Underloading

rwidman said:

.................... My point is, either the majority of boat manufacturers don't know what they are doing or the web "experts" are full of BS.

Which do you think it is?

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For the sarcastic impaired, the answer is, the web "experts" are full of BS. Boat manufacturers have the resourses to select the correct engines and props for the boats they manufacture. Engineers, Naval Architects, computers, test tanks, and actual testing of the first hull.

Marketing determines the engine size...

Models with 2 or more different engine sizes will most likely determine what option sizes will be offered in the next model year....Straight from the regional director of marketing for Sea Ray a few years back.

He also explained why a million dollar boat will use a plastic hinge on an access panel in the engine room and not in the galley of many of those expensive boats....

rwidman said:

For the sarcastic impaired, the answer is, the web "experts" are full of BS. Boat manufacturers have the resourses to select the correct engines and props for the boats they manufacture. Engineers, Naval Architects, computers, test tanks, and actual testing of the first hull.

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And from what I've seen new build's are all checked and certified by the engine manufacturers to be sure they meet all the specs. After all, they are the ones carrying the warranty.

But, I do think there is a real over-propping issue in the industry with new boats. Most appear to be propped for a light to medium load which yields a higher top speed than propping for full load. Higher top speed means better glossy brochures, magazine articles, etc. But once the boat is loaded up with all the typical gear, full of water, and full of fuel, it's suddenly over propped. I think this is the issue folks are talking about.

For decades and decades every year the new cars had more power. If they didn't have more power they were considered old. We've been brainwashed to think bigger is better and more is better. Just in case you wanna pass five cars just before the curve you may need extra power. Like two or three hundred HP. We're only reasonably comfortable when we've got way too much and after we've had it for 10 minutes it's not enough.

It is the nature of human beings. We are the "takers" of the earth. We never stop taking. Enough is never enough. Basically all other animals on the planet stop taking when they become comfortable ... usually when they have had enough to eat. Not humans.

Reflecting on the above it's not hard to conceive of yachtsmen wanting and buying boats with much more power than could ever be used.

Willy has 5hp per ton. That is actually excessive for a FD boat. Not much but some. With a FD boat the appropriate amount of power is within a very small range. On Willy it's about 30 to 40hp. 24hp would be 3lbs per hp and that would be doable but at a slight loss of speed and no reserve. I question if any reserve is needed and I may actually be able to maintain 6.15 knots using 20 of the 24hp available but even I would like to have 30. And I have 40. Theoretically I have almost twice as much as I've ever needed.

But w a semi-disp hull (over 95% of TF members (those that can exceed hull speed)) the range of power required becomes greater w more speed flexibility. UNLESS the boat is dedicated to noticeably less than hull speed. And if it is the owner would be better off w a FD boat. More seaworthy (especially in following seas) and more fuel efficient .. 50 to 100% less fuel burned. Those w close to planing hulls will benefit most in fuel consumption savings (at approx 100%) and those w hulls close to FD but still semi-disp will gain some.

But w the correct amount of power under loading will almost never happen.

This is all my opinion and have no references to claim.

Twisted,

Do you think trawlers will have greater top speed in an over propped condition?

Ron says " the web "experts" are full of BS." Industry, architects, manufacturers, builders and sellers have their customers to answer to. They all need to please the customer first. WE DON"T. Whatever they say or recommend may come back and bite them. I don't have to answer to anybody. I can say what ever I want regarding engineering, design and all that other stuff. If I know the truth/facts I can let it all hang out. So in a way there may be more truth and fact in what a TF member says than someone in the industry. So you could say the industry is full of BS too as their lawyers are controlling what they say.

But you're basically right Ron ... web "experts" are less reliable.

twistedtree said:

But once the boat is loaded up with all the typical gear, full of water, and full of fuel, it's suddenly over propped. I think this is the issue folks are talking about.

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XX

My vessel has an empty weight of 50,000 lbs. Fuel and water add about 10,000 lbs with gear, dinghy etc another 2,000- 3,000 lbs. DeFever built 170 DF 48s with mine at #168. The power and weight combination was very refined by hull# 168 with props and engines correctly sized to allow full RPM without overheating. I can do 6 knots with very low fuel burn or speed it up for that beat the tide or climb big swell need.

