Replies to This Discussion

Permalink Reply by Tom Hruby on June 13, 2012 at 11:30pm

Hi Christopher,

I tried out a hydraulic system on my first dory.  I only had a 36 volt 1.5 kw DC motor, but even then I had problems with the hydraulic system.  It worked fine for about 20 minutes, but then the hydraulic oil overheated and basically stopped transferring power as it became less viscous. I thought I would get enough cooling by putting in a 5 gallon hydraulic tank, but this was not enough.  I would have had to install a large cooling system for the oil with a fan.  I didn't have the space for it so that nixed the whole project.

I also tried a chain drive, but that was too noisy.  I ended up using two V-belts.  This allowed me to tailor the rpms to the prop exactly as I wanted to.  I had a 6 to 1 ratio on the belts with motor rmp at 3600 and shaft rpm at 600.

I suggest you look into V-belts.  7.5 KW is 10 "real" horsepower and a system with 3 or 4 belts should be adequate.  I have seen as many as 5 belts in 20 hp systems and they seem to work fine.  I mounted my motor on a hinged plate I could lock into place, so I could easily remove the belts.  I also used the "link" V-belts so I could put the pulleys on the "down" side of the thrust bearing and save space and still remove the belts if needed. 

For speed control I used the old fashioned resistance coils found in early golf carts.  Most of my cruising was done at full power so the loss of power through the resistance coils while in the harbor were relatively low.

For reverse I used a manual switch that reversed the polarity (this was 16 years ago).  Today however, I would suggest you look into getting a "reversing contactor" that is used in electric vehicles. This  operates off a regular 12 volt battery for the coil and lets you switch the direction of motor rotation.

 

Tom

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Permalink Reply by Tom Hruby on June 13, 2012 at 11:46pm

P.S. I forgot to ask how you will do the voltage switching?  If you plan to do that by using fewer batteries in the bank e.g. first battery for 12 volts, first and second for 24 volts etc., I would recommend not doing that.  This would create an uneven drain on individual batteries in the bank and may cause pre-mature failure of the batteries.  If you have to do this then you would at least need a separate charger/charging system for each battery to avoid problems during the charging cycle. The batteries that are not used at the lower speeds get overcharged during the charging cycle if you have only one 48 volt charger.  

I would suggest you look into a motor controller such as the 48 volt Alltrax used in electric cycles.  This adjust the voltage of the entire bank using a pulse width modulation.

Tom

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Permalink Reply by christopher green on June 14, 2012 at 12:34am

Hi Tom - Thanks for the feedback - not too worried about overheating fluid as I am using a real heavy duty system which is designed to operate at high output levels continuously, but tank size is one of the items I have to finalize. I am committed to the hydraulic drive for the torque and flexibility - have been impressed with systems using small fuel driven motors. The battery switching will be taken from  4 x 12v batteries (about 600AH) linked in series to  48v with mechanical relay to select 24 - 36 - 48V. I like the mechanical fail safe simplicity of this and the control will be mainly by the hydraulic flow lever valve. I will certainly check out out the 48v Altrax as the pulse modulation system does sound interesting. Chris

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Permalink Reply by Tom Hruby on June 14, 2012 at 12:52am

Chris,

When you say mechanical relay to select 24,36,48 volts do you mean selecting the first two batteries, then the third and finally the fourth?  (assuming 12 volt batteries) If so, then the issue of charging them become critical.  All of the information I have gathered over the years say that this leads to degradation of all batteries.  Problems arise both on the discharge cycle and the recharge cycle. If you run on the first two batteries (24 volts) for a time and then switch to 48 volts your batteries will be unbalanced. The first two will already be at a lower voltage and have a different response curve to the demand for power at a constant current the voltage sag in the first two will be much higher than the remaining two that have a different charge level.  This will "age" the first two batteries at a much higher rate than the other two. 

I ran into this problem with my electric vehicle.  I use 20 6-volt golt cart batteries.  Recently one of them started going bad (i.e. reduced capacity) and I noticed that the voltage sag on that battery became two and three time higher than on my other batteries. (e.g. 19 batteries were sagging to 5.5 volts while the bad one was sagging to 2.5 volts for the same current)

 

The same applied to the charging cycle as described in  my previous comment.

So, my thought is that the price of a speed controller is easily covered by the better life-span of the batteries. 

 

When I explored the hydraulic system the other issue that came up was efficiency.  Yes, the hydraulic system is quite flexible but maybe only 80% of the power is actually transferred to the prop vs. 95-98 in a direct drive system.  This becomes an issue if cruising time is important for a given battery set up.  The hydraulic system will reduce your cruising range by 15-20%.

 

Tom

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Permalink Reply by christopher green on June 14, 2012 at 1:25am

Tom 

This is one idea (although will not need the reverse controller) I have the luxury of time not being an issue here, so prepared to investigate options. Take the issues of battery usage - intend to address this with a selective battery management charging. 

