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.
and some (good) folding props
are as good on reverse than forward and much better than some
fixed props on reverse. An example for you is the Danish Gori,
but there are others. The centrifugal force has no problem in
keeping the Gori open on reverse. There are some poor ones, but
why buy a poor one, as there are good ones available?
When I looked for a feathering/folding prop, I'd never met anyone with a
folding prop that could depend upon the blades opening fully in reverse
every time, particularly in an emergency; they all seemed to have to
baby them and learn tricks. I also wondered whether the more modern
transmission gearings make it worse: Our 2GM swings at a maximum of 1200
rpm, quite a bit slower than the anemic engine it replaced.
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.
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.)
If you want to save $$, slow down. We normally power at 5.7+/- knots and
consume 0.36 gph. That's not an unreasonable speed with our 6.6 knot
hull speed. When we went 6.2+/- (enjoying the new prop), consumption was
0.69. When I backed off to 5 knots for a month, consumption over 43
hours was 0.20. To gain just over a knot, we used 3.5 times as much fuel.
--
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/