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Old 14-09-2007, 10:13   #1
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To Turbo or Not to Turbo?

That is the question. I'm looking at a boat (cruising cat -- sail) that is powered by two Yanmar 4JH3-HTE's. The boat was built in '03 and the engines have a bit more than 600 hours on them.

The boat is 47' and displaces 28,000 lbs.

Looking at the Yanmar specs, I was surprised to see that these engines put out 100 hp, yet don't seem very heavy (500 lbs), relatively speaking, since the 54 hp 4JH4E weights 440 lbs.

I know I haven't spent as much time researching diesels as I probably should, so I'm trying to do a bit of catch-up, here. I didn't see much on this engine using the search.

What do folks know about it?

What are the pros and cons of turbo?

If the turbocharger should fail, will the engine not run at all? (I've never deal with turbos before, in any way.)

Since I don't expect to be waterskiing behind my sailboat, I'm wondering if such an engine on such a boat is overpowered, and what are the possible consequences of that?

Thank you all for your help and patience with my ignorance.

ID
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Old 14-09-2007, 11:20   #2
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I have a 4JH2-DTE turbo engine rated 83 HP I believe. If the turbo fails the engine will still run although at greatly reduced output. Would be enough to get home though in my opinion.

I recall a rule of thumb for monohulls of 2 - 4 HP per 1,000lbs. That would put you at 112HP for a mono. Add the extra windage area of a cat and I still think it sounds a bit overpowered at 2x100HP. However you will enjoy the extra oomph while maneuvering in close quarters. You'll also likely have good single-engine upwind motor sailing power & fuel economy. 600 hours on these Yanmars is barely broken in...

My $0.02..... I defer to some of the experts on the forum for more details.

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Old 14-09-2007, 11:27   #3
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The advantage to turbo's is hp and fuel economy. The down side it maintenance and cost.

It's just another gismo that needs to be maintained and if you can't get a rebuilt or new one locally then your down for a while.

During your scheduled maintenance, you'll need to pull the manifold side tubing and check for wetness. It should be totally dry. If not, that's the sign of the bearings loosing their seal and will be due for a rebuild. I haven't seen one just fail unless it wasn't checked on PM's.

Check out the price and availability of the turbo's. But they do seem to last, reasonably, a long time.

If your just going to stay around the Sound, I don't see a problem with them.

I would assume these are 4000 rpm motors. The light weight would mean they use the rpm's more then torque, like the big heavy motors in tugs and fishing boats.

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Old 14-09-2007, 11:42   #4
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Turbo's can fail basically in two ways.

First and most common the seals begin to leak oil into one or both sides, high oil consumption can shut the engine down, or the excessive oil can foul the engine.

Second the turbo can throw some soot, or even metal from the blades, and this can take-out the engine intake valves, then you dead in the water.

Maintained shuould not be a problem.

I have removed the turbos on some engines and installed blanks in the oil supply, return and the air passages, engines put out the naturally aspirated ratings, usually about 60% of turbo rating.

Turbo's increase engine efficiency, and will load the cranks etc more tha a NA engine when operated at more than about 40% throttle.

Operating a turbo engine below 30% throttle for a long time will tend to increase oil consumption and tend to kill a turbo earlier, due to crap on the blades.

For me NA is the way to go in a cruiser.
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Old 14-09-2007, 13:27   #5
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I have never understood why some people install turbo'd engines in displacment hulls. But it does happen.
There is a major difference between a turbo in a petrol engine and a Diesel engine. Diesels do like Turbo's. The more air in to the engine, the more power can be derived and the cleaner burning the engine can become. (This is not true in high performance racing Diesels). A rule of thumb is about a 50% increase in power when a turbo is added. (once again non-performance engines). So the major advantages is getting so much more Hp from a small block weight and at little increased cost. However, another rule of thumb is, the more power you get, the less life you get (from the same engine that is). The biggest disadvantage with turbo's is when the power becomes available. Because the power required to spin up the turbo comes from the exhaust, the turbo does not work so well at low RPMs. So the max power comes at higher RPMs. This gives issues with "power bands", where the engine goes from a low Hp to a high Hp with a small raise in throttle. OK if you are a planning hull and the whole idea is just getting up on the plane and going for it. But not so good in a displacement hull. The are a couple of major issues with lag and overun, but once again, only issues in planing hulls. Heat can be another issue, from both the turbo itself and the fact the engine is now producing more power.
All the above issues vary greatly on age of engine/turbo as well. Many modern engines now have very good turbo's that are water cooled for heat proofing along with Light weight small diameter turbines/compressors for fast spin up. Of course, this also comes at a cost of turbo life. Big engine turbos that are slow reving can last may 10' of thousands of hrs. Small high reving turbo's are only in the few thousand hrs.
And finaly, depending on the type of failure will determin if your engine is able to limp you home or not. Most major turbo failures mean the dumping of large quantities of oil and so engine operation would be out of the question.
If I was buying a motor for a displacement hull, I would buy an NA engine. If I need high Hp to weight for a planning hull, turbo is the way to go.

IMO, a super charger is a better option. More torque down in lower RPM, less of the above issues. Major problem is belt noise. Not often seen on Diesel engines and I am not sure why. Common on the GM two stroke diesels, but for a very different operating reason.
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Old 15-09-2007, 12:00   #6
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Very interesting -- thanks for the tutorials. I've attached the performance graph on the engine from Yanmar. If I'm reading this right, and considering your feedback correctly, it looks like the optimal sweet spot for fuel economy and cruising speed is likely to be in the 2300 to 2600 rpm range.

