The spray rails accomplish two things...the most obvious is that they do a very nice job of keeping the spray off the glass, especially the windshield. This is most important when boating in cold temperatures when there's a cross wind and lots of chop. When the cold sea water drenches the windshield, the glass immediately fogs.
The rails also contribute to better handling, especially in large following seas. They keep the bow from burrowing as deeply into the back of the wave in front as it did before the spray rails were added and therefore reduce the tendency the boat had to swing sideways when the wave behind grabs the stern. (See the write up on rolling chocks.) The extra lift generated (on each side of the front of the boat) by the rails also adds to general stability.
Please take a look at the PDF document by clicking here.
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