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As a newbie to this group I have not followed many discussions yet.  However, I noticed that there have been few discussions of electric propulsion in small boats that we use for messing about.  Is there any interest out there in discussing and sharing stories about electric power?  I built my first electric 24' St Pierre Dory 17 years ago and have been cruising then in Puget Sound since then.  I am now on my third dory.  This one is 26 ft (Lucky Pierre plans from Glen-L marine) and powered by two 36 V trolling motors. 


I would love to share stories with those who use electric power or help answer questions for anyone that might be interested. 


Here are some numbers on my dory.

My hull speed is a little over 5 knots and the two trolling motors can easily get there.  One motor will drive me at 3.8 knots at max power. 

My range is 50 nautical miles since I installed lithium batteries.  It was 20 miles when I use lead-acid batteries. 

I have also installed solar panels (540 watts) that will supplement shore power for recharging.  Four hours of sunlight will give me 1 hr of cruising at 4.5 knots.


For those interested, I have attached a picture of the latest dory. 


Tom post-3782-0-89836000-1429478566_thumb.jpg

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Hi Tom, Welcome to the group.  Lovely dory and thank you for sharing your electric power experience.  I'm hoping to use electric for aux. power for my newly acquired little 5' Core Sound cat-ketch.  Easy (but pricey) solution would be a Torqueedo 1003 with extra battery, but looking at low profile, easily stored, trolling motors too with lithium batt.  Good to see your lithium batts extended your range significantly.  Did they save weight too?  Do you have a preferred brand or good source for test data?  Safe cruising,  Rick

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There was a great book on the subject called Electric Boats by Douglas Little. It was published by International Marine. You can get it from Amazon.



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I will answer Rick and then Chick,

First Rick: 

Yes the Torqueedo is quite pricey and I think over-rated compared to the trolling motors available for a lot less.  I have studied the specs for the Torqueedo and I think their ad campaign doesn't address the real issues in using electric for longer cruising.  Their propeller is slightly larger than that of a 36 volt trolling motor (12" vs 10") so they will be slightly more efficient, but not so much more to warrant the cost difference.  The efficiency of a prop is based on diameter and pitch and rpm.  The calculations are nicely described in the book that Chick mentions and that I often use.  My first dory had an electric inboard motor with a 16x16 inch prop.  I would get to hull speed at about 600 rpm and had an efficiency of about 85%.  The trolling motors and the Torqeedo are in the range of 70-80% because of smaller diameters and higher rpms. So yes, I might need only 1800 watts of power to move my dory with a Torqueedo when my trolling motors require 1900 watts to maintain hull speed. 


Based on my calculations and experience you need about 700 watts of power to move 1000 lbs of displacement at hull speed (My dory is about 2800 lbs and needs 2000 watts to move it at hull speed; if I am willing to go 80% of hull speed my consumption drops to 1300 watts).  A Torqueedo is a 24 volt motor so it will need to a current of 30 amps to move 1000 lbs of displacement at hulls speed.  The largest Torqueedo lithium battery (at a very expensive price) stores at most 2600 watt-hrs of power.  That means at 30 amps the torqueedo will provide about 3 hours of cruising time using their largest battery for a boat weighing 1000 lbs.  If your boat weight 2000 lbs that reduces the cruising time at hull speed to 1.5 hours.  For comparison I had 15,000 watts-hrs of energy storage in the lead acid batteries I first used, and now I have 28,000 watt-hrs in the lithium batteries.  The lithium batteries take up the same amount of space as the lead acids and weigh about 630 lbs. compared to the lead batteries weighing 800 lbs.  The other big advantage of the lithium batteries is that they can be taken down to 10% of their capacity without a reduction in battery life.  Lead acid batteries should not be taken below about 40% of their rated capacity. In addition Lithiums are usually rated at over 2500 charge/discharge cycles whereas the less expensive lead acids are usually good for only about 600 cycles.  Anyway the cost and capacity issues is another whole topic and worth a separate thread if there is interest.  I have done extensive calculations on cost per mile and the lithiums come out close to the flooded golf cart batteries and lower than the AGM type batteries if you do extensive cruising. 