Please check me and see if I'm reading this right: at 2600 rpm, the engine should consume around 153 grams per hp/hr. The curve looks like it is putting out about 37 hp at that speed (37 * 153 = 5661 grams). Assuming a density of 850 grams/liter, that would be 6.66 liters/hr (or, dividing by 3.78 liters/gallon, 1.76 gallons/hr) at 2600 rpm.

Whereas, at 2300 rpm, it would be: 154 grams per hp/hr; 22 hp (3388 grams used) divided by 850 = 3.98 liters/hr (or 1.06 gall/hr).

Am I reading this correctly?

Based on what you folks are saying, if I were doing this from scratch I probably wouldn't go turbo -- something else (expensive) to break and not really needed. But, since this is a used boat I'm looking at, I don't have that option -- it is this boat with these engines, or another boat.

Thank you.

ID
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Old 15-09-2007, 14:32   #7
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Quote:
it looks like the optimal sweet spot for fuel economy and cruising speed is likely to be in the 2300 to 2600 rpm range.
Maybe, maybe not. There is one important figure that is not in the equation. What speed is the hull doing at those revs. (using extreme fiqures to demonstrate) if you were only traveling at 2kts at those revs and the boat was capable of 10kts, then no, it would be better to use a little more fuel and go faster, because in the end, it becomes cheaper. This is one important reason why it is best to match Hp to boat. Once again at an extreme, it is no point in having 1000Hp in the boat if you can still only do 10kts and when a 100Hp motor will also give you 10kts. The The 1000Hp may not be working hard, but it will sure be using the fuel just to turn over at it's rated RPM. Load or no load. A diesel is very different to a petrol engine, in that a petrol engine is almost exponential in fuel usage to load. A diesel is not. It uses "ruffly" a set dosage of fuel per RPM. The fuel increase in loaded and unloaded is about a difference of 20%.
On your graph, the Torgue curve and Hp peak closer to 3-3.2K.
BUT... the engine RPM MUST be matched to the prop and speed of the hull. It MUST be able to rev to close to max RPM when in drive.
So in saying all that, if the boat works OK, I wouldn't worry about the turbo's. And if you ever wanted to repower at any stage, those engines maybe worth enough to allow you to repower with little cost. They are much sort after for planing boats. And seeing as a cat is fast and you need light weight, I see why these engines were chosen. I think they will be fine.
Important rule with turbo's, use really good oil and good filters. Once again I recomend synthetics. The engines like workign, so don't mambypamby them around too much. Light work and low RPM is what causes Turbo's to coke up.
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Old 15-09-2007, 14:54   #8
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It looks like I'm in the same position as ID; I have a 44' steel pilothouse cutter in contract with a Yanmar turbo-diesel auxiliary (4JH-DTE with 2926 hours, removed and rebuilt in 2002). The genset is also a Yanmar, and the two share an oil manifold system that allows clean oil changes. Sea trial and survey are next week, and I've already ordered an oil analysis (engine and gear on both) to be sampled just after the sea trial, while warm. Hopefully, the seller has some prior baseline data.

Any suggestions for things to have the surveyor (or engine guru) check before committing?

Thanks!
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Old 15-09-2007, 15:49   #9
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Thanks, Alan, those are excellent points. Sounds like something to try and figure out in a sea trial: what is the speed of the boat at different rpms? At this point, I have no choice but to assume that the props on the boat were optimized to the hull and engines, though I have no way of knowing that. From the pictures, they appear to be big bronze alloy Flex-o-Folds. Any tips for trying to discover how well they are optimized in a sea trial, assuming we get that far?
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Old 15-09-2007, 18:16   #10
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Quote:
markpj23, he say:
I recall a rule of thumb for monohulls of 2 - 4 HP per 1,000lbs.
I seem to recall reading Eric Hiscock recommending from one-half to one hp per ton of displacement. Seems our generation's expectations have increased…
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Old 16-09-2007, 11:29   #11
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Originally Posted by CaptainJeff View Post
I seem to recall reading Eric Hiscock recommending from one-half to one hp per ton of displacement. Seems our generation's expectations have increased…
Engineeers today push the line to their max these days. That's why we have plastic cars that will withstand impacts AND failure in a lot of products.
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Old 16-09-2007, 13:16   #12
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Wow! they are big props for small high reving diesels. They must be highly geared boxes.
Optimisation is the same in any installation. Find the max RPM of the engine in neutral.(do this manually, not from theoretical paper specs, as the max RPM may have been physicaly limited) Then the max RPM in gear. The difference should be not much more than 10%.
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Old 16-09-2007, 14:11   #13
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Whoa! Is it OK to rev the engine to max, in neutral? I didn't know that -- I've always feared such a thing. Maybe that's a false fear, an old mechanic's myth or something, but I always thought that a great way to throw a rod or lose a skirt. Not so?

Yeah, those looked like big honking props to me, too. I'll have to ask about the gearbox.

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Old 16-09-2007, 21:39   #14
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I'll bet it's geared down to spin larger props....... or it could be just the picture. What size are the shaft's??? (for comparison/reference)
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Old 16-09-2007, 22:59   #15
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Is it OK to rev the engine to max, in neutral?
A Diesel yes, Petrol(gasoline) no. A Diesel engine is govorned and will stop at its' max RPM. It's max running RPM is not it's max runaway RPM should the Govoner not work for some reason. Petrol engines will just rev untill something breaks.
A large diesel in runaway is a little scary to be around.
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