Engine for battery charging (SHTP)

[NATBF]

Hello folks-

I saw someone here (sorry, can't find the post now*) who mentioned using their engine to charge during the SHTP, then... having to rebuild the top end after the race, having trashed the engine by running it under no prop load (only to charge). They seemed to view this is the cost of doing business in the SHTP. I cannot afford to wreck an engine, and have always been told not to run it except under load (i.e., with prop engaged)... but the electrician I'm working with said this was crazy; I shouldn't run the engine at 3000 rpm with no load (it's a Yanmar 3GM30F), but 1800 or 2000 rpm to charge wouldn't wreck the engine and will charge fine.

Is this correct? Obviously, if I can use the engine without destroying it, then I only need solar as a backup. That I can handle.

If someone would, let me know if one can actually charge using the engine without damaging the engine (and if so, what are the trick(s) to avoid damage -- is it just running at 2000 rpm for a Yanmar? Does running at low rpm risk carbon build-up?)

I know everyone probably knows this stuff, but I don't, and a thread search did not turn up the answer (other than that clearly some people clearly use engines for charging.)

Any information/tips appreciated.

PS: I am (very) slowly getting my boat in shape for offshore stuff, and am doing some electrical (charging) stuff right now, which brought this question off the back burner.

* This isn't the post I remember, but @AZ Sailor indicates similar issues:
"ended up putting a lot more run time on the Yanmar 3YM30 power plant than planned. I don't have my log book handy so I can't tell you the total hours, but it might have been twice what what was planned -- so for my 17-day passage, maybe 30 hours or more. This was really not good for the engine. Then, in the inevitable slow down in usage after getting back from HI, things got worse. The injector nozzles fouled and the mixing elbow became so constricted I was surprised the thing even ran at all."

A

Post in thread 'Surprise!'

Mar 18, 2020

I finally got around to measuring all the loads and preparing an offshore electrical budget.

Your comments are encouraged. What did I forget, or what looks out of whack? Thanks!

Morning Star's electrical budget for the 2018 race predicted almost one hour of engine time to charge every day. And that was expecting 7 hours/day of sunshine on rigid solar panels with a total of 225 watts. We had little sunshine for the first 10 days or so, and not a lot over all, and ended up putting a lot more run time on the Yanmar 3YM30 power plant than planned. I don't have my log book handy so...

• AZ Sailor

 

[BobJ]

Surprise! has the same engine (3GM30F). It idles around 800 RPM and charges at 1,200-1,500. Her alternator is a 100 amp Balmar with external regulator. The stock engine came with an internally-regulated alternator which isn't as efficient. As an aside, we never saw alternator output above 40 amps and it was often below 10 amps, but Surprise! has a lot of batteries and an efficient solar array. The batteries don't run down very far.

I agree that running the engine in neutral for charging isn't ideal - diesels are happiest when run hard - and I wouldn't let it sit at idle for very long. But we didn't have any problems using it as a battery charger during the Pacific Cup. We were double-handed but we still used the autopilot a lot, and it has a power-hungry hydraulic drive.

I'd have the engine serviced before the race, including checking the exhaust system for build-up. Though unrelated, I'd also check the tank for sludge, which will come loose and cause lots of grief offshore. Then again when you get back. Depending on how hard you run the engine during the return, your mechanic may have Nurse Ratched-type comments.

 

[NATBF]

Wow, that is super helpful. I have a 120A Balmar alternator (and also an external regulator), and it never occurred to me that I'd be charging at much less than 120A at full rpm. Knowing that it may be so low (e.g. 40A) will definitely impact my power budgeting.

The tip on engine maintenance also appreciated; I knew about tank sludge, and would always get service before a long trip, but the exhaust build-up had not been on my radar.

After seeing your post, I did what I now realize I should have done long ago and looked up the alternator detailed specs; chart below in case others look at this thread later. Clearly no charging at idle (1000 rpm), then there are big changes (+8A per 100 rpm for mine) as one goes above 1500 rpm, plateauing a bit over 2000 rpm. So the question of exactly how high one can run the rpm between 1500 and 200 while in neutral makes a big difference... I think I'll talk with a mechanic about the appropriate maximum. But even at 2000 rpm, I'd only see half the rated output of the alternator...

