12v Power Calculations, Part 2

October 21, 2014 at 12:45 pm

Way back at the beginning of the year I calculated Duet’s power consumption requirements, and I figured that she needed 2 x 75Ah capacity (size) batteries and had a power consumption of a maximum of 50Ah per day.  I was looking at a wind generator and a 20w solar panel for charging. You can see my original blog post and calculations here.

Well, that was all quite pricey and I ended up just keeping my 1 year old 75Ah battery and adding a new 20w solar panel on the lazarette locker lid. In practice it worked better than I thought it would, and actually between leaving Portland and lifting her again in Plymouth 9 weeks later I only actually charged the battery from shore power once.  The solar panel seemed to keep it pretty well charged, topping the battery up surprisingly quickly even on overcast days. But, I was very frugal with the electronics on passage… I kept the VHF off most of the time, used a battery powered GPS and if there was shore power available I made sure that my iPad and cameras were fully charged before we left again.  And then I did a night passage from Helford to Plymouth. And the lights died before dawn as the battery was flat!

Clearly I do need to look at this again… So this first table shows what I think I need for an average coastal passage, or more precisely what I needed for the Helford-Plymouth passage.

It has the following assumptions:

1) Maximum time at sea is 18 hours as my longest passage so far is 15 hours so this is plus a bit for contingency.  On a coastal passage I probably wouldn’t want to do much more than that on my own too often.
2) I want to be using the Garmin 128 GPS, rather than the handheld one as it was very thirsty on batteries.
3) The radio used to transmit has a much higher power consumption than receive or standby.  I will be using the radio at a minimum and haven’t factored for any radio use.
4) In the daytime I am a lot more relaxed about not having the radio on, but at night I would probably want it on standby listening to traffic. I’ve calculated for it to be on for 15 hours. Standby for 13 and receiving for 2. This assumes there’s not that much traffic.  If there’s lots of chit chat or numnuts running out of petrol it annoys me and I turn it off (unless I’m bored ;o)
5) I have already replaced Duet’s old navigation lights with an LED masthead tricolour which tested working OK on the ground, and again when on deck ready to be stepped.  But by the time the mast was raised it didn’t work, and I couldn’t get it going all summer (Grrr!).  I am assuming the fault is with the connection at the top of the mast and I will get it going! It draws a fraction of the power of the old bulb lights.
6) I’ve searched on google for the power consumption of each piece.  This does assume that the wiring is in good condition, as if it’s not the resistance is higher and the unit draws more electric. I know Duet’s wiring isn’t great, but I will be replacing most of it this winter, and the mast was rewired earlier this year. So I’ve gone for best case, as per the manufacturers specifications.
7) There is no electric autohelm allowance on here. I’m still toying with this… and I don’t have any cabin lights, I just use a paraffin lamp.

12v Power Calculation Hurley 22

Coastal Passage Power Consumption

So this is showing a total maximum power consumption of 33Ah a day. As you should never draw more than a third of your battery then 33 x 3 = 99Ah – I need a 100Ah leisure battery.

I would also need to replenish up to the 33Ah.  My 20W panel should replenish 10Ah a day (see previous blog post for calculations), and I also have a 60W charging coil on the outboard which works out to replace 5Ah each hour it’s run. In practice, that night from Helford to Plymouth I actually ran the engine for 7 hours… so if I’d had the engine charging connected I might well have been OK with the lights.  If I add another 20W solar panel to the forecabin hatch, with 40W of solar I should get 20Ah and if I run the engine for just 2 hours on a passage you can see we’re up to a potential 30Ah and are nearly there…

Conclusion: 100Ah battery plus 40W solar plus engine charging needed

But, if you’ve seen my winter 2014/15 maintenance list you can see I want to do some more upgrades and make her a bit more ready for longer, more “offshore” passages. As a base line I’ve done this second table to work out what I would need to give me enough juice to do, say, a 3 day passage across the Celtic Sea… (perhaps, just maybe, leaving on 14th June?)   So this table shows what I would run over a full 24 hour period at sea, but tactics are a little different:

1) I would turn off the radio once clear of the coast, but have allowed 4 hours use per 24 hour period (3:1 Standby:Receive)
2) I would turn off the log/depth and GPS most of the time, only turning it on intermittently for recording in the log as out of sight of land/rocks there is more of a margin for error.
3) I would be using wind vane self-steering and probably turn off the wind instrument  most of the time.
4) I do intend to fit a NASA AIS and would have this on all the time (more on this in another post). It only draws 0.05Ah though.
5) This assumes 8 hours of darkness. In summer in the UK it is less than this.

12v Power Calculation Hurley 22

Offshore Passage Power Consumption

So as an exercise you can see the dramatic effect this has had: I’d need a maximum of 14.2Ah! Times this by 3 then I’d need 43.6Ah of battery, except they don’t come that small so I’d have a 75Ah leisure battery …but 20W of solar alone should replenish 20Ah and easily cover it.

