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  #1  
Old 10-14-2010
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motoring with the hook down

Here is a dumb thought (as most are with me):

If motors are healthier when run under load, is it OK to put it in reverse while at anchor and motor against your ground tackle? (assume it is set well, and you don't rev too high).

This is in an effort to charge the batts
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Old 10-14-2010
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why don't you motorsail? you would be charging the batteries and getting in some sailing at the same time----sounds like more fun anyway. it would not hurt to do it at anchor, just wasting fuel and not having any fun.
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Old 10-14-2010
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You'd best be at the helm and have ground tackle that you REALLY, REALLY trust if you're going to do this.
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Old 10-14-2010
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Quote:
Originally Posted by sailingdog View Post
You'd best be at the helm and have ground tackle that you REALLY, REALLY trust if you're going to do this.
Of course, this would be one way of learning to really, really trust your ground tackle.
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Old 10-14-2010
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Quote:
Originally Posted by Barquito View Post
Here is a dumb thought (as most are with me):

If motors are healthier when run under load, is it OK to put it in reverse while at anchor and motor against your ground tackle? (assume it is set well, and you don't rev too high).

This is in an effort to charge the batts
Considering prop walk, you're likely to just pinwheel around your anchor/anchor rhode until you foul the rhode on your rudder or keel or jerk the hook out of the ground. While some may (will surely) disagree, a bit of fast idling to charge your batteries occassionally won't kill your engine. You want more load, get a bigger charger. Couple that with a Smart Regulator and AGM Batteries that can charge more quickly and you will do no harm to the engine.

FWIW...
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Old 10-14-2010
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I've done it, but there are some preventions:

1) Nobody around - no other boats with my swinging circle, nobody swimming anywhere within sight. Dinghy on shortened painter. No kids onboard, no guests even. I wouldn't want anyone sucked under the swim platform and into the prop. No going on the swim platform when doing this (that means me, since I'm alone when I do this.)

2) Hook really well set. I set the hook at 1300 rpm in reverse for 5-10 minutes while I watch nearby objects and their distant "background" shifting to see if we are sliding back at all, standard practice. Mooring bouys and their background work better than boats that may tack at anchor. With our big 3-bladed prop, that many revs would have us moving forward against a 50knot wind, for sure. (I often cruise at 1600 rpm at 5 or 6 knots.) This also implies that charging be done well below the anchor-setting 1300 rpm. (I've used 850 and 900, sometimes 1000)

3) While attended. Usually I'm doing something on deck or taking care of something. Not napping.

4) Rudder position. With our spade rudder, I find that I can make the boat do 360 degree turns in even 5 knots of wind, if I have the rudder hard over. You'll want to lash it in the center. Not sure if prop walk will affect your boat or not. That's something to take into acount.

5) Instead of doing the above, I find myself coming into our out of the harbor at "best charging cruise". That's the rpm which gives full charging current. Anything over that many rpm gives the same current, so why bother? For us, best charging cruise is about 1000 rpm. We still go 3 or 4 knots, so why rush it when you know you'll just be running the engine later for charging?!

Regards,
Brad

This is what I've done in the past, this is not a recommendation to do any of this. In fact, don't do it!
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Last edited by Bene505; 10-14-2010 at 07:07 PM.
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Old 10-14-2010
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Considering prop walk, you're likely to just pinwheel around your anchor/anchor
BINGO !!!

Engine RPM is not always alternator RPM. Let's consider the optional factory 75A alt off a Yanmar 3YM-30. This alt develops it's rated output of 75A at 5000 RPM but that is the ALTERNATOR shaft RPM, not engine. This engine runs roughly a 2.7: 1 pulley ratio (based on some measurements I took a while ago and from memory). So a 1200 RPM idle yields an alternator shaft speed of 3240 or roughly 63 amps. Bump the engine up to just 1850 and you are at the alts max rated output.

Even at an engine idle speed of just 1000 RPM this factory alternator can put out around 58 amps, only 17 amps less than max rated of 75 amps, or 77.3% of total alternator output, at just idle speed!!!

I have a good friend who will remain nameless. After cruise with them for a few days the outrageous engine speeds in the anchorage, in the evening, were getting overbearing. "what are you doing?" "I'm charging my batteries." . "Do you actually need to run your motor at 2200 RPM to do that?", "Yep.", Do you mind if I come take a look?", "Sure come on aboard."

Long and short was his 70A Balmar reached max rated output at just 2000 alt shaft RPM, it was right in the literature that came with the alt but he never looked or just did not get it and was told the old wives tale that you always need a high motor speed to charge your batteries..

I showed him how to measure his pulley ration and figure out the alt shaft speed, simple enough. What we found was that even his engines idle speed of 900 put his alt shaft speed at 2250 or 250 RPM more than he needed to reach max cold and hot rated outputs of the 70A alternator. Moral of the story is we could barely hear him charging at idle for the rest of the trip... 2200 wasted RPM vs. max rated performance at just 900 RPM.. Wooo hooo...

Of course his alt was only putting about 45 amps into the bank so this then lead to a looooong discussion over drinks about battery acceptance and why a battery monitor is a good idea. He was certain that if we raised the RPM, more current would flow, it did not, trust me he tried it several times before finally "getting it"....

If an engine runs a 3:1 pulley ratio then the engine RPM can obviously be lower.

We can't forget that your bank may not even be able to "accept" the charge current you have available at a given RPM. If the state of charge is high enough increases in engine RPM may add ZILCH to the equation of you are up against battery bank acceptance.
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Last edited by Maine Sail; 10-14-2010 at 05:57 PM.
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Old 10-14-2010
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Something else to keep in mind whilst doing this:

Most boats have the raw-water intake somewhere just ahead of the prop aperture. If you're in reverse for a while with a few revs on and doing a lap or two around the anchor off a shoaling beach with a weedy/muddy bottom under you, you're likely to stir up crap into your engine intake..

Probably not the best thing to do.
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