electric boat, proper rpm and propeller

Discussion in 'Hybrid' started by bairachtn, Oct 28, 2007.

  1. Robin Larsson
    Joined: Mar 2007
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    Location: Gothenburg, Sweden

    Robin Larsson Junior Member

    Rick, You think it would work with a saildrive?
    I already have an old dieselengine with a saildrive, its an Volvo Penta MD11C with the 110S Saildrive. Couldnt find a good site where ypu can see it, you can find some info on www.volvopenta.com and under older engines, couldnt link to it though. I would like to use that one, not buy one more:)

    But I belive that the pros and cons are about the same in the Volvo Saildrive and the new Yanmar one, dont you think? I will rebuild the saildrive I have, all bearings and seals new. And I plan to take away the gearbox/reverse, dont need that. And the upper angle gears. And build new mounts, and put the motor verticaly, connected via an reductiongear.

    Should be much lower losses compared to using a complete Saildrive with reversegear and all. Maybe not much more losses then an axel. And the prop would be in a much better position, so maybe the total effiency would be the same?
    Some people on a swedish sailingforum didnt think it would work good with the saildrive leg, to much mechanical losses they think... I dont know, much better bearings and seals in the saildrive.

    About folding and regen, theres a Swedish company, www.ozmarine.se that sells a system with folding propp and regen, and it seems to work, but of course, a fixed propp would propably be more efficent.
    The best would be a prop like the Kiwiprop och Autoprop, that could be locked in pitched position when regensailing, and be allowed to feather when I dont want the regen. But maybe not to easy to fix..

    Or just a standard pitchable prop, do anyone have a tip on where I could fin one? But then I wont be able to use a saildrive..

    Regards Robin
     
  2. Guest625101138

    Guest625101138 Previous Member

    Robin
    What you propose will work perfectly well. The Perm PMG-132 on 72V should not be too far behind the old Volvo.

    Check the reduction ratio in the drive leg. As you can see from earlier post, 2.6:1 in the Yanmar is still a bit high for the Perm. If you set up an input gear then you can swing as large a prop that physically fit.

    It is not that hard to make a prop to suit if you have the stainless steel. You could at least make one for an initial trial. Would take an hour or two to fabricate and about 4 hours of hand shapping with a grinder. I can give you a design based on 75mm by 6mm flatbar that will have the required strength. It will have efficiency around 84% if you gear correctly.

    With the electric motor on the saildrive it will spin very freely so will not offer much drag unless you are regenerating. You could use lighter oil in the drive leg because the drive will be smoother than the diesel so gear loading will be better.

    Take plenty of photos because I know there will be others interested in what you are doing.

    Let me know if you want to have a go at making the prop and I will work out the blade geometry to suit the gearing you settle on. I will also need to know the speed you got at full throttle with the saildrive and some more detail on the yacht like displacement and basic style.

    Rick W.
     
  3. Robin Larsson
    Joined: Mar 2007
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    Location: Gothenburg, Sweden

    Robin Larsson Junior Member

    Rick,
    Thanks! Yes, I also think it should work great, but as I said, got mostly negative response here in Sweden:) But then again, none of them had used electric motors:)

    I´m just a bit afraid that it would be to much losses on low speeds, when regenerating that is. That the gears will offer to much resistans at low rpm, especially since atleast the lower gear is in oil.
    When motoring there shouldnt be any differens to using a straight axel, since it works with diesel engines.

    Thanks! Yes, I also think it should work great, but as I said, got mostly negative response here in Sweden:) But then again, none of them had used electric motors:)

    I will check the gear ratio, but since I will only leave the bottom anglegear, I think it will be 1:1, but not completly sure, will check next time I visit my parents, thats were some of the boat is:) But only the dieselmotor+saildrive, boom and the sails:) Had no where to put it at home:)

    One downside I see with the saildrive, is that I will have to change oil every now and then. Volvo says 200hours in the manual, maybe I could do with more time in between. But even with 1000hours, it will be pretty often, if I use the regen, and are longdistans cruising. And when rebuilding the top-part of the leg, I will make it possible to change oil from the inside, or maybe even put a small pump and use a filter.

    It is very important for me that the regen works good, I will probably use the regen about 10times more than the motor:) Is after all a sailboat! And I plan to circumnavigate so I will most likely visit Australia!

