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Old 10-09-2015, 11:49 AM   #1
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Higher HP diesel?

In discussion and discussin about the new design in Seapiper 35 Trawler the threads start at one place and meander all over, around and through trailers, rolly, beam, money vs happy etc etc....Can we please focus a minute on the diesel engine? What is the advantage of the optional higher HP Yanmar that drinks more fuel, cost more, (presumably as an added option) runs at higher rpm (spelled h-o-t-t-e-r??) all for the advantage of one additional knot of speed? At some point the prop size/angle plays into additional power (as in fighting wind or currents) so what's the point? Hull design is what everybody says is the deciding factor in how fast the boat goes, so where does engine HP, RPM, come in. and at what point of diminishing return do you quit trying to get bigger, better, thirstier, faster, even flashier, than the guy in the canoe.? Personally, I like daddy Bush's idea "Slow and steady stays the course." Isn't that the basic idea of a Trawler, anyway? I guess what I'm trying to figure out is if the Yanmar would be worth the cost outlay and fuel consumption? Like maybe if you got too close to a dam or a flooded bridge?
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Old 10-09-2015, 12:02 PM   #2
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Both engines will use about the same amount of fuel at the same speed.


If you want to use less fuel slow down but it is nice to have extra power for those times speed is needed or bucking head seas.
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Old 10-09-2015, 12:45 PM   #3
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The nail has been hit on the head:

If you want to use less fuel slow down but it is nice to have extra power for those times speed is needed or bucking head seas.

Our last 2 boats certainly would not meet the classic definition of "trawler" (1996 Bayliner 4087 and 1989 PT52 CPYF)- but they both perform well at 6-9 kts with low fuel burn, and have the ability to get out of harm's way due to the higher HP engines. This works perfectly for us.

Bottom line, you should determine your needs and then purchase based on fulfilling those needs. Never get into the trap of pleasing others- and remember, the guy in the canoe made a choice, too.
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Old 10-09-2015, 12:55 PM   #4
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Both engines will use about the same amount of fuel at the same speed.


If you want to use less fuel slow down but it is nice to have extra power for those times speed is needed or bucking head seas.
I slow down the engine when bucking steep head seas to make the boat more manageable.

I had the option of either naturally-aspirated or turboed engines. I chose NA version of the JD 4045 because the engine provided sufficient power for the hull's full-displacement design. (The builder only offers the turbo-version presently. Does JD still offer a NA 4045?)
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Old 10-09-2015, 12:56 PM   #5
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They are vending a product to a fickle market. Offering brand options and power options seems to be a wise choice, whether it makes sense or not.
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Old 10-09-2015, 01:38 PM   #6
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With FD it's stupid to overpower but most FD boats are overpowered. Mostly because most think they need more power for emergencies like bucking wind, tidal or other currents ect. My opinion is not mainstream as I think 4hp per ton of boat displacement is way more than anybody really needs. So for a FD boat 1.5 times as much power as it takes to cruise at one knot below hull speed is plenty of power.

But the boat in question is not even close to FD. Having said that one could go forth w FD level power but it would be as stupid as overpowering the FD hull. Designers usually design a SD hull for a "speed range" and therefore any amount of power that produces speed in that range is appropraite. It's not entirely stupid to run a bit more or even less unless one gets down to FD power. Then a FD boat (hull) would be the only appropriate way to go. And by the same token installing more power than is necessary to achieve the high end of the speed range will put the boat in a less than optimum configuration re power to hull design and then a hull that's a little closser to a planing design would be better.

So the information necessary here is to determine what the speed range of the Seapiper hull is and start estimating how much power it will take to provide the speeds that match the hull design. Normal cruising speed should take just about exactly 50% of availible power leaving a lot of spare power for whatever reason.

