Wednesday, March 12, 2014

Re: [Electric Boats] Backup option.... voltage pumps in parallel?

 

You should look up how to build a voltage pump. The makeup would tell you that it is a bad idea.

Kevin

On Mar 11, 2014 9:39 AM, "oak" <oak_box@yahoo.com> wrote:
 


I have ordered a couple of these power supplies to play with:

They'll take 10-60V as input, and boost it up to 80V  - as long as the output voltage exceeds the input voltage.
They're rated for 600W.

On my boat, I have a battery bank for the electric motor at 48V, and a second bank at 12V for the "house".

The thought was that perhaps, for short periods of time, in an emergency, could I run the 48V motor from one of these tied to the house battery?

The simple answer should be "yes, at low power".
The more complicated issue is if I want 1000W or so.

Can I safely tie the output of two of the voltage pump power supplies in parallel??
Should I put a diode after each one before tying them together?
Or maybe a very small resistor?

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Re: [Electric Boats] someone check my math please

 

Hi Luke,


Here's something that I posted most recently about prop slip last March:

While I have used a general prop sizing calculator that use 50% slip factors, I know that the propeller shop that sold me a new prop for my 5 ton, full keel, 30' ketch with the prop in a aperture used 55% slip to find the appropriate pitch. I found the following information on the Michigan Wheel propeller website:

Q: What are normal slip percentages for various craft?
A: With propellers correctly selected for the operating conditions, the slip percentages would be as follows: racing hulls 10 to 15 percent, planning runabouts 15 to 25, planing cruisers 25 to 35, displacement cruisers 30 to 40, sailing auxiliaries 35 to 40 and work boats 40 to 80.

I also found this reference on the Castle Marine Propeller Repair Specialists website: "The pitch is the distance a propeller would in theory move forward in the water in one revolution if there was no slippage i.e. a 21" pitch propeller would in theory move forward 21" in one revolution. In practice most propellers do slip the amount being dependent on the boat and as a general guide slippage would be between 8% for a race boat with a planing hull to as much as 55% for a full displacement craft. A 10" pitch propeller on a displacement craft with a 50% slip ration would only move the boat forward 5" per revolution of the propeller. The pitch is the second number in the description i.e a 13 3/4 x 21 propeller is 13 3/4 diameter and 21 pitch."

But enough with secondary research sources, let's discuss a primary data source. Here's the objective prop slip data that I've posted numerous times in this group, Feb 2012 (twice), Dec 2011, Oct 2011 and originally on Aug 22, 2011 (Post #19469). Surprisingly, nobody else here has measured and posted their actual slip numbers. It seems like other reports of primary data sources would be helpful during these conversations since the topic comes up a couple of time a year.

"I purchased my 13 x 15.5 four bladed prop from a local prop shop to replace the 13 x 10 two blade that was already on the boat. You can see both props side by side in "Eric's Serenity" photo folder in this grouphttp://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."

Finding the proper gear ratio is often trial and error. If you hit max RPM without hitting max amps, then you are under-propped or your gear ratio is too high. If you hit max amps without hitting max rpm, then you are over-propped or the gear ratio is too low. With my new prop at full throttle and a 2.55:1 gear ratio, I'm hitting my target RPM and 5.3kW (106A) at 95% of hull speed. That's pretty close to hitting all of my marks and the next gear ratio, higher or lower, would move the balance away from this sweet spot. My performance numbers are pretty good for a 10,200 lb sailboat.

Generally speaking, larger diameter props are more efficient. But typically we're limited in size by the hull design. So many members here have looked for heavy pitch to go with their max diameter prop. In my case, the pitch is greater than the diameter and is considered "oversquare". Oversquare props are considered to trade acceleration for speed, for a sailing auxiliary, acceleration isn't as critical. This prop has given me good "watts to knots" specs and the slippage is much lower than the old prop that was on my boat.
But the optimum prop specs and gear ratios will be somewhat different for each boat. That said, your boat will go fine with the stock prop and a reasonable gear ratio. But higher efficiency (greater speed and/or range) is available by matching the prop and ratio to your motor and your boat.

I hope that this helps.

