glenn, just where is it you found you "facts" that "a medium displacement boat"
has 2,866.66666666 pounds of drag?
btw, "a medium displacement boat" that does 8 knots continuous is rare indeed.
btw-2: that 15% that is so often bandied about comes from a marketing
department at a folding prop company.
btw-3: ------ folding -------- three blade props are rare indeed.
btw-4: the subject of this thread is folding, not feathering (a hugely more
expensive breed of cat)
btw-5, the 170# figures from MIT were for --------- three --------- bladed
props. Two bladed props have much less drag, and two bladed props aligned
behind the keel have even less.
btw-6, when the winds are sufficient to drive ""a medium displacement boat" at
8 knots, you have more than enough wind to drive the boat at 8 knots with a
bucket dragging behind just by putting up anothe 20 square feet of sail.
btw-7: most folding/feathering props are promoted by their marketers as an
improvement in **LITE** airs, not heavy.
As usual, Jax has taken some good data and come up with a completely
erroneous conclusion. The drag from the prop increases as the square of
the speed. Using the MIT figures, at 8 knots you are looking at
something over 430 pounds of drag. For a medium displacement boat that
can add up to 15% or more in improved performance. For a 40' LWL
cruiser that means an extra 20 to 25 miles a day.
Paul L wrote:
Its real easy to test, as I can get my folding 3 blade to stay open. I can
see the boat speed, then have it fold and measure the speed. It is
significant. If the MIT tests say otherwise then they are not testing
reality - wanna guess what errors they made in the test or you in the
interpretation??.
Paul
www.jcruiser.org
"JAXAshby" wrote in message
...
You will definitely will see a decent speed improvement under sail with a
folding prop
no, you won't. folding props only hve value for racing boats, where 2
seconds
a mile means the difference between 2nd place and 6th.
According to MIT tests, a folding prop means a savings of a mere 170
pounds
drag at 5 knots (or 40 pounds drag at 2-1/2 knots, or 10 pounds drag at
1-1/4
knots) over a --------- three ------------ blade prop. ***Much*** less
with
compared to a two-blade, and even less compared to a two-blade rotated
verticle
behind the keel.
Wanna guess just how much powered is required to pull 170# at 5 knots?
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com