I hate having the engine on. Even if I'm anchoring or catching a buoy (or slipping away from same) I'll try and do it without the engine if I think it's safe. But if you have to get somewhere and there's no wind or not the right kind of wind, then you've got no choice. And when you've got the engine going your batteries are getting a good blast of fresh juice. Yum.
But sometimes you have to put the engine on to charge the batteries, and that I really, really hate to do. I have two 70ah batteries, one dedicated to start the engine and the other meant for powering everything else. This scheme ensures you can always start the engine to charge the batteries. No battery power, no engine and so no hope of getting battery power back. Not good. I recently heard of a blue water sailor who was presented with this problem and he had an ingenious solution. He rigged a line from his boom and wrapped it round the prop shaft in such a way so that when he jibed (as he did) the rope yanked on the prop shaft, turning it and thusly starting the engine (he remembered to put it gear!). Sounds risky, but presumably he did his calculations right. I think I'll call the coast guard.
Anyway, it's summer, the sun will be shining constantly, ha ha, and so I have invested in a 40W solar panel. Now I'm something of an expert when it comes to solar panels as I have a 3.4kW installation on my roof at home. GCSE physics tells us that a 40W panel will give me 40watts / 12volts=3.3 amps of current in optimum conditions. But I won't get optimum conditions for many reasons mainly because it will either be dark or the sun won't be shining or the panel will be in shade - i.e. most of the time! The panel will be fixed horizontally on the top of the deck hatch.
They say you should do a power calculation for your boat meaning that you add up the total amp-hours (ah) you think you'll use every day to see if you've got enough juice in your battery to last the course. So here's my simple calculation:
70ah battery means, in reality about 30 ah of useable power before I need to charge the battery. On passage, with all my instruments and with some use of my tiller pilot, I use about 2 amps per hour. Couple that with the need to charge my laptop computer and phone and other bits and pieces, 30ah should see me through one 10-hour passage day. I don't have such luxuries as fridge, power showers & dish washers! A night sail will suck 2-3 more amps for the lights. The upshot is that I'll need to charge the battery fully each day ready for the next which will mean putting the engine on whether I need it or not for at least 1, if not 2, hours. That's what I want to avoid. Hence the solar panel. On a nice day, I reckon I could get 15ah charge from my solar panel. And when I'm not on passage, the solar panel will keep charging (I have a charge controller to prevent over charging). So there's a good chance that this solar panel will obviate any need to use the engine just to charge the batteries, as I will very likely use the engine to some degree every day anyway. We'll see.
I also have a really useful piece of battery management equipment that tracks amps in and out of my battery. It's a Nasa Clipper BM1 monitor as pictured on the right. It's brilliant. Before I got this I never knew the state of my battery and frequently would run it down too low. This tells me how much juice I've got left and how much I'm putting in or taking out at any moment. It'll be even more useful to track the utility of my solar panel. A must have in my opinion.