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Old 11-24-2008, 09:56 AM   #1
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How much power is enough?

Hiya,
** At Mr. Bakers suggestion I would like to ask...how much power is necessary on any particular boat?* Is a 34' CHB, for example, over powered with a 120HP Lehman?*How little power would be necessary for said vessel to operate safetly (keeping in mind the "extra" power needed for wind/current/tide etc.) and ecomomically?
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Old 11-24-2008, 11:41 AM   #2
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RE: How much power is enough?

I think your example of a CHB with an FL120 in it is theoretically overpowered in that it doesn't take this much power to move the boat at a normal cruise speed. However, as FF pointed out, there is a point of diminishing return when you run a lower power engine harder to achieve "x" speed. It will run hotter, pressures will be higher, and so forth, all of which can reduce the working life of the engine. So using a higher power engine and "detuning it" so to speak, can make for a longer-lived engine assuming the engine is still run within the proper temperature range for complete fuel combustion and so forth.

The advantage in having more power than is needed simply to cruise the boat is apparent in the photo of Spray "at speed" I posted somewhere else in this forum. If you have a hull that can take advantage of it, there may be times when being able to push the boat considerably faster than normal is a Good Thing. People usually cite outrunning weather, but there are plenty of other occasions when you might want or need a lot more power. In this neck of the woods, having to buck an 8 or 10 knot current is not unheard of, and running against a four or five knot current, a fairly common occurance here, could be cause to add a fair amount more power if you have it, particularly if you are on a timetable.

Also, since most marine diesels are marinized vehicle or industrial engines, that sort of determines what's available for the marinization folks to use. There was a four-cylinder version of the FL120 which develops 80 or 90hp. This engine was used in a lot of GB32s in the early days of that program. But since I think there are, or were, relatively few land applications for a strong, reliable, small but easily marinized diesel that can move a typical trawler-type boat, the engines that worked best for marine applications tended to be 100 hp or more simply because that's what the base engines developed.
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Old 11-24-2008, 08:34 PM   #3
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RE: How much power is enough?

How much power is enough?

So much that you are afraid to use all of it!
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Old 11-25-2008, 04:38 AM   #4
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RE: How much power is enough?

If you are interested in a displacement boat about 3hp / ton of displacement is about all the hull will absorb.

Do not use USCG "tons" they are a volume not a displacement measure.

Many will power to 5hp per ton , on the assumption the load will still be nice and substantial at cruise speed .

There is usually NO reserve for current , you only go thru the water at the same speed for the same power.

Wind is a problem usually in docking , although the ability to proced in 40K can usually be done.

The big power absorber is CLIMBING waves , which can require 50% more power AT THE SAME SPEED.

Usually most folks do not proced when the G loads get over about 2.5G , so slowing down to climb the hills is the most common, not requiring additional power .

So a "proper" engine instalation will still be a nicely loaded SMALLER engine, rather than a Monster , for "just in case."
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Old 11-25-2008, 07:33 AM   #5
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RE: How much power is enough?

Chop and headwind will knock you back considerably.
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Old 11-26-2008, 04:20 AM   #6
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RE: How much power is enough?

"Chop and headwind will knock you back considerably."


Go out a day offshore and see what a 20K breeze can do with good fetch and plenty of time.

Climbing 8 ft hills slow you down!!!
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Old 11-29-2008, 02:00 PM   #7
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RE: How much power is enough?

