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Old 28-03-2020, 14:04   #1
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Direct solar -> motor -> prop?

With the price of rigid domestic-scale solar panels now ludicrously low, I am seriously considering using three of them to make my bimini. This would give me around 1kw of solar capacity.

I can't help thinking about future ocean crossings, being stuck becalmed under a beating sun, with too little diesel to make any sustained progress... and 1kw of unused solar power sat there...

According to an online calculator, I could achieve 2-2.4kt depending on final drive efficiency. My propshaft is reasonably accessible (centre cockpit boat) so I would envisage a belt drive on to a DC motor mounted in the engine room. The panels would be around 33v nominal each, so I would want to wire them in series and then step down to 48v directly to the motor.

Really just thinking out loud, it might be an interesting project. But perhaps it would be more sensible to buy a Torqueedo and rig up a tow from the tender. And maybe I am vastly over-estimating the amount of time that people get becalmed during trades winds passages.
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Old 28-03-2020, 16:05   #2
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Re: Direct solar -> motor -> prop?

1kW = 1.3hp
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Old 28-03-2020, 17:05   #3
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Re: Direct solar -> motor -> prop?

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Originally Posted by Singularity View Post
1kW = 1.3hp
...in full sun...

OP, the negligible HP produced aside, direct solar drive means no sun = no power. Reduced sun = reduced power. Any shading on panels, like a shadow off the mast = reduced power. This is why most use batteries.
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Old 29-03-2020, 09:08   #4
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Re: Direct solar -> motor -> prop?

Yes I'm fully aware of just how little power we're talking about.
Which is why I am anticipating 2kt or so. It takes remarkably little power to push a boat at a low enough speed.

This would just be something to keep me sane on those days when we are completely stopped but have too many miles ahead of us to consider burning diesel. Since I don't expect it to happen often I wouldn't really want to spend a huge amount on setting it up.

Is it going to be bad for my gearbox to have the prop shaft turned by a motor?
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Old 29-03-2020, 09:36   #5
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Re: Direct solar -> motor -> prop?

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...Is it going to be bad for my gearbox to have the prop shaft turned by a motor?
Depends on your gearbox; my velvet drive is approved to freewheel.

Next you've got to worry about shaft bearing side loads; here many have placed alternators/generators on the prop shaft, such that a ~1hp motor pushing the other way is sort of a force analog and sounds reasonable. But the mind wonders about the energy required to merely turn the shaft/prop through mechanical friction in air...no less water.

On a tangent...when I think about some of these alternate power ideas on my own...I remind myself that gazillions of smart sailor man-hours have been spent at sea becalmed in the "solar panel era" where people have had a hell of a lot of motivation to come up with something that works. Insofar as there's not even a crappy mousetrap available to make all this work, I assume that I'm not going to invent a better one. But more power to you if you can come up with something.
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Old 29-03-2020, 09:37   #6
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Re: Direct solar -> motor -> prop?

Consider mounting a Torqeedo motor on an external mount.
I thought about it quite a bit as an alternative but we're not doing any long passages any more. Wife's uncomfortable at sea.
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Old 29-03-2020, 10:58   #7
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Re: Direct solar -> motor -> prop?

I suppose one downside of the Torqueedo is that it isn't set up with a prop for pushing a heavy boat at a slow speed. I don't know if you can change the prop. On the plus side I would love to have a Torqueedo anyway and it would probably be much simpler than rigging up a Heath-Robinson drive system. Not cheap though.

I'm not looking at putting together something that's in any way polished or suitable for regular everyday use... just cobbling together a basic way of turning sunlight into motive power.
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Old 29-03-2020, 11:28   #8
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Re: Direct solar -> motor -> prop?

FWIW...in the complete brainstorming category...considering the cavitation/slippage of a Torqueedo and/or prop-shaft issues....it'd be interesting to see what (if anything) an industrial air fan could do in a pusher configuration.

Marine environment electricals ignored for the moment, the fan below supposedly moves ~10,000cfm using ~700 watts. 42" diameter, too lazy/stupid to compute the thrust, but I think it'd nudge a boat on calm water.



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Old 29-03-2020, 11:35   #9
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Re: Direct solar -> motor -> prop?

Haha, and imagine the sheer bliss lying behind it on a hot windless day.
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Old 29-03-2020, 12:34   #10
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Re: Direct solar -> motor -> prop?

