When the sight gauge topic came up on the GB Forum (I think) a few years ago, one of the members described his sight tube system, which has only one connection to the tank at the bottom. The tube then goes up the side of the tank and several inches above it, where it has a cap that is vented to the atmosphere. There is no upper connection to the tank itself.
The advantage of this system, according to the fellow who has it, is that it needs only one valve at the bottom, which is one less hole cut in the tank. Don't know how significant an advantage this really is, but it was an interesting alternative to the normal sight gauge system.
Our boat had new tanks when we bought it in 1998.* All four saddle tanks are exactly the same size and shape, and the previous owner must have filled one of them five gallons at a time and marked each level on the tank beside the sight gauge and then put identical markings at the same level on the other tanks.* Must have taken awhile but I'm glad he went to the trouble.
-- Edited by Marin at 01:22, 2008-01-31