    Yes, I would like to test to make a propeller like you suggest, your help is very much welcome:)
    Sadly the boat has never been in the water, and the motor never installed, but I will check with other oners of the same boattype, see if I van get some info aboat speed. The boat is a Wasa 55, bit like a modern Skerrycruiser, and I plan to modernize it further, with a deeper, shorter cord bulbkeel and deeper spaderudder. Also I have a more modern rig for it, swept spreaders and no runners:) And kevlar racing sails...

    Displacement will be around 5 tons, ~11meter Wl, 13.5*2.55 over deck, the keel will be something like 2.5meters deep. Pretty low freeboards, and a rather low coachroof. I will see if I can get up some pics later today.

    Thank you very much for your help Rick, its been great!

    Best regards Robin Larsson
     
  4. Guest625101138

    Guest625101138 Previous Member

    Robin
    I did a rough approximation of the WASA55 hull. See the attached image. It is a long slender hull and low displacement for its length so I consider it easily drive.

    I used this to run Michlet on the hull. There is a pdf file of the drag curve. The pink trace is the Michlet output and the blue trace is the regression line. It is noted that Michlet does not allow for sinkage so it is likely that it is OVERESTIMATING the drag around 8kts because the wateline will increase with sinkage due to the overhang bow and stern.

    It seems that a design point for 4m/s (7.8kts) is reasonable. This has a hull drag around 1300N without allowance for windage, rudder and keel.

    I can design a prop that will give 84% efficiency running at 800rpm. This will require 6.2kW. I recommend that you gear for this speed at full motor rpm. This gives an allowance of 1kW for windage and other items like the drive leg itself and the rudder. So the required gearing for this prop is 4.35:1. This prop would be 550mm diameter and 275mm pitch. Plenty of traction so good capability in heavy weather.

    This is an iterative process. You now have to determine if you can get gearing to suit and will have room to swing a 550mm prop.

    I would not be too worried about the drive leg oil. The recommended frequency is more likely to do with oil contamination than actual breakdown through work. I had a small Bukh saildrive that had the bearings worn out in about 20 hours on the clock. Trouble was I left the boat moored in a creek with a strong current and it worked almost continuously for 6 months until I realsed I should leave it in gear when moored.

    The point of this post is that I believe your application is suited to the Perm PMG-132 motor. Another recommendation is to get a spare set of brushes because these will wear a little even if not carrying current. They should give well over 1000hrs (say 7700nm) under light load but there does not seem to be good data on proven brush life.

    Keep me informed on what you decide on the saildrive. It would be worthwhile determining the existing gearing as it may be satisfactory as-is.

    Rick W.
     

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  5. Robin Larsson
    Joined: Mar 2007
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    Location: Gothenburg, Sweden

    Robin Larsson Junior Member

    Rick,
    Thats wonderful! Thank you very much! Actually I have made an drawing in Delftship on the boat, I´ll post it here, maybe you can use it? But the one you made is not that far from the truth:)
    Yeah, she is long and slender, only abot 2m wide at Lwl. Lwl is more or less 10.7meters, but probably, as you say Rick, around a meter longer when att speed. Originally these boats had a penta engine of 11HP, and that seems to work pretty good, but more power is propably nicve to have when manuvering, and that I will get with the Perm motor.

    This looks like it would work good, aslong as the friction in the saildrive leg dont screw with the regen to much.
    Dont you think that the fixed prop will slow me down alot when sailing, I mean when not using the regen? A pithcable prop, that would be gold:) But to much hassle to build maybe, if I can´t rebuild a Jprop or Kiwiprop, but they cost alot! Specially if one will destroy the first thing one does...
    Maybe I could buy blades from kiwiprop? But, as you say Rick, making the blades is not so difficult.

    If the boat would do about 7.5 knots under "full" power I would be very happy! Then a can do 4knots at a fraction of the powerusage, and could backup the batteries succesfully with a 2 or 3Kw auxillaryengine, maybe just a small Honda on deck. That would be great.

    Rick, if I ever get to Australia, I will take for a sail! And a few bears atleast, must thank you somehow for your help!

    I will check the gearing in my saildrive as soon as I can. But as I remember it, the lower gears, that is between the vertical shaft and the propshaft, is more or less the same size, but I´m not very sure at all.

    The drawings I´ve attached, one is complete with deck and so, but the other is more correct in the hull. I dont know the english word for it, but the grp "piece" that the original leadkeel is attached to, keelsole? is only included in one of the drawings, the keelsole also partialy extends to for the skeg för the rudder. It´s clear in the drawings. The rudder will probably be a freehanging spaderudder behind the skeg.

