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Old 10-21-2019, 12:31 PM   #21
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I use a VFD on a milling machine, and the loads can be all over the map depending on what it is doing. It keeps rpm very well and has been rock solid reliable for ten years plus. Modern ones are probably even better. I think 2hp, 240v, 3ph. Input 240v, 1ph.

I don't see a problem with VFD in this app.
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Old 10-21-2019, 12:46 PM   #22
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Maybe, just seems risky to me. A milling machine should be set up for a proper speed and cut. And, it won't be subjected to the equivalent of climbing a wave for 10 seconds, every 30. At any frequency, if the motor is suffering from lots of slip, the current drain will want to increase. I do acknowledge that a good VFD will watch the currents; but what happens then? Will it make the motor "rest" on the uphill battle? or, trip out, or vastly lower the drive freq, or ...
I don't know the answer, but it just seems problematic to me.

I'll bet someone has tried this.
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Old 10-21-2019, 01:10 PM   #23
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I do not have the boat anymore but it had a great come-home system like you have been discussing. 40 foot Albin.

Generally a 7.5 KW Onan had a truck, belt driven power steering pump with a magnetic clutch. The main propeller shaft at the transmission coupling had a large size 60 chain sprocket sandwiched between transmission and propeller shaft couplings. On top of the velvet drive transmission was a fabricated support for a hydraulic motor with a much smaller size 60 chain sprocket.

I had to use the system one time 40 miles offshore with water in my main engine.

Took the size 60 chain from storage and put the chain around the sprockets with a repair link. Started the generator and switched the magnetic clutch on. Came home at 4 Knots in an 8 Knot boat. Wore out one set of v belts and made the last couple of mile on a new set. Power steering unit and generator used double groove pulleys and a pair of drive belts.

The system was made up of readily available parts or easily fabricated parts. Most of all it worked
Hope this helps.
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Old 10-21-2019, 05:04 PM   #24
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Kind of like this from Mythbusters?
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Old 10-21-2019, 07:02 PM   #25
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Maybe, just seems risky to me. A milling machine should be set up for a proper speed and cut. And, it won't be subjected to the equivalent of climbing a wave for 10 seconds, every 30. At any frequency, if the motor is suffering from lots of slip, the current drain will want to increase. I do acknowledge that a good VFD will watch the currents; but what happens then? Will it make the motor "rest" on the uphill battle? or, trip out, or vastly lower the drive freq, or ...
I don't know the answer, but it just seems problematic to me.

I'll bet someone has tried this.
From all the way back in 2011: Wayback Machine - Electric Boat Design

The Sevcon Gen 4 provides variable voltage, current, and frequency, 3 phase AC to the motor. Hall sensors in the motor tell the controller where the motor’s rotor is at all times. Sophisticated logic control continually monitors motor RPM and torque, and changes current based on the needs of the boat:
  • When the boat is slowed down by a wave, the controller gives more current
  • When the boat rides down a wave, the controller decreases current
  • When the sails power the boat faster than the throttle setting, energy flows back into the batteries.
The controller has a moderating effect on the speed of the boat through the use of Sevcon’s torque/speed control.
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Old 10-21-2019, 07:27 PM   #26
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Not sure installing an independent second shaft, strut and folding prop is really beneficial - it certainly won't be affordable. That little shaft is more likely to be destroyed by a log than the main wheel tucked behind the keel. How about just invest in a spare prop and the necessary gear to change it out underwater?

Hydraulic get-home would make sense if you have a hydraulic windlass, thruster, etc., already setup. Otherwise electric will be tons easier to install. Considering you'll likely never ever need it, why waste huge money on a new dedicated hydraulic system?

BTW, depending on how heavy your boat is, how about buying a used kite sail from some school kid and experiment with that? Going downwind at 1 knot is still better than going nowhere at 0 knots!
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Old 10-21-2019, 07:36 PM   #27
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BTW, depending on how heavy your boat is, how about buying a used kite sail from some school kid and experiment with that? Going downwind at 1 knot is still better than going nowhere at 0 knots!
We met a French couple in the Bahia del Sol anchorage who used an inflatable sail to get across the Atlantic in their powercat. They had to bring it down every 2-3 days to re-inflate the triangular frame but other than that they said it worked like a charm. I've been playing with the idea but that's as far as I've gotten.

Thanks, and thanks to everyone for all the other suggestions above too !!


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Old 10-21-2019, 07:37 PM   #28
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I moderate the Willard boat owners group. Several Willard 40s were built with elecrric get-home motors belt driven to the main shaft. They were built with 3-phase 5hp motors, which really narrows the generator selection when the time comes. I believe they were also spec'd with 12 kw generators which means the boat is over-powered for normal use. Many Willard owners have abandoned the get home engine.