RTF,

For a FD hull FF is right. Now that we are back to talking about boats I can agree with him now and then. 3to 4 hp per ton yes. My willard is at 4.5hp with 37hp and 8 tons. It's hard to select an engine with less than 37hp for an 8 ton boat. I know of a boat that went from Scotland to Puget Sound with 2hp per ton via Cape Horn. Makes me look over powered. I'm glad I can work the engine fairly hard without over driving the hull. I don't see any reason for " extra " power with a FD hull. You don't have to " climb the waves " FF. They will roll under you before you slow down appreciably and I have not had the need for more power in rough going .. I usually slow down 300 rpm. Marin seems to think engines wear out in proportion to thier engine speed .. as though an engine is destined to turn over x number of times in it's life. Probably 98% of engines in our boats have easily 10000 hr lifespan. Even IF Marin is right ( and he is wrong ) who needs more engine life than that. I'm on my old axe of engine loading .. sorry. Many ( or most ) FD yachts are over powered for marketing reasons ... it's hard to convince people they only need 60hp to drive thier 30000lb boat. Most people have cars that have twice that and they are just little things .. or appear to be. FD*boats do vary* in thier powering needs due to design variations like sailboats, fishboats ( not fixed load and people that make yachts of fish boats should be aware of this ), tugs, kayaks and others but most of the boats we are talking about vary little. Bilge keels, stabilizers propeller shrouds and other apendages increase wetted surface and hence drag. Some boats have*a lower*prismatic coefficent*( more pointed at the ends ) and others are more full ( blunt ) and make bigger wakes. That takes more energy .. power. But*most FD yachts*are similar. So it's really very easy to determine appropriate power for a FD yacht.

Eric Henning
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Old 11-30-2008, 04:26 AM   #8
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RE: How much power is enough?

Engines are best contemplated as fuel burners , not rpm makers , in terms of engine lifetime.

Most folks also slow down when proceding at normal cruise would make the ride too harsh.

But the folks that worry about "too small too efficient " power levels always claim to worry about 50K winds and punching into 14 ft seas.

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Old 11-30-2008, 11:13 PM   #9
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RE: How much power is enough?

Quote:
nomadwilly wrote:

Marin seems to think engines wear out in proportion to thier engine speed .. as though an engine is destined to turn over x number of times in it's life.
They do. In fact, I had the chief engineer of one of England's premier engine manufacturers tell me this in almost exactly the same words--- an engine, any engine, is designed for a finite life, he said. It's designed to go around and around so many times and then all bets are off.

It's a no-brainer to realize this. Every engine designer, manufacturing engineer, and professional engine service or overhaul mechanic I have ever talked to about this over the last 40 years (Ford of England, Northern Lights, Land Rover/Range Rover, Fairbanks Morse, EMD, GE, Pratt & Whitney, and Rolls Royce to name some of the ones I still remember) has told me the same thing---- the harder you run them the faster they wear out. Reciprocating, turbine, petrol, diesel, Jet-A, same answer every time.

Obviously there are lots of other ways you can prematurely wear out an engine, but running it hard is a 100 percent foolproof way to hasten the day it wears out. This isn't to say an engine should never be run hard. But even airlines look for ways to use less power on takeoff (safety permitting) which reduces engine wear which reduces maintenance and overhaul costs. If I listed all the airline maintenance directors who have told me this over the last ten years of visiting airlines all over the world this post would be two pages long.

Saying a turbine will wear out in fewer hours than a Ford Lehman because the turbine is running at tens of thousands of rpm where the FL120 is only running at 1600 rpm is stupid because this is comparing apples and oranges. But running the FL120 hard at 2,000-plus rpm all the time will wear it out faster than running the same engine at 1600 rpm all the time (assuming correct combustion temperatures, oil condition, properly functioning cooling system, etc). This is so basic I find it hard that anyone with any engine experience at all doesn't understand this.

*


-- Edited by Marin at 00:18, 2008-12-01
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Old 12-01-2008, 09:02 AM   #10
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RE: How much power is enough?

There is talk lately about using total gallons consumed as opposed to total hours. Hours don't tell the owner/buyer much about the actual use of the engine. (ie) Running at full throttle for long periods of time or cruising at very low RPMs. Engineers think that the total gallons consumed by an engine is a better yardstick as to its health. To do this, however, one must have some kind of fuel flow equipment onboard or an electronic engine thats keeps a tally on such things.

What say you?
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Old 12-01-2008, 10:15 AM   #11
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RE: How much power is enough?

Sounds good in theory. However, how many equipment manuals do you have that say "change the oil at 400 gallons fuel burn"? You'd have to come up with numbers pretty much from scratch, and then they'd probably just be based on a gal/hr and hours run to gallons, which is still pretty much accomplished by watching the hour meter.

