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Old 05-08-2014, 07:20 AM   #21
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Seems like overkill. It's really simple to tweak the analog tachs to get a correct readout.
That's what I thought at first. I should have known when on the sea trials the PO told me the tachs were "a little off" and just needed to be adjusted.

True story is his mechanic tried, and failed, to do so. I had the same results. I could get them set dead on - at one speed and under no load. Once I got underway, they'd be off again. Sometimes a little, sometimes showing twice, or half, of the actual RPM.

With twins, I like to be in synch. I usually do that by ear, and verify with the tachs. But for some reason on this boat I'm not hearing it. So I want good tachs, at least until I learn to find the sweet spot by ear.
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Old 05-08-2014, 08:53 AM   #22
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With twins, I like to be in synch. I usually do that by ear, and verify with the tachs. But for some reason on this boat I'm not hearing it. So I want good tachs, at least until I learn to find the sweet spot by ear.
Is it that important for them to be in synch? Not arguing, just wanting to learn something. I have always thought that running at low RPM (my twin Perkins 4-236's cruise at optimum fuel burn about 1850 RPM) it was not that important, so long as they are reasonably close. Am I wrong?
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Old 05-08-2014, 08:58 AM   #23
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When jwnall's boat (36 Gulfstar, twin 4-236) was mine I had frequent tach issues. Often, but not always, it was sign that the alternator was no longer functioning. Those alternators (delcos) can be rebuilt very economically by an automotive alternator shop.

Just to make sure, you are reving the engine up enough to excite the tachs, right?
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Old 05-08-2014, 09:01 AM   #24
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Greetings,
Mr. j. Aside from the harmonic droning, which after a while gets REALLY bothersome, when the engines are not in sync, it's less efficient. Think about it this way. One engine is running a bit faster than the other therefore the faster prop is essentially dragging the slower prop.
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Old 05-09-2014, 06:46 AM   #25
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One engine is running a bit faster than the other therefore the faster prop is essentially dragging the slower prop.

Not really , both are pushing , the problem , besides the noise/vibration is the rudder drag of keeping the boat going straight .

And the difficulty the AP has in commanding a vessel that wants to turn with the rudder straight ahead.

The wheel belted AP for sail boats are designed for this type helm requirement .
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Old 05-12-2014, 05:19 PM   #26
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I had Aetna tachs on my last boat and loved them, guaranteed accurate + or - 1 RPM. They're on my list to upgrade on Northern Lights.
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Old 05-12-2014, 05:20 PM   #27
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I had Aetna tachs on my last boat and loved them, guaranteed accurate + or - 1 RPM. They're on my list to upgrade on Northern Lights.
I'll look into them, my tachs could use an upgrade on the Gulfstar any idea on cost?
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Old 05-12-2014, 09:11 PM   #28
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IMHO: Tachs on boats should be used for a loose rule o' thumb... e.g. while cruising glancing at them to just get an inkling/average of about how fast the engine or engines are turning. Synchronization with twins via carefully listened to sound or an electronic synchronizer is important... so that no matter what the tach readings both engines are turning props at same speed. Whether single or twin screw, D or P hull design, gas or diesel; boat captain knows by engine sound when it or they are turning at favorable rpm. Boat captain also knows by attitude of his boat in the water and wake its leaving what the most efficient speed(s) are; with trim established by internal weight distribution or possibly using trim-tabs in cases where necessary / wise-to-do. The only time I pay fair attention to tach readings is when I’m on bridge and engine noise is minimal (there I can’t really discern rpm by ear) while idling at lowest rpm for docking or other shift-necessary maneuvers. I always want to be sure engines are lowest rpm so shifting transmissions into forward or reverse causes no internal harm. In actuality... tachs on boats are not really necessary. Tach on our runabout o/b stopped working years ago; I'll know its wiring – someday I hunt it down. Till then, I know when boat is cruising at its best speed and if I’m keeping the engine too high on rpm. Boat Captains pretty much know when their engine “running sounds” are fine and when their boat is “cruising along” OK.
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Old 05-12-2014, 09:33 PM   #29
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I'll look into them, my tachs could use an upgrade on the Gulfstar any idea on cost?

Thanks,

Bill

A few years ago they were about $200.00 each plus what ever you may need for a sending unit. I know my Hinos will require new senders and wiring. They are worth the cost and work to install IMHO
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