, etc. Especially stuff that is up high, like hot tubs. Why would any one put a hot tub full of several thousand pounds of water on the upper deck?? Beats me but some houseboat builders do that. On the other hand if it doesn't bother you and the boat doesn't have any quirks (like slow, long rolls, or snappy rolls that knock your teeth out) then leave it alone. Also check that you aren't storing stuff up higher that could be down low. I once looked at a cruiser that the owner complained about, that seemed very tender. He had 21 cases of soda and beer on the flying bridge. When we moved all that weight down low the boat was fine.

11-25-2014, 03:31 PM
As I reviewed my figures, it looks like I stopped one step too soon. If I was pushing the back of the boat down evenly so that I was displacing water in a rectangle 68 inches wide, 360 inches long, and 2.275 inches high, I would be done at the point where I came up with a height of 2.275 inches. But the area in which I am displacing water is not a rectangle. It is a triangle that narrows to zero at the center of buoyancy (or the middle of our seesaw). That means that the back of my triangle has to be twice as high as a rectangle that would have the same area, like this:


So my stern drops 4.55 inches, not 2.275 inches. If my stern drops 4.55 inches and my bow rises the same, my stern drop relative to my bow will be just over 9 inches. Even deducting the 3 or 4 inches that I'm bow high now, that means I will end up 5 or 6 inches bow high with a full fuel tank.

Does this refinement to my figures sound reasonable?

11-26-2014, 05:38 AM
Do what my neighbor did and buy a couple of blow up swimming pools. Put them on the back deck and fill them with water and see what happens. Unless you know the exact loading of your boat as it exists today, all the math in the world will not give you a definitive answer. My gut feeling says you will even out your boat and make it sit level like it supposed to.

11-26-2014, 08:15 AM
Good advice. When I was working in shipbuilding we use to perform what is known as an inclining experiment. That is, put a known weight at a known distance from the centerline (for heel) or from the CB (for fore and aft) and then measure the amount of heel or trim. So water is a good alternative because you know how much a gallon weighs. Mark the water line before hand both at the bow and stern. Masking or duct tape works fine for this. Then put the weight on board at a known distance aft of what ever you are using for a baseline, or better yet, at the point where the tank will be. Then measure the difference at the water line. Then do your calcs over using the data.