Call up Bob Smith and ask him how many gallons you should burn in a Lehman before you change the oil. That should be funny!
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Old 12-01-2008, 10:23 AM   #12
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RE: How much power is enough?

Coho's fifty-year-old manual for her Cummins HN6 main engine specifies all service intervals in gallons of fuel consumed. Does Cummins still do that?
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Old 12-01-2008, 12:11 PM   #13
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RE: How much power is enough?

Quote:
sloboat wrote:

The operative phrase in your discussion is "the harder you run them". An engine running "X" rpm in neutral will have a different life than one running at rated power at the same rpm.
Absolutely true.* But most people equate engine rpms with how hard an engine is working (although I know this is not necesarily the case).* So with this assumption, the working life of an engine can be related to how fast it is being run.* Obviously one can lug an engine something fierce--- in a boat that would be putting too coarse a prop on it and then running the engine at relatively high throttle settings--- and this can shorten the working life of an engine considerably compared to running the same engine at the same rpm, or even a higher rpm,*with the proper propeller loading.


-- Edited by Marin at 13:12, 2008-12-01
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Old 12-01-2008, 01:47 PM   #14
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RE: How much power is enough?

Quote:
sloboat wrote:
Marin wrote: So the engine is just poking along, not working at all.* If it's maintained properly it should last way beyond what the engineer designed for.**Even a*NA Lehman is typically operated way off the rated power curve most of the time.********
Exactly.* Hence the point that the easier you make life for your engine the longer it will make life easier for you, particulary with old-technology, slow-turning*engines like the FL120, aircraft radials, etc.* But the basic premise of the harder you run the shorter the working life of the engine will be applies to everything.* That's not to say a particular engine that is designed for hard running applications will fail prematurely if operated this way--- it will, or should-- operate reliably for whatever length of hard-running time the engine was designed for.

-- Edited by Marin at 14:48, 2008-12-01
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Old 12-01-2008, 06:26 PM   #15
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RE: How much power is enough?

Quote:
sloboat wrote:
Marin wrote: ... I had the chief engineer of one of England's premier engine manufacturers tell me this in almost exactly the same words--- an engine, any engine, is designed for a finite life, he said. It's designed to go around and around so many times and then all bets are off.*


__________________________________________________ ____

Marin,*

I was responding to the*exchange above...your last sentence in particular.**It's not nearly that simplistic.**
You're right.* There are a zillion variables that affect the performance and longevity of any engine.* But the bottom line is unchanged--- the harder you work a mechanical device that is affected by friction, heat, pressure, etc. the faster it will "wear out."
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Old 12-01-2008, 07:36 PM   #16
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RE: How much power is enough?

Over the years, we have heard it all. Engines, specifically diesels, are designed to work hard. Cruise them at 10-15% below WOT all day. That's what they were designed for. Hog Wash...I'm in Marin's camp on this one. An engine has a finite number of hours it will run. We don't know what that number is but it's there nevertheless. Abusing the engine will approach that number much more rapidly than treating it with tender loving care. Even though my Cummins 330B is rated at 2800 rpm and I've been told that running it at 2450 is no problem, I cruise her at 1800-2000. She sounds better, drinks fuel much more efficiently and there's no vibration. You can almost hear her say how happy she is! <grin>
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Old 12-01-2008, 09:51 PM   #17
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RE: How much power is enough?

One can get into trouble at both ends of the power spectrum. Run an engine hard and it will shorten its service life. Run it too easy and while the life of the engine may or may not be affected, its reliability will likely be reduced with the need for more frequent maintenance.

Willy made a valid point earlier and that was that running an engine hard may not have any adverse effect during the time the engine is in his possession. This is the assumption behind the film industry's mantra, "Trash the rental."

But if by choice or necessity a person runs old or used equipment--- our boat and the de Havilland Beaver we fly are good examples, both have had a number of previous owners--- the treatment the engine(s) received becomes very important.

