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

[Ike]

Glad I could be of service.

JT
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.

[Endurance]

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?

[easttnboater]

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.

[Ike]

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.