I'm following this thread with interest as I've been running a direct low voltage drive for some years.
Yes, it makes sense to run the largest diameter prop you can for low shaft RPM and efficiency. Typically for sailing yachts the P/D will be in the 0.8 to 0.9 ratio for efficiency. Slip decreases a little with increased pitch due to slightly more efficient P/D but then of course for the same speed increased pitch means you travel further and therefore prop RPM is slightly less for that speed. This is all well and good when you have a geared motor and indeed enough room to fit a larger fixed prop with increased or not pitch, if that is what you want on a sailing yacht.
However when running a low voltage direct drive and by that I mean any voltage between 24 to 48V, it's not ideal. Increasing diameter or pitch means the voltage at the motor to its current ratio at the motor will be too little voltage and too many amps. So it then becomes what is the ideal prop for low voltage. Typically it'll be a P/D of 0.8 to 0.9 and a diameter of about 12 max all depending on Kv and Kt of the motor. The moral there is to go with the highest torque per amp and therefore lowest rpm per volt motor (as they are directly related) as long as you have sufficient motor voltage to get the RPM you require for power.
Now back to whether to gear or not. For some, like me, gearing was not an option. This was down to a number of factors. Small double ended yacht so little engine space, lack of funds initially and wanting to experiment without damaging batteries, weight in the right place for trim. That all led me to using the least number of batteries possible, in my case 2 monoblocs at 24V. Therefore to run the equivalent cruise and peak power with gearing I'd have needed 3 or maybe 4 batteries in series. If 4 at say 2:1 I'd have then run up against room, weight and cost issues.
Anyhow as regards efficiency, at the low powers I require, (around 1,000 W at battery for cruise at 4 knots with 4 ton laden displacement, 1,600 for a bit of thrust and 2,300 peak) a 24V direct drive was the answer. As for overall drive efficiency,(win on not gearing, lose on motor/controller efficiency but need less batteries/cost) then for me at these low powers I match a geared 48V drive when doing the serial efficiency calculation. The key is keeping motor voltage up and amps down of course which means a high Kt, low Kv and therefore a prop of around 12" dia max or less (couldn't run more anyhow as an aperture) and a pitch in the region of 9 to 11". I've tried 14 but that was too many amps not enough volts for my needs. I currently run 12 x 11 3 blade 50% BAR but 12 x 10 would likely be best as I rarely use 2.3kW measured at the battery or need more than the 4.7 Knots max speed I have in calm waters. 1,600 W is my max continuous by the way as I only have an 80A rated controller. Should go with bigger but haven't needed and as I run at 40A mostly that is just fine at half controller continuous rating.
Back to batteries. Running 24V to the controller means Peukert comes into play a lot as does voltage sag with lead. The net effect is that a 200Ah, 24V bank gets about 12 miles range to 50% DoD whereas a 200 Ah Lithium gets 20 miles, all at 4 knots, to 80% DoD. High capital cost but as good if not a better cost per cycle than lead. Remember though I work for a company that makes Lithiums and 2 of them is way cheaper than 3 or 4!
Finally it all comes down to how much power you actually need/use (not what you think you need/want), range and cost, if just an auxiliary sailing drive. You also need to be a comfortable sailor when the weathe rpicks up or close to land. I consider I have a sailing yacht, not a motor boat. For me in my small boat limited by space, weight in the right places etc, it was just not an option to go geared. Where on earth could I put 4 x 200Ah or 230Ah lead AGMs! I could (with hindsight) re-glass, re-make the engine space,and the areas under the bunks for a larger system but there is little benefit for my needs and it would be very costly. Just look at this photo in a blog I did yesterday at the size of my engine space. I can only just reach over the 2 batteries to get at the PSS stern gland!
https://www.victronenergy.com/blog/2017/05/10/new-victron-energy-products-introductory-offer/
If you are converting your small boat it's likely that a 36V or 48V geared system is ideal. But for a few of use a 24V or 36V, possibly 48V (subject to range required) direct drive is ideal. My motor is rated for any voltage up to 96V by the way ref http://lynchmotors.co.uk/pdfs/lmc-lem-200.pdf My controller any voltage between 24V to 48V, so I could run any combination of 2V lead cells or 3.2V lithium cells for instance.
As regards thrust bearings, Cedric Lynch that designed the motor said the double row angular contact bearing within the motor is OK to act as a thrust bearing up to a 13 inch prop, although I've seen 12 elsewhere. In fact I assume there he meant for canals and such, so I did load calcs with SKF bearings and I'm happy for my needs, I've a safety factor of around 2.5 to 3.0 with my 12 inch. Having said that I don't want to be surfing off 3m waves with all the shock loading that entails. I've surfed off 2.5m (about the max in my inshore waters) and come to a quick stop as it were in the trough which has tested that theory and all is well, but not ideal if going elsewhere. Bottom line, fit a thrust bearing, but again in my case no space for one. I have 1/4"clearance between the back of my motor and batteries,.
Well, enough of my rambling as that's all that I can think of for this thread for now. I hope it helps so folk can see why sometimes different choices compared to the usual are necessary.
In conclusion my overall system efficiency is around 50% for direct drive, which a good geared drive in my size boat will match. In my case slower motor speed and lack of belt/gearbox also means less noise, although not so easy shaft alignment and in water motor removal, but again there's solutions for that too. For me I'm happy to enjoy my system and around 1hp measured at the shaft is enough for my cruise needs around 4 knots in the calm.
I'm also lucky in the sense I run DC brushed so it's easy to measure motor voltage with my multimeter and current there with a clamp meter at the motor. I did hook up my KTY winding temp sensor but couldn't get it to work. A few quid for an IR thermometer to measure casing and close to shaft temps along with my controller temp read out shows me I'm just fine in my northern climate. I rarely ever see more than 50C at the controller and can put my hand on the motor casing.
More power, more range, means more batteries, means more cost is my motto...
John R
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a New Topic | • | Messages in this topic (21) |
No comments:
Post a Comment