BTW, If you got 40A out of your alternator at 1500 rpm, that indicates a huge benefit from the external regulator; table below suggests one gets about 20A at 1500 with the stock unit. Also, I had not realized that output drops 15% as it heats up, but figure it will probably heat fast so the lower number is most relevant.

Thanks again (and experience from others, especially on how high one can run engine rpm in neutral, would still be very welcome.)


PS: In case anyone wants the plots of Amps / rpm (for a range of alternator sizes), here they are:

https://balmar.net/wp-content/uploads/2017/11/PDS-60-120-DV.pdf

 

[mpolkabla]

I charged my batteries while in neutral twice a day during my crossings and found that 1800 RPM was the sweet spot form my systems which output about 40 amps to my batteries at that RPM. I have a Beta 38 HP engine with 60 Amp rated alternator with external regulator (Balmar MC618) on my Cal40. Worked like a champ and had no issues with the engine being unhappy doing this twice a day racing to Hanalei. Ran the engine under load on the way back of course and no engine issues there either. Keep us posted what you find out about running engine in neutral (without load) for extended periods... I'm curious what the experts say.
MP

 

[BobJ]

That chart is the alternator RPMs, not the engine.

A multimeter is your friend here. The output will drop immediately if the batteries are close to full, so do something to draw them down a bunch so you have some time to measure at various engine RPMs. On the little boat I left the steaming light on all weekend (the only light that wasn't an LED). This was good for a couple late-night phone calls from dock mates, but I digress.

Anyway, you just need to know what engine RPM gets you into charging range with the alternator. We've had a few SHTP skippers who thought their alternator was INOP because they didn't run it above idle. Good fodder for the twice-daily SSB chat: "Just goose it a bit - there you go."

Were you able to print out Surprise!'s electrical budget from that old forum post? Here it is again. I can also send the Excel file to make your own.

 

[NATBF]

Thanks yet again!

I had naively assumed the alternator was driven right off the crankshaft. I'll get in there with a DVM, but frankly better, I suspect, will be using my (soon to be new and capable of telling me Ahr in/out) Victron battery charger.

I had not looked at Surprises' electrical budget yet, but had done my own (which is less sophisticated but comes out similarly, which reassures me I'm not too far off base -- 162 Ahr/day for me.) I don't have the big solar array you do, so want to try to trim that down a lot (LED bulbs here I come, but a big one is the Raymarine chartplotter at 66 Ahr/day, even though it's only 9". I am wondering if there is some way to reduce that (e.g. by turning off the screen) while leaving it running for alarms and so forth. Or maybe I just rely on my Vesper AIS alarm and not the chartplotter, which I can start when I need it? Just starting to think about that.) Any more detailed calculations (for me) will have to wait until I have the new charger and can read current draw when turing different instruments on/off.

But I will have to be charging 1+ hrs/day off the engine, maybe closer to 2 even after I strip things down for efficiency, so still going to talk with a mechanic I guess before running my engine in neutral that much. With luck it'll be fine, as @mpolkabla found.

PS: I got the Excel -- very much appreciated, as you have all the efficiency and other categories I didn't, and now I can just plug into your sheet : )

 

[BobJ]

We didn't use the big chartplotters much, mostly just the laptop. "We" because I have yet to race a SHTP with the big boat - just the PacCup DH.

Besides helping your power budget, adding solar would provide backup if the engine craps out, which engines do. You probably have room.

 

[BobJ]

And again, that 40A output on my chart is a fallacy. It never put out that much because the batteries were never flat enough. We watched the state-of-charge on the panel gauge and ran the engine/alternator when we needed to. My experience with both boats offshore was that I used less power than predicted and needed to charge less than predicted. In my first SHTP on the J/92, I could have raced all the way across and never run the engine. I only ran it to make sure it would.

I need to add Surprise!'s new refrigeration to my power budget - the ice cream will change those numbers a bit!