How to do the math on this kind of thing: You neet to know what the average draw for the device is. If everything is nominally 12 volts, then it can all be done in amps (A). NOT amp-hours (Ah), that is a rating of battery capacity, or for a device, a draw over time, How many amps draw each instant, for how many hours? That's amp-hours. The autopilot uses some average amount of current to steer your boat. It may vary from moment to moment but if you have an average over a longer time like several hours, that is what to use. Say it uses 2A on average. If you use it for 2 hours, that uses up 4 Ah. If your battery is 40Ah capacity and is fully charged, you use up 10% of the (theoretical) battery capacity. Actually you use more than that, by 10-20% or more, but I won't go into the details on resistance losses and device efficiency and charging efficiency here. The lighting coil or alternator generates some average amount of current when you're motoring at a given throttle setting. Say at mid throttle it puts out 3A. If you motor for two hours at mid throttle that generates 6Ah of energy. So in theory you come out a bit ahead; in practice with losses in the system you'll probably at least break even, in this simple fictional scenario. In terms of putting energy from some device into a battery, you want a charge controller to charge a 12V DC battery from a nominal 12V source (PV panel, lighting coil/alternator that provides DC output, DC generator, etc.). The voltage from the source needs to be ~13.5V or more, or no significant charging will happen. So at low RPMs on the motor, you might not get enough voltage. Also at low RPMs you will get less current. The combo of voltage and current determines how much if any energy can be transferred into the battery. For comparison, a 30 watt mono-crystalline PV panel with proper exposure to full mid-day sun and no shading will put out about 1.7-1.8A. So if you manage to get 4 hours of exposure then that's ~7Ah. Best charge controllers (details on request): https://www.flexcharge.com/products.htm PV7D for PV only. NC25A for PV and also any kind of generator/alternator DC source (outboard alternator, lighting coil, wind gen, towed gen, etc.). This is all for lead-acid batteries of whatever kind (wet, sealed, AGM, etc.). For lithium batteries, different charge controller situation. But the power calculations are universal. cheers, John On 11/6/21 5:51 PM, casioqv@usermail.com wrote:
I did some more research on this... the small RayMarine ST1000/2000 autopilots can draw up to 5aH but steering a small boat probably only draw 1-2 aH, especially with the sails down and low tiller effort. A lighting coil on a 2-3hp outboard probably produces around 3aH, which is AC around 10-20v depending on RPM. With a rectifier to convert to DC this can be used to charge a 12v battery that will also stabilize the voltage level, however a voltage regulator would protect from overcharging. It would probably work- you would likely have just enough current for running modern LED nav lights plus the autopilot.
For my old Yamaha 2hp it costs about $40 to get a lighting coil. One can probably be added to almost any small outboard, it uses the flywheel as an alternator in the same way the magneto does to make spark.
Sincerely, Tyler
----- Original Message ----- From: "casioqv" <casioqv@usermail.com> To: "For and about Montgomery Sailboats" <montgomery_boats@mailman.xmission.com> Sent: Friday, November 5, 2021 7:24:51 PM Subject: M_Boats: Re: Outboard generator
Some small outboards can take an optional lighting coil or charge coil, it's an installable option on my 80s Yamaha 2hp. It produces a really small amount of current, designed to run nav lights, but might work with an autopilot depending on it's current needs.
Sincerely, Tyler
-----Original Message-----
From: Dave <scoobscobie@gmail.com> To: For <montgomery_boats@mailman.xmission.com> Date: Friday, 5 November 2021 9:35 AM PDT Subject: M_Boats: Re: Outboard generator
I'm not aware of any outboard that is of appropriate size for an M15 that has an alternator. Alternators usually found as option for motors 5HP or greater. Some 4HP motors have identical powerhead (aka block) as a 5 and 6 HP so an alternator can be added by the owner.
Also, the alternator doesn't start putting out any significant power until the motor is turning higher RPMs ... meaning the M15 is being overpowered (trying to climb the bow wave and dragging the transom deep in the water.
A M15 has absolutely no need for a motor greater than a Honda 2/2.3hp. larger motors are bad for boat trim, aka sailing ability, because of excessive weight hanging aft of the transom.
:: Dave Scobie :: M6'8" #650 :: Baba 30 #233 DEJA VU :: SV SWALLOW - sv-swallow.com :: former owner M17 #375 SWEET PEA - m17-375.com :: former owner M15 #288 SCRED - m15namedscred.wordpress.com <<-- new site!
On Thu, Nov 4, 2021, 11:32 PM Jim Sadler <jimsadler@jascopacific.com> wrote:
Looking for small OB that will power my tiller pilot Capt Jim M15
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