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Old 04-08-2015, 10:18 PM   #41
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Mark,
A 75% load isn't "stressing". It just sounds like it. Unless there's something wrong w your engine.

And the best solution for too much noise is sound insulation .. not underloading.

And I think you made a good choice and are one of the few that aren't overpowered. But most don't have much of a choice as most trawlers are overpowered. I re-powered so I had the luxury of choice.

Thanks Craig I'll check my big computer.
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Old 04-09-2015, 12:02 AM   #42
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otter board
noun, Nautical 1. one of a pair of large, heavy, square or rectangular plates or boards of metal or weighted wood attached to the trawl lines on each side of the mouth of a trawl net to maintain lateral spread during trawling.
And, called an otter board because??? Nothing too fanciful please.
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Old 04-09-2015, 01:21 AM   #43
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Mark,
A 75% load isn't "stressing". It just sounds like it. Unless there's something wrong w your engine. ...
The point I attempted to make was that reaching hull speed using 75/80% load is not stressing the engine.
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Old 04-09-2015, 01:33 AM   #44
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And, called an otter board because??? Nothing too fanciful please.
I have no clue. My own guess is that it's a bastardized pronunciation of a word, probably from another language, that was originally used to describe either this type of fishing or the boards themselves. I've heard the term used in northern Scotland by the langoustine fishermen, along the English channel by the scallop draggers, and in Charleston, South Carolina by the shrimp trawlermen.

I very much doubt it has anything to do with otters, as in the creature.

Here's a definition of this type of fishing but no explanation as to why the term "otter board."

Otter trawling

Otter trawling derives its name from the large rectangular otter boards which are used to keep the mouth of the trawl net open. Otter boards are made of timber or steel and are positioned in such a way that the hydrodynamic forces, acting on them when the net is towed along the seabed, push them outwards and prevent the mouth of the net from closing. They also act like a plough, digging up to 15 cm into the seabed, creating a turbid cloud, and scaring fish towards the net mouth. The net is held open vertically on an otter trawl by floats and/or kites attached to the "headline" (the rope which runs along the upper mouth of the net), and weighted "bobbins" attached to the "foot rope" (the rope which runs along the lower mouth of the net). These bobbins vary in their design depending on the roughness of the sea bed which is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to 0.5 m in diameter, for very rough ground. These bobbins can also be designed to lift the net off the seabed when they hit an obstacle. These are known as "rock-hopper" gear.
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Old 04-09-2015, 06:15 AM   #45
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VT we are slightly overpowered too .. very slightly. What emergencies have you used your "extra" power for?
I've had the proverbial snot beaten out of me a few times in Tampa Bay and the Gulf of Mexico. Experiencing six foot seas with foul weather and stiff winds off Egmont Key was "accelerating" to say the least! Beaching Sherpa actually crossed my mind. That was a day my wife and I still talk about (only time she has ever gotten sea sick).

At WOT and characteristic of a full displacement boat, Sherpa's bow lifts and she produces a relatively hefty wake with little gain in speed.
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Old 04-09-2015, 10:20 AM   #46
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Rule of thumb for max cruising rpm

Quote:
Originally Posted by manyboats View Post
A 75% load isn't "stressing". It just sounds like it. Unless there's something wrong w your engine.
Eric:

The question of what is a "stressful" load on an engine is subject to lots of issues (and debate no doubt) and I believe a 75% load factor isn't appropriate in all situations.

First, I know you said "load factor" and that is probably the right way to look at it, but most boaters have no idea what the load is on their engines. They do know what rpm they cruise at, so most people think of pct of max rpm as the way to express maximum safe cruising rpm.

Second, the correct answer totally depends on the engine. Very slow turning, big iron engines like the Gardner can usually be run at 100% load safely. Other normally aspirated lowish rpm engines like yours and Mark's that max at 2,800 rpm or less can be run at 75% safely. But high output, turbocharged and aftercooled engines like mine and Don's can't.

In my case which is a Yanmar 6LY 370 hp engine that maxes at 3,300 rpm, I believe that a safe cruising rpm is 2,800 which is 85% of maximum rpm and about 62% of maximum hp (the load factor). That is about 230 hp which is 44 hp per liter. The gurus on boatdiesel seem to say that 35-45 hp per liter is the most that you should cruise any engine at. 75% load would be 52 hp per liter.

That is a whole lot more hp per liter than 75% load on your engine which I will bet is more like 20 hp per liter. Mine can do that because of inter cooling, oil jet piston cooling and other design features. Yours will probably last longer in any case, but mine won't wear out until I am an old man ;-).

If we use 85% of maximum rpm as a general rule of thumb for a safe cruising value I think that will put most engines in the ballpark.

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Old 04-09-2015, 10:28 AM   #47
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Well with a hull bottom like this, planing speed is ABSOLUTELY impossible.

