I just spent two days last week attending the fifteenth Chesapeake Sailing Yacht Symposium (CSYS). There was a room full of some of the world''s best yacht designers and researchers. There was a lot of information flowing. I tried to get some answers to some of the common issues that have been discussed on various BB''s either from the lectures or in discussions with presenters and yacht design professionals.
One of the papers dealt with a simplified VPP (velocity prediction program) written on Excel and designed as tool for quick analysis of varying designs. In the follow up discussion members of the audience questioned the use of the particular coefficients of drag for propellers as presented in recent testing results. Practical Sailor had published one such study performed at MIT that suggested that a free wheeling propeller had less drag than a locked propeller. These results were roundly questioned by design professionals who had worked in the field of propeller design. After this discussion I had the opportunity to discuss this issue with a Professor of Mechanical Engineering teaching Naval Architecture and whose students had actually performed the same type of tests as the MIT students. His results are very different than those presented in the MIT study. Here''s what I came away with in this conversation. (I also had the chance to discuss this with a number of other people in the field this includes inf from the general trends of this discussion.)
4 or more bladed, steeply pitched, propellers such as used on big ships, typically have less drag when permitted to free wheel than when they are locked up. This occurs because of the small incident angle of the water flow on the blades (therefore they are not stalled out) and the comparatively small amount of rotational friction of the propeller shaft and bearing when compared to the large amount of drive generated by the blades.
In the case of Sailboat propellers, the pitch is quite flat and so the blades are generally in a partially stalled condition when they are allowed to freewheel. This is even more pronounced when friction is applied to the shaft and the blades are turning slower than the flow of water passing over them. In this circumstance the propeller produces a ball of turbulent water. This ball of water generally has a greater drag than a locked prop. This professor explained the MIT results by saying that the MIT study actually used a propeller that was powered at equal speed as the water to replicate a freewheeling state and so had a very low drag. In his study they allowed the propeller to actually free wheel and then applied increased friction to the shaft measuring the resultant drag. His study concluded that there was more drag in the freewheeling prop than the fixed one. He went on to add that the differences between locked and freewheeling were much smaller in a three-bladed prop than a two-bladed one.
I also had confirmed in later conversation that prop position was found to have a real effect. A two-bladed prop locked in the vertical position and a three-bladed locked with one blade vertical in the down position had substantially less drag than the same props in other points of rotation.
We discussed the idea of using the freewheeling prop to generate electricity. He indicted that in terms of drag this was the worst condition because the partially constrained shaft would produce the greatest drag. Apparently, the propellers designed for water driven generators are specifically designed to have a minimal drag using blades with a very large pitch.
Conclusion: You give up a fair amount of speed if you permit your prop to freewheel. You loose more speed free spinning a two-bladed prop than a three bladed prop but loose the most speed with a partially constrained three-bladed prop.