Ev­ery­thing you need to know about cruis­ing wind gen­er­a­tors

SAIL - - Contents - By Sam Fortes­cue

Wind gen­er­a­tors have a nat­u­ral ap­peal to sailors in need of ex­tra power. They har­ness the same el­e­ment that we rely on to get from A to B, and the tech­nol­ogy be­hind them is well proven and re­li­able. De­spite the grow­ing pop­u­lar­ity of hy­dro­gen­er­a­tors and ever more ef­fi­cient so­lar pan­els, wind gen­er­a­tors are still a com­mon sight on blue­wa­ter cruis­ing routes.

At the out­set, it’s im­por­tant to note the ad­van­tages and lim­i­ta­tions of wind power for keep­ing the bat­ter­ies charged. On the plus side, the gen­er­a­tor will keep pump­ing out power at an­chor or in port, as well as on cloudy days. It is also largely main­te­nance-free and re­quires no launch or re­cov­ery. Th­ese can be pow­er­ful units, churn­ing out 400 watts of power or more—enough to charge 800 amp hours (Ah) of bat­tery ca­pac­ity per day on a 12-volt sys­tem.

On the down side, the world’s cruis­ing routes tend to be down­wind, rob­bing the gen­er­a­tor of some of its power. In fact, the re­la­tion­ship be­tween wind speed and the en­ergy it con­tains is cu­bic, so power de­creases ex­po­nen­tially. To put that in con­text, a boat do­ing 8 knots dead down­wind in 20 knots of true wind would ex­pe­ri­ence an ap­par­ent wind of just 12 knots. A tur­bine might gen­er­ate 40 watts in 12 knots of wind, but most man­age 200 watts in 20 knots. Fur­ther­more, ports and an­chor­ages ap­peal to sailors pre­cisely be­cause they of­fer pro­tec­tion from the el­e­ments, so wind speeds will be lower than fore­cast off­shore.

In ad­di­tion, th­ese days, so­lar pan­els have more or less eclipsed small wind tur­bines for trickle-charg­ing bat­ter­ies when you’re away from the boat. (Al­though, there are still some small tur­bines avail­able, such as the Rut­land 504, which tops out at 5 amps on a 12-volt sys­tem.) So, chances are that if you’re look­ing to in­stall a wind tur­bine, you’re look­ing to in­stall a more pow­er­ful model along­side so­lar pan­els or a hy­dro­gen­er­a­tor.

Al­most all the tur­bines listed be­low claim out­puts of 400 watts or more—the D400 claims in ex­cess of 600 watts. Of more in­ter­est, how­ever, is the out­put at the lower wind speeds that most of us typ­i­cally en­counter (or plan to en­counter) dur­ing a cruise. Per­for­mance in 12 knots or 20 knots of ap­par­ent wind is a much bet­ter in­di­ca­tion of charg­ing po­ten­tial. For ex­am­ple, if you’re head­ing to the Caribbean and plan on beat­ing into 20 knot tradewinds ev­ery day, you’ll get much bet­ter out­put than, say, sum­mer cruis­ing in Maine.

Also, don’t for­get that the power curves quoted by the man­u­fac­tur­ers are usu­ally based on re­sults from smooth, con­stant air­flow in a wind tun­nel. Real-world re­sults can be rather lower. That’s why some cruis­ers with heavy power re­quire­ments opt to in­stall two tur­bines, and most are de­signed to be eas­ily con­nected in par­al­lel through a sin­gle reg­u­la­tor.


If you want to op­ti­mize out­put from a wind gen­er­a­tor, there are a cou­ple of things to con- sider. First, it needs to be as sta­ble as pos­si­ble, be­cause any pitch­ing or rolling will tem­po­rar­ily ro­tate it away from the wind. Sec­ond, it needs clean air—as much of it as pos­si­ble.

To some de­gree, th­ese two re­quire­ments are at odds with one an­other. Wind speeds at the mast­head can be 50 per­cent greater than at sea level due to wind shear, so the higher up you put it, the more power you’ll get. Of course, the pen­du­lum ef­fect of a mov­ing boat means that putting a wind gen­er­a­tor higher up will in­crease the ef­fects of pitch and roll. (This is less true of cata­ma­rans, which pro­vide a more sta­ble plat­form.)

There have been plenty of suc­cess­ful in­stal­la­tions on mizzen masts, but for most peo­ple, it makes sense to mount the tur­bine over­head in the cock­pit. In­stal­la­tion and main­te­nance is much eas­ier, and it is pos­si­ble to man­u­ally turn a tur­bine out of the wind if all other means of brak­ing fail. In ad­di­tion, the ca­ble run is much shorter, so the wire di­am­e­ter can be smaller with­out sig­nif­i­cant volt­age drop. Fi­nally, the weight of the tur­bine, mount and ca­bling can be sig­nif­i­cant, and no one likes to add weight

high up where it will re­duce over­all sta­bil­ity. The ques­tion of volt­age drop is an es­pe­cially im­por­tant one be­cause of the ef­fect it can have on over­all per­for­mance.

“With re­gard to in­stal­la­tion, the big­gest prob­lem that most cus­tomers run into is when the wiring size is not ap­pro­pri­ate,” warns Paul Landino of Primus Wind­power dis­trib­u­tor e-Ma­rine Sys­tems in Fort Laud­erdale. Primus makes the Air Breeze and Air Silent-X tur­bines and is the only major U.S. man­u­fac­turer.


