'Cruising RPM' - What Does this Mean ?

[Dockhead]

Originally Posted by cfarrar

I'm not a diesel mechanic, but I think the 80% rule of thumb (75%, etc) is not about fuel efficiency. It's to maintain a diesel's compression, avoid carbon buildup, achieve the optimum engine operating temperature, etc.

Here's my thinking: my engine's operating costs are a combination of fuel/oil, maintenance, and the amortized cost of a new engine or rebuild. If I plan for a "new" engine after 4000 hrs at a cost of $10,000 (an arbitrary figure that lies somewhere between the cost of rebuild and replacement), that's $2.50 per hour - less than my fuel bill, but not by much. If I also consider the benefits to engine health of running my diesel at 75-80% RPM, I realize that it's penny-wise / pound-foolish to consider only fuel efficiency. I cruise at 75% RPM, just below hull speed, at a reasonable 3/4 gph.

The above reasoning is common in operating aircraft, where higher fuel costs argue for a slower cruise speed (but no lower than a "best range" setting) and higher maintenance costs (per hour) argue for a faster cruise speed (up to the equivalent of an aircraft's "hull speed," to put it simply).

Very insightful post.

My own very subjective and flip answer is that if I could get close to hull speed on 3/4 gph (less than 3 liters/hour), then I would forget all about optimizing fuel consumption

If engine life (and the need for maintenance) were accurately measured by hours run, then your argument is very powerful.

However, in my opinion -- which is just a guess, so FWIW -- hours run is not such a good measure of this. BMW use cumulative liters of fuel consumed as a proxy for engine wear, and their on-board maintenance computers will tell you to change your oil or have a regular service not on the basis of miles driven, or engine hours run, but the amount of fuel you consumed -- oil change every x hundred liters of fuel etc.

I'm guessing that my engine will last longer at 2500 RPM than at 3000 RPM, so I will not be conserving engine amortization by running the engine faster. But if my guess is wrong, then you are right. And you are very right to point out that fuel is not the only cost of propulsion.

[smurphny]

Yanmar diesels are VERY prone to glazing the piston walls/smoking if not run at relatively high RPM. The trick here is to make sure that the prop is matched to hull speed so that at around 2600 RPM, the boat is traveling at hull speed. Running these at 2000 will almost certainly create glazing problems. I have had to re-ring one of these with very low hours due to glazing. Also, if these are stored for longer than usual it is good practice to run them occasionally or the rings will lose their "grip". They seem to lose their purchase on the piston walls. Rev 'er up, they like it!

[Captain Bill]

My Yanmar manual says that ideal cruise speed is 80% of Maximum continious RPM, which on my engine is 80% of 3600 or 2880. At this RPM my engines burn about a little less than 3/4 of a gallon per hour each. This gives me a cruise speed of 7 knots on both engnes. I can get about 8.5 knots at full power at about 5 GPH on each engine. Needless to say I only use max power in short bursts if I want to go any distance. Since I have two engines I find my best fuel consumption solution, short of putting the sails up and shuting them down, is to simply shut down one engine. I keep the rpm around 2900, still get 6 knots and cut my fuel consumption in half. I target refueling at 80 hours of engine run time and I invariably require 56-58 gallons. Of course no tank load is ever done totally at the ideal cruise RPM, but these numbers have been very consistent. The worst case was one time I spent a couple of hours running at high rpm for a couple of hours going directly into a 35 knot wind and an outgoing tide in the Cape Fear river and that tankup took 63 gallons.

[smurphny]

Capt.Bill. Your fuel consumption (3/4 gal/hr)is about the same as I get with my one 3GM30F. Maybe asking Michigan Wheel to calculate your optimum prop size would help. They have a detailed on-line form to fill out and will get back to you with a recommendation. With twin screw, the calculation of best prop has got to be really difficult. I know as soon as I start to push through the water over hull speed, fuel consumption goes up dramatically. If you are exceeding hull speed at a lower rpm, it could mean props are too large/pitched wrong. Oh to have twin screw again! Maneuvering the old Alberg, single handed, under power is often a "fire drill." Putting her in reverse in current and wind is magically unpredictable:-)

[S/V Alchemy]

Originally Posted by Don Lucas

So if the "cruising rpm" is the most effecient for the boat (not the engine), why would people be talking about running at 80% of this?

Because people are inexact about their terms. I have always taken "cruising rpm" to be synonymous with "as fast as I can motor with the least amount of fuel". If we take an arbitrary "average cruiser's diesel" as 50 HP or thereabouts, that number will be revealed in the customary "power band" chart that most engine builders issue.