The one disadvantage of lithiums however is that you will need a battery management system (BMS) since they are very sensitive to overcharging and full discharge.  You can buy batteries with a BMS included but that doubles the price of the batteries.  If you are willing to attach it yourself you are looking at about 5-10% the cost of the batteries. 


I personally have had a good experience with the Mercury electric outboards (called Motor Guide Great White).  These are a bit less expensive than comparable Minn Kota motors.  I have been cruising with the Mercury Marine version for 7 years now without any problems.  The nice thing about the trolling motors is that I can remove the head from the shaft and mount the shaft separately on pivots along the side of the boat (see previous photo).  There throttle only needs two small wires to the shaft and the main power cables can be placed separately.  This you cannot do with the Torqueedo. 


If you want test data, I have been collecting it myself for 17 years so can try to answer questions you may have.  By the way, lbs of thrust have little meaning when you are trying to figure out power needs for cruising.  And, don't fall for the Torqueedo hype about horsepower.  Generally 1 true electric horsepower (=750 Watts) is equal to about 3 hp of a gasoline engine. In that sense the Torqueedo is no different than the trolling motors. So if Torqueedo rates its motor as equivalent to a 6hp gas outboard it is actually putting out about 1500 watts (but at 24 volts that would require a current of 60 amps). Each of my trolling motors is rated at 1080 watts and the max current is 30 amps at 36 volts.  Electric motors are close to 90% efficient at transferring power regardless or rpm.  This is not true for gas motors. 


Hope this helps.


And now Chick.

Yes I have the book by Douglas Little.  It was what got me started in the first place.  It does cover many of the basics, but the technology has now improved significantly with trolling motors and the Torqueedo, and different battery technologies.  Now that I am retired I am considering writing an update that includes discussions of electric power for larger boats.  There is another venue called "Electric Seas" on the web where we do discuss and share experiences, but few people have found it. 



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  • 1 month later...

Thanks Tom for bringing the subject up.  Just from your posts, I feel like I've doubled my knowledge of marine electric outboards...and I've been using an electric trolling motor on my dinghy for years.  I didn't know we had large capacity lithium batteries providing competition for lead-acid.  With all their operating advantages, how do they compare cost wise?

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Hi Bill


It all comes down to cost vs. weight, vs. longevity. 


Sealed lead acid batteries cost about $210 per kwatt-hr.  A 6 volt 200 amp-hr sealed battery costs about $250 and it stores 6 v x 200 amp-hrs = 1.2 kwatt-hrs.  However, you only have about 60% of that capacity that is usable.  Lead acid batteries should not be taken below about 40% of their rated capacity.  If you do, the number of charging cycles you get is significantly reduced.  So your initial costs are about $350 per kwatt-hr of usable energy stored for the AGM batteries.  This is about the same for either 6 volt or 12 volt batteries. The AGM deep cycle batteries at this lower price range have an expected life of about 500 charge discharge cycles (or 5 years whichever comes first).  Thus, over the life span of the battery you will end up paying 350/500 = $0.70 per kwatt-hr used in you electric motor.  There are sealed batteries used in the solar world that are good for about 1000 -1200 cycles but they cost double. 



Lithium iron phosphate batteries cost about $450 - $500 per kwatt-hr of energy.  This includes the needed battery management system. One advantage of lithium batteries is they can be drawn down to 10% of their capacity.  This means that 1 kwatt-hr of usable stored energy costs $500 for lithium compared to the $350 for the lead acids.  However, lithium cells are good for over 2000 charge discharge cycles.  Thus if you look at the cost of power amortized over the life span of the batteries, one kwatt-hr of power to your motors actually costs only $0.25/kwatt hr.  The large capacity lithium batteries have been readily available for only 5 years of so, so we don't have any data on their longevity. One report from a dealer in parts for electric cars found that the cells lost only 10-15% of their capacity after 5 years. 


Another advantage of Lithiums is that they take up about 1/2 the space (volume) of lead acid batteries for that 1 kwatt-hr and weigh less than 1/2.  Lead acids weight about 50 lbs per kwatt-hr of rated capacity regarless of voltage.  Lithiums weigh about 23 lbs per kwatt hr of rated capacity.