The "oversized" (by HP rating but not by CID) 225 HP engines are fully capable of being throttled back with no ill effects and will have a very long life as they are not expected to operate in the upper end of the duty cycle. The CID is less than a 25 year old Lehman 6 cylinder.

The blessing and curse of after cooled engines is they yield improved MPG with improved emissions over the RPM range, but with higher maintenance costs. It is unlikely (impossible?) one could buy a new 35 - 60 trawler style boat today without turbos and after cooling. Therein lies the rub with the over propping debate - technology has run well beyond the discussion.

As Twistedtree points out, over propping can brutally harm today's engines. Correct propping in this century is pretty simple or one can make it complicated by their own choice.

manyboats said:

Twisted,
Do you think trawlers will have greater top speed in an over propped condition?

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If by 'Trawler" you mean only displacement speed boats, then No, I don't think so if you keep everything else constant. It might even run a bit slower if the engine can't rev high enough to achieve max HP.

But if you lighten up the boat, it should go faster.

To what ever extend all of us may be talking across each other, it might have to do with displacement operation versus planing operation. Most of the over-propping, burned up engine stories come from planing boats. That's where high output engines appear most often, and is where they get pushed to their limits. I think many exclude planing boats from their definition of a trawler, but I don't.

I also understand the approach of over-propping a displacement boat to achieve some preferred RPM/cruise speed combination even though it may be above the manufacturer's prop load curve. And I think it's fine to do that as long as you also recognize that you've created a situation where at the top end the engine will be overloaded, and that you take measures (like some sort of throttle limit) to prevent operation is that "danger" zone.

Twisted wrote;

"you also recognize that you've created a situation where at the top end the engine will be overloaded, and that you take measures (like some sort of throttle limit) to prevent operation is that "danger" zone."

Many people have said as much lately and that understanding of over propping is very important to people that never post on these threads and understand only a little about it. They need to know that it's a little complicated to over prop and the only really safe way is to prop to rated engine speed. They should also know that there is some advantage to over propping on an over powered boat and that there are very few that aren't overpowered.

manyboats said:

Basically all other animals on the planet stop taking when they become comfortable ... usually when they have had enough to eat. Not humans.

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Eric, not to interfere with the post but just a bit of trivia. Did you know that a squirrel or chipmunk collects and hides roughly 12 years worth of food each year of their life as a habit and obviously never finds or eats most of it. One of the park rangers told me this and found it interesting. Deep thoughts!!

Well that blew me out of the water.

Seriously hahaha

LaBomba said:

Did you know that a squirrel or chipmunk collects and hides roughly 12 years worth of food each year of their life as a habit and obviously never finds or eats most of it.

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I knew that! I always thought, however, that it is part of Nature's reforestation program.

Greetings,
Well, I see how the squirrel information could quickly relegate this thread to OTDE....tread carefully.

Higher top speed means better glossy brochures, magazine articles, etc

Great , the problem is except for cigarettes and some sport fish few owners operate at "top speed".

Then the boat requires a very different view of what is optimum or efficient.

Anyone operate on the pin these days?

psneeld said:

Marketing determines the engine size...

Models with 2 or more different engine sizes will most likely determine what option sizes will be offered in the next model year....Straight from the regional director of marketing for Sea Ray a few years back.

He also explained why a million dollar boat will use a plastic hinge on an access panel in the engine room and not in the galley of many of those expensive boats....

Click to expand...

I think expecations pf potential trawler buyers and potential Sea Ray buyers are worlds apart. That Sea Ray needs a lot of power to get up on plane and folks expect Sea Rays and similar boats to go fast.

When one decides a trawler is best suted for his/her needs, the decision has already been made that not being able to go fast is not an issue.

FF said:

Anyone operate on the pin these days?

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Probably 90% of the recreational boats sold. I sure maxed out RPM a lot with the multitude of sunny day lake type water toys I've owned over the years - especially heavy ski boats with 6 - 8 people onboard.

I'm suprised everyone missed this...

I would argue that underloading with a modern diesel isn't happening at all!

Won't argue wether its bad or not, I just do not think its happening.