Conclusion: Existing 75Ah battery plus 20W solar should be adequate.

Clearly this is very frugal, and not to everyone’s taste but it’s an interesting exercise. I am definitely cutting my coat according to my cloth, but on a small boat with no diesel inboard and alternator I think I pretty much have to. But I will upgrade to a 100Ah battery, add another 20W solar panel and wire the outboard in, which should just about cover me for the coastal sailing that I’ve done this summer.  I’m pretty sure this isn’t the end of this story though…

Adding another 20W panel should be fairly straightforward, as it just needs to be wired into the regulator. I’ve already decided to put it on the forehatch rather than have a flat one on the pushpit or something. Wiring the outboard is a bit more tricky, but luckily there is already a socket there as the previous engine was wired in for charging so I have a head start at least.

Oh and if you think you’d find my calculation spreadsheet useful, click here to download it and then you can amend it for your own setup.

5 Things that Worked Well (and 5 that Didn’t…)

October 5, 2014 at 10:30 am

So, after a summer of cruising, here are 5 things that worked very well…

#1 Cheapie Battery Monitor off eBay

Hopefully you’ve seen my previous post on what this is, but I was rather pleased how well this worked! I used my multimeter a couple of times to compare results, and the multimeter always showed a couple of points above the plug in, which I’m OK with as I would rather it under reported than over reported… When the solar panel is charging it can over-read, but they do say that you should leave a battery half an hour to settle after charging before taking a reading.

Cost: £4

12v Voltage Meter hurley 22 duet

#2 Barton Winchers

Once I’d got these on, they were great.  They worked really well with 3 turns of my sheets round them (without even trying to get the sheet into the groove on the top).  The only time they slipped was when I clearly needed to reef.

Cost: £60 odd… But worth it I think!

Barton Winchers Hurley 22


#3 Oilie Stowage

After a few weeks of cruising I realised that what with the vaguaries of the British Summer I needed to have my oilies to hand from the cockpit. If I left them on the quarter berth they would without fail end up on the floor and out of reach from the tiller.  So I put this together from a piece of decking teak bought at Beaulieu that I just varnished, and then added some brass hooks from the Pound Shop.  I stuck it on with some 2 part epoxy glue and it was pretty life changing to be honest.  Funny the little things…. Pretty proud of the mitre too. It fits nicely and looks good.

Cost: £1 for wood, £2 for hooks, £6 for glue

Coat Rack Hurley 22

#4 Cider Jug

I’m not going go into detail but hopefully you can guess its use?  Much safer than dangling over the stern, use a cider jug in the cockpit and then tie a sheet to the handle and just lob the lot over the side (downwind). Perhaps more useful to the singlehander, as with crew where a modicum more privacy might be appreciated?  Don’t forget to bring it back aboard after 5 minutes or so… It will slow you down 0.3 of a knot (I know this for a fact).

I also found that initiating its use was generally likely to invite a search and rescue helicopter flypast. Maybe it was a question on my CG66? I must amend that…

Cost: Free

Cider Jug Hurley 22

#5 Galley Storage

I wasn’t sure if these would be secure enough.  They were.  Even though the compass on a few occasions became unfastened from its stowage and ended up on the floor, the cutlery and salt and pepper never did! Thank you IKEA.

Cost: 60p for cutlery holder, £3 for salt and pepper holders

Galley Stowage Hurley 22

#5 Galley Storage


And now 5 things that were a bit disappointing…

#1 The Tender

I bought a secondhand inflatable from Bussell’s before I left Weymouth, as every cruising yacht needs a dinghy, right? Well, er no I don’t think so.  I towed it about for a couple of weeks and then stowed it behind the mast before I left Portland to go round The Bill. 400 miles and 14 harbours later it hadn’t moved.  At 2.3m it is over 1/3 of the length of Duet and it is a small tender! In all the harbours I have been to I either had a walk ashore mooring, was on a buoy and there was a water taxi service, or I was at anchor and probably wouldn’t have felt comfortable going ashore and leaving Duet on her own anyway.  Admittedly the water taxi’s weren’t *that* cheap (from £3 to £5 per person return) which can quickly add up if you are crewed, or want lots of trips ashore.  But if you have crew, then presumably you would also have assistance to get the thing blown up and launched. Not sure it works for a singlehander.

Okay, so it could also be deployed as a liferaft in the event of a sinking, but I’m not even sure I could have done it on my own, and I don’t think the cockpit is big enough!  Blowing it up from the water doesn’t even bear thinking about. Besides, I have not so far been that offshore.  And I have a PLB on my lifejacket…  The dinghy’s going on eBay.

Cost: £150

Dinghy Stowage Hurley 22

#2 Bungee Self Steering

So I got this working twice, when the wind was forward of the beam.  All the other times, either the sea state or the wind was too high and I had to hand helm. I need better power and a tiller pilot, or a wind vane.