Fair winds,
Eric
Marina del Rey, CA

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[Electric Boats] Re: 5KW or 10 KW

 

While searching for something else, here's what I posted about my "at the dock" experiment a while back:


On another note, I did a full throttle run at the dock. Average amp draw was 158.8A DC at 47.24V or 7.5kW! Compare that to the 5.1kW noted above at full throttle in open water. Since I was starting with 80Ah already used of my 128Ah of usable capacity, the 160A load was rapidly draining my battery pack. After more than 5 minutes at full throttle, the motor windings had climbed to 85C, well under the manufacturer's rated limit of 150C, although the temp had not stabilized yet. I decided to stop the experiment as I quickly approached having only 30% of my usable capacity left and I didn't have too much time to get some electrons back into the pack before I had to leave. Perhaps I can do this again with a fully charged pack to see where the ME0913 motor temps will stabilize while pulling 7.5kW in the bilge of my boat. In spite of what some of the component vendors might say, I'm pretty sure that running that motor in boat at 10.5kW will cause it to overheat in relatively short order. I appreciate that my vendor conservatively rated my entire drive system, I'm sure that it will continue to perform well for many years. 

Obviously, the forum history remembers things better than I do....
Fair winds,
Eric
Marina del Rey, CA


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[Electric Boats] Re: 5KW or 10 KW

 

Hi Brian,


I've seen that rating info from Motenergy, but I think that it has a big assumption built in.  For rating purposes, Motenergy states that the max continuous current for the ME0913 is 125A AC into the motor or 180A DC into the controller, as you stated,   But which controller?  I assume whatever Motenergy sells.  But if these numbers are correct and the AC figure is RMS, that controller is losing over 2600W at that setting (less than 70% efficient?), which points to a serious cooling problem for that controller.

However, I think that most of us are now using Sevcon Gen4s with these motors for marine installations.  I know that my Gen4 barely gets warm to the touch when run at 50A and running at 100A did not produce any significant heat at the controller.  I know that the same motor using the older Millipak controller needed a big heat sink on the controller and there were still occasional controller heat problems.  But my Gen4 is mounted flat on a wood bulkhead (no air gap) and I have never had any heat issues.  The controller temp has been such a non-issue that I haven't bothered logging the displayed controller temp when doing performance testing.  Given this data, one can deduce that the Gen4 is much more efficient than earlier AC motor controllers.  I can only say that my offhand observations concerning DC current going into the Gen4 controller versus the AC current coming out of the controller, observed using a Sevcon ClearView display, show that the DC and AC amps are pretty close indicating pretty high efficiency.  This is supported by the observed low controller temps while in use.  

So in practice, at full throttle in open water (6kts in my 30' 10,200# ketch), I pull 106A from the batteries.  If I remember correctly, the displayed AC current is close to 100A.  Full throttle tied to the dock (0kts in my 30' 10,200# ketch) increases the load and my drive has pulled close to 140A from the batteries in those conditions, though only for a few minutes because I started the experiment with a mostly depleted battery pack.  Controller and motor temps were still reasonable,  Of course this is a worst case scenario, with my presently installed motor, reduction, and prop, the boat just can't pull that sort of current while moving.  I guess that I owe the group a high load run tied to the dock for 30-45 minutes or more to see how the controller and motor behave, including DC current, AC current, controller temp, and motor temp.  Maybe I'll get around to it this summer.  If so, I'll publish the results here.  

Fair winds and smooth seas,
Eric
Marina del Rey, CA


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Re: [Electric Boats] someone check my math please

 

Luke,

1000 rpm and 13" pitch is about 7 knots with no slip.
Ned

On Mar 12, 2014, at 12:47 PM, <luke.wolbrink@gmail.com> wrote:

 

Almost done installing everything on my Tartan 27 sailboat and spring is running out fast. I've installed a HPM5000 motor from golden motor (5kw) on my 7400# sailboat with LWL of 22' and beam of 8.5 (not sure at the water). I've replaced the propeller with a 13x13. The max RPM is 6000, so I'm going with a 5.83:1 drive ratio to get my prop speed down around 1000. From what I've calculated using Douglas Little's book this seems to be about right. However it seems too slow. any feedback on my math would be appreciated.


thanks in advance,
Luke


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Re: [Electric Boats] someone check my math please

 

I'm not used to slip % being that high. Mine is in the 3% range, but instead of swinging a 13 inch pitch prop at 1050 rpm, I'm spinning a 18 foot pitch prop direct drive at 108 rpm.

Bob

From: "luke.wolbrink@gmail.com" <luke.wolbrink@gmail.com>
To: electricboats@yahoogroups.com
Sent: Wednesday, March 12, 2014 11:12 AM
Subject: Re: [Electric Boats] someone check my math please



Thanks bob, I've been using 45% slip in my calculations which i understand is fairly typical. that would give me nearly hull speed. Anyone know if this is a safe assumption?  Thanks for the verification.




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Re: [Electric Boats] someone check my math please

 

Thanks bob, I've been using 45% slip in my calculations which i understand is fairly typical. that would give me nearly hull speed. Anyone know if this is a safe assumption?  Thanks for the verification.

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