Here's why... Lets take the Cummins 5.9L engine as our for example.

That engine is available from Cummins in HP ratings of everything from 220 to 370HP.

The differences between the engines are all add on components. Stuff like turbos, aftercoolers, and MAX RPM ratings.

None of this has anything to do with the basic block, pistons, etc... that are the components involved in "wet stacking".

If my 47' boat had 220 HP cummins engines I do not believe it would be over powered enough to have engine problems.

Now, add on all the stuff that makes the engines capable of producing 370 HP, but run the boat at the same speed as you did with the 220HP engines and you have the exact same cylinder pressures as you had with the smaller engines. Remember at low RPMs the turbos are not producing boost, etc... everything is the same.

I think thats why we don't see underloading related engine failures in modern marine diesels.

ksanders said:

I'm suprised everyone missed this...
I think thats why we don't see underloading related engine failures in modern marine diesels.

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I didn't, see post #23 and #36. A month ago I had a discussion with Cat and Cummins guys on this exact issue. They are "largely" in your camp, or better stated you in theirs.

sunchaser said:

I didn't, see post #23 and #36. A month ago I had a discussion with Cat and Cummins guys on this exact issue. They are "largely" in your camp, or better stated you in theirs.

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Sorry Sunchaser for missing your posts.

It seems at least on this subject we are thinking alike. I feel a little smarter (or luckier) now that you confirmed this line of reasoning with some factory trained techs.

ksanders said:

I think thats why we don't see underloading related engine failures in modern marine diesels.

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I sure am not going to pretend that I am well versed in the technical workings of the modern diesel engine. I do believe that the "under loading problem", so prevalent on this forum, is really overblown. ksanders and Sunchaser's posts make total sense to me. Having a tiny little engine that allows one to achieve hull speed is just one part of the equation. Having more hp for hanging some extra stuff on the engine and the ability to achieve more speed (even at the expense of doubling the fuel flow) in certain instances, makes even more sense to me. At trawler speeds, even the big engines are pretty economical.

Now, add on all the stuff that makes the engines capable of producing 370 HP, but run the boat at the same speed as you did with the 220HP engines and you have the exact same cylinder pressures as you had with the smaller engines. Remember at low RPMs the turbos are not producing boost, etc... everything is the same.

"everything is the same."

Perhaps but many MFG will have slightly different internal builds .

The Turbo engines frequently have trapizoidial rings , rather than square cut , which do suffer from under loading.

So running a 200hp rated engine at 100hp may be OK , but its brother built to 380 hp specks may suffer .

Mechanically put what is the under loading issue? I can think of only two.

1. The Tempuratures of the mechanical parts in the engine aren't high enough to keep unwanted solid or gummy deposits from forming around the rings and preventing the rings free enough movement to ride up and down the cylinder in full contact w the cylinder wall controlling oiling and gas sealing duties per/re the design of the ring.

2. Insufficient combustion pressure to exert on rings like "Keystone" style rings the needed pressure to force the ring to exert enough pressure on the cylinder walls to prevent glazing of the cylinder walls that would reduce the ability of the rings and cylinder walls to seal gasses and allow the engine to perform normally.

A bit wordy and I'm sure many could do better but my point is probably made. If wer'e going to discuss this, much too much argumentative water has gone under the bridge without identifying the actual problem ... Or perceived problem.

#1 and #2 above is very basically the problem as I see it. What more is ther'e really to it? Does the sideways pressure of the piston against the cylinder wall get into the picture or perhaps it's the driving force re glazing?

Want other ideas/opinions.

Engine temperatures are easily dealt with, a thermostat maintains the jacket water temperature and reducing the flow rate or bypassing part of the raw water flow will keep lube oil temps up if you have a seawater cooled oil cooler.

The cylinder pressure difference between 50% load and full load is not all that great.

A turbocharged engine may suffer overheating of valves from extended periods of low loading (<50%) due to reduced charging pressure and the cooling produced by valve overlap.

All the rest is mythology and dock talk.

"A turbocharged engine may suffer overheating of valves from extended periods of low loading (<50%) due to reduced charging pressure and the cooling produced by valve overlap."



