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Old 05-09-2016, 08:46 PM   #61
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Hawaii.
Mount a 4 stroke OB on the back with a bladder tank?
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Old 05-09-2016, 08:48 PM   #62
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Sharing is tricky, spitting is not.
Wifey B: I'm not an electrical expert, but apparently you're saying there is a simply way that if you spit the generators do magic things? Do you spit just saliva or tobacco or gum or? How far must you be able to spit?
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Old 05-09-2016, 08:52 PM   #63
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Wifey B: I'm not an electrical expert, but apparently you're saying there is a simply way that if you spit the generators do magic things? Do you spit just saliva or tobacco or gum or? How far must you be able to spit?
Spitting combined with large hammer can work miracles when it comes to some things mechanical.

But in this case I should have typed splitting.
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Old 05-09-2016, 10:12 PM   #64
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I don't think I have room for a wing engine - at least not without getting rid of the 20kW generator. I am thinking that electric would be the most compact and simplest to install. Motors are still quite expensive but the price and performance is really improving as they are gaining popularity for transportation in general. Maybe in another 5 years or so it will be a no brainer.
Ok, we must have a different layout. On the Krogen 42, the genset is midline aft of the main engine. The battery bank to port and the wing engine to starboard. Hard to believe the Krogen 54 has less room.
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Old 05-09-2016, 11:09 PM   #65
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Ok, we must have a different layout. On the Krogen 42, the genset is midline aft of the main engine. The battery bank to port and the wing engine to starboard. Hard to believe the Krogen 54 has less room.
Ok - I take that back. There probably would be room - on the port side in my case. I could move a couple of things around and fit one in. I wouldn't want to though - it would be hard to work on and make other systems harder to service. I'm sure it could have been put in easily when the boat was built. Much more work now.

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Old 05-09-2016, 11:18 PM   #66
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Mount a 4 stroke OB on the back with a bladder tank?
You're joking, right?
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Old 05-10-2016, 03:33 AM   #67
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Richard-your idea using an electric motor may not be such a bad one, and may not be as expensive as you might think. Some years ago we looked a doing a one off build designed as a diesel-electric hybrid for 57' trawler. The drawback is that you may need a larger generator. Your 20kw generator will produce about 27 HP. That is not going to move you very fast. A good naval architect or engineer can give you an idea of how it would do. In our design, the Siemens engineers we worked with figured we needed between 275 and 300 HP or 205-224KW. That would drive two shafts, i.e. two electric motors, and move a boat designed at ab0ut 85,000 lbs a comfortable 8-9 knots. The actual parts of the system are not cost prohibitive. The Siemens motors we examined, about 140 HP each, were around $4,000. The control unit for two motors was about $3,000. Motors that size are not very large and the ones we looked at were around 150lbs.

If the power from your 20KW is sufficient, you should be able to mount an electric motor parallel to the shaft and use a chain drive, assuming your transmission will allow the shaft to freewheel. You could gear it 1:1 or maybe gain some efficiency by playing with different drive gears.
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Old 05-10-2016, 07:56 AM   #68
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With a hydraulic drive it is easy to install a pressure modulating setup so that the prop can spin at various speeds. Varying the speed of an AC or DC motor in the 20 + KW range can certainly be achieved but becomes costly and requires a fair amount of moisture proof space.
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Old 05-10-2016, 10:57 AM   #69
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Richard-your idea using an electric motor may not be such a bad one, and may not be as expensive as you might think. Some years ago we looked a doing a one off build designed as a diesel-electric hybrid for 57' trawler. The drawback is that you may need a larger generator. Your 20kw generator will produce about 27 HP. That is not going to move you very fast. A good naval architect or engineer can give you an idea of how it would do. In our design, the Siemens engineers we worked with figured we needed between 275 and 300 HP or 205-224KW. That would drive two shafts, i.e. two electric motors, and move a boat designed at ab0ut 85,000 lbs a comfortable 8-9 knots. The actual parts of the system are not cost prohibitive. The Siemens motors we examined, about 140 HP each, were around $4,000. The control unit for two motors was about $3,000. Motors that size are not very large and the ones we looked at were around 150lbs.