Cost: £5

#3 My phone.

I got my smartphone wet rounding The Bill and fried it. I bought a cheapie replacement phone in Bridport, but I then had no easy internet access for the weather etc.  Annoying as I actually have an Aquapac Stormproof cover, but wasn’t using it. Lesson learnt.

Cost: £hundreds…

#4 The Rigging Tuning

I messed about all summer with the rigging tension, and it’s still not right. There’s too much pre-bend now, and I think I need to slacken it all off and start again! The mast’s coming down for the winter anyway… There will be more on this I’m sure.

Cost: FREE

Hurley 22

#5 The Compass

Duet came with a big bracket mounted compass that is fixed just below the companionway in the cockpit on a removable bracket.  It looks retro and cool. However, it makes getting in and out of the companionway in anything other than a flat calm more tricky.  It’s also showing nearly 20 degrees deviation, and is invisible in the dark (no backlight or glow in the dark markings).  Sadly it has to be replaced (but I actually have a plastimo bulkhead compass on my day boat so I might just swap them.)

Cost: FREE (came with boat)

Would love to hear what you found this summer…

Dolphins and Moonlight. On the same passage.

September 21, 2014 at 9:52 pm

I got as far west as Helford before my time started to run out… I’d decided to keep Duet in Plymouth for the winter, and I really wanted to do my first solo night passage to round off my season.

So I decided to do Helford to Plymouth overnight.  It’s a straightforward run, with the Eddystone Lighthouse to keep me company most of the night. At 42 miles I figured it would take about 11 hours at least. As there were no headlands to round and the tide atlas said the streams weren’t that strong I decided to ignore the tide and left at 5pm so I would have a good few hours to get off past Falmouth and the lobster pots before nightfall at about 8.30pm.  This should get me to Plymouth after 6am, when it would be light and I decided to head back to Cawsand to anchor and catch up on my sleep.

The forecast was for variable becoming NW 4, then 5 later and it was pretty calm as I motored out of the Helford River. Engine on then.  But at least I could lash the helm and relax a little.

Old Gaffer.  No horizon.

Old Gaffer. No horizon.

As I got a little way off Falmouth I found I was motoring through acres of sea birds bobbing about on a mirror sea.  I thought they were Guillemots, but it turns out they are actually Manx Shearwaters. There were acres of them and I amused myself playing wildlife photographer for a bit:


Manx Shearwater

Manx Shearwater



Manx Shearwater

More Manx Shearwaters…

And then the dolphins came… Please forgive my “whoop” in the video.

The dolphins are coming!



Just after sunset, but before the light was completely gone some wind came in from the North West and I was soon humming along to a f4 and a really big moon (turns out it was the harvest moon), still amusing myself mimicking the shearwaters taking off (if we ever meet, do ask me). However, in the dark I realised that the light behind my handheld GPS did not stay on permanently and I could not see the compass as it was unlit and did not even glow in the dark! Effectively I was sailing blind and for the first time ever I wished I had a chart plotter.

There were also lots of fishing boats on both sides of me that kept me on my toes all night, presumably they were out of Polperro and Looe and running at right angles to the shore as they seemed to keep crossing me at right angles and then turn and run parallel to me before going off back inshore. I found their presence a comfort though as I wasn’t totally alone but could see their course and direction clearly with their lights, and with the engine off I could tell from their engine volume how close they were. Yes they kept me alert in the small hours!

But the moonlight on the sea was pretty mesmerising.  I do love night sailing.  Was pretty exhilarating on my own too:

But sure enough the wind went a bit over the top, and the sea was starting to feel quite big in the dark, so at about 3am I dropped the main and continued under a reefed genoa. We were well into a Force 5 and the moon had gone so it was pretty dark now. I was also a bit ahead of schedule and by 4am I was less than 10 miles off Plymouth but I didn’t particularly want to approach in the dark. About this time I checked the battery and it was running low even though I’d turned the VHF off long ago and was running only with the lights and the windometer (there will be another blog post about this!). I definitely didn’t want to approach in the dark without lights! So it was a little before first light at 5am that my navigation lights died completely and I went into ‘stealth mode’. But in any event I put the engine back on as it was clear that I couldn’t point high enough to get me into Plymouth which was now dead upwind. Dawn also showed me I was now in a fairly significant swell and the wind was still a good Force 5. Of course with no lights on the fishing boats all were now on a collision course with me, and I turned to run parallel with one trawling at right angles to my course turning back to my course once I was well astern.

But you know, we bashed to windward, the dawn broke and I was exactly where I expected to be, and now visible again to the fishing boats… Anchored at Cawsand at 8am with no power for the depth sounder feeling pretty pleased with myself all in all. First solo night passage, and the longest of the season.

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