Of course, your mileage may literally vary. In some ways, figuring out "the sweet spot" is part intuition and part measurement, because there are too many variable, like attempting to "sail to one's rating"...good luck with that!

Short of using a flowmeter and a stopwatch and physically counting the minutes it takes to burn a litre or gallon at a given RPM, I think this is only ever going to be an approximation. Sea state, boat shape, drag you can identify (prop) vs. that chunk of seaweed you can't see around the keel are all going to screw with the results.

I have expectations that an RPM of 1,900-2,100 is going to burn in my new Beta 60 approximately 0.8 U.S. gallons of diesel per hour, and will yield in a flat sea a forward speed of 5.2-5.4 knots. I base this on what I achieved with the previous 52 hp engine driving a different prop and with a slightly different gear ratio.

Should the seas kick up, but with no wind, the speed could drop to 4 knots. I don't really care, because in the sort of conditions that I would motor, range will trump speed unless I am trying to power out of danger or to get in ahead of weather, anchor firmly, etc.

The brilliant aspect of diesels is that they are quite efficient at getting effort out of a small amount of fuel. They do not, however, like changing RPMs and will get greedy at higher speeds. So phrases like "80% of cruising speed" are just marketing guff, because every engine in every boat is going to react both generally (they all seem to have a sweet spot in the low 2000s) and specifically (the last 1,000 RPM will kill your fuel budget and that's an expensive extra knot of speed to purchase...but how expensive? You'll have to measure for that engine in your boat!).

[Tellie]

Originally Posted by cfarrar

I'm not a diesel mechanic, but I think the 80% rule of thumb (75%, etc) is not about fuel efficiency. It's to maintain a diesel's compression, avoid carbon buildup, achieve the optimum engine operating temperature, etc.

Here's my thinking: my engine's operating costs are a combination of fuel/oil, maintenance, and the amortized cost of a new engine or rebuild. If I plan for a "new" engine after 4000 hrs at a cost of $10,000 (an arbitrary figure that lies somewhere between the cost of rebuild and replacement), that's $2.50 per hour - less than my fuel bill, but not by much. If I also consider the benefits to engine health of running my diesel at 75-80% RPM, I realize that it's penny-wise / pound-foolish to consider only fuel efficiency. I cruise at 75% RPM, just below hull speed, at a reasonable 3/4 gph.

The above reasoning is common in operating aircraft, where higher fuel costs argue for a slower cruise speed (but no lower than a "best range" setting) and higher maintenance costs (per hour) argue for a faster cruise speed (up to the equivalent of an aircraft's "hull speed," to put it simply).

I mostly agree. Except that 4000 hrs on a well maintained and operated diesel is not a lot of hours.

[S/V Alchemy]

Originally Posted by defjef

The other point we don't see discussed with respect to engine wear is the nature of the engine wear. A diesel run at no load and idle will show more wear than one run under load at a few thousand RPM. So few entire rotations can equate to MORE engine wear. I would guess that the same would apply running the engine constantly at the "red line". Each of those three engines running the same time would show different wear with the idle and the redline engines showing more wear.

That is in essence why I've bought a replacement engine prior to passagemaking. We bought a 20 year old boat with the original engine but with only 1,100 hours logged. That's 80-90 per season...pretty low but representative of the Great Lakes. Certain aspects of its otherwise reliable starting and running made me think that a lot of these hours were idling to charge the batteries. Throw in an unproveable and unlogged winterization history, and I opted for new over used, because a rebuild would've cost me only a grand less than new.

But why rebuild at 1,100 hours (1,300 hours after two years with us)?

Because diesels are killed faster by low hours, in my view, than by anything else. Switching on a cold diesel and powering out to head to wind, and then shutting down, is very wearing and the diesels around here show it. Diesels, once on, like to stay on, and they like to cruise at a nice, warming RPM. Switch on, run cold lake water through them and then switch off 10 minutes later? Not so much.

[Bash]

Cruising RPMs designates the point where giving the boat more throttle no longer makes economic sense. On most displacement boats this will be around 80% WOT.

On my Yanmar 4JH2E, cruising RPMs with a three-blade Flex-O-Fold were 2,800.
On my Yanmar 4JH3-TE, cruising RPMs with a three-blade Flex-O-Fold are 2,800.

Of note: the turbocharging didn't make a lick of difference.

