PROP
Prop is 6x12x7/8" R.H.
Davey Stowers
Outlaw '74 30-1
Lake Erie
David
Have the same power in my '82 32' with the standard 2-blade prop. Just after annual haulout (ie clean bottom and house cleaned boat - read lighter) will get 6.5 kts at 3200 rpm. By the end of the year, just before haul out (dirty bottom and too much junk on board) I'm lucky to squeeze 5 kts.It has been recommended to me by those in the know that the diesel should run as the factory recommended. For this engine it is 3400 rpm continuous and 3600 rpm max. At one point in time I had a Campbell 3 blade sail prop on the boat and the most I could get out of the engine was 2400 rpm with no improvement in speed. I was advised that this was puting undue pressure on the engine by making it 'work too hard' and not allowing it to run at proper rpm and temperature. When I returned to using the standard 2 blade prop, the engine also ran according to Yanmar spec.I normally cruise at 3000 to 3200 rpm at about 6 kts with a clean bottom.
Frank Walczak'Felicity' C&C32
David E. Homa wrote
I have a 1985 C&C 29 with a Yanmar 2GMF, 13HP. Can someone tell me what recommended cruising RPM and what max throttle should be, and what kind of flat water speed I should expect at each of these engine speeds? I have a standard 2 blade fixed prop.
Thanks in advance.
David E. Homa C&C 29 '85
It sounds like someone put a left hand pitch prop on a right hand motor. We have a 1980- 30 with the Yanmar 2QM15. It had a 13 1/2 x 9 fixed prop on it when we bought it and would go 6 knts at 3500. I replaced it with a 15x10 folding prop at the suggestion of a couple of prop "experts" and it bogged down at just over 2000 and put out a lot of black smoke, at about 5 1/2knts. Since then I have had the prop cut down to about 14 diameter and slimmed down in blade area. Now 6 knts at 3000, but still a little blacksmoke. This winter I am going to taper it a little more and make the blades thinner, because of the cut down, they are rather fat. Should be a few evenings with the grinder (and postal scale).
I had a talk with a prop vendor at the Annapolis show - apparently there were different transmission ratios for the Yanmar diesels. One has a reduction of around 2.5 to 1, the other is about 2.1 to 1. In the discussion, we determined that I probably had the 2.1 ratio and the advice I received from the prop experts assumed the 2.5. Thus, on a Yanmar 2QM15, (2.05 or 2.1 to 1) the 13.5 or 14 X 9 prop would be about right, on the 2GM15 (with 2.5 to 1 ratio), the 15 X 10 would be about right. This clears up a lot of confusion I have had over the four years I have been playing with this boat. I hope it is helpful to those of you thinking about changing props. Your Yanmar book should have the ratios listed, and I think the ratio is stamped on the plaque on the transmission, if not you can turn the engine over by hand and watch the output shaft - count the turns.
3/4 of a knot is impressive! Gary Beck, VP of Martec, told me not to blame the worn prop for lack of boat speed. Mine has been done twice, so it can't be done again. He suggested that I live with the lower boat speed, and think about replacing in a season or two (when the blades actually touch. BTW, he is very helpful.
Steve & Suzanne S/V Pony Express www.angelfire.com/pe/ponyexpress
Has anyone ever finished their prop with teflon? There is a local person who does it; would it keep barnicles, etc. off?
Never used teflon - but have had great success with STP (automotive engine oil additive)
I used Aurora Bottom Wax with VS721 which is more slippery than tefelon on the props of a power boat and lost 5 mph. When I got it off, the speed returned. You might want to try teflon on the shaft & hub, but I would not use it on the blades. The VS721 is so slippery that barnacles can't attach.
Richard
Are you saying that slippery props are slower? Why is that?
Joe Della Barba
Hi Joe,
The slippery finish causes a molecular pressure boundary layer which seals the microscopic pores in the base metal. This percipitates a decrease in the drag coeifficient which interupts the normal fluid layer formation. Correspondingly, the hydrostatic laminar confluence is reduced by the square root of blade tip speed, so long as it is below the speed of sound, to the fourth power, in salt water. In other words Joe, the slippery prop slips in the water so it doesn't go as fast. And if you don't believe me, just walk those two blocks over to Fawcetts and ask the experts :)
Do Dah - Hank Evans
The Teflon trademark belongs to DuPont, I spoke to them some years ago. In order to apply teflon it is painted on as a liquid and then cured by baking at about 450F. This will produce a result like a teflon skillet. I have never heard of someone doing that to a prop, but it can be done. DuPont will also sell the teflon resin (the liquid) and you might be able to talk them out of a gallon sample. Anything that can be "painted" on without the high temperature baking is a misuse of their trademark and they do not sanction it. A paint can have teflon *powder* or balls or dust in it, but it cannot and will not give you a "teflon finish" comparable to the real process. DuPont is quite insistent about this. In theory you might also be able to use Teflon as a bottom coating--if the hull did not combust from thr 450F baking process. On a prop, you might have a problem because cavitation is to some extent normal on a prop, and that would probably abrade/boil off the teflon from the blade tips. Still, if you have a kitchen oven and call DuPont for that gallon, you can try it and let us know if it works.
Thanks, Hank, for the simple explanation of this phenomenon, now perhaps someone could go into a detailed physics-based explanation, then Stu could post it on the photo-album website under the "What you never realized you needed to know about sailing" category. I suspect it could submitted to the "Nature" or "Science" journals.
Good Job Hank!
Dave