About 2/3 throttle produces 7.0 kts. WOT produces 7.2 kts. Bow comes up a few inches, stern drops a few inches and the wake gets huge. I'll bet 100 more hp would only push the bow up a few more inches and bury the stern even more. It's all about the hull form. Displacement hull, absolutely.

You better like life in the slow lane with one of these, because that's all you will ever see.
Indeed Larry. Have you seen the boat on WBO w the 80hp Cummins engine? He claims 8 knots at WOT. BIG wake. Several other W30s have the 55 Yanmar and from all accounts it's a really good engine. Doug in AK does.
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Old 04-09-2015, 10:58 AM   #48
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Old 04-09-2015, 11:02 AM   #49
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David very well put above.

Sorry I usually don't think of turbo'ed engines while talking on TF. My bad I suppose but I assumed most here are NA.

Yes I'm thinking many think that 75% of throttle lever movement = 75% load and most of the rest think 75% rpm = 75% load. Whereas it's 75% of max fuel consumption.

My Mitsu is rated at 3000 and I've never run it for any length of time at more than 2500. But I do run 2500 at times. I think 2300 (where I almost always run) is basically 50% load at 1gph. I think the engine burns 2gph max.

And I think there's stressing of the engine and stressing of the skipper. Some worry about both. But it need'nt be. Most manufacturers specify a safe working rpm to run but here there's so many over propped boats that 85% of rated rpm won't apply. I totally agree that a properly loaded engine would have no problem w 85% of rated rpm. I'm not criticizing anything .. just talk'in the concept around a bit.
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Old 04-09-2015, 02:00 PM   #50
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Yes I'm thinking many think that 75% of throttle lever movement = 75% load and most of the rest think 75% rpm = 75% load. Whereas it's 75% of max fuel consumption.

Perhaps in a true work boat with a pro sized engine.

Remember engines operate at a range of RPM , with a different power output at each RPM , from 1200-1500- on up to the pin.

Operating at 75% of the rated output at a rated RPM is fine for any engine.

The measurement need not be from screaming full throttle at full RPM.

Because of the Advertising folks and the reality that some 80 Hp engines cost more than a 120Hp engine , the 120 is common , even if half that would be fine.

If a 120hp rated engine is in a boat that needs 40 hp , 2.5 GPH or so it can be done at a nice quiet RPM and still be near 75% load , for that RPM.

One reason for CPP.
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Old 04-09-2015, 03:09 PM   #51
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Here is a bit of information of engine loading for a displacement hull. This diagram is for my boat and the Volvo-Penta D2-40 diesel I installed. The black line is the maximum power curve for the engine as a function of rpms. For this engine the maximum power output is 39.6 hp at 3,200 rpms. The red lines are the power required to move the boat in calm conditions (solid red line) and rough conditions (dashed red line). The power curves for the boat as a function of engine rpms reflect the actual power needed to push the hull, the propeller details (diameter, number of blades, pitch, and propeller efficiency) and the transmission gear ratio. While this curve is specific to the way I have my boat set up, the general form of the power curves is the same for all displacement hulls. For my boat with the engine running at 2,400 rpms which is 75% of rated rpms, the maximum power output of the engine is about 32 hp. At that engine rpms, moving the boat requires about 15 hp, so running at 75% of rated maximum engine rpms loads the engine to 47.5%. When the prop is correctly chosen, the engine is at 100% output at maximum rated rpms. To load my engine to 75% of its rated power, I would have to run at about 2,880 rpms. At any engine rpms lower than 2,880 the engine would be loaded less than 75% and above 2,880 rpms the engine would be loaded above 75%. 2,880 is 90% of maximum engine rpms. Basically, in order to know how heavily loaded your engine is at any given engine rpms you need to know the specific power curve for your boat. Assuming your prop is properly selected the curve will have a form approximately described by:

PHP = C x rpm^2.7

where PHP is the power at the prop, rpm is the engine rpms and C is a constant chosen so that rpm raised to the power of 2.7 gives the engine output at maximum engine rpms.

When a boat is under propped, the boat's power curve is below the engine output curve at all rpms. When a boat is over propped the boat power curve intersects the engine curve at less than the engine's rated maximum rpms. Looking at the diagram, you can see that my boat is slightly under propped in calm conditions and slightly over propped in rough conditions. This is all discussed in a simple fashion by Dave Gerr in his book "Propeller Handbook". For a more comprehensive discussion I would suggest Larsson and Eliasson's book "Principles of Yacht Design".

Another thing I often read is that people test their engine by running with the boat tied to the dock. when you do that you change the power curve for the boat so that the engine is at maximum output power for all rpms. This occurs because you are trying to move an immovable object. Running for long tied to the dock will often overheat the engine. Basically running tied to the dock is not useful except to test that your transmission cable is correctly set up.

