|MadSci Network: Engineering|
Howdy, Sorry it's taken me so long to get back to you on this question. I myself have been involved in many arguments on this subject. While I'm sure you've heard both sides of the argument, and opinions either way, hopefully I can give you an answer that will help. Simply stated, whether the prop freewheels or is locked, you are creating drag. When the propeller is allowed to freewheel some of the force propelling the boat is transferred into the rotational motion of the propeller, thus creating drag at the propeller, and unless you can disengage the driveshaft at the aftermost shaft seal, at all the bearings between the propeller and the transmission (and for that matter, at the transmission itself). On the otherhand, if you lock down the propeller, you are creating a vortex behind the propeller caused by the water being forced to follow the blades and continuing in a helical pattern after leaving the blades. Also, because the water will not be flowing past the propeller at an angle of minimum slip, you will be creating a low pressure area aft of the blades, effectively pulling back against the foreward motion of the vessel. Now as for remedies to this situation, there are two. One, you can go with the method used on the earliest steam screw ships. A retractable propeller. Not very easy to deal with, but creates the least drag possible. Or, you can go with a system that's availible at most marine supply stores. A "feathering propeller." There are two types of feathering prop, and I have no experience with either one of them, so you'll need to do some checking up on the technology. The first type is a two bladed type, which supposedly has the least drag under sail... and also the least propulsive force under auxiliary propulsion. This type has blades which fold back against each other when the engine stops, and have a very low drag coefficient. The other type I have seen, are three bladed propellers which actually feather much like an adjustable pitch aircraft propeller. Obviously, you have some drag from the blades being in the water flow, but it is considerably less than a normal propeller in the water. Anyhow, I hope I have answered your question to your satisfaction, and if you have further questions, feel free to email me. John Metcalfe 3rd Mate US Merchant Marine
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