Jim

I have a smaller engine than you but, I have found that if I am in a hurry and want to minimize the time to destination I use 2000-2200 RPM and with a clean bottom that will get me about 6.5-7 KTS. I have also found that this is not the most comfortable way to travel and if you are in a hurry you have selected the wrong mode of transportation anyhow.

On longer legs I know often use something near the 1500 RPM range, tweek it for the smoothest RPM, this get me about 5-5.5 KTS in smooth water no wind but, it is much quieter, smoother and less stressful. The fuel burn if far lower and if there is any kind of breeze using the mainsail the appearant wind along with the engine often gets you back up to the same speed as you would have listening to everthing rattle and vibrate in the more stressful higher RPM harmony.

A correction dated 9/22/09: I do not know what I was drinking when I made this post. My RPM numbers were off by 1000 RPM. 3000-3200 will produce 6.5- 7 knots in smooth water no wind. 2500 RPM produces 5-5.5 kts.

I have the M35BC engine as well and use the RPM setting that I think appropriate at the time, keeping in mind that:

1. fuel consumption goes up significantly when you start to "push it"
2. the noise and (thankfully minimal vibration) increase with higher RPMs
3. running a diesel at low load most of the time is not great for the engine

With that said, I will motor-sail at low engine load for 10 hours if I feel I need to (occasionally running up to 2500 to load it up a bit). I will push it at 2500 RPM for a few hours straight if I feel it prudent to make speed. I will run anywhere in between as I feel like it.

To answer your question more specifically, in no wind, calm seas I will usually run at 2200 RPM, which is about 75% of WOT.

[QUOTE=Steve Frost;1427]Jim

I have a smaller engine than you but, I have found that if I am in a hurry and want to minimize the time to destination I use 2000-2200 RPM and with a clean bottom that will get me about 6.5-7 KTS. I have also found that this is not the most comfortable way to travel and if you are in a hurry you have selected the wrong mode of transportation anyhow.

On longer legs I know often use something near the 1500 RPM range, tweek it for the smoothest RPM, this get me about 5-5.5 KTS in smooth water no wind but, it is much quieter, smoother and less stressful. The fuel burn if far lower and if there is any kind of breeze using the mainsail the appearant wind along with the engine often gets you back up to the same speed as you would have listening to everthing rattle and vibrate in the more stressful higher RPM harmony.[/QUOTE]

So, I have to ask what prop you have that returns that kind of speed. I have a 3 blade Max Prop, pitched at slightly less than 10 inches. At 2500 RPM, I get about 61/4 knots with a clean bottom.

It must be the rum in the liquor locker weighing me down:)

[QUOTE=bcam;1434]So, I have to ask what prop you have that returns that kind of speed. I have a 3 blade Max Prop, pitched at slightly less than 10 inches. At 2500 RPM, I get about 61/4 knots with a clean bottom.

It must be the rum in the liquor locker weighing me down:)[/QUOTE]

I'm with you. I might see those speeds on the GPS, but not through the water. I generally run the RPM around 2200, but push it up to 2500 for while every so often. I tend to think "babying" an engine is not a good idea, but neither it running it too hard constantly.

Not only are tachometers notorious for being incorrect, but knotmeters are rarely accurately calibrated. Consequently, these two potentially quite large sources of variability add to other considerations, such as prop type, bottom condition, sea state, winds, etc, that make comparisons of rpm/speed between two vessels problematic. Ditto sea state and wind - and also currents - in finding a truly quiet sea in which to compare with GPS SOG.

[QUOTE=lcbrandt;1437]Not only are tachometers notorious for being incorrect, but knotmeters are rarely accurately calibrated. Consequently, these two potentially quite large sources of variability add to other considerations, such as prop type, bottom condition, sea state, winds, etc, that make comparisons of rpm/speed between two vessels problematic. Ditto sea state and wind - and also currents - in finding a truly quiet sea in which to compare with GPS SOG.[/QUOTE]

Larry,

Even though the boat spends most of it's time in the salt, several times a year we go through the locks and get some fresh water into the cooling system and the holding tank. My observed speed comes from those times. No current and no sea state, but the question of tachometer accuracy remains. The speed reading comes from the GPS/SOG.

The bottom gets wiped on a regular basis, so that shouldn't enter into it either.

Regards,
Bruce

For motoring RPM see post in:
[URL="http://www.c36ia.com/forums/showthread.php?t=251"]Diesels "don't like" to be run easy[/URL]

I have a 2000 C36 MkII under contract, but the max speed we could get in relatively calm water with no current was 4.5 or so knots under full rpm (with excessive vibration). It has a Max-Prop PY that I suspect is mis-pitched but am not sure.

Please post your max motoring speed and typical rpm -- also, does anyone have any tips on vibration ? Ah, yes, the engine droop was a bit small too so I don't think it's loaded sufficiently in gear.

First post here. :)

We have an M25XP and a 3-blade max prop. We cruise at 2400 RPM and get roughly 6 KT on smooth water & no wind. Our engine runs smoothest at 2400-2600, has a rough spot at 2000-2200, so I avoid that range. We've had it up to 2800+ for short periods. Our engine has over 2600 hours on it, so it is no spring chicken.

Fred Aylstock
S/V Neptune's Court #1023
Washington - Puget Sound
1989 C36

I have an Autoprop on my 1999 MKII with 35BC engine.

Most experts seem to say you should cruise at 80% (Some say up to 90%) of WOT. First off, almost all C36 tachometers are not accurate until they are calibrated. You need to find out for your tach, what WOT (Wide Open Throttle) is. The 35BC engine is limited by an internal governor, so the quick and dirty method with this engine is to warm up the engine then put it in neutral and push the throttle to maximum for a few seconds and check the tach. The best way to calibrate your prop is to follow the directions posted here on the association site.