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Old 04-09-2015, 03:21 PM   #52
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TDunn, Glad to see you here, your experience / knowledge is welcomed.
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Old 04-09-2015, 03:22 PM   #53
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For my 2400-RPM JD 4045, idle speed is one-third of maximum RPM, and 75% (1600) RPM is a "gentle cruise" compared to my "normal" cruise of 1800. "Max" cruise of 2200 to reach hull-speed for a gain of one knot over "normal" cruise. Don't consider relative RPM for measuring engine load.


Relative fuel consumption is how to measure engine load. Fuel consumption at wide-open (2400 RPM) throttle is 4 gallons an hour, so my "max" cruise at 3 gallons per hour is 75% load and "normal" cruise at 1800 with 1.7 gallons an hour for 42.5% load.
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Old 04-09-2015, 03:38 PM   #54
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Mark that isn't correct since your maximum fuel burn is also a function of engine rpms. At lower engine rpms your maximum fuel burn is less because maximum engine output is also less than the rated horsepower (which is given for maximum rpms).
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Old 04-09-2015, 04:05 PM   #55
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Mark that isn't correct since your maximum fuel burn is also a function of engine rpms. At lower engine rpms your maximum fuel burn is less because maximum engine output is also less than the rated horsepower (which is given for maximum rpms).
I disagree. The amount of fuel consumed determines how much work the engine is doing. The only times RPM matches relative work is when the engine is off (zero percent load) and when it is at maximum RPM (100-percent load).
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Old 04-09-2015, 04:52 PM   #56
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You are right Mark, fuel burn determines how much power the engine is putting out. However, it is not a measure of engine per cent loading unless you use the engine's maximum fuel burn at the rpms you measured fuel consumption at. If you look in your engine manual you will find a diagram showing engine power AND fuel burn as a function of engine rpms. That fuel burn is for maximum engine output at any given rpm setting. So if that diagram says your engine's maximum fuel burn is 4 gph at 1700 rpms that is the fuel burn you divide your measured fuel burn by to get percent engine power, NOT the maximum fuel burn at maximum rpms. What I am saying is that an engine has a maximum power for each rpm setting. That is the power (or as a proxy, fuel burn) that you should use to calculate % output. The way you are doing it calculates percent of the engines rated output at 2,400 rpms. If you look at the power curve for a JD4045T you will see that the rated hp at 1800 rpms is about 67 hp. Based on your WOT fuel consumption of 4 gph, the 1,800 rpm max fuel consumption is about 3.62 gph, so your 1.7 gph is equivalent to about 47% loading. It is a small difference, but it does reflect that the engine has a lower maximum power at 1800 rpms than it does at 2400 rpms. The effect will be larger at lower rpms.

Here is the link to the 4045T engine power data I used.

https://www.deere.com/en_US/media/im...045TF290_A.png

Based on your reported fuel consumption at WOT, I think you are a bit under propped. I base that statement on the fuel consumption data on the graph at the link I posted.
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Old 04-09-2015, 05:14 PM   #57
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Old 04-09-2015, 05:33 PM   #58
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TDun,
The method you describe determines % of fuel burn and engine loading at rpms other than max. And it looks like a good method of determining that. But it dosn't determine % of the engines power ... just at a certian rpm. When I say I'm running 50% load at 2300 I'm saying at 2300 the engine is at half it's potential output .. turning my propeller. Never thought of talking about 50% load at any specific rpm. But now that I think of it I can't determine what would be most meaningful. If one is to say "I'm running 50% load at 2300rpm" it would be nice to know the other person has understood. It would have a lot of bearing on what concept is most used as expressing engine load. When the tech ed of PMM says 75% load if no rpm is specified one should assume it's 75% of max power .. and that would be at rated rpm. But if I say I'm at 50% load cruising at 2300rpm (3000 rated) I'm not at 50% load. My engine is burning 50% of the max fuel it can burn at 3000 and I'm saying my 50% load point is 2300. It all boils down to % of what.

If an engine manufacturer says to run my engine at half power what should I do? It looks like I'd need to ask him "half power at what rpm". Or perhaps I should just take my pick. But I could ruin my engine. Half power of a 40hp engine means running it putting out 20hp to me. Or 50% load .. same thing as I see it. But by your definition 50% load would be considerably less power output.

Very good point and my head's swimming around a bit and it's hard to keep focus.

Now David's recommendation of 85% of max rpm for sustained operation looks more usable but I'm having fun thinking of Marin running his FLs over 2100rpm from Bellingham to Friday Harbor. I'm seeing him looking for holes below the engines where the oil and rods went.

TD your boat looks much like TAD's "Yellow Cedar" .. one of my favorite boats.
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Old 04-09-2015, 06:12 PM   #59
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My consumption figures were taken from JD engine documents (which are consistent with my gauges). JD also wrote in their publication that load is based on fuel consumption.
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Old 04-09-2015, 08:11 PM   #60
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TD your boat looks much like TAD's "Yellow Cedar" .. one of my favorite boats.
If it was out here, Eric, I'd be inclined to call it a "Lake Union Dreamboat." But I'm sure there are differences between those and TD's boat.
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