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Old 08-08-2018, 07:53 PM   #21
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I propose that it's a get home option, not a cruising option and if you have to anchor and wait for fair weather, so be it. Also, 2-3 kts should be acceptable if your fuel supply will allow the Gen to run long enough for you to get home.

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Old 08-08-2018, 08:40 PM   #22
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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.
I like the idea except:
The propeller is to small
Probably need a 7HP+ unit
Doubt the lower unit was designed to have a multiple hour duty cycle

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Old 08-08-2018, 08:46 PM   #23
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What about an electric motor (smallest possible depending on the size of the boat) powered by the generator that would drive the prop from a gear on the inner boat shaft? Main engine as an issue, put the prop in free wheeling, and electric drive will power the prop. Not sure I am clear about what I mean

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Old 08-08-2018, 08:58 PM   #24
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What about an electric motor (smallest possible depending on the size of the boat) powered by the generator that would drive the prop from a gear on the inner boat shaft? Main engine as an issue, put the prop in free wheeling, and electric drive will power the prop. Not sure I am clear about what I mean

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That could be done with an electric motor, gearbox reduction and likely a chain drive. This assumes the transmission, shaft, and propeller weren't the reason primary propulsion was disabled.

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Old 08-08-2018, 09:06 PM   #25
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That could be done with an electric motor, gearbox reduction and likely a chain drive. This assumes the transmission, shaft, and propeller weren't the reason primary propulsion was disabled.

Ted
Ted, this is what I meant! Thank you to clarify

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Old 08-08-2018, 10:32 PM   #26
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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. I’m 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.
We aren't talking about a "reasonable" cruising speed, just making progress towards civilization. 3 knots it actually pretty good, 75 miles/day. The question is what does it take to push a small trawler at 3 knots? Pretty sure 5 - 7 hp will do it, but I haven't tried that. Against any wind you'd be screwed, against a light current you could still make headway.
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Old 08-08-2018, 11:17 PM   #27
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That could be done with an electric motor, gearbox reduction and likely a chain drive. This assumes the transmission, shaft, and propeller weren't the reason primary propulsion was disabled.

Ted
I think you could do that with a cog belt, which would be easily untensioned/removed. The complication is you would be driving the transmission (and many of them don't like this), and the prop and shaft have to be serviceable. None of those are a problem with the outboard.
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Old 08-08-2018, 11:37 PM   #28
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Get a transformer to drop the voltage off the gen then rectify to DC and you have your DC power to run a DC outboard with no batteries needed.
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Old 08-09-2018, 06:45 AM   #29
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The big issue with the small outboard gas or electric, is the diameter of the propeller. Simply, you need a significantly larger propeller turning at a much slower RPM to keep the propeller from cavitating each time the boat hits a small wave. A few outboard manufacturers make somewhat bigger propeller units with slightly higher gear ratios for pushing small sailboats. We need to get to about a 16" propeller and much taller gears to turn it much slower.

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Old 08-09-2018, 07:24 AM   #30
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I would look at belt/chain drive to existing main shaft from a DC or AC electric motor.

The max size the genset could handle with normal alternator or charger capacity.

The main shaft or prop on a single is rarely compromised so bad that it can't be used separate from the engine.
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Old 08-09-2018, 07:44 AM   #31
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Bay Pelican, a 42ft Krogen made 4.5 knots in relatively calm waters using a 27 hp wing engine with a max prop.
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Old 08-09-2018, 08:06 AM   #32
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Lou, You were on a good track, just adding tidbits...
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Old 08-09-2018, 08:11 AM   #33
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I think you could do that with a cog belt, which would be easily untensioned/removed. The complication is you would be driving the transmission (and many of them don't like this), and the prop and shaft have to be serviceable. None of those are a problem with the outboard.
In an emergency, I might accept the cost of tranny repair/replace.....or buy, modify the tranny that it will work.

Singles with protected shafts and props, be it the first point of damage usually survive well enough to be put back into service as long as not so large to be unmanageable with common tools.
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Old 08-09-2018, 08:55 AM   #34
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Not sure what size boat we are talking about "getting home". My experience with a 47ft Selene (60K lbs) is that a 65HP Yanmar Saildrive wing engine (folding prop) managed about 4 kts in calm seas. Figured it would probably help in rougher conditions but would be very slow. So for sake of argument let's say that the minimum HP that is even worth considering is 25HP. That converts directly to 18.6KW. Now figure a (best guess) 80% power conversion from generator, direct to electric motor (no battery), to propeller. So to achieve the rated 25HP, you now need a 23KW generator hard wired to the motor. Proportionally, a 5KW generator would at most provide about 5HP. Hard to see it doing much of anything - can barely move a large dinghy. Batteries will not help because the charging cycle wastes power and is too slow to keep up with the power demand.
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Old 08-09-2018, 08:59 AM   #35
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I thought there was an option with Dickson stern thrusters to rotate them 90 degrees and use them for propulsion...
Dickson Thruster - Stern Thrusters
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Old 08-09-2018, 09:21 AM   #36
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Chrisjs is right. When I looked it always came out to be needing too much hp for wing propulsion for it to work ideally as a gen or perfect gen size and not enough hp for propulsion. Weather hydraulic or electric.
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Old 08-09-2018, 09:44 AM   #37
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Singles with protected shafts and props, be it the first point of damage usually survive well enough to be put back into service as long as not so large to be unmanageable with common tools.
I'm not sure I can swing hand tools underwater (50 deg water) with enough force to do any good, even if I had the dive gear onboard.

At low enough speeds, essentially all drag is viscous friction. With this scheme you'd need to forget about a convenient retrieve in time to make your dinner reservation. That's wing engine territory. I'm pretty sure I can push this trawler (34') by hand at 2 knots in calm conditions, and that's not 160 lbs. Even 2 knots is a long way in a day. Many have crossed the Atlantic at that speed. You'd need calm conditions. I guess I need to borrow a trolling motor and try it. Trolling motors have flat pitch, built for static or low speed thrust. Most outboards are not.

I don't think anything involving batteries is workable - they take for longer to charge than discharge in this application.
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Old 08-09-2018, 10:03 AM   #38
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Lou, You were on a good track, just adding tidbits...


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Old 08-09-2018, 10:43 AM   #39
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Swing hand tools underwater?


Ok, enough said.......
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Old 08-09-2018, 11:52 AM   #40
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Not too long ago there was a very nice Willard 40 listed at Boatshedseattle.com that was equipped with an electric get-home drive. The listed displacement was 33,000lbs.

From the listing:

"An 7.5 HP 3 phase 220v electric motor is installed with belt drive to the shaft providing for "get home" power in the event of a problem with the drive engine. The Onan MDJE-18R diesel generator provides 7.5KW 3 phase power for the get home motor and 5KW single phase for house loads. The get home motor provides steady drive forward at about 5 knots in calm water."

IIRC, to use, the belts were placed around the pulleys and were tensioned with the large adjusting screw in the foreground. The motor base was hinged on the opposite side. Not the easiest 'engagement' but better than oars I guess.

I read somewhere that some W30's were also fitted with this option from the factory, but I was never able to substantiate this.
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