With respect to marine diesels it's been my observation that the main proponents of the "run them at 10% below WOT all day" theory are people who have nothing to lose if you do this and your engine fails. Steve D'Antonio is not responsible if I follow the advice he expouses and one of our Lehmans burns a hole in a piston or whatever. The "run them as though you hate them" proponents can spout this theoretical stuff with no consequences to them. It looks cool in the magazine articles and people hang on this crap like it's gospel.

But the people I know or have known--- back country pilots, long-time commercial fishermen, log truck drivers--- whose livelihoods actually depend on their engines operating reliably for a long, long time with minimal maintenance and repair expenses---- every one of them has told me--- often fairly emphatically--- "take it as easy on your engine as you possibly can."

Given the choice of giving credibility to magazine writers, armchair engine "experts," and the like who expouse a lot of theoretical formulas based on Lord knows what or following the advice of people I've met who make a living by keeping the engines they depend on running reliably as long as they possibly can, I'm going to stick with the people who've who've gotten 20,000-plus hours out of a Ford Lehman 120, or racked up many thousands of hours in reciprical-powered airplanes without so much as a hiccup from an engine, or my friend in Hawaii with the tuna fleet who gets pissed off if he doesn't get at least 30,000 trouble-free hours out of the diesels in his 70' longline boats. The "secret" all these folks have told me when I've asked them about it, is to take it easy on the engine.

As Bob Munro, a founder of Kenmore Air and arguably one of the best Beaver pilots ever in addition to being an outstanding engine (aviation and marine diesel) mechanic said when I asked him how to maximize the life of an engine, "A piston is only going to go up and down so many times. The easier you can take it on the engine, the more times that piston will go up and down."

-- Edited by Marin at 01:36, 2008-12-02
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Old 12-02-2008, 05:03 AM   #18
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RE: How much power is enough?

. " But the basic premise of the harder you run the shorter the working life of the engine will be applies to everything"

Not from my experience ,

The Tiny Kubotas and Yanmars (30ish HP) that are installed on reefer trucks get an incredible number of hours (20,000+), of usually HARD work 24/7and 50% more is done by some!

The same engines in rag boats ,or on boat/RV gen are dead at 5000 to 7000, from lack of use and usually severe underload much of the time .

An industrial (say 500+ rated HP 24/7) will usually get over 20K to 40K!!! running 80% rated load and 90% rated rpm.

Light loading brings far more problems than running the rated load, and far SHORTER service life.

Also usually far higher fuel burn and more overhauls till replaced.

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Old 12-02-2008, 11:26 AM   #19
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RE: How much power is enough?

FF, I think you are comparing apples to oranges. They are the same engines but the recreational application is maintained in a recreational way....IOW, not very well. The engines that get 10s of thousands of hours are on a very strict maintenance program because their longevity directly affects the operator's bottom line. "They" also most likely operate a "fleet" of those exact same engines and have a very tight relationship with the manufacturer on how the engines should be run and how they should be maintained. Again, capitalism at work. The manufacturer wants to keep his relationship with the operator so that operator will keep buying his product. Recreational operators do not get that kind of support or experience.
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Old 12-02-2008, 01:29 PM   #20
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RE: How much power is enough?

Quote:
Baker wrote:

The engines that get 10s of thousands of hours are on a very strict maintenance program because their longevity directly affects the operator's bottom line.
Also, engines are not created equal. One would hope that an operator selects an engine that is designed and suited for the type of work it will do. So the little diesels that are running the refrigeration systems on trucks and railroad cars are designed with this kind of 24/7, constant speed operation in mind. There are a lot of engines that would not hold up for the required length of time under this same application, so it would be dumb to try to use one of them.

Actually, running something like a compressor system is pretty easy on an engine. The load doesn't vary all that much, the rpms are constant, and the hours are predictable with relation to the strict maintenance programs John mentions. In fact, this is what the Ford Dorset engine that was later marinized into the FL120 proved to be ideal for. It was terrible in its intended application as a truck engine because it was underpowered and did not hold up under the constantly varying high loads and higher rpms of highway use. But it proved to be an ideal engine at lower load and constant rpm applications like pumps, generators, cranes, and other industrial and agricultural uses. Which is what made it ideal (at the time) for marine applications.

-- Edited by Marin at 14:30, 2008-12-02
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