When I replaced my lead acid batteries (15 kwatt-hrs of total energy) with lithium batteries in the same space I was able to get 28 kwatt hrs of stored energy and thus more than doubled my cruising range.   So my lead acid batteries were good only for about 10 kwatt hrs of cruising while my lithiums are good for 25 kwatt-hrs of cruising within  the same space and weight. 


If you cruise in a cold climate be aware that lead acid batteries have  a reduced capacity when it gets cold.  At freezing a lead acid battery has only 1/2 of its rated capacity, while the CALB CA lithiums (the brand I have) are reduced only by 8% at freezing.


So you will have to juggle the different variables to meet your needs.  If you cruise only a few dozen times a year in a warm climate, and have the space and can carry the weight, then the lead acid batteries are just fine and more cost effective. 


If you are interested in the large capacity lithium iron phosphate batteries you will need to explore the web sites that sell parts for electric vehicle conversion.  I have had good experience with the CALB 180 amp-hr cells in both my boat and in my electric pickup truck.  


One thing to note about lithium batteries.  The "cells" are only 3.2 volts so you will need to put together more of them to achieve your desired voltage.  Lead acid cells are 2.1 volts but the manufacturers put them together for you in 6.3 or 12.6 "batteries."  Also note: the lithium iron phosphate cells (LiPO4 or LiPo) are the not the same as used in electronics or the Tesla.  LiPo cells are much more stable and don't suffer from the temperature (and in extreme cases, explosions) that the lithium ion cells do. 



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Thank you very much for the additional data Tom.  Really want to go electric on my 15' cat ketch.  Easily driven hull 350# + 800# crew and gear = 1150-1250#. Hope to use your numbers to help plan speed and endurance projections with a maybe a great white 50 and 1-2 20-25# lithiums.  Rick

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I am somewhat confused.  I was unable to find the specs for a Great White 50.  All they now have is a Great white 82 lb/24 volt and an 105 lb/36 volt. 


Just to get you started here are some numbers that I come up with.

Assuming you want to more 1200 lbs at hull speed you will need a motor that produces about 900 watts.  The 36 volt Great White I have consumes about 1080 watts at maximum power (30 amps +-).  I don't have the specs on the 24 volt version but I expect its max power is slightly below 900 watt. 


Lithium cells are rated at 3.2 volts so you would need 12 cells to make up the nominal 36 volts.  The actual voltage of the pack is closer 40 volts when fully charged.  The trolling motors were designed for lead acid batteries, but the slightly higher voltage should not cause problems.  I checked with Motor Guide and the max voltage for the motor is 49 volts but they refuse to commit themselves to saying that lithium batteries are OK. However,  I have had lithiums for two years without any problems. 


If you go with the 24 volt motor you will need 8 cells. 


Your comment about 1-2 20-25# lithiums confusing.


The smallest "pristmatic" lipo cell is 40 amp-hrs and they go up from there to 60, 100, and 180 amp-hrs. 


At 40 amp-hrs you will have 40 amp-hrs x 38 volts = about 1.5 kwatt hrs of energy.  If your motor is consuming 1000 watts/hr your cruising time will be a little less than 1.5 hrs.

You do a similar calculation for the 24 volt motor. 


For example, assume you motor consumes 800 watts at maximum throttle.

8 of the 180 amp-hr cells would give you about 4.8 kwatt hrs of stored energy and about 6 hrs of cruising time. 


Here is a sample of the CALB lithium cells available and a reasonable pricing .  This is from http://evolveelectrics.com/calb/




CALB 40Ah - 3.2V
LiFePO4 Battery
CALB 70Ah - 3.2V
LiFePO4 Battery
CALB 100Ah
CALB 100Ah - 3.2V
LiFePO4 Battery
CALB 130Ah
CALB 130Ah - 3.2V
LiFePO4 Battery
CALB 180Ah
CALB 180Ah - 3.2V
LiFePO4 Battery
CALB Busbars
Laminated copper busbar to fit each CALB battery variant.



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