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Take a look at the January 1, 1995 and October 1, 1993 issues of Practical Sailor. There are detailed results of propeller tests conducted at M.I.T. The articles are quite technical.
The 1995 article provides detailed graphs of prop drag. The folding and feathering props have perhaps 2 pounds of drag, while a 2 blade fixed prop has 6 pounds of drag at 3 knots, 12 pounds at 5 knots, and 34 pounds at 8 knots. Of course this doesn''t mean a lot, what you want to know ''s what is the speed loss with a fixed prop.
The article doesn''t address that directly, but says that "a computer model for one boat showed that while drag could slow a boat up to about .8 knot, over a wide range of wind speeds and angles the average speed loss is only about 1/3 knot."
By some back of the envelope calculations, it appears that a two bladed fixed prop increases frictional resistance by 20-30%. At 3.5 knots on a boat with a 25 foot waterline, frictional resistance is about 75% of total drag. At 5 knots it''s about 50% (the rest is wave making drag). The biggest impact on speed will thus be in light airs when the boat is moving slowly.
If the Practical Sailor tests are accurate, it appears that, in light air, a folding prop could provide a 10% speed increase. That means getting into port an hour later on a 10 hour 40-50 mile sail. This is more than 60 seconds per mile (albeit in light airs). Even the 1/3 knot average cited in the article would be 6-7% of a 5 knot boat speed, or about 45 seconds per mile.