In my opinion, best option along these lines is a hydraulic pump off the PTO of a generator - I believe ABT makes a shaft drive hydraulic get home hydraulic pump motor. Many generators over 6kw have PTOs.
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Old 10-21-2019, 07:43 PM   #29
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I moderate the Willard boat owners group. Several Willard 40s were built with elecrric get-home motors belt driven to the main shaft. They were built with 3-phase 5hp motors, which really narrows the generator selection when the time comes. I believe they were also spec'd with 12 kw generators which means the boat is over-powered for normal use. Many Willard owners have abandoned the get home engine.

In my opinion, best option along these lines is a hydraulic pump off the PTO of a generator - I believe ABT makes a shaft drive hydraulic get home hydraulic pump motor. Many generators over 6kw have PTOs.
Hi Pete !

I'll have to see if our Onan 8 kW has a PTO.



-Sven
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Old 10-21-2019, 07:50 PM   #30
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Hi Pete !

I'll have to see if our Onan 8 kW has a PTO.



-Sven
In the case of my Westerbeke, the hydraulic pump is coupled not to a pto(gears) but directly to the end of the crankshaft opposite the electrical end. Very simple set up that likely delivers around 70% of the engine BHP to my propeller drive shaft. I doubt you would get over 40-50% of total engine power to the shaft converting the mechanical power output of the engine to electrical power for the motor and then back to mechanical power for the shaft. Heat increment/power loss at both sets of brushes seems inefficient to me.
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Old 10-22-2019, 05:52 AM   #31
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"In my opinion, best option along these lines is a hydraulic pump off the PTO of a generator -

You bet , The hyd could also power the bow thruster (no short operating tine) and the windlass (no white smoke from large loads) .

If hyd is installed , use it as much as you can !!!
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Old 10-22-2019, 07:16 AM   #32
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I think the key to making this work, either via electric or hydraulic, is the power requirement for acceptable propulsion. It doesn't take much to make a boat move in calm water, but that goes up a bunch (I don't know how to quantify how much) in rough water. And if you need a get-home motor, there is a very good chance that the need is accompanied by rough water.


To me, the threshold is 4-5kts in calm water. Anything below that and I think you will not make progress against any sort of weather.



The OP said 5hp which is very doable with an 8kw generator as a power source. But I don't know what the 5ph is based on.


I looked at something along these lines for our 68, and concluded it didn't really work. Why? Because it took more power to move the boat than any reasonable size generator could produce. The standard wing engine that drives a shaft and folding prop is 160hp. That seems like a lot, but it moves the boat about 5kts. I suspect there is quite a lot of loss in the off-center prop and shaft, and in the small, folding prop. For comparison, the folding prop is 24" diameter where the main prop is 46" diameter. When you size a generator to produce something even close to 160 hp, you end up in the 100kw range which is way more than you would ever need or want on a boat that size. Again just to put it in perspective, our generators are 12 and 25kw. The 25kw could probably power around a 20-25hp get home motor which is a far cry from the 160hp that it takes to move the boat 5kts.


Now on smaller boats, the propulsion power requirement is less, and perhaps the natural sizing for a generator doesn't drop as rapidly as the propulsion power need such that for some size boat a reasonably sized generator could also provide get-home power. I don't know, but it seems this is what's required fro this to actually work acceptable.


BTW, I know of a couple of boats that have done hydraulic get-home drives powered off the generator. Both do the job, but just barely. But both also drive the main shaft and main prop which is much more efficient that an offset folding prop. I think that's what makes them work, though their performance is marginal in my opinion. But as a retrofit you have constraints, and I think they took the best possible approach. And there is of course a down side; if the main is disabled because of a drive line problem, say a destroyed shaft seal, fouled prop (seems to be the most likely case), or other disabling of the prop, then the get-home is useless too. This is where the independent shaft and prop comes into play in many get-home schemes.


Trade-offs, trade-offs, trade-offs.
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Old 10-22-2019, 08:04 AM   #33
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I've been studying hydraulics the past few days. My boat is going to have a simple open loop system. I agree that driving a hydraulic pump off the genset crankshaft and powering a hydraulic motor would be the better solution. Chain to driveshaft coupler would be the easier way, IMHO. The chain wouldn't take long to install with a master link set up, if you have easy access to the coupler. Any hydraulic shop should be able to do the math to get the components needed to make the conversion.
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Old 10-22-2019, 08:50 AM   #34
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Just one word, and its not "plastics". Its hydraulic. If the genset doesn't do it, find one with a PTO option. I've designed in VF drives for some electric motors, but all with essentially fixed loads. I wouldn't think an ac induction motor, or its drive, will be happy with prop loads underway. From recurring high negative to high positive loads due to seas. Something will overheat on a passage. Hydraulic motors are the ticket for this application.