    Regards Robin
     

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  6. Çemberci
    Joined: Apr 2007
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    Location: Istanbul/Turkey

    Çemberci Senior Member

    size of propeller

    Mister
    2 bhp 1500 rpm 1:1 Reduction
    for 6 knots speed your propeller size is
    Diameter 228 mm
    2 bladed
    Oktay Çemberci
     

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  7. cahudson42
    Joined: Dec 2007
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    Location: Orlando, FL

    cahudson42 Junior Member

    Nicholas,

    Perhaps you are well along with your electrification project and everything is set, etc. But if not, here is an additional thought to go with Rick's Outboard idea and his comments on keeping weight down vs. adding batteries for more HP.

    Remember at 2 HP you are going to need a Battery Bank able to supply about 1500w - at minimum, as that is at 100% efficiency. If you use a typical flooded deep cyle - say 12V 105A at about 25 Kg each - and draw 30A/hr from each (a lot) - you are going to need four of them weighing 100Kg total!
    12v x 30A x 4 = 1440W)

    Rick can calculate the drag of your Hull at various speeds (Delftship?) and he may find that dropping HP to 1 does not really effect the top speed that much - as typically when you start to go over 4 - 6 kn - particularly in a Planing Hull - resistance goes up dramatically.

    At 1 HP you now have only 50Kg of batteries.

    Plus, it is now practical to use 'submersible direct drive motors' - yes, trolling motors, mounted as an outboard would be.

    For about $110, you can buy (here in US at least) a 'speed coil' low tech trolling motor capable of generating about 1/2 HP continuously - particularly if you couple it with your own a 24V PWM controller. (You can 'load it up' to its full 34A current rating by increasing the voltage above 12V as necessary).

    2 of these motors coupled with a drag link for steering control gives you 1 HP. The motors only weigh about 10Kg each (No bevel gears, no long shafts, no 'stuffing boxes' etc. etc.).

    You could continue with your JavaProp approach also - should you wish to maximize prop efficiency by fabricating your own props for the trolling motors.

    Just another thought thrown into the pot:)

    Regards,
    Chris
     
  8. Guest625101138

    Guest625101138 Previous Member

    Chris
    What you say about the drag is correct. It comes down quite a lot if you reduce speed just a bit. So for normal motoring, range can be extended dramatically.

    The little motors would need to be geared to be useful as auxiliury drives. Occassionally the auxiliary is called into service when the boat is in trouble. Under these circumstances you need to overcome significant forces from strong wind, flapping sails and decent waves while making progress against a strong current. The thrust is a function of the installed power, the propeller diameter and blade geometry. The Perm motor suggested can develop 15kW for a few minutes at twice rated torque throughout its rev range. It will also have a decent sized prop so plenty of traction.

    On the batteries I would recommend sealed gel L-A batteries. I have two small sealed batteries for testing and one advantage is that you do not get corrosion around the terminals from the acid. Maybe different in the bottom of a boat having to contend with salt but having acid spills would add to the problem.

    Rick W.
     
  9. cahudson42
    Joined: Dec 2007
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    Location: Orlando, FL

    cahudson42 Junior Member

    Hi Rick,

    Great! I'm learning as always. Did you mean 1.5Kw? My experience is the same - 200% - 400% - even 600% (6 HP for two motors) for a (very) short time.. Gets Nicholas over any short-term problem.. As long as the controller can deliver the volt-amps.. And at $110/motor - you carry a spare.

    For Nicholas's boat - maybe 250 Kg - I was thinking the two 12V trolling motors at 1 HP nominal total would be OK for the primary propulsion. And I was thinking that they would not need to be 'geared' - as a 24V PWM controller sort of 'self gears' allowing the full current rating of the motors to be obtained (or exceeded) by slightly increasing the voltage (see: http://www.cfnet.net/tm.

    In my own planning I thought the sealed (AGM?) batteries too expensive. Instead, I put the conventional flooded in plastic battery containers - and they seem fine.

    I use WALMART (belive it or not) EverLast 12V 105 Ah Marine Deep Cycle made by Johnson Controls - and priced at $60 each. Unfortunately, I've yet to see them again at our local WALMART. Hopefully, they are still available (Comments anyone?)

    I would be interested in exploring a prop redesign for the 'direct drive' trolling motor.

