Originally Posted by mrkeith
i m sure there are many qualifying factors, but i m curious how much speed one would expect to pick up on a larger sail boat (40footish) by switching from a 2 or 3 blade fixed prop to a comparable folding prop? thank you
I have a 33 foot cruiser racer with a fin keel, spade rudder and cutless bearing strut with a typical 12 x 6 Michigan Wheel on a direct-drive Atomic 4. I was sold at a deep discount a Gori folding 11 x 8.
I noticed several interesting effects:
My prop walk was reduced, but my stopping power remained the same, once compensation for the "fold/unfold" process of gear shifting on the prop was taken into account.
I needed to add more throttle under power to get the same speed. Not an issue as I was near hull speed at just over half-throttle with the old boat, and I rarely motor with intent.
My "dead slow" speed: minimum throttle in gear, roughly 700-800 RPM...dropped from 2.5-2.7 knots as per GPS/knotmeter to 1.5-1.8 knots. This has more than compensated for the less responsive reverse, as my too-fast dead slow put me in the habit of "dead sticking" in neutral and coasting into my dock or mooring anyway. A controllable, slower speed is a boon.
My sailing speed in a given range of say, 10 knots, is up between 1/2 and 3/4 of a knot. The boat gets into the groove faster and accelerates faster, noticeably so. I have even had to rethink my sail changes (I have hank ons), because what I call my "apparent wind speed bands" have altered: because the drag has lessened, I am over-canvassed somewhat earlier than I used to be. This I can well live with, however.
My new boat is a 13 ton steel motorsailer, about as far from the old boat as you can get. However, the experience of losing this drag (and I intend to sail more than motor) has been so positive that I will likely replace the 18 x 13 three blader with a VariProp four-blader of similar size. I would have gotten an AutoProp, but would have needed a half-inch out of the rudder stock, something I won't alter.