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#1
posted to rec.boats.cruising
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Running a large diesel slow
As we continue to look at trawlers and motor yachts, they seem to come in a wide range of diesel engines. We've seen some 44' - 47' boats that use engines ranging in horsepower from a single 125 to twin 450s. They seem to have only small "relative" differences in LWL (to me meaning similar hull speeds). They have differences in displacement, but the differences don't seem to track very closely to engine HP. If these boats are operated below planing speed as displacement hulls, then shouldn't they require around the same amount of power to push them through the water? In reading engine specifications (power and fuel consumption) it appears that large diesel engines running at a relatively low RPMs use around the same of less fuel than smaller diesels that are run harder. Or at least the horsepower to the prop seems to cost about the same fuel consumption regardless of engine size. The engine RPMs for a given HP seems to fall at different points on the torque curve for larger HP engines than for smaller ones, especially for turbocharged engines. (I don't know if this makes any difference or not.) (OK, thanks for following. This is what I'm leading up to....) So is it more economical to operate large diesel engines at low RPM or small diesel engines at higher RPMs? Is there a long term problem with either running a large diesel under gentle RPMs or a smaller diesel closer to its continuous duty capacity? Is there a maintenance advantage (extended oil, belt, filter, etc., longer engine life) to running a diesel slower? Are these questions even meaningful to boat ownership and maintenance? TIA and happy New Year to all!! |
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#2
posted to rec.boats.cruising
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Running a large diesel slow
Cal:
You are going through exactly the analysis that I did before buying our new trawler. As you have noted there are true displacment trawlers- Krogens for example and fast trawlers- Sabres, Mainships, Albins to name a few. The true displacment trawlers have naturally aspirated diesels- the venerable Ford Lehman, or lightly turbocharged, low rpm diesels- the John Deere in the Krogen. The fast trawlers have somewhat larger engines, but mostly they are heavily turbocharged with high boost pressures, common rail fuel injection, etc which yields up to 70 hp per liter. In my view if you don't go overboard, you can operate a fast trawler slow at displacment speeds and enjoy the benefits of the fast engine when you need it. Yes, the Krogen is a very efficient displacment hull and the John Deere is a very efficient modern diesel. But a fast trawler such as my Mainship 34T with its 370 hp Yanmar can operate at displacement speeds just fine. I run mine about half the time at displacement speeds and another half at about 35 hp/liter which just gets me into planing territory at 12 kts. The other consideration is running the high horsepower engine too slow and at too light of a load. At some point- less than 10 hp per liter in my view, a modern high output diesel will not reach operating temperatures. Then you will get sooting, unburned fuel washing the cylinder walls, glazing, high oil consumption, etc. But if you stay at 10 hp per liter or greater, particularly if you run at fast cruising speeds for the last ten minutes of the day, then you will be ok. Some trawlers, the Sabres and Albins come to mind, put such large twin engines that you can't really run them a displacment speeds and stay at the 10 hp per liter rule. These might work if you only ran one engine, however. And back to your original question about engine efficiency. My Yanmar 370 peaks at about .05 gal/hp/hr at 2,800 rpm which is my fast cruising speed. Slow it down to 1,700 rpm and it drops to .06 gal/hp/hr. I suspect there are two reasons for this. First it takes a lot of parasitic horsepower to simply turn over a big block and that parasitic horsepower is a bigger part of the total at 1,700 rpm (60 hp) than at 2,800 rpm (200 hp). Secondly an injector that can squirt enough fuel for 370 hp won't be very efficient at 60 hp. The common rail piezo injectors have improved on this however. An engine designed for displacment speeds only, say 3 liters and 100 hp would be about 20% more efficient than the 370 hp Yanmar in my boat. So, I probably answered more questions than you asked. Enjoy your search for a new trawler. David |
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#3
posted to rec.boats.cruising
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Running a large diesel slow
Cal Vanize wrote: As we continue to look at trawlers and motor yachts, they seem to come in a wide range of diesel engines. We've seen some 44' - 47' boats that use engines ranging in horsepower from a single 125 to twin 450s. They seem to have only small "relative" differences in LWL (to me meaning similar hull speeds). They have differences in displacement, but the differences don't seem to track very closely to engine HP. If these boats are operated below planing speed as displacement hulls, then shouldn't they require around the same amount of power to push them through the water? In reading engine specifications (power and fuel consumption) it appears that large diesel engines running at a relatively low RPMs use around the same of less fuel than smaller diesels that are run harder. Or at least the horsepower to the prop seems to cost about the same fuel consumption regardless of engine size. The engine RPMs for a given HP seems to fall at different points on the torque curve for larger HP engines than for smaller