If the power from your 20KW is sufficient, you should be able to mount an electric motor parallel to the shaft and use a chain drive, assuming your transmission will allow the shaft to freewheel. You could gear it 1:1 or maybe gain some efficiency by playing with different drive gears.
I agree that 20kW is a bit low for a get home. David Gerr's formula shows that I could make 3.5 kt with 20kW. And that's power at the prop. With the various losses in the system I'd be lucky to make 3kt. That's marginal even for a get home system. Boats my size typically have something like a 45HP wing engine. That would achieve up to 5kt.

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Old 05-10-2016, 10:59 AM   #70
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With a hydraulic drive it is easy to install a pressure modulating setup so that the prop can spin at various speeds. Varying the speed of an AC or DC motor in the 20 + KW range can certainly be achieved but becomes costly and requires a fair amount of moisture proof space.
At only 3 kt (see my previous post) I wouldn't need a throttle. Forward and reverse would be sufficient.
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Old 05-10-2016, 11:37 AM   #71
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At only 3 kt (see my previous post) I wouldn't need a throttle. Forward and reverse would be sufficient.

I was on a Northern Marine 57 and its 20KW smooth water hydraulic drive speed was 4.5 kts. Then how to do it from a prop and shaft standpoint. Lots to think about as you well know. No argument from me though that 20KW is pretty small. What is your RPM to achieve 3, 4 and 5 kts currently? That will give a pretty good replication of power needed for a get home.


BTW, an FPB 64 with a JD 4045 get home will make 7.5 knots comfortably.
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Old 05-10-2016, 11:48 AM   #72
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I was on a Northern Marine 57 and its 20KW smooth water hydraulic drive speed was 4.5 kts. Then how to do it from a prop and shaft standpoint. Lots to think about as you well know. No argument from me though that 20KW is pretty small. What is your RPM to achieve 3, 4 and 5 kts currently? That will give a pretty good replication of power needed for a get home.


BTW, an FPB 64 with a JD 4045 get home will make 7.5 knots comfortably.
The only number I have in that range is at 1000 RPM I'm making 4.8kt. The engine is a JD 6068T - the power curves only begin at 1000 RPM and aren't easy to read. Propeller power may be 15-20HP at those revs?

It may be hard to predict. I'd definitely have a propulsion expert do the calcs.

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Old 05-10-2016, 12:18 PM   #73
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I was on a Northern Marine 57 and its 20KW smooth water hydraulic drive speed was 4.5 kts...
That's a 120,000 lb boat - I just looked it up. This is surprising and encouraging. I don't know how well Gerr's forumula works down at the low end. I suspect it's really designed to figure out how much power is require to push a boat speeds in the 1.0 - 1.3 range of S/L.

Thanks

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Old 05-10-2016, 01:57 PM   #74
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The only number I have in that range is at 1000 RPM I'm making 4.8kt. The engine is a JD 6068T - the power curves only begin at 1000 RPM and aren't easy to read. Propeller power may be 15-20HP at those revs?

It may be hard to predict. I'd definitely have a propulsion expert do the calcs.

Richard
Easier to calculate required HP in those situations by measuring fuel consumption. Take the fuel consumption and multiple by 15 HP per gallon. 15 is a better reflection of HP per gallon as the engine is running very inefficiently at that RPM.

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Old 05-10-2016, 02:30 PM   #75
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Easier to calculate required HP in those situations by measuring fuel consumption. Take the fuel consumption and multiple by 15 HP per gallon. 15 is a better reflection of HP per gallon as the engine is running very inefficiently at that RPM.