[chris07732]

This is a great thread!! I have a new Yanmar 4JH4-TE, 75hp, installed, only 8 hrs on it before pulling for the winter. I delibrately over propped the boat with the anticipation that I'll adjust prop OD or pitch to achieve that sweet spot of RPM vs speed vs NVH that was my goal for the new engine. Based on my sea trials in flat water before pullng for the winter, I saw hull speed (7.9kts) at 2500 rpm. I could power thru hull speed to 8.1 kts but only to 2750 rpm.

WOT therefore in my installation is 2750 rpm. Yanmar demands to see max 3200 rpm or warranty is voided due to the glazing issues.

My problem is that I'm comfortable with this setup as it is. As I see it, I can cruise at 2200-2300 rpm, 5.5 - 6 kts with engine sufficiently under load to avoid glazing. I have reserve power, 2200 rpm to 2750 rpm, to go faster/harder into seas and if conditions get worse and I have hard time maintaining hull speed due to more wind/seas, there is more power on tap above 2750 rpm to accomplish that. Since in rough conditions, it takes more power to maintain hull speed and since my WOT which is 2750 in flat water, will go up until I achieve hull speed into the waves/wind.

Another criteria someone's suggesting comes into play is 80% max rpm is good cruising rpm. 80% of my 3200 max is 2650 rpm. If I can cruise at 2200 rpm and no glaze, I thinking I'm good and NVH will be less at 2200 rpm than at 2650 rpm.

Deep down, I believe that I'm sensitive enough to technical issues that I will not to try run my motor sailer above hull speed and therefore operate the Yanmar in glazing conditions (full throttle/fuel delivery, but not achieving max rpm, possibly generating black smoke, etc). At least, that's my story to Yanmar and I'm sticking to it. I haven't heard back from them yet.

[SkiprJohn]

Aloha Chris,
You might be comfortable with the prop as it is but your engine might be more efficient with your prop pitched less. You'll still get the same top speed on the boat when needed and your engine will have more power with more rpm to punch through whatever you need to.
kind regards,

[Captain Bill]

Originally Posted by chris07732

Deep down, I believe that I'm sensitive enough to technical issues that I will not to try run my motor sailer above hull speed and therefore operate the Yanmar in glazing conditions (full throttle/fuel delivery, but not achieving max rpm, possibly generating black smoke, etc). At least, that's my story to Yanmar and I'm sticking to it. I haven't heard back from them yet.

What are you expecting them to say? You've already told them you know more than they do about their product and you're going to do things your way. You've overproped your boat despite warnings in your owner's manual not to do so. On top of that you've confessed to doing something that will void your warranty. Most customer service organizations operate on the principal of never arguing with the customer and be nice at all times. They probably just made a note of your blown warranty in the database and went on to help other customers.

[NY GIN]

This kind of goes along with this thread ...... a1972 38' Egg harbor, 22,000 lbs with twin 454s. What would be the optimal "trawling" vs "cruising" speed and what kind of GPH can I expect?

Is it feasible to assume cruising at around 14 knots would give me a burn of about 25 GPH?

How about trolling around 6 knots, 4 GPH ??

[Liam Wald]

Originally Posted by Tellie

I mostly agree. Except that 4000 hrs on a well maintained and operated diesel is not a lot of hours.

Agreed. Properly run and cared for a Yanmar should easily do 8,000-10,000 hours. On the other hand I did an engine repacement on a boat that was 30 years old and had less than 300 hours on the diesel. I doubt that this engine was ever run long enough at one time to get up to temp. That same engine, if used for a pump or generator could have probably run at 80% power non-stop for 2-3 years or 17,000 to 25,000 hours. And then do it again with a top end re-build!

[biminijimini]

Although most of you are talking of sailboats my boat is a power cruiser with a planing hull. I run the engines (gas) at 75-80% {3200RPM}of full throttle much of the time. That's what the cruiser was built for - that's how I run it. The engine manufacturer has recommended long term periods of RPM suggested ratings. They are there for a purpose. I follow their recommendations.

[Tellie]

Originally Posted by Captain Bill

What are you expecting them to say? You've already told them you know more than they do about their product and you're going to do things your way. You've overproped your boat despite warnings in your owner's manual not to do so. On top of that you've confessed to doing something that will void your warranty. Most customer service organizations operate on the principal of never arguing with the customer and be nice at all times. They probably just made a note of your blown warranty in the database and went on to help other customers.

BINGO!!
I'm always amazed at folks that are smarter than the manufacter.
But then mechanics have to make a living too. Ya have to wonder why Yanmar, Volvo, Cat, etc. spends millions in R&D when all they really have to do is ask a sailor.