Cool!!! I found some real experimental data with a discussion of the strengths and weaknesses of the mathematical modelling that has been used in the past to estimate the drag of propellers.
Here's an interesting article just published in 2007. MacKenzie, P.M. and Forrester, M.A. (2008) Sailboat propeller drag. Ocean Engineering, 35 (1). pp. 28-40. ISSN 0029-80: I quote: "The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag." Citation: http://strathprints.strath.ac.uk/5670/ Citation: http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf Sailboat propeller drag MacKenzie, P.M. and Forrester, M.A. (2008) Sailboat propeller drag. Ocean Engineering, 35 (1). pp. 28-40. ISSN 0029-8018 Preview PDF 1733Kb Official URL: http://dx.doi.org/10.1016/j.oceaneng.2007.07.004 Abstract All but the smallest classes of modern keelboats are fitted with inboard engines and consequently, when making way under sail, the craft experience parasitic drag due to trailing propellers and associated appendages. The variety of screw configurations used on sailing boats includes fixed-blade, feathering, and folding set-ups, with blades numbering two or three. Although the magnitude of the resultant drag is thought to have a significant influence on sailing performance, the published literature having regard to this problem is sparse. Here, the aim was to evaluate the drag effect of fixed-blade propellers of types commonly used on sailing craft. The results of towing tank tests on full-scale propellers are presented for the locked shaft condition; these are presented along with reconfigured data from the few previously published sources. For the case in which the propeller is allowed to rotate, tests were conducted on a typical screw with a range of braking torques being applied. It was hypothesised that the performance coefficients of the Wageningen B-Screw Series could be used to characterise adequately the types of screw of interest and that these could be extrapolated to enable prediction of the drag of a freewheeling propeller; an assessment of this formed part of the investigation. Item type:Article ID code:5670 Keywords:drag; propeller; sailboat; towing tank; mechanical engineering; Subjects:T Technology > TJ Mechanical engineering and machinery Department:Faculty of Engineering > Mechanical Engineering Deposited by:Strathprints Administrator Deposited on:12 Mar 2008 Last modified:21 Oct 2008 23:43 ________________________________ From: Clyde Thorington <[email protected]> To: [email protected] Sent: Wednesday, September 9, 2009 11:46:56 AM Subject: Re: [IC27A] Re: new noise This same topic came up on the C30 list and some retired rocket engineer showed through equations, parens, to the umpteenth power that rotating prop does not cause any more resitstence then fixed. The best fix for the Hurth tranny was to keep in reverse while sailing to prevent undue wear. Clyde Thorington ex C27 now C30 San Jose, CA --- On Wed, 9/9/09, Judith Blumhorst, DC <drju...@blumhorst. com> wrote: From: Judith Blumhorst, DC <drju...@blumhorst. com> Subject: Re: [IC27A] Re: new noise To: ic...@yahoogroups. com Date: Wednesday, September 9, 2009, 11:28 AM I googled it the issue once and didn't come up with any real experimental data... just a lot of opinions.... so, my carefully considered answer is: I have no answer. :^) Curious minds want to know..... Fair winds, Judy B ____________ _________ _________ __ From: "Jenngsjohn@ aol. com" <Jenngsjohn@ aol. com> To: ic...@yahoogroups. com Sent: Tuesday, September 8, 2009 3:01:31 PM Subject: Re: [IC27A] Re: new noise Interesting comment on freewheeling prop adding more resistance than one that is locked. Would like to hear more comments from our C27 "experts". What say you Dr. Judy? John Jennings 83 C27, 5267 Inboard with FREEWHEELING PROP In a message dated 9/8/2009 5:49:14 P.M. Eastern Daylight Time, leroy.sailorman@ yahoo.com writes: --- In _ic...@yahoogroups. IC2_ (mailto:IC27A@ yahoogroups. com) , "dsavlin" <dsav...@... > wrote: > > I do the same. Lock tranny in reverse. I saw a post that suggested a freewheeling prop adds more resistance compared to one thats' locked under sail. Counterintuitive ? //leroy > Dunno how significant this is, but in my Universal M-15 manual, it > states in all caps, underlined, and bold letters, never to put the > transmission in forward gear while under sail; always use reverse gear, > if you do not want the shaft to spin free. > > On Persephone, we do this to reduce wear and tear on the cutlass bearing > and the packing gland. It probably costs us a small amount of speed but > we're pure pleasure cruisers so we don't care. Our cutlass and packing > gland are both still in fine shape after 7 years of ownership. > > David > Persephone, '88 C27 > > > --- In _ic...@yahoogroups. IC2_ (mailto:IC27A@ yahoogroups. com) , "new_horizons_ , <new_horizons_ new> > wrote: > > > > If it is not your prop shaft spinning free, it is most likely your > cutlass bearing rattling within its strut, which indicates that it's > failing and needs to be replaced. I had the same noise when mine was > failing. It sounded very much like the prop was free-spinning under > sail. > > > > --- In _ic...@yahoogroups. IC2_ (mailto:IC27A@ yahoogroups. com) , "sharon_speller" unclesharon@ wrote: > > > > > > I had a new mystery noise coming from the stern cowling that is > connected to my blower hose. The noise happens when i am under sail, > engine off. All batteries are off. If im at the dock nothing. it sounds > like a motor whirling. the noise is coming from behind the engine. Near > the transmission and prop..... I think. > > > Could it be the prop? never had it happen before today. > > > thanks. > > > > > > [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] [Non-text portions of this message have been removed]
