Re: [Cal_Boats] Cal 39 question #2

Peter - Your furry bottom can easily shave 2 kts off your boat speed. In addition to the "fur" you likely have barnacles both on your bottom and your prop. When I first started beer can racing I was unaware of how much impact this growth could have until motoring out to a race one day and could barely make 5kts when I could usually do 6.5. After that we had the bottom cleaned weekly. Paul DavidOwen <wi… [at] mariposasailing.com> wrote: > > >Peter, > >A two bladed prop, of the proper size is more efficient than a 3 >bladed prop of the proper size, so it is all in the details. The >problem would be if the two bladed prop gave up some thrust area >because there wasn't room to swing a large enough diameter prop, which >is of course not unusual on sailboats. The three bladed prop can fit >more surface area into a smaller diameter, even though the extra blade >interferes with the efficiency of the other two. I think generally a >3 blade is nice around the docks and maneuvering under power, but the >two blade is a better choice for cruising speeds and certainly for >sailing. > >If you know the diameter and pitch of the prop and can find the >reduction ratio of the transmission, you will find a number of sources >online to tell you whether it is the proper size or not. Then you can >go ahead and diagnose if your transmission is slipping or not. > >I'm not sure that a one knot difference is unreasonable with a 10 knot >headwind and against an opposing current. It makes a bigger >difference than we suspect. 2600 rpm sounds high for only 3-4 knots, >however. What is that engine rated for as max rpm? > >I don't know much about Velvet Drive transmissions, but most of them >can be damaged by running them low on fluid. Also, a friend just had >a very expensive repair made to his (A Freeport 36 with 50 HP Beta >engine and velvet drive) because the shift linkage was replaced and >improperly adjusted. Not sure that has anything to do with your >problem, other than that I now know that they are sensitive to such >things. Best of luck. > >Wilkie > > > >On Jan 2, 2011, at 8:39 PM, C. Peter Audet wrote: > >> >> Our Perkins 4-108 with 2 blade prop and Velvet Drive transmission >> only manages 3.2 knots at 2600 rpm, into a light chop of 1-2 feet, >> opposing current of less than 1 knot, wind of less than 10 knots on >> the nose, along with a somewhat "furry" bottom. Turning 180 degrees >> gave us 4.2 knots. I'm concerned that the transmission is wearing >> out (1978...), I don't think it is the 2 blade prop, unless Rafael >> confirms that he had similar performance all along, too. The >> transmission leaks a bit and I keep topping it up regularly, but if >> it had run on low fluid for a while could that wear things out >> prematurely? Seems like a possibility. Any ideas? >> Thanks, >> Peter >> >>