I decided that I should try and answer my own question so yesterday I conducted some experiments. Before I describe the experiment and the results I think that I should reiterate that I have not suffered from a great deal of water getting into my boats lockers but have to admit that the worst conditions I have sailed in are a Force 5 and short Solent chop. My experience has been that the BayRaider is a very dry boat especially when sailed upright.
My experiments were conducted with the boat on her trailer and she was level. I was interested in Grahams washback and having looked at the locker wondered if the locker lid was dividing the water flow as it flowed down the end grooves and directing some of the water along the inboard lower edge and into the locker.
I paced a video camera and some lights inside the locker and used a large bucket of water as my simulated wave. I threw the water down and along the locker lid with my aim point being the middle of the lid.
The video shows that the lowest corners of the closed lid impede the water while it is draining away. Some of the water attempts to drain down the inside edge of the ends of the lid and then meets the lower edge of the lid and is forced around the corner. At this point the lip of the locker is only approx 2c.m. high and the water spurts into the locker over the lip. Mean while a pool of water is formed on top of the lid. This water quickly fills the drain reservoir and then shoots under the lid into the locker.
I conducted a second test with the locked wedged open a c.m. or so. This allowed a greater flow rate down the side grooves and reduced the amount of water entering the locker from either the lower corner or the top edge.
Water gets into the locker when it is present in quantities that allow it to flow over the upper and lower lips. The use of seals will restrict how much water gets in but I would recommend that lower edges of the locker lid should be shaped to allow water to escape at a greater rate. I also think that the lower lip of the locker should be raised in the lower corners to prevent any water that does get around the corner from entering the locker.
For the heavy weather sailors some additional form of plumbing to help carry the water away could be employed. Either from the top reservoir down through the locker and out through a skin fitting or scuppers running from the locker top through the air tank to the sea.
The seals discussed in this thread will help but if the flow of the draining water is not improved then the water may still find its way in. Also a completely sealed locker will make it difficult to right the boat in the very unlikely event of a capsize.
At the end of the day how far do you want to go, or how many buckets of water per minute to you want to drain away?
Note: The greater the angle of heel the slower the water will flow away.
The locker cam video may be viewed here:
http://www.youtube.com/watch?v=Utx5lk57TU8best wishes
Simon