This subject comes up frequently, someone wants to use a product that won't rot when rebuilding a boat, and those product are available, at a price, and sometimes special ordering. There are cores that will add strength like plywood does and not rot, these tend to cost a great deal and depending on where you live are hard to get, but they do what they say they do. Then there are cores that won't rot, but don't add any strength, these don't cost quite as much, but you can have the same problem buying them. With these cores (foam is a good example) you need to increase the laminate thickness to carry the load the core can't, so more glass is used.
Wood will rot over time if it gets wet, but is low in cost, strong, easy to work with, and available in just about every small town in the country. One problem that happens in production boats is that since wood is so strong they use less glass, sometimes not even covering the wood, so it gets wet and rots. If it had been covered properly it wouldn't have gotten wet and rotted away, or at least not as quickly.
The easiest and most cost effective method is to use wood and then count on it rotting away, this will put it in the same catagory as a foam core, it's a nice shape, but offers no strength, so the laminate needs to be thicker. with the thicker laminate it makes no difference if the wood rots, plus, there is much less of a chance that it will rot because it will be protected by a thicker laminate. Now you have the best of both worlds, just don't put a bunch of unsealed screws holes into it.
I like hollow stringers, but it's more of a hassle to make them.