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Not just easier but far more accurate.
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Old 05-10-2016, 04:36 PM   #76
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Easier to calculate required HP in those situations by measuring fuel consumption. Take the fuel consumption and multiple by 15 HP per gallon. 15 is a better reflection of HP per gallon as the engine is running very inefficiently at that RPM.

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My Floscan indicates that it's burning 1.2 gph at 1000 rpm and 4.8 kt. That equates to 18HP by your formula. If that's true then it's evidence to suggest a 20kW generator may be enough. The accuracy of the Floscan is always a question. But these numbers line up with others I've seen on similar boats.

Thanks for the suggestion

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Old 05-10-2016, 10:58 PM   #77
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At only 3 kt (see my previous post) I wouldn't need a throttle. Forward and reverse would be sufficient.
And at 3 knots, depending on the prevailing wind/current, you may be going in reverse without being in reverse.

I hope this isn't a derail that breaches etiquette, but once I was salmon trolling at about 2.5 knots against a 3+ knot current, so I was actually going backward. So, our lookout was in the cockpit with everyone else as we literally watched the scenery pass us by backward.

Another vessel was traveling at about 25 knots on a crossing heading. Because the current was dragging him backwards too, he was on a near collision course. Fortunately, I happened to be at the helm at the time. To avoid a collision, I put the boat hard into reverse, so he missed us by a few feet.

Lesson learned, but if he had hit us (my bow could not have hit his, it would have necessarily been the port side of his boat hitting my bow), under the ColRegs, whose fault would it have been?
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Old 05-10-2016, 11:51 PM   #78
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And at 3 knots, depending on the prevailing wind/current, you may be going in reverse without being in reverse.

I hope this isn't a derail that breaches etiquette, but once I was salmon trolling at about 2.5 knots against a 3+ knot current, so I was actually going backward. So, our lookout was in the cockpit with everyone else as we literally watched the scenery pass us by backward.

Another vessel was traveling at about 25 knots on a crossing heading. Because the current was dragging him backwards too, he was on a near collision course. Fortunately, I happened to be at the helm at the time. To avoid a collision, I put the boat hard into reverse, so he missed us by a few feet.

Lesson learned, but if he had hit us (my bow could not have hit his, it would have necessarily been the port side of his boat hitting my bow), under the ColRegs, whose fault would it have been?
Yes - 3 knots may be too slow even for a get home. Although further anaylsis is suggesting I may be able to do better than that. Even so, you don't fight currents and sea state in a get home situation. If I'm half way to Hawaii I'm going to pick whichever direction gets me somewhere. Plenty of small sailboats spend days averaging 3 knots on the way to or from Hawaii. I watched a youtube video of one that took about 45 days to do Hawaii to Washington State. That's an average of not much over 2 knots!

On your bizarre colregs question I'm guessing that if it was his port side hitting your bow then it was crossing from your starboard. If so, then it was the stand on vessel and you were at fault. It really doesn't matter that there was current - you were both in the water and moving relative to one another. I don't quite see how the current was pushing him towards you if you were in the same current. Surely the current makes no difference.

I was in a sailboat race with a friend in SF Bay and we anchored because we were being pushed backwards by the current (this is allowed as long as the engine is not started). A boat was pushed back into us by the current. In that case it was the other boat's fault - you are the stand on vessel if you're anchored!

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Old 05-11-2016, 06:34 AM   #79
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While 3K is slow for cruising as an emergency get home its still about 70 miles per day.

About what most rag boats cross oceans at.

And at just above idle the fuel burn will give the longest endurance , should the fuel supply be low.
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Old 05-11-2016, 08:05 AM   #80
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You're joking, right?
Only a little.

You wouldn't need a very big outboard to move you along at get home speeds. 15 - 20hp? You may already carry one for your dink.

And this solution would be a lot cheaper than adding a wing engine. Especially if you only have a real need for the wing engine for this one long trip.

I've seen it done on other boats. And while they didn't have nor need the fuel capacity you might, it did work.
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