I just checked the manual and the max rpm is listed as 3000 and cruising rpm as 2000 to 2500.

The Autoprop is a bit more efficient at lower rpm than other props, so I generally cruise at 2100 to 2400 rpm and get upper 6's to low 7's depending upon how dirty the bottom is.

We have a 1985 M25 and a 3 blade (fixed) 15X9 prop.

We do 6 at what the tach says is 2200 rpm and 6.6-7 at WOT. the tach reading about 2600 rpm. I have been told that our tach is off and should be recalibraded, but my speeds are true, checked with multible GPS's.

Bob

2003 MKll, I run the engine mostly at 1700-2000 which gives me 6.5 kts depending upon current and weather. Once in a while I run it higher up to 2100 but not much beyond. I use a three blade prop.
John Meyer Hilbre-2135

Just a fyi. I bought a optical tach from harbor freight last Friday and found that I was off 10% from the actual to what the tach on the boat read. But what I found was the stop on the throttle was backed off as the little safety wire had rusted which allowed the stop to backout. So it might be worth all to check,

Randy

Those optical tachs are great. You can see if your outboard engine is making full RPM, too.

[QUOTE=fwa;1839]First post here. :)

We have an M25XP and a 3-blade max prop. We cruise at 2400 RPM and get roughly 6 KT on smooth water & no wind. Our engine runs smoothest at 2400-2600, has a rough spot at 2000-2200, so I avoid that range. We've had it up to 2800+ for short periods. Our engine has over 2600 hours on it, so it is no spring chicken.

Fred Aylstock
S/V Neptune's Court #1023
Washington - Puget Sound
1989 C36[/QUOTE]

I have the same engine & prop - similar results. What is the setting on your prop and what boat speed do average?

Bob Anton
S/V Lunea #624, 1986 TR
Newport RI

Rule of thumb is 80% of max rpm. Diesel Engines run better and are more effecient at 80%

[QUOTE=LCBrandt;1437]Not only are tachometers notorious for being incorrect, but knotmeters are rarely accurately calibrated. Consequently, these two potentially quite large sources of variability add to other considerations, such as prop type, bottom condition, sea state, winds, etc, that make comparisons of rpm/speed between two vessels problematic. Ditto sea state and wind - and also currents - in finding a truly quiet sea in which to compare with GPS SOG.[/QUOTE]

I have just recalibrated the tach on Fresh Aire II. A mechanic measured the revs at the engine with a strobe to be 980rpm at idle and 3,050rpm at full throttle. The tach was reading 700rpm at idle and 2,600rpm at full throttle.

Once you know what your true full throttle rpm (in neutral is) then calibrating the tach is really easy. Simply remove the screws from the engine instrument pod and remove the front (after first removing the wheel for access). At the back of the tach is a little hole marked "Calibrate" and a platic screw deep inside -a jeweller's screw driver works. Turn the screw clockwise to increase the readings (as I had to) and then run the engine to full throttle. Keep on adjusting this screw till the tach reads the same as the max engine revs. The idle revs should then reflect what the engine also says.

That's it!

As was said, the M35B series manual specifies a cruising RPM of 2000-2500. I use a 16" Martec Autostream stainless steel feathering prop 3 blade) and see ~6.5-7.0 knots @2500 RPM. I adjusted the prop pitch to allow the engine to just reach the rated 3000 RPM.

Operating below 2000 RPM might save fuel, but may have the potential for greater maintenance expenses/reduced engine life. Cruising at 2400-2500 RPM yields an average 0.80 to 0.85 gal/hr, in my experience. (The average includes lower RPM maneuvering.)

It is perhaps worth noting that the engine's peak torque is at 2460 RPM, which is an operating speed often considered to be a good balance between fuel consumption and performance.

Indicated speed anomalies.

I had made a post in December of 07 indicating I was getting 5.5 kts at a very low rpm from my little 25XP. I am now confused by what I posted and feel I was off on stated RPM by 1000 RPM.

I have stopped taking that medication now and am feeling better.

I have noted an anomaly since my last haulout that may explain my optimism. The last two times I hauled and painted my bottom I noted a hefty increase in speed, I would expect this may be due to the increased efficiancy of the boat after emptying my wallet. After my last haul out I noted an even larger increase in indicated speed right after putting the boat back in the water I took a group out for a tour of the bay. On a nice power reach I noted the speedo posting consitant 11-12 with an occasional dash to 15. I thought to myself that is some great bottom paint. Now I have done 14kts in this boat before but we were overpowered and surfing then and my eyes were really big according to may son as I was spinning the wheel to near its stop running out of rudder control.
This time we were moving at a nice pace, probably near hull speed but certainly not 12kts. After a month or so the speedo settled down to a normal reading. I am perplexed as the little paddle wheel trasducer just generates pulses as it spins that the knot meter counts. I have no clue why it would see higher speed after haul out then settle back to normal.

On my most recent trip motor sailing north up the coast for seven hours I used an RPM of 2650. This produced the least resonate buzz and the engine hummed along happily, it gave me a speed of 5.5 to 6 kts confirmed by GPS.

Interesting, Steve. Did you have a GPS aboard to compare your SOG (speed over ground) with the STW (speed thru water)?

FWIW, I don't know anyone in our cruising grounds who uses a paddle wheel speed device; they foul too quickly here. We all use GPS speed (SOG) and just realize that we don't always know how fast the current is running, except by estimation.