MadSci Network: Engineering

Re: When a yacht is sailing, will it go faster if the propeller can freewheel?

Date: Mon Jul 30 00:04:06 2001
Posted By: John Metcalfe, Staff, Computing and Information Services, Texas A&M University at Galveston
Area of science: Engineering
ID: 993555044.Eg

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 

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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