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sanibel sailor 17-01-2020 13:00

Planning/engineering for focused solar/battery system
I am engineering a small, very focused electrical system. I am planning a 300 mile adventure race in Florida in March aboard an engineless Windrider 16 trimaran. I'd like to be able to use my ST1000+ tillerpilot for say 6 hours a day. No other electric draws. Tight budget and no long term plans after this race.

Trying to figure out solar adequacy and battery size.

It says the unit pulls 0.5 to 1.5 amps, (0.04 on standby) but that is for a boat up to 6600lb. I hope that a very light boat will allow a short tiller arm, less motor excursion and consumption will be much reduced. I will need to adjust the gain way down.

1) Am I overly optimistic to hope for 3 amp hours a day usage?

I have a 20 watt rigid solar panel. It will be soft mounted and so I can adjust it to some extent to optimize output. Most likely limited to keeping it out of sail shade rather than perpendicular to the sun. At max output, around 1.5 amps. I will be using a cheapo eBay solar controller.

2) Is it reasonable to expect to get 3 or more amp-hours per day out of it in March in Florida?

Planning on getting a small Sealed Lead Acid battery, perhaps 18AH size, to run the tillerpilot. I will need to make or adapt a battery box as I don't think they are readily found in this size. Weight and volume are important. I don't want too big nor too small. As the battery is only $30 or so, I am willing to accept a short battery life associated with deeply discharging it, but it has to last the week or so planned trip. I will also have a separate small 3-5AH SLA battery charged to keep cell phone and handheld GPS going.

3) Does this seem like an appropriate battery capacity for this scenario?

john61ct 17-01-2020 15:52

Re: Planning/engineering for focused solar/battery system
Best to actually measure the Ah per day with a wattmeter / coulomb-counter, and go for a battery 3-4x that size.

Check out the small LFP units sold as motorcycle starter batts.

Yes 20W panel should put 5-7Ah per day into a 12V battery but only in very good insolation conditions.

Pete7 18-01-2020 13:46

Re: Planning/engineering for focused solar/battery system
We are a bit further north than you, but we never saw more that 1.6A from a 45w solar panel mounted flat on the deck even in good sunshine. I think you need more and a PWM solar controller to keep an eye on things. They are cheap as chips now. We just bought one of these to measure the freezer. Not fitted yet but cheap enough and will enable you to see what is going on and how the solar is keeping up. Must be something similar states side.

Are you only racing during the day?

Ignore Johns advice (most do) Lithium motor cycle batteries are twice the price of an similar sized AGM and since you have no plans after the race, that is money that can be spent elsewhere.

Buy yourself a cover, the autopilots are not waterproof!


Originally Posted by sanibel sailor (Post 3057455)
I hope that a very light boat will allow a short tiller arm, less motor excursion and consumption will be much reduced. I will need to adjust the gain way down.

Don't think it works like that. Are you aware they are full of plastic cogs and little elastic bands. How fast does this tri go? they work well at 5 knots, we used one for a decade (ST2000+) might be difference at 15 knots.

Photo from Chip Fords website:

Paul L 18-01-2020 16:09

Re: Planning/engineering for focused solar/battery system
You are getting good advice from Pete7.
Will you have time to do any test runs with the electrical setup before the race?
I'd estimate something like 1.5a draw at 50% duty cycle at 6 hours to get 4.5ah per day usage. Probably a little high, but better to error on the safe side. The entire race sounds like about 10 days, so thats a consumption of around 45ah.
If you could mount two 20w panels, one on one side of the boat and one on the other, this will reduce the time screwing around with moving them while racing and give you much more charging in the hours before and after being on the water. If you can get 1a for 6hrs back into the battery, that should keep you pretty safe with a 20ah battery.

sanibel sailor 19-01-2020 10:48

Re: Planning/engineering for focused solar/battery system
I will make a cover. Gonna be a wet ride.
I already have the pilot, panel and a PWM controller.
Found a plastic box this AM that will fit the battery and controller perfectly with an area to run wires out.
I am planning on hand steering during day and napping, when safe, on autopilot at night. Much is weather and progress dependent.

Pete7 20-01-2020 04:33

Re: Planning/engineering for focused solar/battery system
Certainly going to be fun. I would put a go pro or something on a pole at the back and film yourself each day. Let us know how you get on.


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