    The trolling motor I refered to was the MinnKota Endura 34 lb (34A) thrust. In tank tests, the aftermarket Kipawa draws 34A at a lower voltage than the stock prop does at 34A. Meaning - the Kipawa draws 34A at LESS HP since the voltage is less. But this assumes equal efficiency and slippage for the two props - which is likely incorrect.

    There is probably a definite opportunity to develop an optimized prop for the trolling motors - particularly given that with PWM - the revs of the prop can be precisely controlled.

    Your thoughts?

    Regards,
    Chris
     
  10. Guest625101138

    Guest625101138 Previous Member

    Chris
    The 15kW was correct. Robin is planning on a 72V system and the PMG - 132 motors are usually current limited to around 200A. They get up around 90% efficiency. So these motors are very powerful for their 11kg weight.

    Most electric motors will reliably develop around 2 times rated totque throughout their speed range so are much more flexible than combustion engine. However for a design point to get the most from the motor you aim to have rated current aligning with full voltage for the application. This becomes the design point for the motor.

    Hence for the prop design you need to know the drag on the hull, the speed of the motor at full voltage and current, the rated output power and the drag v speed curve for the hull you are powering.

    If you give me an idea of some of these things I can do a prop design that will maximise your speed at rated power. If you decide you would like to be able to utilse the full available power from your motor for a short time then the prop could be designed for this.

    On batteries - I understand the sealed gel give better cycle life than sealed AGM. But both types are considerably more expensive than a flooded battery. Remember a yacht will be doing a lot of heeling. The WASA 55 is narrow so will probably sail well on its ear. Need to watch acid spillage.

    Rick W.
     
  11. cahudson42
    Joined: Dec 2007
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    Location: Orlando, FL

    cahudson42 Junior Member

    Hi Rick,

    Thanks! Sorry I was still thinking of Nichlas' boat when thinking 1.5Kw..

    My application will be something like the attached Delftship fbm - an 'Aluminum Electric Launch' - or 'Poor Boy ELCO'

    I'm planning on 2X 12V MK Endura Trolling Motors - either 34 lb thrust (34A @12V) or possibly 46lb thust (46A @ 12V). These will be driven by PWM 24V 'scooter' controllers, so I am not limited to 12V. I can increase (decrease) the voltage so whatever prop used draws 34A

    I'd read somewhere that the 12V Endura is about 1200 - 1400 rpm. But I've no tachometer, so have not actually measured it. So thats 100 rpm/V (Anyone know for sure?)

    I got an aftermarket 'Kipawa' prop for a 34. It is advertized as giving 'more speed' at rated 12V. A three-blade rather than the stock 2-blade.

    The Kipawa will draw 34A at less voltage than the stock - more like 11.5V rather than the stock at closer to 13V. This seems as expected with its higher advertized pitch. I could measure the dia. and pitch of both if it would help as a starting point.

    As I understand it, brush arcing excluded, I am mainly limited by the 34A current rating as current is the determinant of heat losses in the motor..

    So I'm wondering if I would 'get more watts to the water' by going the 'other direction' from the Kipawa - like a prop drawing 34A at 18V turning 1800 rpm?

    Thoughts?

    Regards,
    Chris
     

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  12. Guest625101138

    Guest625101138 Previous Member

    Chris
    The launch is not too bad as a hull. I trimmed it bow up by half a degree and sat it lower to extend the waterline. The displacement in this trim is 1030lb.

    If you take the proposed motor at 34A, 24V with 75% efficiency it might deliver 612W. Now we should be able to get a prop better than 80% so power to move is 480W. This will give around 5.4kts on the hull.

    This speed is getting you into serious wavemaking for the hull. For example halving the power gives 5kts. So you are gaining very little screwing the motor to the max on this hull.

    To get the best result you should match the hull to your specific requirements unless you can get the hull cheap or you like some other aspect.

    To go faster you need to go longer and reduce beam. The reduction in beam does not necessarily mean a reduction in stability. This hull has very good form stability. The KMt is 9ft. Do you need it to have such good form stability. Remember you can set the batteries down low in the middle of the boat. This will get the CoG low.

    I had a look at a longer boat with a lower KMt to see what might be possible from optimisation. The attached shows a hull. The above water part is only there to show the waterline. It could be reshaped how you like. This hull should do 6.5kts with the motor flat out. Speed dropd to about 5.2kts with half power. The downside is that this boat is over 22ft long.

