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Old 10-13-2015, 11:28 AM   #21
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My own thoughts, from the "single vs. twin" thread and from the "twin diesels on one screw" thread:

Fewer engines should be less expensive than more.

A single often gives better access for maintenance and service. (At least ours did.)

I'd like to go back to a single-engine boat. (Not likely to happened anytime soon, but I can dream).

Having said that, since we're inshore with towboats everywhere, I'd likely be happy enough with that, as is.

Having said THAT, if I were going offshore more often, longer distances from the towboats... a get-home (or at least get-within-tow-range) sounds prudent.

An option that uses the same shaft/prop as the main seems OK at first, but there's also risk that "the problem" (that may arise) isn't the engine at all, but rather the gear, the shaft, the prop... Still, a "small" diesel ought to be much less expensive than a larger "twin" would be. Could potentially be located so as to not lose all the improved access space?

An option that includes a whole 'nother gear/shaft/prop seems OK at first, although that also means exercising that system periodically. (Not difficult.) And the time and expense of yet another system to maintain.

Using the genset diesel seems OK at first, but then there's the potential problem of underloading under normal circumstances, or else the genset diesel might not be large enough to be particularly useful. (Might be some balancing act that's possible, there.)

The option of two different size gensets is intriguing. I assume that means fixed-rate RPMs (e.g., 1800) for both, even when the larger is being used as a get-home? In any case, I'd guess the larger could drive either the main shaft, or a separate shaft, depending on the specific installation...

I'm of three minds.



But doubt I'll have to "solve" this one any time soon for our own purposes. Just examining the ideas...

-Chris
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Old 10-13-2015, 12:49 PM   #22
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Richard: It is a fairly simple procedure. Just a sprocket that is attached to the main shaft. The chain is placed around both the hyd. motor and the shaft sprocket and connected. Open a valve to keep the dripless seal cooled and you are back underway. In heavy seas though things start flying around and heads are bumped, etc....
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Old 10-13-2015, 02:18 PM   #23
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I have a Dickson hydraulic stern thruster with the get home option that runs off the generator. It's rated at 5hp so won't push it very fast but will push it. Haven't tried it yet since the boat is relatively new to us. The stern thruster works fine and I prefer it over the bow thruster in tight situations.
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Old 10-13-2015, 03:09 PM   #24
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Chris: I think your "three minds" put it all in the proper perspective. In most any marine system back up and redundancy only go so far. You have looked at the problem in one direction- having a common shaft. It goes the other way too, both the main and the gen sets use fuel that originally came from a common tank. So it is possible that the main goes down from a fuel related problem and the gen set running the get home only works while its own day tank is used (about 10 hrs or so). You can plan and create redundancy and you still have to hope for the best.

Horse Power wise I have a 210 HP Main turning a 46" Prop the Gen Set/Get Home is off a 20 KW so that is about 28 HP or so- enough to move it in the right direction.
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Old 10-13-2015, 03:38 PM   #25
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One of my concerns would be duty cycle. If I really need a get home motor it'll be because I'm out of tow range. If I'm half way to Hawaii then I could need to run it for 1000 miles. I'm not sure if want to do that with an electric motor.

Richard
You can. We converted our original sailboat to purely electric power (ok, serial hybrid with a Honda 2000 that we've never needed) and the duty cycle on the motor is rated for full throttle 24/7/365.

We just put in offer on a trawler that has a farymann "genset" with a chain sprocket to the shaft as a get home. Basically, it's a get home engine with a giant alternator. One of my project thoughts was to use that electric drive system (still have it) to replace the get home engine. Then sell the farymann and either get a quieter generator or use the Honda and a bunch of solar.

Electric power is absolutely the cats meow in every category EXCEPT energy density. Batteries keep getting better, but it's going to be a real long time before they're in the same league as diesel.
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Old 10-13-2015, 04:02 PM   #26
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You can. We converted our original sailboat to purely electric power (ok, serial hybrid with a Honda 2000 that we've never needed) and the duty cycle on the motor is rated for full throttle 24/7/365.

