On Oct 16, 4:29 am, "Dennis Pogson"
wrote:
Frogwatch wrote:
Anybody have experience with these? I am about to replace my fixed 3
bladed prop on my 8.5M S2. This looks like a potentially good
candidate replacement because they have already figured out which prop
works for a given boat and because they say they can put a larger prop
on with their "Extendo" system.
I am a little worried about the plastic of this prop not being able to
take my aggressive barnacle removal method. Is it easy enough to
remove that re-painting it every so-often works?
My old bronze prop has succumbed to corrosion and many impacts with
oyster beds and with a Laser mast in the water.

Now, one of the advantages of a 2 blade prop is that you can align it
with the keel for lower drag. How does one do this? Do you simply
mark the prop shaft inside the boat and then rotate it till your mark
is in the right place? This really seems like a PITA considering the
gawdawful engine installation on most boats. Even on my boat it
requires removing an engine panel and peering down into a dark space.
Is there any other system? Would the prop left in neutral auto-rotate
to the lowest drag position (I have never had a two blade prop so I
dont know) and stay?
Could I attach a magnet to the shaft so that it engages a magnetic
reed switch (non-contact) to tell me when it is in correct position?
Can you get it in correct position by "bumping" the start switch?

Ever heard of a thing called paint? You can even mark a prop shaft with it.
I won't charge for this piece of wisdom as I learned it about fifty years
ago. (and the piant is still there!) You could easily fit a small glass
spyhole in the cockpit floor. However, some sailors would argue that
allowing the shaft to rotate in neutral causes less drag than locking it in
the upright position, particularly on a modern fin/skeg design.

Dennis.

Dennis.

My primary motivations for being interested in CDI is that I hear so
many stories of ppl who get new props and they just are not right and
thye just live with it due to the cost of hauling to change them. CDI
seems to have gotten it right and their props are supposed to be easy
to change if not. They can be changed in the water .
Another reason for considering a two blade instead of simply replacing
my existing 3 blade is that I have been in a few races lately on
other boats and I might want to do club races in mine.

On Tue, 16 Oct 2007 08:07:09 -0700, Frogwatch
wrote:

I cannot believe that most sailors
take off their engine cover to peer down at the prop shaft whenever
they sail to align the prop with the keel.

I figure most sailors fart it off, and live with whatever drag it has.

Casady

On Oct 16, 5:07 am, Frogwatch wrote:
... tsmwebb points out that the flow is rarely directly fore and aft and I
admit this had never occurred to me, however, is he correct? If he is
correct, I would not expect a 3 bladed prop to cause much loss of
sailing speed. ...

Well, I think it's true. You can approximate the lateral
deflection of the keel wash by multiplying the distance from the
trailing edge of the keel to the prop by the tangent of you leeway
angle. My guess is that on your boat a degree of two of leeway angle
will send the wash wide of your two blader if it is locked up and
down. However, there is more to it than that. Your boat also has a
somewhat modern keel. The "standard" section on fin keels when your
S2 was designed was the NACA 63xxA. At small leeway angles this
section is very low drag. As a result, it has a very narrow and
relatively short wake. My recollection of the S2 8.5 is that the keel
is a couple of cord lengths ahead of the prop. At the speeds you are
likely to be going and if your keel is in good condition there really
shouldn't be a significant amount of keel wash left when it reaches
the prop even in the zero leeway angle case. So, I don't think your
prop will be in the keel wash in most cases regardless of leeway.
That's the bad news, because the speed of the water relative to the
prop in the keel wash is less than the free stream speed and drag is
primarily a function of that speed. But, that' still not the whole
story. The drag of the prop is proportional to its effective area
times the square of the speed of the water flowing over it.
Generally, two bladed props have less area than three blade props so
they have less drag. There are other complicating factors, but I
think it's a good bet that the two blade prop will be less draggy
under sail than the three blade one but it will not be as good as a
folding prop or a prop that is hidden behind the keel.

-- Tom.

On 2007-10-16 11:15:38 -0400, Frogwatch said:

Another reason for considering a two blade instead of simply replacing
my existing 3 blade is that I have been in a few races lately on other
boats and I might want to do club races in mine.

The 2-blade will probably be considerably less drag as I recall the CDI
blades being fairly high-aspect compared to the older 3-blade props.
They can afford to be because of the improved foil shape.

If you club race, you'll get a few seconds allowance for NOT having a
folding/feathering prop, but more important, in normal sailing, you'll
find yourself being able to sail in lighter air than you're used to.

We went from 2 blades to feathering 3-blade (to get rid of a harmonic
vibration) and move in amazingly light air. The only justification I
could give for getting that expen$ive prop was to amortize it over the
number of years we expected to have the boat, but it's proven to be
worth the investment -- to me.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/