Post by Glenn Ashmore20 miles a day is a 15% improvement on a base 5.6 knot cruising speed
and you are correct that folding props make the most difference in light
air.
Reducing drag has the advantage over adding sail area in that it does
not add to heeling moment.
BTW, Gori, Martec, Brunton/Varifold and others make 3 blade folders.
Volvo even makes a 4 bladed folder.
Now, you can continue to rant and make an ass of yourself as usual but
that is all I have to say about it.
For what it's worth:
Buddy of mine with a steel Wallstrom-designed (partner to Brewer) 1979
ketch popped for a three-bladed AutoProp, a feathering design. He had
to haul out for many reasons: 1) to confirm the exact geometry of his
hull around the shaft; 2) to cut back his rudder at that point for the
install; and 3) to get the prop itself on. 1) was because it's
essentially a custom casting, and NOT cheap (about $3,500 Cdn.).
After three seasons now, he's happy as a clam with his decision and
outlay. He says the following:
Advantages:
Bigger prop, better bite and power curve. He's got a 35 HP Volvo
(probably 15-25 HP too small for a 28,000 lb. boat), but according to
him, the power he transmits to the prop is greatly increased. His top
speed without redlining has gone from 6.5 to 8.2 knots, or
approximately hull speed.
He says he gains 1/2 knot due to the self-feathering action, a
significant gain in typical light Lake Ontario air for this
essentially blue-water cruiser.
He backs down (after a quick rev to feather out the blades) far more
effectively now. He stops far more rapidly.
His docking is far more controlled. He can move his heavy, trad.
keeled boat like a minivan now. It's quite interesting to see.
Disadvantages:
The cones in his transmission have been wearing far more quickly and
have required replacement. He can handle this himself as a repair, so
it's more an annoyance than a tragedy, and spares from Volvo are
pricey.
He figures that shifting the gears with a bigger, heavier prop is
causing wear and tear beyond the engine spec. You can hear the "clunk"
of his shifting outside the boat, actually. He is consulting with a
marine engineer to determine a better course of action, but he is
leaning towards a shock-absorbing coupler and a thrust bearing
designed to isolate the engine transmission more effectively from the
sheer inertia of this otherwise fine prop.
Overall, he is so happy with the performance, however, both under sail
and power, that he would consider getting a new diesel to make full
use of the prop, rather than sticking with the small but still viable
diesel that perhaps doesn't have the beef at the back end to cope with
God's Own Feathering Prop <G>.
Hope this little tale helps.
R.