From the pics of the Seapiper it looks closer to a planing hull so more power can probably be used but re the above a nice relaxing cruise speed that you will use most all the time should be at least 50% of installed power. Wanderinstar, most boats w two power options have the higher power option as an afterthought to attract customers to the boat that don't think the designer knows what he's talk'in about and/or are of the opinion that one can always use extra power. It's not true. One can only reasonably use so much. Like Mark above I always slow down when the wind blows and seas mount. Only if you have extreme windage can you use more power.
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Old 10-09-2015, 01:52 PM   #7
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I'm part of the "less is better" camp on horse power. Usually, more hp means more expensive engine, more space consumed below deck, more weight, more expensive to maintain & repair. Others may like the option of more speed--which can be justified, but it's not on my list of priorities...
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Old 10-09-2015, 02:05 PM   #8
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Higher HP Diesel

Wow! Lots of info. Thanks. I have never had a trawler so I don't really know much about how they handle. Wind, current, seas, etc. Only been on the open seas with a heavy cruiser and Spruance class destroyer, so I guess those handling experiences don't really cross over. Been fishing by hours, not days on East ICW around Georgia, and on Lake Michigan. This river thing got my blood goin so now I'm achin to get to it. Too late to start this year so thinking maybe July or August next year. Maybe spend early summer around Lake Michigan getting the feel of things and then head towards Chicago.

this is the link that got me to thinking about power.
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Old 10-10-2015, 09:38 AM   #9
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"With FD it's stupid to overpower but most FD boats are overpowered. Mostly because most think they need more power for emergencies like bucking wind, tidal or other currents ect."

Its my belief that most trawlers are "Overpowered" because instead of industrial or big truck engines the boat selling folks chose auto or farm equipment which carries a different , far higher HP rating.

Sure its nonsense, a DD can be operated at max rated power for its entire life , no one would do that with a Yanmar or Ford tractor engine.,

The simplest concept is 3 cubic inches makes one long term engine HP

15 HP burn 1 GPH , buy what you need.

A modest sized NA engine will not suffer from overloading or under loading and be the simplest to maintain.

Less fun strutting the dock proclaiming 450HP tho.
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Old 10-10-2015, 10:34 AM   #10
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Lots of boats are "overpowered" if you look at max engine output and compare it to actual cruising hp needed. But that is the wrong way to look at it.

What you really want is engine to be in a sweet spot on its performance map when the boat is in a sweet spot in its hp vs speed curve.

A Yanmar for example might be rated at 3300rpm, but it's best efficiency is actually at part load down around 2000rpm and mid load, well below its rating. So is that boat overpowered??

How much fun is it on a nice hull speed cruise with that beast down there doing 3grand? Sounds like a weedeater!!!

Good rule of thumb is to size engine so cruise speed is around max torque rpm for that engine.
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Old 10-10-2015, 11:36 AM   #11
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Ski,
Re the Yanmar you look at a different engine then or the manufacturer hasn't done his homework or completed the job of designing the boat.
Also the differince between the sweet spot you mention and running a few hundred rpm away is usually fly stuff not to mention that the everyday "sweet spot" is simply where the engine vibrates less. Burning 2.7gph v/s 2.6gph will not be even noticed by anybody but a Passagemaker.
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Old 10-10-2015, 01:28 PM   #12
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A Yanmar for example might be rated at 3300rpm, but it's best efficiency is actually at part load down around 2000rpm and mid load, well below its rating. So is that boat overpowered??

Good rule of thumb is to size engine so cruise speed is around max torque rpm for that engine.
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Old 10-10-2015, 02:52 PM   #13
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Ski,
"Cruise at max torque" ...
Is that for max efficiency or is there more to it?
I've heard that many times so there must be something to it. However I would think loading would be more important than small differences in efficiency. Most of the engines we're running were designed for duties other than powering boats. As I recall someone said a long time ago that most gen sets run at 1800 and that's why so many have max torque at 1800. What do you know about that? I think my engine is in that catergory but I run it at 2300. My 2300 is a result of choosing the right amount of power, propping the engine for rated rpm (3000) and then driving the hull at about one knot below hull speed. Fuel burn wasn't considered important enough to put on the chart. But my fuel burn isn't important enough to consider 10% this way or that. To me that seems more like a Passagemaker issue.
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Old 10-10-2015, 05:19 PM   #14
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If you look at max torque vs rpm curves, each has a peak. And it tends to vary with engine size. A two liter might be 1900-2100, a six liter 1700-1800, a 14 liter about 1400-1500. As engines get bigger, they have a harder time breathing efficiently at high rpm, and also the injection has to cover some distance in order to use the available air. And with turbo engines, the torque peak can be moved up in rpm depending on how that system is designed.