ones, especially for turbocharged engines. (I don't know if this makes any difference or not.) (OK, thanks for following. This is what I'm leading up to....) So is it more economical to operate large diesel engines at low RPM or small diesel engines at higher RPMs? Is there a long term problem with either running a large diesel under gentle RPMs or a smaller diesel closer to its continuous duty capacity? Is there a maintenance advantage (extended oil, belt, filter, etc., longer engine life) to running a diesel slower? Are these questions even meaningful to boat ownership and maintenance? TIA and happy New Year to all!! Cal, you hit on a bunch of points, let me see if I can cover them. First off, trawlers are displacement hulls, adding lots more power does not equate to lots more speed. I've seen plenty of lobster boats that crused at 15 knots with say a 200 Hp engine. Someone decides to add a 400 Hp engine, and only gains three knots (provided the hull continues to operate in the displacement mode, some start to climb up on a plane on the keel, and fall off, resulting in a scary ride). My point being, as you noticed, don't expect to see a dramatic increase in speed due to much higher power, that's just the way displacement hulls behave. On running engines at low RPM's, diesels, like gas engines, have their "sweet spot" where they run best. On older style engines, running them for extended times at low RPM's can cause problems. When you do this it's best to give the boat/engine a good run to burn off any soot that may start to build up. Sportfishing boats run into this after extended periods of trolling, when they make the run home the engines typically run poorly for a short period of time while everything is burned off. On newer style common rail engines I suspect this is not as much of an issue. Diesel's run best under load at rated RPM's. While the engine speed and economy varries from engine to engine a propeller is most effeciant at slow speeds, so the best combination is a large propeller spinning as slow as practical. For a trawler a slower turning engine makes the most sense, your just not going to be able to use the extra RPM's a faster spinning engine has, they will gear the engine, or prop the boat, with less reduction and a smaller prop (beware, they tend to under prop such engines on purpose, to me that's a dead giveaway that their not going to hold up). For your application, a reasonable size engine, turning at lower RPM's is your best bet, a common rail setup would really give you the best of both worlds. It will give you good fuel burn numbers, good speed and long term reliability. I would stick with engine manufacturers like Cat, Cummins (I would avoid the over 300 Hp 5.9 engines) or Lugger, I would avoid the lightweight high RPM engines, I haven't seen then hold up as well. The right engine should outlast you. I hope this helps you. John |
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#4
posted to rec.boats.cruising
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Running a large diesel slow
Cal Vanize wrote: As we continue to look at trawlers and motor yachts, they seem to come in a wide range of diesel engines. As a confirmed old school sail guy I have advocated a specific "trawler" type to all. its really a buy of a llife time opportuity. Do a Google search for ship brokers and look at the "gulf shrimper" vessels. I defy you to compare the following numbers for a MOTOR BOAT: 1997 to about 2000 Steel 75' x 22' x 12' of course may vary up to 85' x 24'x 13' Single 540 HP Cat.... 3412 rated at 100% continous HD.... Industry standard. 20 GPH Got problems???? Just go to any real boat yard that works on real work boats, not "yachts" and get a fair deal for fair work. Now get this........................... $100,000 to about $120,000 Why on earth would anyone buy some fiberglass, candy ass, yacht when yas can get a real boat for 1/5 the price????!?!?! Bob |
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#5
posted to rec.boats.cruising
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Running a large diesel slow
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#7
posted to rec.boats.cruising
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Running a large diesel slow
Hi,
The smaller the diesel engine the harder it has to work to achieve the hull speed of the boat. (Square root of the waterline length X 1.34) You need the minimum size diesel that will achieve this. If 50 horsepower does it then that is what you need. Diesels must be worked hard or there are several issues with glazing and smoking. A hard worked diesel engine is a happy engine. If you want a planing fast boat then you must buy more horsepower. However a Trawler type 'Slow Boat' only needs minimum power. They are cheaper to run per hour as well. Oliver Fleming "Cal Vanize" wrote in message ... As we continue to look at trawlers and motor yachts, they seem to come in a wide range of diesel engines. We've seen some 44' - 47' boats that use engines ranging in horsepower from a single 125 to twin 450s. They seem to have only small "relative" differences in LWL (to me meaning similar hull speeds). They have differences in displacement, but the differences don't seem to track very closely to engine HP. If these boats are operated below planing speed as displacement hulls, then shouldn't they require around the same amount of power to push them through the water? In reading engine specifications (power and fuel consumption) it appears that large diesel engines running at a relatively low RPMs use around the same of less fuel than smaller diesels that are run harder. Or at least the horsepower to the prop seems to cost about the same fuel consumption regardless of engine size. The engine RPMs for a given HP seems to fall at different points on the torque curve for larger HP engines than for smaller ones, especially for turbocharged engines. (I don't know if this makes any difference or not.) (OK, thanks for following. This is what I'm leading up to....) So is it more economical to operate large diesel engines at low RPM or small diesel engines at higher RPMs? Is there a long term problem with either running a large diesel under gentle RPMs or a smaller diesel closer to its continuous duty capacity? Is there a maintenance advantage (extended oil, belt, filter, etc., longer engine life) to running a diesel slower? Are these questions even meaningful to boat ownership and maintenance? TIA and happy New Year to all!! |
