On Sat, 25 Oct 2003 18:07:58 +0000, Jere Lull wrote:

Lauri Tarkkonen wrote:

Looks like you have not learned much about props. Some fixed blade
props are very poor on reverse, especially if they are optimized to
give the best performance forward

Lloyd mentioned that.


Of course all the data I have is theoretical - YMMV. The "testimonials" I
read about how much better even some fixed-blade props were in reverse
against folding props were written by folks who replaced their folding
props - so obviously they were not happy with them, or at least felt they
had to justify their decision.


If you have a good three bladed propeller it can be much more
quiet while motoring than a two plade folding propeller and
can be worth the exstra cost.


We had to replace our fixed two blade because of harmonic vibrations at
high power. The whole boat shook, which can't have been a good thing.
The 3-blade is MUCH quieter and smoother; not quite as efficient, but
not as bad as I expected.


I will never go back to a 2-blade, period. A writeup at WestByNorth tells
why: http://www.westbynorth.com/choose.htm


Auto-prop:


This is the same as a feathering prop, but instead of a fixed pitch,
the pitch varies with speed, torque, etc. similar to the "torque
convertor" on a ski-doo. This allows max power at all settings, best
fuel economy, fastest cruising and WOT speed, etc. The downside of
course is the cost.



You might not be able to recover the cost difference in the fuel
saving, especially if you are sailing a lot.


Overall cost savings don't much enter into this sort of purchase.
Cheapest by far would be a fixed prop. It would take us a very long time
to consume $1500 worth of fuel, much less save that much on fuel. (In
1067 hours, we consumed 341 gallons.)


Agreed!
I can see some non-monetary reasons for going to a feathering or folding
prop, but can't see the advantage of the auto-prop.

I'm now torn between the 3-blade fixed Campbell Sailor and the AutostreaM.
I like the AutostreaM's stainless-steel: at the very least, it would Look
Kewl on my SS shaft (and I know the corrosion and marine-growth
characteristics of SS). I also like the ability to change pitch - I now
feel that PITCH is the primary variable to get optimum power/speed from
the prop. I suspect that a C.S. fixed would give marginally better
performance than an AutostreaM AT OPTIMUM PITCH. But since the A-S is
easily adjustable, I think I'd be able to achive optimum pitch much faster
and more easily.

I'd also like the feathering advantage, although I was jokingly adding up
all the "fasters" I could get: feathering gets me 1-2 knots faster, the
Sail Guy said a new headsail would get me at least a knot, clean bottom
would give me an extra 1/2 to 1 knot...and I was going 6 knots in an 8
knot breeze. So add that all up, and I'd be going about 9-10 knots!

Lloyd Sumpter
"Far Cove" Catalina 36

Lloyd Sumpter wrote:

On Sat, 25 Oct 2003 18:07:58 +0000, Jere Lull wrote:


Overall cost savings don't much enter into this sort of purchase.
Cheapest by far would be a fixed prop. It would take us a very long time
to consume $1500 worth of fuel, much less save that much on fuel. (In
1067 hours, we consumed 341 gallons.)

Agreed!
I can see some non-monetary reasons for going to a feathering or folding
prop, but can't see the advantage of the auto-prop.


The AutoProp's an unusual beast. After some discussions this past
season, I might consider an AutoProp for a trawler or other
mostly-powered boat: the blades keep a constant *relative* pitch, giving
better thrust and keeping the engine better loaded at reduced RPMs.

I'm now torn between the 3-blade fixed Campbell Sailor and the AutostreaM.
I like the AutostreaM's stainless-steel:

I like the *idea* of SS as well, but our SS shaft gets considerably more
growth than the bronze blade, even at the hub. And bronze seems more
suited to hard knocks. I've been thinking of switching back to a bronze
shaft....

I'd also like the feathering advantage, although I was jokingly adding up
all the "fasters" I could get: feathering gets me 1-2 knots faster, the
Sail Guy said a new headsail would get me at least a knot, clean bottom
would give me an extra 1/2 to 1 knot...and I was going 6 knots in an 8
knot breeze. So add that all up, and I'd be going about 9-10 knots!


They apply at different speeds. Once the wind's up and you're pressing
against hull speed, you're wasting power intentionally. I honestly have
noticed the speed bump up a half to full knot when I locked the blades
at 4-5 knots. The bottom makes the most difference at lower speeds, as
can the sail. Adding all of the effects together can make the difference
between 2-3 knots and not being able to move at all. THAT can surprise
neighboring boats.

--
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/