[Public-list] Propellers

[jim mennucci]

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.

While I'm covering stuff that goes beyond the average layman understanding 
let me comment on Mike Lehman comment a couple emails ago that halfing the 
boat speed halfs the load on motor.  Halfing boat speed actually reduces the 
laod by a factor 4.  Its boat speed squared  as I mentioned above which is 
not linear as Mike inadvertantly advised.


Jim Mennucci
Emerald City


----- Original Message ----- 
From: "Gordon Laco" <mainstay at csolve.net>
To: "Alberg 30 Public List -- open to all" <public-list at alberg30.org>
Sent: Tuesday, July 12, 2005 11:11 AM
Subject: Re: [Public-list] Propellers


A free-wheeling creates more drag too.

Gord #426





> You should not have you engine in neutral when sailing and the prop should
> not be turning. Without the engine running, there is not adequate oil
> circulation and you could damage your transimission if you the the prop
> 'free wheel'.
>
>
>
> Mike Lehman
> ~~~_/)_/)~~_/)~~~
>
>
>
>
> ----Original Message Follows----
> From: Rob Alley <1ralley at comcast.net>
> Reply-To: Alberg 30 Public List -- open to all <public-list at alberg30.org>
> To: Alberg 30 Public List -- open to all <public-list at alberg30.org>
> Subject: Re: [Public-list] Propellers
> Date: Fri, 08 Jul 2005 16:41:45 -0400
>
> Three questions:
>
> 1.  Who makes the machinery prop, please?
> 2.   Do you know--how many RPMs did you get with your old prop in neutral
> when sailing? 3.   How many RPMs does the new prop turn when sailing in
> neutral at the same speed?
>
> Thanks
>
> Rob
>
> Dudley Baker wrote:
>
>> All
>> Much has been written on proper propellers, but here is my additional  2
>> cents:
>> - Last summer, (first season for me with the boat), speed and power were
>> awful with existing prop
>> --- I have a Universal MX-25, 2:1 (23 hp).  Prop was a 3-bladed 13 X 9
>> Sailor.
>> -  In the end, I decided to change out the prop for a 3-bladed 13 X 12
>> Machinery.    What a
>> difference!!.  At a very comfortable 1200 rpms, I am moving at 5 knots.
>> Max speed is over 6 kts @
>> 2800 rpm
>> - If you want performance go to the machinery "mickey mouse ears" shape!!
>>
>> Dudley Baker
>> 297 Heron
>>
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