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Old 08-19-2016, 09:17 AM   #21
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I may be way off since all my engine knowledge has been based on automotive, but I drive a 8000Lb Suburban with large tires and pull a boat so the small block that was in it had 230 HP and 360 FtLbs about.

FWIW, I'd suspect curb weight is more like 5600-lbs or thereabouts. Maybe the 8000 is GVWR?

I drive a Suburban too, and we've just been shopping for wife's new ride; seems to me the Tahoes/Yukons/Escalades are in the 5400-lbs (curb weight) area...

-Chris
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Old 08-19-2016, 09:26 AM   #22
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Warwgn,
Torque is an instantious force and does no work at all.
And trawlers don't give a whit how much rpm is involved getting the job done. 500rpm or 5000rpm as long as it has the hp to get the job done. Torque can reduce the amount of time it takes to get up to rpm but hp keeps the prop turning and does the work.
You want to run you're trawler w big prop and use a 120hp OB? Fine .... better have a deep reduction gear .. maybe 6-1 ?? Job done.
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Old 08-19-2016, 09:34 AM   #23
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I think fuel being burned is a very good way to measure load. Read Tony's Tips article "Props move boats. Engines turn props". I think that is the name or similar.
Exactly ...
I have a 40hp engine (actually rated my Mitsu as 37) run at 2300rpm and burn 1gph at 50% load. Percent of load has nothing to do w rpm. Other than they both rise and fall together.

Fuel burned is probably the only practical way to determine load percentage.
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Old 08-19-2016, 09:56 AM   #24
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Originally Posted by Nomad Willy View Post
Warwgn,
Torque is an instantious force and does no work at all.
And trawlers don't give a whit how much rpm is involved getting the job done. 500rpm or 5000rpm as long as it has the hp to get the job done. Torque can reduce the amount of time it takes to get up to rpm but hp keeps the prop turning and does the work.
You want to run you're trawler w big prop and use a 120hp OB? Fine .... better have a deep reduction gear .. maybe 6-1 ?? Job done.
I fully agree Eric. And to take it a step further to whoever was suggesting an outboard as it relates to "work being done". The same amount of work may be done but an outboard is less efficient than a diesel....therefore not a good comparison.
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Old 08-19-2016, 10:06 AM   #25
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I think fuel being burned is a very good way to measure load. Read Tony's Tips article "Props move boats. Engines turn props". I think that is the name or similar.

I will go back and look at that one again, thanks. I know with my particular engine, 1400 rpm is about 10% of max fuel consumption. Seems to be a big difference.
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Old 08-19-2016, 10:21 AM   #26
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Originally Posted by ranger42c View Post
FWIW, I'd suspect curb weight is more like 5600-lbs or thereabouts. Maybe the 8000 is GVWR?

I drive a Suburban too, and we've just been shopping for wife's new ride; seems to me the Tahoes/Yukons/Escalades are in the 5400-lbs (curb weight) area...

-Chris
Its an 86 K20, been modified quite a bit with lift, and 37" tires, bigger running gear, lot of extra metal welded in and the extra weight of a 496 iron head big block over a small block. Its a big heavy beast compared to a stock suburban. But that is why I repowered it. Oh and the GVWR is 8600 for my year and model, at least that is what the internet says;-)
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Old 08-19-2016, 10:27 AM   #27
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Is there a way of telling when your semi-displacement hull is "on the money? I run at 8.5 knots at 2460 RPMs (measured mile) with two 136 horsepower Volvos, and it feels just right.
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Old 08-19-2016, 10:57 AM   #28
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You are right. I have the QSB 5.9L Cummins. It can be rated from 230 to 480hp. It would be easy to derate mine to 330hp with an ECM change. The point though, is that I have a lot more horses available than I actually need to run at displacement speeds.
Except, your boat and engine weren't paired or designed to be only run at displacement speeds. They were designed to run at greater speeds some of the time.
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Old 08-19-2016, 11:22 AM   #29
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Except, your boat and engine weren't paired or designed to be only run at displacement speeds. They were designed to run at greater speeds some of the time.
Very true. The PO said that he ran the boat at 10 knots most of the time, but then he had a lot of money to burn. I am happy that I have that option, it is just that I haven't yet felt the need to take advantage of it. However, I see a lot of SD boats running up and down the Sound digging giant holes in the water running at way above displacement speed. They certainly like the power they have available. (although I'm not all that happy with the wake they toss at me as they pass withing a boat length of me.)

