[Public-list] Propellers

[George Dinwiddie]

Jim,

Maybe it depends on the prop design, but I've had people who I took to 
be knowledgeable tell me that a free-wheeling prop has a drag comparable 
to a disk of the same diameter.  They told me this was based on 
empirical measurements.

jim mennucci wrote:
> 
> 
> A free-wheeling prop has more drag compared to what?  Compared to a lock 
> propeller hiding in the deadwood--maybe.  Compared to a locked propeller 
> exposed to the normal inlet flow--no way.  A locked propeller is a lot 
> like a sea anchor--its a flat plate with flow normal to it.  Its twisted 
> at the pitch angle so its really a sine or cosine function but still its 
> what they call pressure drag which is a function of boat speed squared 
> and projected area--its really the local velocity of the inflow but for 
> this exercise lets just use boat speed.   If your racing and your props 
> locked while someone else has a folding prop then you at a big 
> disadvantage.   Allowing a prop to windmill actually solves the pressure 
> drag problem.   The prop rotates but it doesn't generate any lift, and 
> if there's no lift there no drag or torque.  There's friction in the 
> transmission which retards the free-wheeling rotation which does 
> generate some lift and drag but this can be pretty small.  So saying 
> that there's more drag when windmilling has to have caveats.

-- 
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   When I remember bygone days                         George Dinwiddie
   I think how evening follows morn;            gdinwiddie at alberg30.org
   So many I loved were not yet dead,           http://www.Alberg30.org
   So many I love were not yet born.
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