Rick, is this in reference to maximum output at crankshaft?

Thanks. SteveH

I'm surprised Rick. I thought coolant temperature had little to do w lube oil temp, piston or valve temps. I thought it was the low temps of piston and ring temps that drove this under loading issue. I see "if you have a seawater cooled oil cooler." ... I would think the oil cooler would be counter productive to benefit an underloaded engine. In fact if I were to constantly under load an engine I'd consider removing or blocking the oil cooler.

Rick says "A turbocharged engine may suffer overheating of valves from extended periods of low loading (<50%)" Well that includes just about everybody on this forum that has a turbocharged engine.

SteveH said:

"A turbocharged engine may suffer overheating of valves from extended periods of low loading (<50%) due to reduced charging pressure and the cooling produced by valve overlap."



Rick, is this in reference to maximum output at crankshaft?

Thanks. SteveH

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Yes, brake horsepower

manyboats said:

I'm surprised Rick. I thought coolant temperature had little to do w lube oil temp, piston or valve temps.

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If the oil cooler is cooled by jacket water then it stays at the correct temperature as long as the thermostat works.

If the oil is cooled by seawater then it might overcool.

Coolant temperature has a great deal to do with piston and valve temperatures ... all the things that go sideways when the raw water impeller fails or the freshwater goes away ... it is the dreaded "overheating."

Valves are cooled by jacket water passages in the head and by charge air during valve overlap. A turbocharger with a worn compressor can lead to valve failure.

Larger engines use staged turbochargers so that charge air flow matches the load more closely. Very large engines with multiple turbos are now being fitted with kits to block off all but one so the engine can operate for extended periods at outputs as low as 10 percent.

I thought it was the low temps of piston and ring temps that drove this under loading issue.

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Underloading problems are a combination of high piston speeds with low loading as occurs in generator engines. It is not such a big deal with propulsion engines except when the power output is very low for long periods since low loads are not experienced at high piston speeds.

I see "if you have a seawater cooled oil cooler." ... I would think the oil cooler would be counter productive to benefit an underloaded engine. In fact if I were to constantly under load an engine I'd consider removing or blocking the oil cooler.

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It depends on how the oil is cooled, I wouldn't remove it, I would restrict coolant flow or bypass it. You need to keep the oil hot enough to maintain the correct viscosity so it lubes everything and doesn't waste a bunch of power to just pump it around. A jacket water cooled oil cooler keeps the oil hot ... or should.

Rick says "A turbocharged engine may suffer overheating of valves from extended periods of low loading (<50%)" Well that includes just about everybody on this forum that has a turbocharged engine.

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Maybe 50 percent was a bad choice of power level, maybe I should have written 25 percent. You make a good point though since we are not bombarded with posts by owners of turbocharged boats telling us about how their engines were wrecked by underloading. so that alone speaks to the reality of this perceived underloading "problem."

VERY informative Rick.

My little Mitsubishi warms up really fast. Three or 4 minutes. Marches right up to 180 to 185 and in five it's 190degrees. Does this at 1100rpm no load. 2300 rpm and heat soaked it's still 190. At 2500 for awhile it climbs just a little as in 3 or 5 degrees. Any idea why my engine is so stable coolant temp wise? Only 2 unusual things. A steel exhaust manifold w lots of extra coolant space and hence coolant because the heat exchanger is not in the manifold. I put it on the aft bulkhead.

Greetings,
Mr. m. "Any idea why my engine is so stable...". MY guess: Well designed and efficient cooling passages and a thermostat that works.

Could be RT but it almost seem too good to be true. Perhaps it has to do w where they put the coolant temp sensor or the position of the thermostat re the position of or the flow of the water pump or ....................... ?
My engine thanks you for the compliment.

manyboats said:

Could be RT but it almost seem too good to be true.

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I think RT nailed it. The sensor is probably located at the hottest point, your cooler works well and the thermostat is effective. That is about as good as it gets.

If you really want to see what is going on, clag some thermocouples or RTDs on the various coolant inlets and outlets and collect the outputs in a datalogger for a few hours at varying loads.

The first half hour or so after start up will be very enlightening for you. After that things will stabilize and not be very exciting.