[Public-List] Prop while sailing, locked or free to spin ?

Sat Apr 23 12:42:24 PDT 2016

Gord

Jim Mennucci here...go to dip my oar again

To the windmilling vs locked propeller drag issue--which case is higher or
lower?  There are arguments made by many writers that show,
amazingly, opposite conclusions--so who's right?    My experience is the
windmilling prop is the lower drag case.  I worked on a windmilling case
with a number of MIT hydro-acousticians (Jake Kerwin protégés--who happens
to be Mr. Propeller by the way) and they showed that a windmilling prop
has  lower drag compared to the locked shaft case.  So Gord while you may
believe your sources, my sources say its the other way.

Go to: eprints.cdlr.strath.ac.uk/5670/  for a rigorous discussion on just
that subject.  They have test data to substantiate their conclusion and
further developed reasonable analytical prediction models that the test
data corroborated.

 Everyone should know: propeller design and operation is an arcane science

On Fri, Apr 22, 2016 at 12:03 PM, Gordon Laco via Public-List <
public-list at lists.alberg30.org> wrote:

> But of course it should be kept in mind that a windmilling prop generates
> drag which is calculated as a function of the swept circle... Whereas a
> locked prop generates drag calculated as a function of the blade area
> alone.
> A locked prop is significatly better with regard to sailing performance...
>
> Gord #426 Surprise (hard at it preparing PICTON CASTLE for sea here in
> Lunenburg)
>
>
> On 2016-04-22, 10:54 AM, "Wes Gardner via Public-List"
> <public-list at lists.alberg30.org> wrote:
>
> > I just heard from Chris, for my motor/trans, he says I can let it turn...
> >
> > Wes
> > Griffin's Faith #196
> >
> > On Fri, Apr 22, 2016 at 10:30 AM, Wes Gardner <wesgardner1952 at gmail.com>
> > wrote:
> >
> >> From what I can tell, some transmissions don't like being turned while
> the
> >> motor is not running and some don't care.  Mine, from what I can tell
> from
> >> Chris Oliver is a "splash type" trans.  I will hazzard a guess that mine
> >> wouldn't care (as long as the fluid is at the correct level).  Chris
> >> commented on this while we were reinstalling my motor - he says that
> these
> >> types of transmissions are fairly sensitive to both "too high" and "too
> >> low"  fluid levels...I have a Vetus 3.10.
> >>
> >> Hopefully others will have more...
> >>
> >> Wes
> >> Griffin's Faith #196
> >>
> >> On Fri, Apr 22, 2016 at 10:23 AM, Jonathan Bresler via Public-List <
> >> public-list at lists.alberg30.org> wrote:
> >>
> >>> Folks,
> >>>
> >>> Have heard from list members to lock the prop while while.  Several of
> our
> >>> racing members do so by putting the A-4 into gear after rotating the
> shaft
> >>> to "hide" the two blade prop behind the deadwood.   Some folks use a
> >>> vise-grip on the shaft with the handle against the hull to stop the
> shaft
> >>> and prop from spinning.
> >>>
> >>> While sailing last night, I came up with a slightl different way to
> lock
> >>> the prop.  Wrapped the rubber strap from a rubber strap oil filter
> wrench
> >>> around the shaft till the handle was held against the hull.   Worked
> >>> great.  Getting it off, required a good steady pull on the handle to
> spin
> >>> the shaft backwards a few rotations.  Had picked up a pair of these
> strap
> >>> wrench at Advance Auto in their discount bin for $5.00, I seem to
> >>> remember.  Useful for removing the oil filter, as well.
> >>>
> >>> So what do you do ?
> >>> Lock the shaft or not?
> >>> If you lock the shaft, how ?
> >>>
> >>> Sample:
> >>>
> >>>
> https://www.buysend.com/en-US/Product/Details/20985?U=2pc-silverline-500mm-6
> >>>
> 00mm-mechanics-plumber-rubber-strap-wrenches/&gclid=Cj0KEQjwl-e4BRCwqeWkv8TW
> >>> qOoBEiQAMocbP9E1Gluu36O8RLAr6XhNvZTNK4zs6HU8NoPloic7aY8aAs0c8P8HAQ
> >>>
> >>> --
> >>> Jonathan M Bresler
> >>> S/V Constance Alberg 30 #262
> >>> Annapolis/Eastport MD
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> >>
> >>
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