MaxProp Pitch vs RPM, Redux

BCC Forum
1

Ok. Marty and others, once again the issue of MaxProp pitch vs Yanmar RPM.

What follows is an exchange I just had with David Swain at www.yanmarhelp.com. I find it hard to believe that the pitch is set all that much too high as it is more or less in line with what others on the BCC list have reported using in the past. Is it possible that my tachometer is reading waaay low? Or should I go a lot lowere than the present 18 degree setting, to 14 or lower instead of the 16 I intended to try next time I haul out?

Opinions welcomed.
Scott

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Quoting Scott to YanmarHelp

Another question. As purchased, the boat (a Bristol Channel Cutter) was set up with a 15" MaxProp set at 22 degrees of pitch. Engine RPM/speed curve topped out around 2400 RPM, so last year I reset it to 18 degrees pitch and now it cruises happily around 2200 RPM and tops out around 2750 -2800. I am inclined to reduce the pitch further to 16 degrees to increase the max useful RPM. What do you advise? What RPM and length of operation at that speed would be pushing the limits for good practice with this setup? Up until now I have never noticed any black smoke at all, unless pushing hard at top RPM, not something I routinely do.

Regards, Scott

Yanmarhelp.com Posted: Jan 27 2006, 10:32 PM

Administrator

Scott you were massively over-proped you should me able to reach 3600rpm If your are using the Yanmar tacho that’s about 3700 as they under read by about 100rpm at the top. probably why you’ve experienced the head gasket failure. You definitely need to decrease pitch further. Not sure how much adjustment you have but you need to pick up another 800-900rpm!

Regards,
Dave Swain

Marine Power Ltd. - UK Yanmar Dealer

2

Well,today we fired up the newly rebuilt 3GM30F and did what should have been done first of all – checked the accuracy of Itchen’s tachomenter, and all the previous discussion was rendered irrrelevant. The Yanmar tachometer readings were a little bit high at the low end and extremely low at the high end! I also noticed that the Yanmar tachometer needle rose smoothly as engine RPM increased, but when I throttled back a little bit that the tachometer needle tended to stay at the same place and only indicated a lower RPM when the speed dropped much further – “sticky needle syndrome” but only when speed was lowered.

Here are some numbers. In each case the first one is the boat’s tachometer and the second is the mechanic’s electronic RPM strobe instrument.

700/540 (idle); 1000/994; 1100/1545; 1500/2100; 1600/2200;1800/2430; 1900/2600; 2000/2725; 2200/2960; 2300/3100; 2400/3215; 2500/3380; 2600/3540; 2700/3610; 2900/3809 (full throttle, no load).

So it seems to me that our MaxProp setting is about right and that our actual normal cruising engine speeds have been pretty much in the right ballpark.

Marty, can this Yanmar tachometer be adjusted or fixed or should it or the sensor be replaced?

Regards, Scott

3

Yanmar tachometers are very accurate, more so than most alternator driven units, if it getting old and acting up replace it.

Your tachometer test, is partially valid, only to the extent you were able to verify it with another, hopefuly digital infrared tachometer and a good one at that; I suspect with the erratic performance noted the tachometer is bad.

However, this does nothing to validate weather or not your propeller is pitched correctly, only that your have a malfunctioning tachometer.

Replace the tachometer, re-run tach test, then load test underway as described in an earlier email.

Marty

BCC Forums bccforums@samlmorse.com wrote:
Author: Itchen
Username: Itchen
Subject: Tachometer inaccuracy (was Re: MaxProp Pitch vs RPM, Redux)
Forum: BCC Forum
Link: http://www.samlmorse.com/forum/read.php?5,4419,4430#msg-4430

Well,today we fired up the newly rebuilt 3GM30F and did what should have been done first of all – checked the accuracy of Itchen’s tachomenter, and all the previous discussion was rendered irrrelevant. The Yanmar tachometer readings were a little bit high at the low end and extremely low at the high end! I also noticed that the Yanmar tachometer needle rose smoothly as engine RPM increased, but when I throttled back a little bit that the tachometer needle tended to stay at the same place and only indicated a lower RPM when the speed dropped much further – “sticky needle syndrome” but only when speed was lowered.

Here are some numbers. In each case the first one is the boat’s tachometer and the second is the mechanic’s electronic RPM strobe instrument.

700/540 (idle); 1000/994; 1100/1545; 1500/2100; 1600/2200;1800/2430; 1900/2600; 2000/2725; 2200/2960; 2300/3100; 2400/3215; 2500/3380; 2600/3540; 2700/3610; 2900/3809 (full throttle, no load).

So it seems to me that our MaxProp setting is about right and that our actual normal cruising engine speeds have been pretty much in the right ballpark.

Marty, can this Yanmar tachometer be adjusted or fixed or should it or the sensor be replaced?

Regards, Scott

4

Thanks Marty, comments inserted below.

On 2/2/06, BCC Forums bccforums@samlmorse.com wrote:

Am I right in assuming you mean only the instrument panel unit? Is the
bell housing mounted flywheel/ring gear sensor bullet proof, or should
both dial and sensor be replace together?

Right, we went with what was available and I hav enp way of knowing
the accuracy/precision of the mechanic’s digital device. But we did it
twice and got consistent readings.
In the interest of brevity & bandwidth I didn’t report info from past
speed versus RPM runs I did before and after the last MaxProp pitch
change. But, when I correlate the hopefully more accurate mechanic’s
data with my old Yamar tachometer speed/RPM logbook entries I get
figures pretty close to what you and YanmarHelp.com advise for RPM at
various load conditions, MUCH closer than the puzzling figures I was
getting with the old Yanmar tachometer. I just seemed bizarre to me
that by using a pitch setting close to what is used by others, I would
be getting RPM readings in the “Massively overpropped” range. Sure,
all boats even from the same mold may differ somewhat but probably not
that much. All my figures were taken with clean bottom and
barnacle-free prop.

Clearly the tachometer has to be repaired or replaced. Another tach
test with a different tester would be nice when we have a chance. I
always note engine RPM and speed in Itchen’s decklog anyway, so I have
lots of old data and new data will accumulate.
This morning someone said that there are calibration screws in the
back of some tachometers -true? Have not had time yet to pull
Itchen’s panel to check ours.

Scott