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Old 08-07-2018, 10:38 PM   #1
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Get home propulsion - strange ideas

This might be a walk on the wild side, skip to the next post if it isn't of interest.

I've got this small trawler and I come from a sailing background. In sailing, as long as the hull is still floating, there is usually a way to make it move: motor if the rig is gone, or jury rig something from the remains of the rig. Larger trawlers sometimes have twin engines, or a wing engine, or a PTO on a large genset that can be used for emergency propulsion.

The scenario I'm talking about is you are in a remote part of the PNW and a gremlin gets into the engine or a tree gets into the prop. As near I can tell, a small trawler will simply drift downwind until it hits something. There is the dinghy of course, and one could rig it to push slowly towards immediate safety. However it is gas powered, and I would usually have a very limited supply of gas - a few hours at most. A small light diesel outboard might solve the issue - you could motor for days - but such things don't exist.

Like most small trawlers, I have a 6KW diesel genset running off the main fuel tanks. It would run for weeks on the fuel onboard. Without significant redesign, a PTO isn't possible. But - there exist several vendors of electric outboards in the power range required. In principal, these could be powered nearly indefinitely by the genset. They are light, relatively small to stow, produce no drag when stowed. From vendors specs 5 KW or so will give you around 175 - 200 lbs of thrust. Hardly neckbreaking, but might move a 32-34' trawler at a few knots in calm-ish conditions. With time you could run hundreds of miles this way, if needed. 100 miles puts you much closer to civilization.

The only technical difficulty is these outboards run on 48 - 60 VDC, so either the motor would need to be changed out, or a suitable power supply constructed to convert 40 amps of 120VAC to 100 amps DC. The whole setup would cost a few thousand, far less than any other solution.

What's wrong with my thinking?

The easiest path would be to swap the DC motor for an 120V AC motor, and simply plug it into the wall plug (let's ignore the safety aspects for a moment, those are solvable). A variation would be to mount a 120V motor inside, allowing belt drive to the main shaft - but this requires the main shaft and prop to be functioning (as well as transmission and other issues). I kind of like the idea of the outboard as it is completely independent.
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Old 08-07-2018, 11:02 PM   #2
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I'm gonna grab a cold one and see what happens. Interesting idea. A different tack (OK, bad pun!) might be to drop anchor when you get to a shallow enough area and get on the VHF to call for an assist.
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Old 08-07-2018, 11:15 PM   #3
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Plenty of places up there where there is sparse VHF coverage, no cell coverage, and a tow ordered on the sat phone is going to take a 1st mortgage. This scenario is not all that likely, but I am always attracted to the idea of self sufficiency, when possible.
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Old 08-08-2018, 01:33 AM   #4
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If you have a large enough house bank ( 8 x 12 volt ), you could take 4 batteries in series to run the outboard....and use the generator to charge the other 4 batteries. Use 4 while you charge 4, then switch.
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Old 08-08-2018, 04:20 AM   #5
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120vac 35lb thrust https://canadianpond.ca/product/kasc...ller-de-icers/


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Old 08-08-2018, 04:36 AM   #6
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Not available in the US...yet. Probably bigger than you might want, not going to be cheap, but a sign of things to come, and what's available now


https://powerboating.com/yanmar-debu...esel-outboard/

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Old 08-08-2018, 06:29 AM   #7
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A quick rule of thumb is 20lbs of thrust is created by 1 HP.

A 200lb thrust electric motor would be about 10HP.

Perhaps a new expensive DC motor can do that , but most cont. duty AC motors are REALLY !! heavy.
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Old 08-08-2018, 07:19 AM   #8
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I like the idea of the electric outboard and looked at several different manufacturers a few years ago. The two issues I saw were propellers to small to be effective on anything but flat windless water and my concern about the durability of the lower unit under full load all the time. I just don't think they're going to survive a 100% power load for hours on end.

My conceptual solution would be to build an outboard style lower unit large enough to house a hydraulic motor direct coupled to a propeller. When needed, transom mount the unit, the hydraulic hoses have quick disconnects for easy portability, and run the unit off a hydraulic pump mounted to the front of the generator. While a person could take an outboard, remove the motor and just mount the hydraulic motor in place of it, I wanted to eliminate the unnecessary gearbox (and lost power there) and be able to turn a bigger flatter prop.

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Old 08-08-2018, 09:59 AM   #9
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Quote:
Originally Posted by Benthic2 View Post
If you have a large enough house bank ( 8 x 12 volt ), you could take 4 batteries in series to run the outboard....and use the generator to charge the other 4 batteries. Use 4 while you charge 4, then switch.
Trouble is recharging the batteries takes far longer than discharging them. It would be more efficient to run wires.