    So if you want to rethink your design constraints along these lines before you settle on the design conditions let me know.

    I have read that the 12V motors will run on 24V but not unless loaded. I think they will throw windings unloaded. So you might not like running it on the full 24V. My controller drops less than 1V at 10A and batteries fully charged are above 24V.

    Have you looked at scooter motors? These are low cost. These are rated for higher voltage but not suited for submerged operation. I have a 300W motor that pushes a light catamaran at 5kts.

    Rick W.
     

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  13. cahudson42
    Joined: Dec 2007
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    Location: Orlando, FL

    cahudson42 Junior Member

    Thanks again Rick.

    I would be very happy with 5 kts at half power. What I'm trying to get to is a lightweight version of something like the ELCO Electric Launch:

    http://elcoelectriclaunch.com/19 ft launch.htm

    - without all that wood, glass, and weight by doing it in Al. The ELCO 19' top speed is 5 kts - with 3 HP. If I get that with a small trolling motor I will be very, very happy:)

    The beam is there for several reasons, 1. a wide enough floor so a wicker chair (or two) can be placed for 'full size' wife, 2. Stability - so she can get in and out easily, and 3. lower draft requirement as our Dock Space is maybe 12" late Summer.

    The boat will be used on Lake George, NY for leisurely evening cruises with wife in calm water, and for fishing (by me).

    I was thinking of using 2x 34lb motors on the transom - one starboard and one port. I have two 4-quadrant electrically reversible PWM controllers, and was hoping to get away with 'differential' (tank) steering - eliminating any rudder or need to rig up something to reorient the motor(s).

    If I am reading your Post correctly, it seems I shouldn't need to 'stress out' the 2 motors at all - as I should easily get 300W/motor (34A @12V = 408W 'nameplate rating') - and 5 kts.

    I briefly considered scooter motors - then abandoned the idea when I looked at the trolling motor economics. $110 for a 'fully submersible direct drive' is hard to beat in my opinion. I can treat them as 'disposable' and even replace them at the end of the season or 200 hrs - and still have $1/hr cruising cost. Plus they are light enough and cheap enough I can even carry a spare..

    Thanks for the tip on 'unwinding' at higher RPM. Sounds like I should stay well away from 24V to the motors.. (I have been running them at 12V - 14V anyway because of the 34A current max.)

    Any further thoughts or suggestions greatly appreciated!

    Regards,
    Chris
     
  14. bairachtn
    Joined: Oct 2007
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    Location: Greece

    bairachtn Junior Member

    Mr Chris(cahudson42) thank you very much for your advice...but at this time i have started my project and it is on another way different from yours( i am going to put an inboard motor), i appreciate very much your interest on me.

    Mr Rick...hello again...i would like to ask you a favour...is it possible to send me the .fbm file from Delftship program which results the first drawing of my boat in the 1st page(in case i am wrong and this drawing was not a result of Delftship) ...i want to "play" with this a little...
     

  15. cahudson42
    Joined: Dec 2007
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    Location: Orlando, FL

    cahudson42 Junior Member

    Thanks for reply Nicholas. Best Wishes for your Project!

    Rick,

    FWIW, I tried to get the pitch of the two trolling motor props. The stock 2-bladed has an angle of 16 degrees, and the prop is 9" in dia. If I multiplied/divided right, I get about 8 kts theoretical, then divide by 2 for slippage = 4 kts at 1200 rpm (approx).

    The Kipawa '314' aftermarket 3-blade has an angle of 20 degrees. Also 9". So theoretical about 10 kts - divided by 2 = 5 kts at 1200 rpm.

    The blades of the stock are quite flexible compared to the Kipawa. I remember reading elsewhere posts claiming the stock blade tips 'straighten out' at full RPM - and also reading that the stock prop top speed was about 3.5 kts.

    I probably should plan 'on the boat' tests with both - but I'll likely start with the Kipawa and its theoretical 5 kts/1200 RPM. With the Kipawa, I ran static Tank Tests and got a 33A draw with only 10V going into the motor. I would think that with 'water going past the prop' at speed, the Kipawa 34A max draw should occur at a voltage higher than 10V. We shall see..

    A custom prop may or may not be worth it (?) but a larger diameter prop is doable. No 'cavitation plate' to worry about, and the skeg is well behind the prop.

    Comments suggestions appreciated as always!

    Chris

    P.S. - perhaps this discussion should be moved to its own thread? I don't want to impact/confuse your other two discussions...
     
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