CD-27 prop

[Wayne Brown]

I have a '83 CD-27 with the Westerbeke 13 engine in it and a 12 RH 11 prop. I haven't had ocassion to use max power until the other day. The engine is rated at 3600 rpm and I was surprised to get almost 4000 rpm. To me that indicates I need more pitch. Does anyone with the same engine know what prop is on their boat? Can a old prop be repitched and is that a good idea?

Wayne Brown

[Boyd]

Hi Wayne:

The max rpm of a diesel is set at the injector pump. No matter how light the load the engine will not exceed that rpm. Westerbeke can verify what that value should be. For instance my 21-A is limited to 3200. I suspect your tach is in need of calibration. The proceedure is on the Westerbeke web site. For instance, changing the alternator will greatly affect the rpm readings.

I suggest you get an optical tach and verify what your engine is really doing before you start messing with the prop.

If you do determine that your engine is in need of more pitch a good prop shop can repitch your old one. They can safely change it by several inches.

Its possible your old prop just needs a good tuneup to bring it back to original specs. If you have hit something it could be out of pitch. Last time I hauled there was a palm tree branch wrapped on the prop. I had no vibration, just a slight reduction in performance. These things happen.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[John Vigor]

I have a '83 CD-27 with the Westerbeke 13 engine in it and a 12 RH 11 prop. I haven't had ocassion to use max power until the other day. The engine is rated at 3600 rpm and I was surprised to get almost 4000 rpm. To me that indicates I need more pitch. Does anyone with the same engine know what prop is on their boat? Can a old prop be repitched and is that a good idea?

Wayne Brown

Wayne, I have the same engine in my 1983 CD27. The engine is actually rated at 3,000 rpm maximum, not 3,600, so, as Boyd suggested, you should have your rev-counter checked.

My boat also came with a 12-inch diameter prop, 11-inch pitch. I found I couldn't get maximum revs with that prop, so I sent it to a prop shop and they changed the pitch to 9 inches for me. It seems perfect now. I cruise at just over 5 knots at 2,300 rpm and about 6 knots at 2,800. At 6 knots, the temperature gauge shows 185 degrees, so I know the engine is not struggling, as it used to with the 11-inch prop at top revs.

If I open up the throttle any more after 2,800 rpm, I get black smoke from the exhaust, which indicates overloading, but I figure that getting within 200 rpm of rated maximum is about as good as it gets in real life. A few barnacles on the prop, or a fouled hull, will cut the revs by at least as much, if not more.

I'd be very surprised if you are actually getting 4,000 rpm from your 13 hp Westerbeke with an 11-inch prop, particularly since the engine is governed to 3,000 rpm maximum according to the workshop manual.

And yes, an old prop can be repitched. You might want to try changing yours to less pitch, rather than more, once you've got your rev-counter sorted out.

John Vigor

[Andy Denmark]

Boyd is correct. The tachs on most engines are notoriously inaccurate. An optical tach is the only certain way to get accurate readings (well, almost --- read further down). Governor max rpm settings are also widely disparate from one to another. I think they are only roughly calibrated" when the engine is built and tested.

To further complicate matters, props from different manufacturers or different styles from the same manufacturer will behave differently. There are numerous postings on this board about how certain manufacturers give faulty information about the rpm's they are able to achieve when they change props. Going from a Campbell to say a Michigan "sailor" prop of the same diameter and pitch will result in fairly serious differences in rpm curves. This is often wrongly interpreted as "faulty" info from the mfr. A 2-bladed prop will have entirely different pitch than the same diameter 3-bladed one (on the same engine), usually 2 or 3 inches less pitch.

Since this is a guessing game with lots of variables the best bet is to work with what you have. I wouldn't be happy with John Vigor's black smoke (classic overload indication) nor would I be happy with over revving, either. Somewhere in between is the happy medium.

In the FWIW column, there is a new tach on the market called the Tiny Tach that attaches to one of the injector lines and simply counts the times the injector fires. This seems like a truly accurate way to measure rpms. All you need to know is the diameter of the tubing and installation is a snap. It has some other neat features like setting service points, etc. I have the info on where this thing is available but can't put hands on it right now. It's under 1 frogskin for the entire package.

My CD-27 came from the factory with a 12 X 10 Michigan (std) 2-bladed prop. It turned upwards of 3600 rpm out of the box. Next haulout I repitched to 12 X 12 and the rpm's came within 100 of optimum. That's where it's been ever since. We did some prop tests with 3-bladed vs. 2-bladed, measuring thrust against rpm curves, etc. and generated some good data. (same engine, same boat, flat water w/ no current, etc.) I also tested with a 2-bladed Michigan "Sailor" of the same pitch & diameter, some 3-bladed props, a Campbell, all with different results. It's hard to do this unless you own a boatyard and have access to a number of props (I had both).

Best I can tell, there isn't a magic formula that you can plug numbers into that will give you a one-shot, accurate, predictable outcome when it comes to props. Year to year adjustments toward the optimum goal will ultimately help to get it right. Too much pitch yields black smoke, too little gets rpm's too high. Cruise at 80% of whatever your max (loaded) rpm's are (not what the specs say). Skene's has a nomogram that is a good shot but our props are so small that they are at the extreme end of the data curves and difficult to interpret accurately.

If this clarifies anything for anyone I'll be surprised. If you're close and want to tweak the pitch next haulout, then go for it. I think that's the bast way to go. Don't be upset with the prop mfr or your boatyard if it isn't exactly right the first time as their recommendation is probably an educated guess. While this will raise the hackles of the engineer types on the board (and I am one, too) it's what I've arrived at after a few years of trying to figure it out.

Hope this helps.
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Sc2 Replays

[M. R. Bober]

When we replaced the prop on RESPITE, the OLD oldtimer at the prop shop told me that in his opinion (50+ years in the prop business) the true test was to see if you could reach--in forward gear--within 100 rpm of max engine speed in neutral. He also warned that many of the high ouput alternators made tachometer readings unreliable.

He also tried an extra inch of pitch because we had a Campbell rather than a Sailor prop (higher chord.) In the end, you "pays your money ans you takes your chances." A good prop shop will offer to make adjustments if their recommendation proves faulty.

Mitchell Bober
Sunny Annapolis (home of slow pitch CD props,) MD

[Andy Denmark]

In response to several emails, I have chased down the Tiny Tach URL:

http://www.tinytach.com/tinytach/diesel.php


Also, for those who asked, my engine is a Westerbeke 13.
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Vaporizer Questions

[Andy Denmark]

I mistakenly reversed the 2-blade vs 3-blade pitch thing in my earlier posting. (Thanks for pointing this out, Earl.) Going from a 2-bladed prop to a 3-bladed prop will see a reduction in pitch for the 3-bladed prop. This is assuming the 2-bladed prop was correctly pitched to begin with.

Some think that going to a 3-bladed prop will resolve the pitch/rpm problems and sometime it does. Often, though, it does not.

Another misconception is that one will somehow get more thrust from a small (low horsepower) engine by going to a 3-bladed prop. Not so. More thrust will result if the pitch is closer to correct than with the "old" prop but there is only so much that can be done to get that horsepower translated to thrust; the old "can't squeeze blood out of a turnip" philosophy.
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Charlie sheen intern