I'll apologize up front if this sounds too much like a school science class!



Again, to float, you need to weigh less than the water you displace. So, take some object of a given volume, and put it in the water. It will push some water aside as it sinks in. If it weighs less than the water it has pushed aside to make room for itself, it will float. If you increase its weight for the same displacement, it will sink more, keep adding weight and it will sink altogether. As it sinks in, you will be displace ing more, but your reserve is going down. Now, if you add something to the object that increases volume with light weight (like an exterior foam float, tube, cushion, etc.) you increase the amount of water that will be pushed aside, but the material you have added weighs less than the water it is displacing. So, you have increased displacement with little weight, therefore adding to "bouyancy". If the material added was something like, oh say, lead, you would have increased displacement but what you added would weigh more than water, and you reduce bouyancy.



You can set up an experiment to illustrate this with a measuring cup with water in it, and a few small objects like a piece of cork and fishing weights and such. When you put things in the water you will see the water level rise in the cup - an cheesy way to measure displacement.



So, adding something like foam to the inside of the mast does not increase displacement, but does increase weight, because foam weighs more than the air normally inside a mast. This would reduce bouyancy.



Adding a block of foam (or a tube, noodle, mast float, cushion, etc.) to the outside of the mast increases displacement, and adds some weight. But, due to the nature of the material, it weighs significantly less than the water it can displace to make room for itself. You have increased your displacement but added little weight - this would add to bouyancy.



Your other alternative would be to make your mast lighter for its given volume - a fair less practical solution unless you feel like spending for a carbon mast with the same dimensions as your metal one. ;-)



Hope that helps!