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Old 09-12-2018, 04:18 PM   #1
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Trolling bags

Im using two 38 trolling bags on my 38 twin Lehman 80s C&L trawler. Using them to get down to 2-3 knots for salmon trolling.

My question is is it better for the motors to run both motors at a slower rpm like 1000-1100, or should I run one at a little higher rpm 1300-1400 and alternate the motors? I dont care about fuel consumption Im more concerned about making the motors happy while slow trolling for hours on end.
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Old 09-12-2018, 06:04 PM   #2
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I commercial fished salmon and tuna with Detroit twins. I ran on one engine for salmon trolling, about 2 knots. I locked the other prop for drag. I ran what we called flopper stoppers off my trolling poles. Mounting in the back holes caused drag. I changed props between salmon and tuna.

Some people drag a small sea anchor or a basket. Sometimes more than 1.

Speed is critical with salmon. Also currents effect the speed you run. If you catch fish in one direction and can't 180 later at the same speed, there's a current at play.
I ran at about 700rpm for up to 14 hours. If I was far out, I drifted at night and ran the next day at about 700 again. It didn't bother Detroits at all. I think you could alternate on the engines and when you run back in should make up for the slow speed.
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Old 09-12-2018, 06:49 PM   #3
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Personally I would run one and alternate.
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Old 09-12-2018, 07:24 PM   #4
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I commercial fished salmon and tuna with Detroit twins. I ran on one engine for salmon trolling, about 2 knots. I locked the other prop for drag. I ran what we called flopper stoppers off my trolling poles. Mounting in the back holes caused drag. I changed props between salmon and tuna.

Some people drag a small sea anchor or a basket. Sometimes more than 1.

Speed is critical with salmon. Also currents effect the speed you run. If you catch fish in one direction and can't 180 later at the same speed, there's a current at play.
I ran at about 700rpm for up to 14 hours. If I was far out, I drifted at night and ran the next day at about 700 again. It didn't bother Detroits at all. I think you could alternate on the engines and when you run back in should make up for the slow speed.

Those would be the same Detroits that one always hears "need to be worked so always run them at 1800" or similar crying.

When I was commercial fishing as a kid, we ran a 6v71 trolling for salmon. We ran hard to the fishing ground, usually 15 to 20 hrs non-stop at full throttle, then 14 days at idle, getting 2 knots and hauling salmon, then 15 to 20 hrs at full throttle. Unload, load ice, repeat, for 4 months.
Never considered that to be hard on the engine. The whole fleet behaved the same way, with a large variety of engines.
We also used a triangular drag board, about 3' on each side, made of 3/4" plywood with chains from each corner and towed vertically behind, to slow us when in a strong following wind, or when the species of fish wanted slower speeds, as idling the 6v71 was sometimes not really slow.
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Old 09-12-2018, 09:46 PM   #5
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we ran a 6v71 hard to the fishing ground, usually 15 to 20 hrs non-stop at full throttle, then 14 days at idle, getting 2 knots and hauling salmon, then 15 to 20 hrs at full throttle. Unload, load ice, repeat, for 4 months.
How many hours between oil changes?
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Old 09-13-2018, 01:05 AM   #6
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Those would be the same Detroits that one always hears "need to be worked so always run them at 1800" or similar crying.
Same ones. When I fished in the 1970s, most salmon/tuna boats had 671 inline. Detroit 471, 371, 12v71 were also common. 53s in smaller boats. Cummings was getting some action. No rice burners I remember. A few Cats in bigger boats. They could run #3 diesel. At the time I didn't know the 6v71 existed. 671 meant an inline. Some boats had WWII built 671s. Some of those had never been rebuilt. We all trolled about 700 rpm or below. Most had props about 36" and were hard to slow down without a prop change.

I have 671s in my current boat and run at 1800. I ran them at 1800 when I first bought the boat and they had 20,000+ hours before overhaul. My favorite marine engine. No circuit boards, sensors, or injector pump. I got a lot of years on the ocean in ships and boats. I think the 71 series are the most reliable of engines their size.
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Old 09-13-2018, 05:47 AM   #7
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The hassle with running 2 stroke DD light is the fuel consumption does not drop off as much as an ideling 4 stroke.

Engine longevity is not harmed , in colder areas of the US DD ,bus and truck were idled all night for years , the only cost an extra gallon or two overnight.

In hot climes buses would be idled at about 1200RPM all night so the air cond would keep the bus cool. The 60-80 HP required was minor , so the only price was a couple of gallons of fuel.

One of my "dream boats" would be powered with a landing craft tranny ,(2 engines for 1 shaft) with both a 6-71 and a 2-71 or 3-71 installed.

The 6-71 could be selected if speed were needed , inshore where fuel is plentyfull, and the smaller engine used for long range cruising at hull speed where 40-60HP would be enough.

For a better offshore performance two 3-71 would be used so there is a supply of parts underway , and both at once would give 6-71 performance inshore,,,,"perchance to dream".
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Old 09-13-2018, 03:17 PM   #8
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Iv been salmon fishing for many years. I use Amish outfitters trolling bags on my smaller inboard. I setup the trawler the same with bigger bags. With one motor and no bags at 1000 rpm I get around 3 knots. I like to get down to 2 when I want.

I was told the Lehman’s don’t like low rpms for long periods they carbon up. Is that true? The bags allow me to go very slow with one motor at higher rpms. So alternating motors with the bags in seems to be the smart thing to do - provideing what I was told about the low rpm with Lehman’s is true.
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Old 09-13-2018, 05:52 PM   #9
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Trolling bags

The Lehman mechanic I spoke with said extended running at low RPMs is hard on the velvet drive dampner plate. Many commercial trollers have a trolling valve on their transmissions. Im not sure how that works.

Edit: Tony Athens discusses this here:

https://www.sbmar.com/articles/marin...rolling-valve/
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Old 09-14-2018, 06:36 AM   #10
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We have ZF trolling valves on our ZF gears.

Works by creating a longer loop for gear oil; activating the trolling valves causes gear oil to take that longer route, reducing oil pressure, so the clutch plates slip. (Or something like that.)

Our Teleflex adjustment controls allow very fine changes in speed (rotate), or a course adjustment or very quick activation/deactivation (push/pull).

ZF told me it's OK to troll on one engine, with the other engine off (no lube), as long as I keep speeds down. Our norm is around 2.5 kts plus/minus, and he said that's fine. Dunno what upper speed limits might be; didn't think to ask at the time.

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Old 09-14-2018, 07:49 AM   #11
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Lehmas like any older make engine will carbon up at really low RPMs. I run around 1650 cruise RPM and had a tiny bit at 3000 hrs when I pulled the head.

If I ran up near 2000 ok nce and while, I might burn it off, but there is a camp that believes Lehman's are longer lived when run below 1800, but many have experience that disputes that.

I can see the dampner plates and or BW tranny's being damaged when idled for long periods ( there is a warning in some BW operator manuals), so if may be a concern if you can hear the rattling at trolling speed.
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