Flemming Torp wrote:
"Roger Long" skrev i en meddelelse
...

OK, here is the answer you are looking for.

Unless you have a highly unusual powertrain set up and
strangely pitched prop, determine the position in which
the most blade area is shadowed by keel and hull. Mark
the shaft inside. Stop the shaft in that position. Sail
the boat. It's very unlikely you'll go faster doing
anything else.

Now we are getting close to 'basics' Roger ... but, but ...
as I wrote in the introduction, it has so far been very
difficult to get hard evidence from the log when trying to
let the propeller run and have it locked, as the speed of
the boat is a function of so many things, and I'm convinced
that there is not a big difference - so may be my question
is of a more theoretical type, as reliable data are hard to
get in the real world ... In a bassin, it might be easier
... I have seen som reports, where different kinds of
propellers - folding with two blades, folding with three
blades, fixed with three baldes etc. were compared ... and
the result indicated differences in 'thrust' and in speed
up to between ½ - 1 in worst case ... But I have not seen
any reports on the comparison between a locked and a free
wheeling propeller ... but I have certainly got a lot of
input ... also from the aviation world, that I know nothing
about ... thank you.

But your final proposal is very logical, operational and
easy to implement ... when the water gets warmer, we might
do what you have recommended ... or do as Larry - use the
energy coming from the rotating propeller - og invest in a
folding propeller ... time will show, and thank you so much
for your keen interest in learning me some physics ...



Roger's advice is correct, but it does sidestep your original
question. Locking a prop in the "shadow of the hull" is generally a
winner. Even with a 3-blade prop you should have one blade lined up
with the hull. With a two blade prop the gain can easily be enough to
win a race, or come in before dark on long passage.

I've heard of two studies that addressed this. One, a publicized MIT
study showed that free-wheeling had less drag. However, this did not
directly model yacht props so it really doesn't apply. (In fact,
their prop wasn't actually free, it was powered at a speed that
minimized turbulence.) Other study, which I've heard of third hand,
"proved" that locked was less drag for yacht props. Dave Gerr, in his
"propeller Handbook," says rotating is less drag (unless you can lock
it behind the keel) but he doesn't give a reason or cite a reference.

Consider two cases: first take a hypothetical "flat prop" with zero
pitch. Obvious, it won't spin and it will have a lot of turbulence,
indicating a lot of drag. Now give it a bit a pitch. It will start
to spin, but the small pitch will mean that it has to spin very fast
fast to match the boat speed. Since it can't, there will still be a
huge amount of turbulence. Will it be less than the flat blade? Hard
to say, because there are a variety of factors - the shape of the
turbulence is different and the rotation means that a larger volume of
water is disturbed. This approximates the yacht prop where the free
rotation is very unlikely to match the boat speed and the turbulence
will be high and quite complex.

Now consider a feathering prop, where the blades are lined up with the
flow and cause no turbulence. Give them a some pitch - as long as the
flow is smooth there will be little drag. Increase the pitch
(actually a smaller number since feathering the pitch is infinite) to
the fastest spin with low turbulence. The drag will still be low at
this point. Now lock it - the turbulence (and the drag) will jump
up. Here's a case where the free rotating prop will have less drag
than the locked one, but it probably does not resemble a typical yacht
prop. It does come close to large ship props, so its not surprising
that there are some studies and anecdotal evidence from other forms of
props, that say free wheeling is less drag.

My point is (as I mentioned in my first post) that its impossible to
determine intuitively the drag for the case of a yacht prop, where the
turbulence is high whether locked or not, and analogies from other
type of props simply don't apply. The issue of the heat buildup in
the transmission is a "red herring." It is a significant side issue,
since yacht transmissions do have a lot of friction, and thus heat
problems, when free wheeling, but the dominant factor is the
turbulence. This approach tries to measure the energy extracted from
the water by the work done by the shaft, but this is only valid if the
turbulence is low and the prop is working in a relatively efficient
mode. When the turbulence is high, most of the energy goes into
heating the water, and this is rather difficult to measure.