Then you go look at the BSFC map. Turns out the sweet spot for best efficiency is usually real close to the full power max torque rpm. But this max efficiency is usually at something less than full power, but similar rpm. What we are seeing there is that if engine is breathing and burning well enough to make max torque at full, it likes being there at part load for best econ.

Trouble is, now add in the boat power curve. Hp vs speed. Unless overpropped, engine will not be at the sweet spot in the BSFC map. Load for a normal boat too low there. But that's ok, draw a vert line straight down and BSFC might not be best, but still dang good.

For your smaller Mitsi, 2300 is absolutely fine.
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Old 10-10-2015, 07:41 PM   #15
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Thanks Ski,
Well stated,
But what's "full power max torque" ?
Seems to me it's either max torque or max hp.
So you're saying it's a fuel burn thing. How can very small amounts of fuel economy trump basic engineering principals? Seems like a fad and a bandwagon getting together to me.


If a person w a trawler has a paranoia about fuel burn now w fuel cost at nearly half price perhaps a smaller boat is in order.
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Old 10-10-2015, 07:49 PM   #16
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With my NA JD 4045, maximum torque is 1400 RPM where the engine will operate "at temperature." That's about 650 RPM over idle, 400 less than normal "cruise", 800 less than maximum cruise, and 1000 under "wide-open" throttle at four gallons an hour moving a 14-ton vessel. An efficient "one knot below hull speed" at less than two GPH is at 1800 RPM, 400 more than maximum torque.
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Old 10-10-2015, 07:56 PM   #17
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What is the advantage of the optional higher HP Yanmar that drinks more fuel, cost more, (presumably as an added option) runs at higher rpm (spelled h-o-t-t-e-r??) all for the advantage of one additional knot of speed?

Is this achieved with a different electronic chip set on the same engine?


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Old 10-10-2015, 08:08 PM   #18
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Mark and Eric- I was discussing load matching in somewhat hypothetical terms; it is impractical to set up a trawler engine to run at it's and the boats most efficient point and still meet the loading specs around full power. Unless you have a variable pitch prop!!! But both of you guys are close enough to have happy engines.

Mine is similar, peak torque around 1700, but I cruise at 1800-2100.

Eric- Full power max torque is a curve that should have come with your engine. It's a plot of rpm on x axis, and torque on y axis. The engine is held at various rpms while at full power and torque measured on a dyno. Each engine has a unique curve and the peak on the curve is usually somewhere around 70% of max rpm.
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Old 10-10-2015, 08:48 PM   #19
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Ski,
I don't have that on paper but it's probably on the Mitsubishi web site that I have bookmarked on my crashed i-mac. Miss that thing.

OK so this sweet spot is a junction where power and torque curves meet. Kind of a best of both worlds thing? Looks like Mark's running his Deere at a little lower load than me. It would be interesting to see all our boats cruise rpm as a percentage of max power rpm. It would not however indicate a percentage of engine load. I often have viewed it as X # of rpm down from max rated. But that's not apples and apples unless all engines considered have the same max power rpm.
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Old 10-11-2015, 07:41 AM   #20
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"I often have viewed it as X # of rpm down from max rated. But that's not apples and apples unless all engines considered have the same max power rpm."

The engine doesn't matter that much as a few hundred RPM will lower thre HP the propeller can absorb by a large amount.

SO full tilt less 10% in RPM is usually safe from a load point , but is seldom efficient.

BMEP graphs for the engine build are hard to find , so the simplest is to use the torque peak as desired cruise RPM.

If you go to slow , a bigger prop, using more of the HP avilible at that RPM is the solution.
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