On the efficiency issues of Hydraulic, I  accept - as with any transmission  - there will be some loss of efficiency. Hydraulic systems may trade off 20%, which is why I chose an DC motor with power in hand for driving my hull. My last Atalanta project was way overpowered at 10hp. The major plus sides from the hydraulic system is the massive gain in torque efficiency and the wonderful efficiency of instant control. A secondary benefit is the reduced wear on the D C motor, which is not being reversed - direct driving etc. Chris

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Permalink Reply by Tom Hruby on June 14, 2012 at 1:55am

OK got that. So you plan to charge each 12 volt battery separately.  That is good.  However, you still have the problem of uneven discharge rates on each battery if you use the different speeds.  The first battery in the system will be the first to degrade and will need to be replaced more frequently than the others.  As the batteries age, the discrepancy will increase and the battery will degrade faster because the internal resistance in the battery also increases.

 

With regards to the efficiency. It's not so much the power as the cruising distance.  The main issue with electric systems is mostly longevity.  How far can you cruise on a charge? In my original dory I had a cruising time (range) of 2.5 hrs at full speed.  A 20% reduction meant I could only cruise  2hrs.  In my new dory I have  cruising range of 5 hrs because I doubled the numer of batteries.

 Also lead acid batteries degrade with time and lose capacity.  So  a battery rated at 200 amp-hrs when new may only have 150 amp-hrs after two years.  My electric truck had a range of 35 miles when the batteries were new, but now after two years on this set my range is only about 15 miles. A 20% difference is quite significant in this case.

BUT it all depends on what you need.

Tom

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Permalink Reply by christopher green on June 14, 2012 at 12:57am

Tom - Just to clarify my system, the DC motor is non reversible and driving a hydraulic gear pump. Ahead and astern control will be via a hydraulic spool valve which will provide speed control and also have a neutral (i.e. with the motor still running) - driving directly from the DC motor is not in the plan. Chris

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Permalink Reply by Tom Hruby on June 14, 2012 at 1:19am

Chris,

I am confused.  If the hydraulic system provides the speed control why also alter the voltage to control the speed of the motor?

 

As I mentioned, controlling speed by selectively adding batteries (voltage) that are in series is prone to result in a quick battery death.  All of the information on battery systems is unanimous on this.  Even in the old days three speed golf carts changed speed by switching through different arrays of resistors to change the voltage rather than switching the batteries in the series.  Batteries in a series need to be as closely matched as possible. That is why the battery folks also recommend that you never replace one battery in a series.  You need to replace all of them at the same time or the entire system will degrade quickly.  

Tom

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Permalink Reply by christopher green on June 14, 2012 at 1:45am

Tom - Sorry for your confusion, hopefully this will clarify - The hydraulic speed control when just pottering around will not require the full 10hp maximum speed of the DC motor. The hydraulic flow control is delivering whatever power is selected to the prop. So for major power usage the higher voltage will be selected - just as you will with your direct drive system.  As for the battery control - I am reviewing the battery charging and maintenance options and will take account of your comments before making any final decisions - thanks again for your input. Chris

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Permalink Reply by christopher green on June 14, 2012 at 2:18am

Tom - The Atalanta is a 26ft 2 ton displacement sailing boat for which the auxiliary motor provides very minimal propulsion. Hardly use the motor except when maneuvering in or out of  difficult moorings etc. Therefore range is not an issue and will hardly dent the deep cycle battery bank in terms of percentage of use between charging.  The hydraulic system will also provide a lift arrangement for the Atalanta's twin 400lb keels - had a hydraulic keel lift on the last Atalanta on a separate system, very useful to reduce draft from 5ft 9" to 1ft10" in seconds when creek crawling. Thanks again, Chris

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Permalink Reply by Tom Hruby on June 14, 2012 at 4:32am

Chris,

OK, so now you should look into the rpm vs. volts for your electric motor.  Electric DC motors have a rpm/volt number that is important to help you figure out your speed reductions.  The motor will draw the maximum amps at whatever voltage you have.  Your speed will be controlled by the rmps at that voltage not by the power required to move the boat.  If your motor gets 3000 rpm at 48 volts it has 62.5 rpm/volt. If you want 1800 rpm you will need 28.8 volts. Your 7.5 KW motor draws 156 amps (7500/48 = current).  It will draw that same amount at 28.8 volts but only have a lower rpm.  That is why I suggested the controller.  It draws the needed power from the batteries based on current and changes that to the current voltage for the motor but at a higher current.  For example the motor in my electric vehicle has about 30 rpm/volt.  If I am driving at 2000 rpm the motor will be running at 66 volts and may need 200 amps to get the power to go up a hill.  However, I will only be using 100 amps from the batteries because they are at 120 volts.

So another problem you will face is your batteries.  Most lead acid batteries do not like to be drawn at a rate of more than 1/4 of "capacity"  So a 200 amp-hr battery should not be drawn at more than 50 amps continuously.

Tom

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Permalink Reply by christopher green on June 14, 2012 at 5:06am

Hi Tom - As discussed - may consider using an intermittent control system (pulse type) you mentioned earlier - have been impressed with this from an engineering elegance point of view. Concerning battery drain - on my calculations given less than one hour use motoring between charges, battery depletion is unlikely to be a major issue, but thank you for the rigorous analysis. Most of the light motoring, domestic use and  keel lifting will be covered by wind and solar charging with mains charging top up at marinas or back on her home berth. It will be interesting to record the overall engine efficiency when the much better torque delivery is offset against transmission cost (max 20%). I shall record the performance data and post here when I have it. IChris

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