Interesting idea... I'd say you are going to be all set up to test it, given that you are intending the generator, the fridge, and the charger & large battery bank in any case, and it sounds like you already have the trolling motor too? So I'd say give it a test before a big cruise and see if/how it works. Worst case I think would be if you have to motor for a long time. Test out whether the genny (via the charger & batteries) will run the motor at "high enough" thrust for hours if necessary (fridge off, since you are testing "emergency" propulsion scenario, all power to propulsion system). And, is the motor rated to run for hours at "high enough" thrust? In theory, from your numbers below, you could use position 3 constantly (how fast does that move you?), but with actual efficiency losses in the system (10-20% from generator to motor via charger & batteries) that might be on the edge...try it and see... If you have a clamp-type DC ammeter, then you can take direct readings on your DC cables with everything running - how much current out of the charger to batteries, how much from batteries to trolling motor, are you breaking even, getting a bit into the batteries, or still drawing down batteries. Solar electric panels that would fit on and stow in your boat are pretty much useless for electric propulsion and/or refrigeration needs. Unless you are deploying a large raft panels all around your boat when at anchor (and somehow stowing them when underway) and it's sunny all day every day. I would say the one issue is, you are probably underestimating the power usage of the fridge. If it is the Dometic CF-50 or anything similar, with average power usage rating of 45W (at 70 deg. F steady temp, so if your average temp is higher, it will use more, or if you want to keep it colder than whatever their test temperature is), then at 50% duty cycle (likely in practice, conservative estimate at worst) that's 540WH/day which at 12V nominal is 45AH/day, meaning a couple hours a day running the genny (at full 26A output, with a 10%+ loss via charger & batteries) just to support the fridge. Let us know how the testing goes...:-) The SailPro 6 is massive overkill for an M17, as you have noticed (hull speed at 1/4 throttle!...and 60+ lbs. on the stern...and needs high RPMs to put out much charging current). A lightly used one came with my M17, and I sold it for about 1/3 of what I paid for the whole boat-motor-trailer rig (a nice leveraging of surplus resources :-). The comment about energy density of gasoline is relevant...if you weren't already planning on all that other gear (fridge, genny, & big battery bank) then a Honda 2.3 would be way more efficient... :-) cheers, John S. On 03/05/2017 05:22 PM, Wilson Frye wrote:
All,
Will any M17 owner who has used a trolling motor as auxiliary propulsion for leaving and approaching the dock, and for short term auxiliary propulsion when winds are light, share what you may have learned from the experience?
I have a 6hp Tohatsu SailPro. It drives the boat at hull speed in benign conditions with very little throttle (perhaps quarter throttle). it sips fuel. It has a 6 amp alternator that is advertised as capability for recharging batteries.
Fact is, the alternator is inadequate for battery charging. We will be cruising with a 50 quart Dometic refrigerator this year, and as a result we’ll be taking a 1000EUi Honda generator and a Noco Genius 26000 (26 amp capability…@14VDC that’s 364W) along to charge the 200AH of deep cycle batteries to keep them near topped up from day to day.
Today it occurred to me that if my 45 pound thrust 12V trolling motor will push the boat at a reasonable speed when needed, why not leave the Tohatsu at home?
Here’s my thinking… - The outboard uses about a half gallon an hour at 6K RPM…about 5 knots under good conditions. - The generator’s 0.6 gallon tank will run the generator at full load for 7 hours. - Useful energy in the batteries is about 40 percent of capacity to avoid discharge below 50 percent…which is about 80AH. - The battery charger will provide up to 26A when bulk charging the battery if it is significantly discharged. - The trolling motor will draw 40A if run at position 5 (maximum speed), 32A at position 4, and about 24A on position 3. - The Dometic draws about 3A when running to maintain temperature, after cooled down, and it’s duty cycle is likely to be about 40-50 percent of the time while cruising in summer in the Gulf. - The Honda generator is quieter than the Tohatsu. - The outboard is a heavy load on the transom when not in use. - Solar panels are not mounted and are stored when underway. - On days when the wind is unreliable, on the trolling motor we can move about 20 miles (assume 3.5 knots) in 6 hours. If we run the generator long enough to bulk re-charge the batteries for their overnight use, about an hour, before getting underway, then the charger should be able to complete the absorption phase of charging during the six hours of re-positioning (is the assumption that the charger will provide 20A to the trolling motor while charging the battery a good one?)
And the overall effect will be that I will use about a third of the fuel using the generator and charger and trolling motor in this fashion, rather than the Tohatsu. And be able to keep the batteries charged efficiently without running the generator for extended time at anchorage on days that we move to a new location (solar will be used if at anchor all day), and be able to leave that heavy outboard off the transom.
Thanks for any comments on the feasibility of doing this…
Will
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