A week long Cruise

[Brad Young]

We are getting ready to go on our first week cruise.    We will be sailing from San Carlos Mexico to the Midriff Islands ( about 110 NM ). Take a look at http://www.tucsonsailingclub.com  It is pretty remote, therefor I will be cut off from my power cord. No generator or solar panels (yet)

I would like to know if there is a rule of thumb on how long it takes for the alternator to recharge the batteries.
say at 1k RPM or 2k RPM


1)I Have one 90 amp alternator
2) 4  6v golf cart batteries (house)
3) 1 12v starter battery

Thanks
brad

[Stu Jackson]

Brad, the BIG part that's missing is what your energy budget is.  We do not know if you have, for instance, a fridge.  Energy Budget is here: http://c34.org/bbs/index.php/topic,3976.0.html

Battery Acceptance, the amount of amps you can put back into a bank, is here (followed by discussions about other issues on that thread): http://c34.org/bbs/index.php/topic,4787.0.html

Most all reliable sources on batteries indicate that for cruising you end up using between 50% and 85% of your house bank capacity.  No lower than 50% to avoid damaging the batteries, up to 85% because, as in your case with no generator or solar, the last 15% of the charge takes forever (and shorepower) to put back into the house bank.

Doing the math:  4 6V golf carts, say 450 ah house bank time 35% is 157.5 usable amp hours.  With a fridge, your daily average use is about 100 amp hours.

You also didn't mention if your alternator is externally regulated or whether it is internally regulated.  In either case, you're not going to see much more than 50 Amps out of your 90 amp alternator.  And that's at startup - as the bank gets fuller, less will be accepted by the bank.  Even a deeply discharged 450 ah house bank wouldn't have much more acceptance than that.  It's NOT a linear relationship as discussed in the acceptance link.

Here are your options:

1.  You'll have to drastically cut down on your daily electrical usage (hard to do with a fridge - although I've had some success in turning it OFF at night and covering it up with a wet towel or other insulating material) [We did this successfully for a week in 2004, stayed on the hook for three nights, but motored at cruising speed the other few days.]

2.  Run your engine for at least two to three or more hours every day (less if you can cut down on fridge use) - see below

3.  Eliminate the fridge altogether

4.  Buy a generator (the Honda eU1000 will power your battery charger, the 2000 will do that and make hot water).

You should install a battery monitor - at least then you'll know what's coming and going re- your house bank, for sure.  If you do, you must read this: http://c34.org/bbs/index.php/topic,4922.0.html

There is no "rule of thumb" for 1K or 2K as you asked.  Why?  First, it's based on battery acceptance: the deeper the discharge, the more you can put back into the bank - this is a steadily diminishing law of returns, because the more you've drawn down the more you have to replace; second it's not too good for the engine to run it at 1000 or 1500 rpm; third, it's based on your house bank and alternator sizes per the acceptance discussion.

The slower you run your engine, the less any alternator will deliver and the longer you have to run the engine.  You'd be better pulling up the anchor and go motoring around at cruising speed than sitting on the hook.  I also wouldn't leave a running boat unattended.

I wrote this as part of a long 2004 Summer Cruise story I wrote for our local Fleet 1 Telltale:

Helpful Hints:  Electrical Systems:  For extended cruises for those with OEM stock alternators and no external regulation, there is inherently a steadily diminishing law of return in battery charging even if you motor a lot everyday.  If you do an energy budget, even if you motor for eight hours a day, you will be drawing down your batteries, regardless of how much you've increased your house bank.  You WILL need to stop and plug in every week or so.  Do the math.  If you really want to stay out "forever," add solar and a new high(er) output alternator and smart regulator.  We minimized use of electric lights with our lamp oil trawler cabin light and lamp oil anchor light hooked onto our Garhauer dinghy davit, but most of the energy use is from the fridge.  With our large 315 amp hour house battery bank and a separate start battery, we were very comfortable being out in two different situations.  The first was for a few days without motoring or plugging in.  We turned off the fridge at night, so we only used 30 to 40 amp hours a day and could go three or four days without running the engine.  The second was knowing we'd be motoring for at least five hours or plugging in at a marina.  So, before you go, make sure your electrical system is up to snuff so you can really relax and enjoy the peace and quiet without having to worry about your boat systems."

Followup comments:

The "Do the Math" referred to doing an energy budget.  Also assumes you have refrigeration, rather than an ice box, and that you plan to use it.  You could go longer, much longer, with just using the icebox as an icebox, even if you do have refrigeration - just turn it off!  

If you've topped off your house bank before you leave, then make a running total of how much gets drawn daily, and then how much you'll be able to put back in with a stock alternator.  Unfortunately, that's not very much because even with a 55 amp alternator, with no external regulation, the internal regulator will not put out very much.  Without getting into the details, covered elsewhere and in references, say you get 20 amps (which is pushing it).  Motor four hours you get 80 amps, which barely keeps up with the fridge load.  We still have our stock alternator with an AutoMac, so we get a little bit more out of the alternator.  I have a new Blue Circle Alternator and MaxCharge ready to go in as soon as I can find a window to start boat work amidst all the sailing.  :)

PS - I've since removed the AutoMac, installed the 100A alternator with the MaxCharge regulator.  The "steadily diminishing law of returns" still applies if you "do the math."  If you stay between the 50% and 85-90% as "cruisers" do, it can work for you.

You'll also find an anchor light is a BIG draw.  We use a lamp oil fired fresnel lens anchor lamp we hang from the backstay - saves a LOT of electricity unless you've switched to LEDs.

[sailaway]

Brad Just got back from the BVI. The charter company told use to run the engine 1.5 hrs  at 1800-2000rpm in the morning and an 1.5 hrs at evening. Not 3 hrs at one time. We had 2 frigs,5fans, two heads with shower pumps.hope that helps. charlie

[Kyle Ewing]

When I sail the Mac (333 miles under sail) I end up on average running the engine 3-4 hours at 2000 RPMs every 24 hours, 1 to 1.5 hours at a time.  I have a Blue Circle 100 amp alternator and ARS-4 external regulator with 4 old golf cart batteries.  I watch the battery monitor and run the engine when charge gets below 60%.  I watch the acceptance rate and turn the engine off after it's been in the absorption phase for 30-45 minutes because the amp output is very low.

My power consumption under sail is much higher than at anchor--auto-pilot, radar, instruments, nav lights are all being used plus the typical refrigerator, lights, radio, etc.

From the cruising guide (a nice guide!), it looks like you'll be moving every day or two.  You'll certainly experience the diminishing law of returns and find it takes 12-24 hours to bring your batteries back up to full charge when you plug in, but I'd think if you motor sailed a few hours a day you'd be okay.