Not keen on outboards either, but mostly due to the shallow prop position at the "end" of the boat. Obviously, seas are not usually flat.
My choice would also be a hydraulic solution, but you'd need a power takeoff (pto) on your genset to run a hydraulic motor. If that was possible, you could turn the main shaft by a belt or chain. I have seen this done for get-home power.
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Old 10-22-2019, 12:01 PM   #35
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I've been studying hydraulics the past few days. My boat is going to have a simple open loop system. I agree that driving a hydraulic pump off the genset crankshaft and powering a hydraulic motor would be the better solution. Chain to driveshaft coupler would be the easier way, IMHO. The chain wouldn't take long to install with a master link set up, if you have easy access to the coupler. Any hydraulic shop should be able to do the math to get the components needed to make the conversion.

Keep in mind that a hydraulic drive system will require cooling. This typically means a heat exchanger with hydraulics on one side, and raw water on the other. And the raw water will need a pump, inlet, and discharge. Circulating pump can be AC powered, DC powered, or hydraulically powered. Most of the ABT systems use a hydraulic pump to circulate raw cooling water since it will run anytime the hydraulics are active. Without cooling, it will overheat very fast. So hydraulics aren't necessarily all that simple to add. And have you priced a PTO for a generator? $5k, and now perhaps more.
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Old 10-22-2019, 12:33 PM   #36
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Keep in mind that a hydraulic drive system will require cooling. This typically means a heat exchanger with hydraulics on one side, and raw water on the other. And the raw water will need a pump, inlet, and discharge. Circulating pump can be AC powered, DC powered, or hydraulically powered. Most of the ABT systems use a hydraulic pump to circulate raw cooling water since it will run anytime the hydraulics are active. Without cooling, it will overheat very fast. So hydraulics aren't necessarily all that simple to add. And have you priced a PTO for a generator? $5k, and now perhaps more.
My ancient Vosper hydraulic stabilizers have the heat exchanger inline in the engine raw water intake - no additional pump required. I just had the exchanger replaced as part of replacing a bunch of hoses and pumps on my 50-year old boat in prep for heading 500 nms to Ensenada where my boat is being refit. The exchanger was only about $150 or so.
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Old 10-22-2019, 12:48 PM   #37
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Going with the kite idea. Here is a example which is a little more complicated than I would like.

https://youtu.be/Qy3IayvR_SQ
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Old 10-22-2019, 12:50 PM   #38
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Another sail option.

OMEGSAILS : Kites for Stand Up Paddling, canoeing and boating
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Old 10-22-2019, 02:39 PM   #39
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Keep in mind that a hydraulic drive system will require cooling. This typically means a heat exchanger with hydraulics on one side, and raw water on the other. And the raw water will need a pump, inlet, and discharge. Circulating pump can be AC powered, DC powered, or hydraulically powered. Most of the ABT systems use a hydraulic pump to circulate raw cooling water since it will run anytime the hydraulics are active. Without cooling, it will overheat very fast. So hydraulics aren't necessarily all that simple to add. And have you priced a PTO for a generator? $5k, and now perhaps more.



My hydraulics are going to be in with my other keel cooled accessories. An open loop system doesn't usually build up heat unless flow control valves are used or there is a high duty cycle with the equipment. With that being said, even though my system will only power my winches/windlasses, dinghy crane, bow thruster, and possibly a get home option. The get home option will win out because it would be a 100% duty cycle system. The rest wouldn't need cooling because they are only used in short bursts. The fluid running through the system wouldn't build much heat. The hydraulic system I'm planning is an on demand system, not a continuous running system. I will have an accumulator and a pressure/flow switch to activate the hydraulic pump's clutch when the system's pressure or flow drop. A continuous running system would require cooling even if the hydraulic equipment is low duty cycle. I won't activate my hydraulic system until I have a need for it. It will mostly be laying in wait until needed. I will have full instrumentation for my hydraulic system.
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Old 10-22-2019, 03:24 PM   #40
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My ancient Vosper hydraulic stabilizers have the heat exchanger inline in the engine raw water intake - no additional pump required. I just had the exchanger replaced as part of replacing a bunch of hoses and pumps on my 50-year old boat in prep for heading 500 nms to Ensenada where my boat is being refit. The exchanger was only about $150 or so.

I had my Grand Banks set up the same way, for the same reason (no need for another pump). But that approach wouldn't work for a get-home where the main engine is presumably dead.
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