Simple solution

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Old 29-03-2020, 12:58   #11
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Re: Direct solar -> motor -> prop?

You would have Max about 1 HP less losses, those losses are among other things friction, there is a lot of friction in your driveline, not much to a 40 HP motor. But a real killer to a 1HP motor.
Besides if your using your Solar to try to drive the boat, what’s running the fridge and other accessories?
Air propulsion is horribly inefficient. That’s why air boats have big HP motors for a small lightweight boat, 1/3 the HP of an outboard will outrun an airboat.

The external electric motor and prop whether Torquedo or trolling motor would be best, a trolling motor is meant to push a relatively heavy boat at very slow speeds.
You may even move more than you would drift.
But a much better plan is of course extra Diesel on board, you’ll go far further and faster with 5 gls of Diesel than you will with Solar. I move about 5 kts at a half gl an hour, less fuel burn slower but I’ve never gone slow long enough to know what the fuel burn is, so a 5 gl jug will carry me 50 miles and run me all day (10 hours).
Carry six on deck and that adds another week of day running
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Old 29-03-2020, 13:33   #12
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Re: Direct solar -> motor -> prop?

study the power rudder of these . Look at style of boats and speeds . Hours run time etc. Then remember that sail boats are designed to run on wind power.
https://www.duffyboats.com/
May give some more ideas. I would love to see one running. I have played with a small paddle wheel powered by solar . Still playing with building one again. I just move slower each day called A G E I have the advance state O L D A G E good luck
Oh I have used trolling motors on my 23' trailer sailor 2 80 # thrust ,,1 80# thrust,, and 1 55# thrust. Never checked how much power or how long batteries lasted diesel is more power and faster. Solar may be quitter and cheaper. as to weight I don't have the diesel motor or fuel now so was trade off for weight.
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Old 29-03-2020, 14:20   #13
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Re: Direct solar -> motor -> prop?

We're on the same page. I'm already in the midst of making my solar bimini (1200w). I cannot comment on your belt prop project.

I would not shell out all that dough on a Torqeedo. If I were you, buy two 24v trollers, ck out Caroute (brushless, have their email address if you like), Newport and don't forget Minn Kota. I have learned the key is your battery system. The money you save on the Torqeedo with twin 24v trollers (super low cost used) you can plop into lithium batteries. Or, heavier, less efficient new AGMs. Some experienced EV boaters still used flooded too. Whichever, you need lots of em and the highest AHs and also a good BMS (so cool you can use bluetooth on some).

I feel like with what's going on right now it is more than ever a smart reason for electric, or at least hybrid for complimenting/redundancy.

Oh, and as others have said - shading is a big issue. I'd consider ensure you can really get your boom out of the way, obviously off to one side, but I've seen a couple mod so it can be pulled up against the mast. Also, adding tilt option for the panels helps.

Sailing Uma is a great resource/vlog for electric cruising. Right now they are finally back in a marina plugged in. Why? In cloudy, cold UK. You could supplement your EV system with a genny, like the big, new, electric cats. Thunderstruck motors is popular for inboard motors. But again, if I were you I'd go cheaper with this experiment with two 24v saltwater trollers and see how your boat behaves before you invest much larger sums of money and time. Nice thing there too, is it is a small enough trolling motor to use as your dinghy outboard as well.
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Old 29-03-2020, 15:07   #14
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Re: Direct solar -> motor -> prop?

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Originally Posted by SY Kelpie View Post
And maybe I am vastly over-estimating the amount of time that people get becalmed during trades winds passages.
I think so, but were are you going?

If you can mount the panels though, I would do that to run fridge and a separate freezer etc to make life on board comfortable, it is after all going to be your home for some time.
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Old 29-03-2020, 15:21   #15
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Re: Direct solar -> motor -> prop?

I did just that on my 2010 Atlantic crossing. Totally becalmed, we made less then 2 knots with a Minkota trolling motor lashed to my stern boarding ladders. Biggest problem was lack of suitable gauge wire. The AWG10 I had was heating up and having a significant voltage drop. Ran the motor ot 2/3 throttle. I would not consider using the existing prop shaft, as the friction would eat up a significant amount of the available energy. Electric outboard of some kind may give you some progress. I have a AVI video file of the prop twiddling and a tiny stern wake behind my easily driven (in light air) Yankee 30., but this kind of file is not allowed here.
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