We just put in offer on a trawler that has a farymann "genset" with a chain sprocket to the shaft as a get home. Basically, it's a get home engine with a giant alternator. One of my project thoughts was to use that electric drive system (still have it) to replace the get home engine. Then sell the farymann and either get a quieter generator or use the Honda and a bunch of solar.

Electric power is absolutely the cats meow in every category EXCEPT energy density. Batteries keep getting better, but it's going to be a real long time before they're in the same league as diesel.
Interesting. Is anyone selling an electric get home system?
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Old 10-13-2015, 04:12 PM   #27
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Interesting. Is anyone selling an electric get home system?
I believe James over at Electroprop has something. I'm not sure if it's necessarily badged as a "get home" because most of the larger-than-launch sized electric boats are all sailboats. It's much easier to stomach a 20 mile range under power when you have sails. Check out his site.
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Old 10-13-2015, 04:59 PM   #28
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For the best noisemaker efficiency simply copymyhe newest Honda units.

The engine is run at variable speed to create DC that is inverted to AC for 120 or 240 use.

Simple to do , but the main cost becomes the inverter about 4KW each.
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Old 10-13-2015, 05:17 PM   #29
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Originally Posted by Britannia View Post
One of my concerns would be duty cycle. If I really need a get home motor it'll be because I'm out of tow range. If I'm half way to Hawaii then I could need to run it for 1000 miles. I'm not sure if want to do that with an electric motor.

Richard
Quote:
Originally Posted by questionmark View Post
You can. We converted our original sailboat to purely electric power (ok, serial hybrid with a Honda 2000 that we've never needed) and the duty cycle on the motor is rated for full throttle 24/7/365.
Quote:
Originally Posted by questionmark View Post
I believe James over at Electroprop has something. I'm not sure if it's necessarily badged as a "get home" because most of the larger-than-launch sized electric boats are all sailboats. It's much easier to stomach a 20 mile range under power when you have sails. Check out his site.
??

Big difference between 1000 miles out and a 20 mile range...

This must be about some nifty combination of genset, batteries, maybe solar too?

-Chris
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Old 10-13-2015, 08:27 PM   #30
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??

Big difference between 1000 miles out and a 20 mile range...

This must be about some nifty combination of genset, batteries, maybe solar too?

-Chris
20 miles is the battery range of the Catalina 30 at 4kn. More if you go even slower. That's battery only. If you size the generator or genset properly, range is limited only by fuel.

The idea is to run the "get home" from the generator. I wouldn't bother with a large battery bank for that mission, just a small one to smooth power draw and handle torque bursts (like stopping.)

The displacement electric guys figure a KW per ton of displacement will get you about "hull speed." That's generally for easily driven sailboat hulls. Easy to do if you have a bigger generator.
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Old 10-14-2015, 07:50 AM   #31
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20 miles is the battery range of the Catalina 30 at 4kn. More if you go even slower. That's battery only. If you size the generator or genset properly, range is limited only by fuel.

The idea is to run the "get home" from the generator. I wouldn't bother with a large battery bank for that mission, just a small one to smooth power draw and handle torque bursts (like stopping.)

Got it, thanks for additional insight.


Quote:
Originally Posted by ulysses View Post
Chris: I think your "three minds" put it all in the proper perspective. In most any marine system back up and redundancy only go so far. You have looked at the problem in one direction- having a common shaft. It goes the other way too, both the main and the gen sets use fuel that originally came from a common tank. So it is possible that the main goes down from a fuel related problem and the gen set running the get home only works while its own day tank is used (about 10 hrs or so). You can plan and create redundancy and you still have to hope for the best.

Ah. Right. Fuel, tanks, and delivery systems too. Yep, good reminder. Now I'm of four minds.



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