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A quick rule of thumb is 20lbs of thrust is created by 1 HP.

A 200lb thrust electric motor would be about 10HP.

Perhaps a new expensive DC motor can do that , but most cont. duty AC motors are REALLY !! heavy.
5 HP is about all a 6KW genset is likely to run for long. The 5 - 7 HP electrics are claiming that thrust - could be marketing puffery. Maybe reworking the DC motor to run at higher voltage, then just rectifying the AC would be a solution.
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Old 08-08-2018, 10:02 AM   #10
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Originally Posted by Keysdisease View Post
Not available in the US...yet. Probably bigger than you might want, not going to be cheap, but a sign of things to come, and what's available now


https://powerboating.com/yanmar-debu...esel-outboard/

I've no doubt that a small diesel outboard could be built - in fact there are some odd ones overseas. Keeping the weight low is the challenge. If I could buy a 15-20 HP which was only a little heavier than a 4-stroke gasser, I'd use that on the dinghy and call it done. In the US, I think the EPA won't let it happen.
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Old 08-08-2018, 10:24 AM   #11
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Maybe just build your own is an easier idea. Here is a small vertical air cooled diesel, in the pull start it weighs about 80 lbs. Clone that onto an outboard leg and it's heavy, but still manageable perhaps.
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Old 08-08-2018, 10:52 AM   #12
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Why not get two 12v trolling motors and make some bracketry so they can click onto the transom. Steer by changing power on each, or cycle on and off.

Oversize your charger or build a 120Vac/12Vdc converter with a big transformer and big rectifier. Or go 24-36-48V, the transformer and rectifiers get smaller.

I towed a 36' sea ray with dead engines against the wind with a dink and a 2hp Evinrude. It was very slow, but it moved.
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Old 08-08-2018, 01:42 PM   #13
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I've been on the water for about 60 years and lost propulsion twice. Both times with twin mains. Both propulsion losses were in the shaft/prop. Motored in on one engine.
Since you come from sail, in 1921 US sub R-14 ran out of fuel SE of Hawaii and rigged sails. Five days later made Hilo. The US Submarine R-14 - Under sail
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Old 08-08-2018, 02:54 PM   #14
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Outboard motor on swim platform? Works as long as you have enough gas.
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Old 08-08-2018, 03:28 PM   #15
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Outboard motor on swim platform? Works as long as you have enough gas.
Gas is the issue - lots of diesel, not much gas.

It looks like Minn Kota will sell you pretty much just the motors. 24V and claimed to be 160 lbs of thrust. Also in the literature but not shown online is a 36V 200 lbs thrust version.

I'm away from my nautical library at the moment, in no wind I wonder what 160 or 200 lbs of thrust would do to a 20,000 lb trawler.
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Old 08-08-2018, 03:45 PM   #16
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I always find these really interesting discussions.

Iíd suggest starting with a basic energy analysis. Nothing fancy, just how much power do you need, and how much have you got. Thatís typically the first tumbling block.

How much HP is required to move your boat at 3kts? What about 4 or 5? I think 3 is the bare min for a useable get home. That would probably hold position is bad conditions, and make progress otherwise. For kicks, say thatís 20hp, or about 15kw.
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Old 08-08-2018, 03:52 PM   #17
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Oops, hit return too soon.

Right there you can see that your gen is way too small to be of much help. You 6kw gen 100% dedicates to propulsion would give you around 8 or 9 HP. Is that enough to move your boat at a reasonable speed?

Then the other issue is getting the power from the gen to your propulsion machine. Iím not sure what to power of something like a torquedo is, but maybe 3kw? So you would need 2 to match your gen output, and maybe as many as 4 to get acceptable propulsion.

Speaking of torquidos, it might be reasonable to have two battery packs per motor, and charge one while draining the other, swap batteries, and repeat.
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Old 08-08-2018, 05:21 PM   #18
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Hows about a propane outboard? That is if you have propane on board already.
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Old 08-08-2018, 05:22 PM   #19
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I went down the same road. Just shy of 10,000nm with no issues(knock on teak) it would have been a waste of money. But here was the discussion 3+yrs ago:

wing engine/generator combo

This is new, similar to something I looked at but failed due to corrosion, would think its OK w/ reputable company:

https://www.torqeedo.com/us/en-us/pr...M-1250-00.html

Still limited by gen though, and would need an industrial voltage conveter to handle load.

Good luck, will be watching....
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Old 08-08-2018, 06:04 PM   #20
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What about using a modified hydraulic bow thruster? You could use quick connect lines and have pumps on both the main and gen.
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