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#8
posted to rec.boats.cruising
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Running a large diesel slow
On Mon, 01 Jan 2007 18:16:19 +1000, Oliver Fleming wrote:
Hi, The smaller the diesel engine the harder it has to work to achieve the hull speed of the boat. (Square root of the waterline length X 1.34) You need the minimum size diesel that will achieve this. If 50 horsepower does it then that is what you need. The rule of thumb for this is 2hp per ton of displacement. So a 20 ton boat would need 40hp to reach hull speed, and a 50hp diesel to operate at 80% capacity (another rule of thumb). But this doesn't take wind or waves into account. Matt O. |
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#9
posted to rec.boats.cruising
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Running a large diesel slow
On Mon, 01 Jan 2007 12:34:08 -0500, Matt O'Toole
wrote: The rule of thumb for this is 2hp per ton of displacement. So a 20 ton boat would need 40hp to reach hull speed, and a 50hp diesel to operate at 80% capacity (another rule of thumb). But this doesn't take wind or waves into account. Or hull shape, bottom condition, engine/propellor configuration, etc |
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#10
posted to rec.boats.cruising
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Running a large diesel slow
"Oliver Fleming" wrote in
: The smaller the diesel engine the harder it has to work to achieve the hull speed of the boat. (Square root of the waterline length X 1.34) You need the minimum size diesel that will achieve this. If 50 horsepower does it then that is what you need. Diesels must be worked hard or there are several issues with glazing and smoking. A hard worked diesel engine is a happy engine. If you want a planing fast boat then you must buy more horsepower. However a Trawler type 'Slow Boat' only needs minimum power. They are cheaper to run per hour as well. Oliver Fleming Dead on and very well said. I see overpowered Nordic Tugs PLOWING down the ICW through Charleston on the Florida Express trying, in vain, to drive them like a bass boat with those overpowered "Express" engines. All they do is make a huge bow wave to crash into everyone's boat and dock near shore. With that big a bow wave, they must be just guzzling diesel. It certainly doesn't make speed. On the other hand, my friend Dan lived aboard a 1980-something Hatteras 56 FBMY with twin 8V92TA (twin turbos) train engines in it driving 32" screws, Naiad Stabilizers and hydraulic planing fins under the swim platform you had to climb down to. Dan is NOT a boat captain and could barely sidle it up to a dock with all this power. He was deathly afraid of planing it. So, there are these HUGE 735hp twin two-stroker beasts idling along at 1200 RPM in "trawler mode" for hours on end. The book says they should have had all kinds of problems...coking, carbon deposits, all those awful things, glazing, you hear about. Dan never worked them at all...just idling around. The only time they were "worked hard" was when I was at the helm, offshore and away from the fishermen far enough where the monster wake wouldn't sink anyone. It was a sight to behold with 55 tons riding right up on plane. I trimmed the tabs, energized the Naiads I'd just fixed others had given up on, and after the big exhausts cleared their throats of the accumulated carbon in them (leaving a black trail that looked like a steamship blowing its tubes.. (c  ...she ran great! I never saw any evidence of anything dastardly when I throttled them up to full. All the rest of the time they ran at just above idle in the ICW or around the harbor. The diesel engineers pulled both engines apart when Dan sold her for the new owner to assess their condition. Their condition was outstanding, as I suspected because they always started dead cold on the second compression they came to, even in what little winter we have here. They found one weak spring in the injection and replaced it. Far as I know, the new owner I've seen when it passes through Charleston, never mentioned any problems with them, a lot of hours later. He wanted me to make a drawing of the extensive power distribution system I had installed for the electronics suite from a separate panel. I mailed it to him in the BVI. I don't think running them at so little load so slow hurts them at all.... My feeling for a trawler is it should have just enough HP to climb its way up a fairly good sized ocean swell when it's rough outside. 50hp isn't it, unless you're going to stay in the ditch and never go outside. I'd want the extra HP to make sure I wasn't going to slide back any more than is absolutely necessary. But, I think twin 400hp V-8's is stupid. The boat builders must be doing that to satisfy the demand of buyers whos lives move at the speed of jets. Those people shouldn't be buying a trawler in the first place....or a sailboat... Everyone is in WAY too much of a hurry to "get there".... |
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