Engine alternator? or water and wind power? - a comparison

by Larry Janke on 10 Jan 2010 How much do you need to use your boat to make utilising wind and water power a distinct advantage?

If you rarely sail, you may decide that one more piece of gear to break down is just not worth it, and if you are crossing oceans, there's no question - but where is the cutoff point, and how do you work it out?

Here Larry Janke, from SE Marine, compares alternator against the Duogen:

The following calculations were made using a hypothetical 100 amp hour per day usage, and an assumed 500 engine hours per year. Of course if your usage is greater the numbers change proportionally. The numbers in these calculations may vary according to your location, but the comparisons should hold.
 
A survey of several dealers indicates that the average yearly cost of the engine typically found in a sailboat equipped with a high output alternator and regulator including maintenance amortized over 15 years is about $1000.00 per year, assuming 15 year or 7500 hour life.
 

Yanmar engines from 21-54 horsepower average cost is say $10,200.00. A Balmar 60-YP-100 package consisting of a 6 series 100 amp alternator, a MC 612 regulator and alternator/battery temperature sensors will add about $ 1000.00 to the installation and maintenance will be about $250.00 per year for oil, filters, belts, zincs, antifreeze etc. Not counting any labor if you do not do the work yourself.

Total cost for 15 years, $15,000.00

$15,000/7500 hours = $2.00 per hour operating cost not including fuel
1.93 horsepower is required to produce 100 amps in a perfect world.
100 amps x 14.4 volts = 1440 watts. (1440 watts/746 watts per horsepower) = 1.93 horse power. However the world is far from perfect, in reality it will take at least twice that amount of power or 3.9 horsepower to produce our desired 100 amps because of alternator/battery inefficiency and belt loss. 

A new diesel engine or one with relatively low hours and in good condition will use 0.44 pounds of fuel per hour to produce 1 horsepower. (0.44lbs x 3.9 HP)  =1.7 lbs/horsepower hour. Diesel fuel averages about 7.2 pounds per gallon. (7.2 lbs per gal/1.7 lbs) = 0.24 gallons per hour an older engine or one in not such good condition, will use even more so let’s round it to 0.25 gallons per hour.
 
Let’s assume a hypothetical battery bank of 600 amp hours at the 20 hour rate. If we use 300 amp hours, (50% being the maximum we should discharge a battery), we need to return about 350 amp hours (350/3days = 115 AH/day) to satisfy the first law of thermodynamics (you don’t get anything for free). When it is hot our 100 amp alternator will produce about 84 amps at 4000 alternator RPM or about 1600 engine RPM assuming a 2.5 to 1 pulley ratio.

(350 amp hrs/ 84 amps) = 4.2 hours to produce our desired recharge. (4.2 hrs x 0.25 gal) = 1.05 or about 1 gal per recharge of 350 amp hours $5.00 per gallon is a conservative estimate of diesel fuel cost throughout the world.
Per hour cost of engine operation = $3.20 ($2.00 amortized engine cost + $1.20 fuel)
If you only use 100 amp hours per day, a low estimate, you will have to run your engine 4.2 or more hours every third day or 122 times per year at $3.20 per hour or $1600.00.
 
A DuoGen 2 Water and wind generator with regulator and diversion load costs about $3800.00 or less depending upon the choice of shaft length and diversion load. Let’s assume that in a year you sail 25% 0f the time and sit the rest. At 6 knots boat speed the DuoGen 2 will produce 192 amp hours in 24 hours of sailing, or about 60% more than our required 115 (100 amp hours per day usage plus the additional 150%).

So now, unless you need to motor for propulsion, you do not have to run the engine at all. Another money saving benefit since you do not have to cycle your batteries, they will last substantially longer. When you get where you are going, and convert the DuoGen 2 to wind mode you will continue to save money.   

In a 15 knot wind and operating ½ time or 12 hours per day the DuoGen 2 will produce 75 amp hours per day, or 65% of our required 115. As a practical matter, you should use less power when not underway and some Cruisers report that the DuoGen in wind mode provides all the power they need.

Here's a letter I received:

The DuoGen is working great! My diesel engine went out during the crossing from French Polynesia to American Samoa. When we anchored in Pago Pago, I ran solely on DuoGen wind power for four weeks -- fridge, DVD movies and all! On passage, we run computer, radar, fridge, et al, and the hydro has kept us topped off the whole time. I just can't say enough good about the DuoGen.
Chris Burns
s/v Wind Dancer
Currently in: Apia, Samoa

But even if we still need to supplement with some engine time (a couple of solar panels wouldn’t hurt) we should only have to make about 135 amp hours every 3 days.
100 amp hr per day usage x 1.15 =115 amp hours to be returned
75 amp hours from DuoGen in wind mode= 45 amp hour deficit
300 amp hours/45 = 6 days between recharges at 50% or 42 days x 4.2 hours =176 hours
176 hours x $3.20 = $564

DuoGen 2 a regulator and diversion load will cost about $3800.00 USD ($3800.00/ 10 years) = $ 380 per year or $120.00 per year less than using your engine driven alternator, not to mention wear and tear.
In other words you will pay for the DuoGen 2 in a little over 3 years, compared to using your engine. . After that it is free with no noise and it doesn’t smell bad. Again since we are not cycling the batteries so deeply our battery life will go up.  

If for example we are using Rolls 4000 series batteries, (1350 cycles at 50% discharge), and we only discharge to 75%  battery cycle life will increase to over 2000 cycles or about 33% longer life. Translated into dollars, at $250.00 per battery, the cost per cycle is now $0.12 instead of $0.18.

If we add 150 watts of solar panels, we should get about another 60 amp hours per day so we are home free. The panels and regulator should cost about $ 800.00 and if we divide that by 10 years we have added $80.00 per year to the equation or a total of $460.00 per year as against $1600.00. Further since engine hours can be greatly reduced, maintenance costs and headaches will go down and engine life will go up. 

This is not an argument against installing a quality high output alternator; someday you are going to need one, but with high fuel price, and environmental impact, the numbers listed above multiplied times thousands of boats is significant. 122 gallons of fuel per year multiplied by 10,000 boats is a staggering 1,220,000 gallons of fuel, or $6,100,000.00 at $5.00 per gallon.

For more information on this subject, go to the www.semarine.com!SE_Marine-Website, or contact Larry Janke at info@semarine.com

Copyright 2008, Laurence L. Janke. All rights reserved. Duplication, redistribution or any other use is allowed only upon the express written permission of the author.