Here are two opposite examples I have experienced in two separate boats. First one was early in the boating season still debris on the water. Launched the boat and proceeded to the local party spot. On the way lost speedometer reading thinking just hit something plugging the pitot tube. No problem. After sitting there with friends several hours we all decided to go to another cove several miles away. On the way we got into a bit of competition in some moderate lake chop. Boat ran good, speed normal, but just did not seem right as to the way it took the chop. After several more hours of sitting we all went back to the launch ramp. I really don't remember why I checked but I opened the engine hatch and found that I had a bilge full of water. It was up on the bottom of the engine and the "automatic bilge pump had never come on". Ran it manually for quiet some time before back to normal. Upon exploration found that the speedo hose had come off of the transom assembly and had been filling the bilge all day. That changed that boats center of gravity balance point or whatever you wish to call it.
This was on a single engine 260 Baja.

Now to the second example being in our present boat a 36 Outlaw that I happen to have a real good idea of where the COG is. If running along at a steady state, I have a passenger go from the cockpit area forward into the cabin I can definitely tell the difference in the way the boat feels. It does not matter if it is one of my grandchildren or an adult. It is noticeable to me at the helm. Also the farther forward they go the more pronounced the change is.

Mark

This is exactly why we haven't switched to small blocks, the boats top speed would exceed its capabilities

V-Bottoms:
The most important discussion on COG is with step bottom boats. The step is a pivot point when you are turning. If you do something that affects the COG, you could cause the boat to pick a different pivot point in turns; and this is when you are likely to go swimming. There is no set rule, and this is a long conversation.

Catamarans:
Some concept, but you now are concerned about how the boat packs air, at what speed and at what volume; according to what is needed to keep the boat light, but not TOO light. We know what happens in this case as well.

Doesn't have more to do with the relationship between center of gravity and center of hydrodynamic lift. Unless you build trailers - then you only worry about COG.

If you tried running it with the same trim and tab settings yes, but what about adjusting the attitude of the boat with trim adjustments for the new weight placement?

Trim and tab adjustments move the center of lift. They also scrub efficiency, that's why race boat people worry about weight placement so they can run the boat with the most efficient trim and tab settings versus the conditions.

I would consider center of balance to be the point at which the boat is equally balanced front to rear or say the point by which you could lift the boat up by a single strap without it tipping forward or backward. When I think center of gravity I'm thinking of weight being placed higher or lower within the boat. A good example would be a lifted truck, you haven't necessarily changed the center of balance but you have raised the center of gravity. Most companies seem to go to great lengths to mount heavy items as low as possible to keep the center of gravity low to improve stability.

Some years ago I owned a 1987 12 meter with 900's and number three's , boat ran real flat, was wet all the way to the beak. I decided to pick the boat up with a single strap with a travel lift, til I found C&G, it was right behind the drivers and pass bolster . The boat would balance its self perfectly. I then de rigged the boat, rolled it over on its side, took out 3/4 of inch from the back of the hull and tapered it to C&G just behind the bolsters, basically built I big rocker in the boat took me 4 months of countless hrs. Rerigged the boat, learned a lot . The boat now carried the nose or beak like a champ, porpised at low speeds with out taps, reduced wetted surface by 20 ft. Now the disappointment it only picked up 2 mph, That's when I learned you can't make something heavy fast with out lots of power. My experience with C&G

That is very interesting, do you think now props could have helped? If you have any pics I'd love to see them. How fast was it?

right behind the bolsters, that's what 12-14 feet forward of the transom on a 36 foot boat 12 feet is a third / 33 percent of loa center of balance