"Ronald Raygun" <no.spam@[EMAIL PROTECTED]
> wrote in
message
news:H%n8k.15420$E41.14045@[EMAIL PROTECTED]
> Duncan Heenan wrote:
>
> > I usually sail with the prop locked (by putting it in
reverse gear - fixed
> > 2 bladed low tech prop on a sail drive). This avoids the
annoying noise of
> > the gearbox turning over and is something that a lot of
sailors do.
> > However, the handbook for the Volvo MD 2030, says to
leave it in neutral
> > when sailing, and several people have told me that I am
significantly
> > increasing drag by locking the prop. What do the URS
gurus recommend
> > (Please don't recommend a change of prop, as I want to
stay on topic.)
>
> There are 3 issues here.
>
> One is what is best for the engine. If the manual says to
let it spin,
> it must do so for a reason, though it would be interesting
to know what
> the reason is. Do they say why?
>
> Another is the noise. If it bothers you, lock it.
>
> Finally there's the speed thing. Indications are that it
makes so little
> difference that folk aren't sure whether locked or
unlocked drag is bigger.
>
> I expect Ian will know the answer. My intuition tells me
that locked drag
> is bigger, because although some power is being extracted
by the prop from
> the water flowing past, and expended in overcoming the odd
bit of friction
> here and there, and in stirring (and probably slightly
heating) the gearbox
> oil, it's so little as to be negligible compared with the
effect that
> presumably the overall force "seen" by the water ru****ng
past a fixed blade
> is bigger than with a spinning blade.
>
> Things might be different if one were seeking to extract a
serious amount
> of power from the prop. One might want to decouple it
from the engine,
> and rig up some fancy gearing to drive a reverse windmill
to help the
> boat sail faster...
>
>
Google found this which seems to make some sense and agrees
broadly with what others have said
http://books.google.co.uk/books?id=VLg6Lx5yRP0C&pg=RA1-PA220&lpg=RA1-PA220&dq=locked+propeller+drag&source=web&ots=jwixLiA42g&sig=-9Dn2K_kytoDWD3M7RFJniPeP5c&hl=en&sa=X&oi=book_result&resnum=1&ct=result
Practically I would think that differences in performance
that can't be measured it isn't im****tant, though I suspect
that a competitive, high performance, well sailed, fully
instrumented, polished racing yacht would have a better
chance of detecting differences (in the unlikely event that
the owner could be persuaded to fit a fixed cruising prop
for a trial of course). Performance may not be just about
boat speed, Performance % and VMG meters might reveal a
different perspective to speed alone.
Personally I lock my 3 blade on my N26 long keel in the
possibly erroneous belief that it will save wear and noise.
ChrisR
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