I could be wrong, but my understanding is the prop walk is caused by the fact that the prop shaft on a sailboat is almost never horizontal (except for a Sail Drive). This has the effect of increasing the pitch on one side of the prop while reducing it on the other. It's possible (I think) the effect is exacerbated for props with increased pitch (smaller diameter). Comments?
Gary S/V Expresso '75 C&C 35 Mk II East Greenwich, RI, USA ~~~~~~~_/)~~~~~~ On Fri, May 15, 2015 at 12:54 PM, Martin Kane via CnC-List < cnc-list@cnc-list.com> wrote: > Michael > > I expect you are right about the impeller vs screw analogy, because while > the 12x6 is correctly sized going forward given the 2.21:1 gear box, vs the > 2:62:1 that is most common in 2GMs, it needs to spin faster in reverse than > a prop that was properly sized for a 2.62:1 gearbox. > > > > So its seems that I’ve solved the inability to hit max rpms and cruise at > ~80% cont max, but created a different problem. > > > > Financial practicality aside, I’m wondering if a feathering prop where the > forward and reverse pitch can be set independently might work. Any listers > have experience with this style of prop and are they 2 or 3 blade? > > > > > > Martin > > > > martink...@sympatico.ca > > > > 905 848 0925 > > > > *From:* Michael Brown [mailto:m...@tkg.ca] > *Sent:* Thursday, May 14, 2015 5:21 PM > *To:* cnc-list@cnc-list.com > *Subject:* Re: Stus-List Prop Walk C&C 29-2 > > > > You could plug your specs in here and see what the recommendation is: > > http://www.vicprop.com/displacement_size.php > > I would have guessed a Gori 15X10 to be a little big, maybe the 14X9.5 is > OK. > > The 12X6 seems too small. If it cannot "bite" the water and stay connected > it will churn - cavitation - and the effect may be prop walk. Acting more > like > a centrifugal impeller than a screw. > > Michael Brown > Windburn > C&C 30-1 > > Date: Thu, 14 May 2015 16:10:22 -0400 > From: Martin Kane <martink...@sympatico.ca> > To: <cnc-list@cnc-list.com> > Subject: Stus-List Prop Walk C&C 29-2 > Message-ID: <blu437-smtp450fc0619106756cf69f2abd...@phx.gbl> > Content-Type: text/plain; charset="us-ascii" > > Boat was significantly over-propped when I acquired her. Largely because > transmission ratio is 2.2:1 not 2.62:1 as stamped on the casing. While > the > props that came with the boat (a Gori 15X10 and a 3 blade fixed 14X9) were > too big, prop walk was not an issue. > Just installed a Campbell Sailor 12x6 and the prop walk is severe (in > spite > of the additional tip to hull clearance). It is basically impossible to > turn > the bow to port in reverse . The bow swings to starboard and then once > there > is some movement through the water will crab sideways, but the bow will > not > swing around to port. I have not found any combination of throttle and > rudder position that makes the situation any better. > > I need to replace the prop to avoid getting into trouble in close quarters > at some point. Question to C&C 29-2 owners: Has anyone found a prop that > works well going forward and in reverse. > > Thanks in advance. > > Martin > Recalculating C&C 29-2 > Mimico Cruising Club > > > _______________________________________________ > > Email address: > CnC-List@cnc-list.com > To change your list preferences, including unsubscribing -- go to the > bottom of page at: > http://cnc-list.com/mailman/listinfo/cnc-list_cnc-list.com > > >
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