backs like a pig
 

"Roger Long" wrote in message
...
"dbraun" wrote

You may want to know that there is a
prop shop called Bay Propeller (510-337-9122) that has a technique for
modifying fixed props. They tap the blade tips over to create little
tiplets. They claim it eliminates prop walk AND increases thrust. I have
no personal experience with their results, but it would be worth a try.

This is a very cool idea and there are some sound physics behind it. I
doubt that it completely eliminates prop walk but it might will reduce it
to the point that it isn't noticable. Prop walk has a lot to do with the
vortexes thrown off the tip if any airfile and tiplets reduce them.
That's why you see them cropping up on the wings of most new aircraft.

I'm not endorsing or claiming that this does work, just that the rational
is solid enough that I wouldn't reject it out of hand.

--

Roger Long

It sounds interesting, that's for sure.

--
"j" ganz @@
www.sailnow.com

backs like a pig
 

On Mon, 15 May 2006 14:43:40 -0700, "Capt. JG"
wrote:

Do you think there would be a significant difference between a 3-blade
feathering prop and a 2-blade folding prop? I guess my question is why would
you prefer that prop?

FYI, most of the sailing is on the SF bay, so lots of wind, strong currents,
etc.

It's hard to say without actually trying the swap. You might be able
to find a diver who can swap the props underwater at a reasonable
price.

Is there racing involved? If yes, I would choose the folding prop
regardless. In order to get the prop to fold reliably it is necessary
to mark the shaft when the hinge pin is vertical, and manually align
the shaft to that point every time you shut down the engine before
racing. Otherwise the lower blade will flop downward and defeat the
purpose of drag reduction.

backs like a pig
 

On Fri, 12 May 2006 10:30:00 -0700, "Capt. JG"
wrote:

Back to more questions somewhat related to the Perkins 4-108. This sailboat
is a pig to back up. It really wants to prop walk to starboard. According to
the owner, it's got a fixed, 3-blade prop. I believe the boat also has a
folding prop aboard in a locker. Would that help? Would tuning help? What do
you think?

Prop walk is a function of hull geometry, aperature size, prop pitch,
etc. One boater's solution may not solve yours.

However: I run a direct drive Atomic 4 on a '70s light cruiser-racer
with a fin keel and a spade rudder on a tiller. I used to have severe
prop walk with the stock Michigan Wheel 12 x 6 R.H. fixed two blader.
Just prior to launch I installed a Gori folding prop of 11.5 x 8
dimensions. It has made a significant difference in my handling under
power for the better, given me a good 1/2 knot increase in speed under
all conditions, better control and "bite" under low speeds, and best
of all, GREATLY reduced prop walk.

A friend of my with a very heavy steel 40' ketch with a full keel,
cutaway forefoot (36,000 lbs. in the slings) claims that cutting a
slightly larger aperature in his rudder allowed him to swing an
AutoProp custom cast for his boat. His frankly underpowered vessel (35
HP Volvo) can now dock like a minivan and makes 8.2 knots, a full knot
better. The thing sails better as well.

Call me a convert.

R.

backs like a pig
 

On Sat, 13 May 2006 14:30:49 -0400, Wayne.B
wrote:


I think in the end that the best cure is technique. Once water starts
flowing along the keel and rudder, prop walk is minimized. Give the
boat a quick burst of power in reverse to get it moving a little and
then shift to neutral. Apply more reverse as needed in short bursts,
and use the rudder to compensate for additional prop walk.
Many people are reluctant to build too much boat speed in reverse but
it really does a lot to minimize control issues of al types.


I know the OP wanted physics and not art, but the prop walk of my old
fixed two-blade was so nasty (and contrary to my port side docking
situation) that I learned quite early to just walk the boat off the
dock in neutral and "coast" until I could put it in forward, and to
"dead stick" the boat back to dock by throwing it into neutral,
bleeding off speed in a sharp S turn and basically crabbing right into
my slip.

So even though I can back down straighter (after a slight pause as the
blades open), I still don't use a lot of reverse thrust. I fell out of
the habit.

R.

backs like a pig
 

"Wayne.B" wrote in message
...
On Mon, 15 May 2006 14:43:40 -0700, "Capt. JG"
wrote:

Do you think there would be a significant difference between a 3-blade
feathering prop and a 2-blade folding prop? I guess my question is why
would
you prefer that prop?

FYI, most of the sailing is on the SF bay, so lots of wind, strong
currents,
etc.

It's hard to say without actually trying the swap. You might be able
to find a diver who can swap the props underwater at a reasonable
price.

Is there racing involved? If yes, I would choose the folding prop
regardless. In order to get the prop to fold reliably it is necessary
to mark the shaft when the hinge pin is vertical, and manually align
the shaft to that point every time you shut down the engine before
racing. Otherwise the lower blade will flop downward and defeat the
purpose of drag reduction.

No racing involved unless the other boat is unawares. :-) There are plenty
of divers around, but I suppose it would be just as easy to swap them during
the next haul, which is coming up in June.

--
"j" ganz @@
www.sailnow.com

backs like a pig
 

In article ,
"Capt. JG" wrote:

Do you think there would be a significant difference between a 3-blade
feathering prop and a 2-blade folding prop? I guess my question is why would
you prefer that prop?

We need three blades to combat a vibration. The feathering prop (two or
three blade) will back more reliably. You have to be more deliberate
when backing a feathering prop. Going forward, the difference in drag
won't be noticeable.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

backs like a pig
 

"dbraun" wrote in message
lkaboutboats.com...
wrote:

Back to more questions somewhat related to the Perkins 4-108. This
sailboat
is a pig to back up. It really wants to prop walk to starboard.
According
to
the owner, it's got a fixed, 3-blade prop. I believe the boat also has a

folding prop aboard in a locker. Would that help?

First off, put me in the camp that recommends technique as a means for
neutralizing prop walk. You seriously do not want to eliminate prop walk
as it is the only thing that will turn the stern of your boat predictably
and reliably. People's biggest beef is that it will turn the stern in only
one direction and I can see that if you're viewing boats as watery
versions of cars, then this would be a little upsetting. If you want to
see what prop walk can do, check out the Dashew video "Dashew Offshore".
They are able to spin their vessel 360 degress with ZERO headway. All
technique.

That said, I can understand where some people may not be in the right
frame of mind to learn technique (think reluctant sailing wife being given
"verbal hints" when backing down an anchor). You also mentioned that the
vessel was being sailed in SF bay. You may want to know that there is a
prop shop called Bay Propeller (510-337-9122) that has a technique for
modifying fixed props. They tap the blade tips over to create little
tiplets. They claim it eliminates prop walk AND increases thrust. I have
no personal experience with their results, but it would be worth a try.

David

As a follow up to this, I gave them a call. Unfortunately, they no longer
can do this work (it's not very expensive, about $160 for a 3-blade prop).
There is a patent issue. The guy I spoke with said he would be happy to
refer me to the patent holder (a former employee), but that it might be a
couple of months before he's going to be able to accept clients (the patent
holder that is).

They said that they had excellent results from the work, but of course this
is all heresay at this point.

--
"j" ganz @@
www.sailnow.com