New to diesel power

[Croation]

I am looking at a boat that is advertised as 22 knots cruise, top end 31 knots. Can I run this boat for extended periods at somewhere in the middle of that? I realize they are saying 22 is the most efficient, but if I am not in the fuel conservation mode, will I be putting the hurt on the Yanmars at a higher RPM.

[Orion Jim]

Accelerate your maintenance schedule and expect a diminished lifespan for all components based on running at max RPM.

[Croation]

Thanks, Jim!

I assume you mean that on a linear basis? As my question was more of running a bit harder than cruise, but less than top end? Thanks, Mike

[Cheechako]

Yanmar's should be fine. You'll find the sweet spot in that range and like it.

[hamburking]

Speed vs RPM is NOT linear!!!

There is the whole issue of getting up on plane (or "on the step"). The previous owner understands the boat...I'd listen to his comments.

However, even not knowing much about your setup, you likely could rev much lower and achieve a slower speed in comfort, safety, and fuel economy...until you are actually out there going 5 knots all day which can be really, really, sloooowwww.... ask any sailor, LOL!!!

[a64pilot]

22 kts is not most efficient, it’s just a number picked that fuel burn vs speed was considered acceptable. Most if they don’t care about fuel burn and range will cruise at a speed that on plane they leave a relatively flat wake with the trim tabs all the way down on average, but please don’t run around just on or almost on plane, bow high and rolling a huge wake. Besides being an ass to fellow boaters, your destroying property, cumulative damage to docks, tied up boats and sea walls etc.
Unless just running around and coming back to the Marina later, you’ll do like most and run at Trawler speed, it’s actually quieter, more comfortable and the fuel burn difference is astonishing. Yes it takes longer, but if your in a hurry a boat is not the best vehicle of choice.

Sportfishing boats in a tournament is a different thing, all tournaments begin with a boat race, and then the center consoles with over 1,000 HP of outboards rule. Idea is of course you only have so much time to fish, and travel time takes away from fishing time.
We fished the Jacksonville King fishing tournament a few times.

[a64pilot]

Originally Posted by Croation

I am looking at a boat that is advertised as 22 knots cruise, top end 31 knots. Can I run this boat for extended periods at somewhere in the middle of that? I realize they are saying 22 is the most efficient, but if I am not in the fuel conservation mode, will I be putting the hurt on the Yanmars at a higher RPM.

Yes, the harder they are run, the shorter their lifespan will be. People will argue that of course, some will even tell you that to make them last, you need to run them hard, that Diesel’s like it.
That’s not true, Diesels will usually tolerate it better than gas motors, but they don’t “like” it. A lot depends on how “souped up” a particular motor is, if it’s not highly turbo charged etc they will tolerate it a lot better than one that is highly turbo charged.
Highly turbo charged means how much boost pressure there is, many engines are mildly turbo charged and these will tolerate high RPM’s better than one that the boost is really turned up.

However if your realistic about how many hours you will actually put on a boat most often you will find out that your not going to wear one out. When you discover that your burning several hundred dollars an hour of fuel, many decide to not burn so much and either back way off or sit at the dock more often.

Often depending on where you are of course you may find the most pleasurable times on a boat is early morning or late afternoon when the wind and waves are down and its peaceful and maybe then motoring along at no wake speeds is sort of relaxing, and your fuel burn is in the tens of dollars an hour too.

[Squanderbucks]

Engine manufacturers rate their engines for use in different categories like recreational- commercial- continuous duty. Etc. each has a different RPM range for the same basic engine. If you plan on running near top rated RPM the for non commercial use there is usually something said like - to run not more than 8 hours out of 10 at top speed. Then must run at least 2 hours before pushing back up.
All this is presupposed that the engines are maintained in top shape and propped do the engines are not over loaded ie can reach top rated RPM plus 100-150 RPM when fully load as planned to operate.
Generally there is a cruise range. Slow cruise and fast cruise. Speaking of on plane. Of course slower is ok if not “pushing” water. My boat travels along at 9 mph, slow cruise at 2200 and what I consider my fast cruise 2500. Top speed is at 3000 RPM. Fuel not a concern then I could fast cruise at 2650 or so.

[Chuck Hawley]

As I recall from the Yanmar owner's manual for a far smaller diesel (3GM30), there were at least three RPMs specified. One was maximum RPM (which I think was 3600), one was continuous RPM (3300?) and one was in-between (3400?)

What's extremely helpful is to know your fuel consumption as you change engine speed. With a diesel, it's a little more difficult since you have to subtract the fuel that is returned to the fuel tank, but I think that you'll find that almost all boats have a "hockey stick" deflection in the fuel consumption, and below that point becomes the sweet spot (with some experimentation).

The charts that show fuel consumption vs. RPM or horsepower will be helpful, but an engine at 3000 RPM can be lightly loaded or heavily loaded, and will burn different amounts of fuel. Measuring fuel consumption directly, or experimenting over time, is probably the best route.

Chuck

[GeneM]

I have two Cummins 5.9L 370HP diesels. They both died, so I replaced them. I've come to believe that letting a diesel sit a long time is what causes the most wear. I also had bad exhaust risers that leaked back.

Sitting allows light rust to coat the cylinder walls, which eats into the crosshatching on every start. Running slow is the next issue. That gumms up the crosshatching, which allows blowby because crosshatching is smoothed over.

So, get your engine specs and find MCR (maximum continuous rating). That's what you can do all day for weeks. This is also called (top of the green), the top of the green area on the rpm gauge.