For planning hulls I imagine it would be different as well. You would want enough power to quickly get up on a plane, but then be able to dial it back to stay on a plane yet still be running at a reasonable % of load.
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Old 08-19-2016, 12:30 PM   #30
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Mark, you see to me saying that "load" is based on fuel consumption. ...
Yes, comparing actual fuel consumption rate to the maximum rate the engine is designed.
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Old 08-19-2016, 12:43 PM   #31
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Is there a way of telling when your semi-displacement hull is "on the money? I run at 8.5 knots at 2460 RPMs (measured mile) with two 136 horsepower Volvos, and it feels just right.
ancora,
What on earth is "right on the money"? Hull speed? That's too fast for best economy ... MPG. Not knowing what you mean "on the money" may be about .75 of a knot below hull speed. However you've got a long boat and w that much length work'in for you more speed would be fully appropriate depending on how you are re buying fuel.
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Old 08-19-2016, 12:51 PM   #32
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For folks that think a diesel is a diesel might wish to view;

https://www.morganscloud.com/2015/06...gine-fuel-map/

When you get to the fuel map part it will explain why there is no fixed HP/Gal output at any loading but in the bullseye .

Interesting when using google and "diesel BMEP chart" one can read the entire thing, going to directly , far less info.

????
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Old 08-19-2016, 09:29 PM   #33
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True, you can't separate torque from horsepower, but the formula above does not mention what torque actually is. Torque = force x lever arm length. The reason diesels make such high torque is that they are long stroke, high compression engines. The long stroke gives a long lever arm on the crank journal. This is why we are able to cruise and turn a high pitch prop at such low RPM. Gasoline engines, on the other hand are much shorter stroke engines. They have a shorter lever arm and much less torque (especially in the low ranges). You can have both a diesel and a gas motor rated at the same BHP, but the diesel will develop the same torque as the gas motor @ 1600 RPM while the gas motor needs to be screaming @ 5000 RPM or more to make the same torque. Which engine would you want in your boat?
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Old 08-19-2016, 11:01 PM   #34
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.. You can have both a diesel and a gas motor rated at the same BHP, but the diesel will develop the same torque as the gas motor @ 1600 RPM while the gas motor needs to be screaming @ 5000 RPM or more to make the same torque. Which engine would you want in your boat?
Which explains nicely why my turbo 150KW/400NM diesel car so easily conquers hills. First diesel car owned, but I rented several in Europe; I`m sold, performance and economy together. Applying that to a boat as Dave does, diesel is an easy decision.
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Old 08-20-2016, 04:41 AM   #35
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Trawler Dave hit it right on the money.

I would only add that TQ initially will be the primary moving force in any vehicle but it's HP that keeps your speed up/constant once your are moving. Coincidentally TQ can and does break many things....especially if you are running a high gear load...things like axles, propshafts, transmissions and gear tooths come immediately to mind....or in my bonehead case sometimes bolt heads too..

@bruce..now go try yourself a full size diesel truck and tune it....My Cummins truck tows our 8k lb trailer across 14k foot Rocky Mountain passes like it's not even there....600hp/1200ft lbs TQ...crazy..
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Old 08-20-2016, 08:17 AM   #36
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Exactly my point, a trawler needs a lot of force but low RPM. I need a lot of power but don't want to work to hard for it, that is why a diesel is the right power plant, it puts out lots of torque at low RPM, so you can put a large high pitch prop on it and still be able to turn it at low RPM using less fuel and less wear and tear on the engine.
RPM does not affect fuel use. Only load does.
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Old 08-20-2016, 08:20 AM   #37
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I will go back and look at that one again, thanks. I know with my particular engine, 1400 rpm is about 10% of max fuel consumption. Seems to be a big difference.

If that is measured OK.

If it is from the engine curves true only if you use prop curve and reach rated WOT.
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Old 08-20-2016, 08:24 AM   #38
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To understand how unrelated RPM is to fuel use consider that out of gear your engine can rev all the way to max but will still use very little fuel.

Only when in gear will load require fuel and that fuel required will be based on loading not maximum engine rated vs rpm curve.
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Old 08-20-2016, 08:25 AM   #39
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Good discussion, especially on horsepower and torque. I like the three cylinder analogy and the weightlifter. We have a planning hull, so the engines are running at about 80% of WOT. This same engine is Used by a lot of fishermen in my area who also run on the high-end with many hours and no problems . I probably won't get trawler life out of the engines, but they should still outlive me before a major overhaul is needed.
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Old 08-20-2016, 11:19 AM   #40
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Trawler Dave,
Sorry to burst your concept of torque but long stroke has almost nothing to do w torque. Look at the Buick straight eight and compare it to the V8's that came after. Extreme lomg stroke straight eight had far far less torque than the V8's. I've observed that in many other engines as well.
The force (torque) is re to the force on the piston top and the lever arm created by the stroke via the crankshaft. You either have big force on the piston crown and small leverage via the short stroke or small bore small force on the piston crown and big leverage on the long stroke crankshaft for X amount of torque. Amount of oxygen, fuel and compression also gets into it but the force on the piston top and the mechanical advantage of the stroke is primary to torque.

The above is just opinion but if you care to look you'll find lots of short stroke engines with above average torque output.


Fletcher500'
Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.
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