If the spec sheet doesn't have an MCR, find the WOT (wide open throttle) rating. That is what the engine rpm should do at max throttle. MCR is usually about 80% of WOT. They usually say to not run more than 1 hour at WOT, but you can probably do more.

Running at MCR is best for the engine if it is set up well. The cylinders are clean, the turbos get lots of oil, the heat exchangers are tested (not overheating), and the injectors are not gummed.

I do run often at slow speeds (not on plane) mostly to save fuel. But about every 4 hours I run at WOT for 15-30 min.

I heard of a guy that had lots of smoke and ran all the way to the Bahamas at WOT. That run fixed everything.

If you can't get to WOT, consider getting smaller props. You are lugging the engine with too big props. Lugging means overworking, which should show up as high exhaust gas temp. Not getting to WOT with a clean hull means engine problems or over propped. If over propped, you will likely get better consumption and more speed with less pitch.

[oleman]

If you are lucky enough or unlucky enough to own a diesel made since around 2001 and made by CUMMINS anytime the engine is running it is computing fuel consumption and the consumption can be displayed on a laptop with a free app from Cummins, nice touch!

[a64pilot]

I’ve actually graphed three different planing boats, one 45’ Diesel Sportfisherman, one 36’ gas Sportfisherman, and one 21 ft CC with an outboard, the two Sportfishermen had flow scans and the outboard fuel consumption was calculated by the motor and was picked off its Can Bus. But was amazingly accurate. The Garmin 740s tracked fuel consumption and its calculated fuel burn was always with a could of percent of actual. You check this when you filled up of course.
All three graphs were pretty much identical, fuel consumption on the left side and vertical, speed on the bottom. The graphics were pretty flat but on a slope until close to planing speed, then the graph was nearly vertical, after on plane the graph wasn’t so steep again, but on all three boats at no point was there ever any higher speed that resulted in better mileage than a slower speed. Before I did it, was certain that just after getting on plane there would be a sweet spot where you got the best fuel mileage and going slower or faster would decrease mileage, but I didn’t see that, in every case faster speeds cost more fuel, as in decreased mileage, not just burn rate.
All three boats had trim tabs and they were optimized for that RPM by adjusting them until the highest GPS speed was reached.

In my opinion the “best” speed was the lowest speed that she would run a nearly flat hull on plane, going faster just ate more fuel, and if you needed range, your at Trawler speeds, ideally slow enough so there is very little wake.
What People don’t understand is that 100% of the energy to make that wake, comes from their fuel tank, and it you watch how that wake will bounce other boats around etc., it’s a whole lot of energy.

[Simi 60]

Originally Posted by GeneM

I have two Cummins 5.9L 370HP diesels. They both died, so I replaced them. I've come to believe that letting a diesel sit a long time is what causes the most wear. I also had bad exhaust risers that leaked back.

Sitting allows light rust to coat the cylinder walls, which eats into the crosshatching on every start. Running slow is the next issue. That gumms up the crosshatching, which allows blowby because crosshatching is smoothed over.

So, get your engine specs and find MCR (maximum continuous rating). That's what you can do all day for weeks. This is also called (top of the green), the top of the green area on the rpm gauge.

If the spec sheet doesn't have an MCR, find the WOT (wide open throttle) rating. That is what the engine rpm should do at max throttle. MCR is usually about 80% of WOT. They usually say to not run more than 1 hour at WOT, but you can probably do more.

Running at MCR is best for the engine if it is set up well. The cylinders are clean, the turbos get lots of oil, the heat exchangers are tested (not overheating), and the injectors are not gummed.

I do run often at slow speeds (not on plane) mostly to save fuel. But about every 4 hours I run at WOT for 15-30 min.

I heard of a guy that had lots of smoke and ran all the way to the Bahamas at WOT. That run fixed everything.

If you can't get to WOT, consider getting smaller props. You are lugging the engine with too big props. Lugging means overworking, which should show up as high exhaust gas temp. Not getting to WOT with a clean hull means engine problems or over propped. If over propped, you will likely get better consumption and more speed with less pitch.

I'll just leave this here.

To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings.

.The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less

https://www.sbmar.com/articles/low-s...arine-diesels/

[jmschmidt]

A steady 5 knots into the wind in a straight line from point A to point B in a power boat will beat a sail boat every time. DDW maybe not.

[ranger58sb]

Originally Posted by Croation

I am looking at a boat that is advertised as 22 knots cruise, top end 31 knots. Can I run this boat for extended periods at somewhere in the middle of that? I realize they are saying 22 is the most efficient, but if I am not in the fuel conservation mode, will I be putting the hurt on the Yanmars at a higher RPM.

Not sure they're really saying that. Maybe, maybe not. Find Yanmar's published fuel corve, have a look at their numbers. Sometimes "advertised" cruise speed is just where the fuel curve hasn't yet started to climb higher faster. Often doesn't actually take into account resulting boat speed, which in turn is a factor when computing efficiency too.

In general, the more fuel you burn over time, the faster the engine will wear out. That could be a few years in a sportfish being flogged in tournaments.. or it could be 50 years in a boat puttering along at hull speed most of the time.

It's not uncommon to hear rules of thumb:
- run at 80% (or sometimes 90%) of WOT
- run at 80% (or sometimes 90% of load
- run at "200 off the top"
- et cetera.

All of those are just starting guesses...

Rummage around to see if you can find an owners club for that boat. If you find one, ask there for folks with experience with that engine in that boat.

-Chris