Get­ting the power from the tur­bine’s al­ter­na­tor and safely into your bat­ter­ies sounds like the easy part of the process. How­ever, not one of the main man­u­fac­tur­ers uses the same terms or tech­nol­ogy, and each is con­vinced the oth­ers’ claims are pure snake oil.

At one end of the spec­trum is Mar­lec, a Bri­tish com­pany that man­u­fac­tures the Rut­land 914 and 1200 tur­bines. They use so-called Max­i­mum Point Power Track­ing (MPPT), which em­ploys a DC-to-DC con­verter to con­stantly search for the volt­age and cur­rent in the sys­tem that will op­ti­mize wattage (watts = volts X amps), an ap­proach that has been shown to boost out­put from pho­to­voltaics by as much as 30 per­cent. They also use a clever tech­nol­ogy called Pulse Width Mod­u­la­tion (PWM) to re­duce tur­bine speeds as the bat­tery comes up to charge and needs less power. Sim­ply put, PWM short-cir­cuits the wind­ings with in­creas­ingly long pulses, cre­at­ing a grow­ing brak­ing force.

Scep­ti­cal of this, how­ever, is Peter An­der­son, a sailor who de­signed and built his own marine­spe­cific tur­bine, mar­keted as the D400. “You can­not pro­duce a struc­tured out­put when you have a vari­able in­put as with a wind tur­bine,” he says of PWM tech­nol­ogy. And after five years of de­vel­op­ment work with re­searchers in Delft, Hol­land, he con­cluded: “We were never able to demon­strate any in­crease in over­all yield with MPPT, and if any­thing sys­tem per­for­mance was re­duced.”

Klaus Krieger of Su­per­wind, on the other hand, lies some­where in the mid­dle: “With a prop­erly laid-out gen­er­a­tor, op­ti­mized for low wind speeds, we do not see an ad­van­tage of MPPT reg­u­la­tion for small wind tur­bines. The ad­van­tage of gen­er­a­tor ef­fi­ciency for the most part is com­pen­sated by the loss of the MPPT elec­tron­ics it­self. How­ever, a Pulse Width Mod­u­la­tion reg­u­la­tor al­lows us to charge the bat­tery to 100 per­cent, be­cause it will pro­vide the bat­tery ex­actly the charg­ing cur­rent it can ac­cept at any stage of the charg­ing process.”

Si­len­twind takes a sim­i­lar view, al­though it still makes use of the DC-DC con­ver­sion ele-

ment of MPPT tech­nol­ogy in its tur­bine. “Our 12/24 volt gen­er­a­tors start charg­ing at a wind speeds of less than 2m/s (4.2 knots) be­cause the boost func­tion of a DC/DC con­verter raises the out­put volt­age in low wind con­di­tions,” says Si­len­twind’s Stephanie Silva. “The Si­len­twind starts charg­ing at 2 volts, pro­vid­ing a charge power of 3 to 5 watts.” The com­pany claims this is an enor­mous ad­van­tage in pro­tected an­chor­ages, but while smart, it only yields tiny amounts of ex­tra power.

Note that the PWM cir­cuits in the Su­per­wind and Si­len­twind have a small but im­por­tant dif­fer­ence. While the Si­len­twind uses pulses to short the al­ter­na­tor and ap­ply a vari­able brak­ing ef­fect, the Su­per­wind di­verts the pulses into a dump re­sis­tor, which burns off the ex­cess power as heat, “so that the wind tur­bine is al­ways elec­tri­cally loaded” ac­cord­ing to Krieger. Clev­erly, the Su­per­wind’s reg­u­la­tor also draws its power from the tur­bine, rather than from the bat­tery, so no power means no cur­rent draw.

As for Primus Wind­power’s tur­bines, they have a built-in charge con­troller that aims to max­i­mize power com­ing out of the tur­bine. Ac­cord­ing to head of sales Ken Ko­ta­lik, his com­pany de­lib­er­ately avoided MPPT or PWM tech­nol­ogy be­cause: “We like to keep it sim­ple—re­li­a­bil­ity is key. Al­most all of our tur­bines are cou­pled with so­lar pan­els, so the job of our units is bulk charg­ing. Adding fancy elec­tron­ics in­creases vul­ner­a­bil­ity and raises the price.”

An al­ter­na­tive to the ex­ter­nal reg­u­la­tors that some of the man­u­fac­tur­ers of­fer is a stand-alone “dump” type reg­u­la­tor, like that men­tioned above. It can be added to any sys­tem and con­stantly mon­i­tors the power com­ing in from the tur­bine, as well as the state of charge of the bat­ter­ies, di­vert­ing any ex­cess into the re­sis­tor. It has the ad­van­tage of keep­ing the tur­bine at full load, where its blades are turn­ing at their most ef­fi­cient rpm.

Be­ware of re­ly­ing on the tur­bine’s built in “reg­u­la­tor” alone. This is usu­ally a sim­ple elec­tronic brake trig­gered when the tur­bine is pro­vid­ing too much power and bat­tery volt­age rises too high. If your bat­ter­ies are close to full charge, this will of­ten re­sult in an un­sat­is­fac­tory stop-start rou­tine. The bat­tery volt­age drops back as the wind gen­er­a­tor brakes; then the reg­u­la­tor restarts the gen­er­a­tor. This is far from the true reg­u­la­tion that your bat­tery needs, care­fully di­al­ing back the amps as the bat­tery comes up to full charge.


Tur­bine blades are an­other key area of dif- fer­en­ti­a­tion be­tween units, with some im­pres­sive claims. The blades op­er­ate on a sim­i­lar prin­ci­ple to an air­craft wing, al­though there are some dif­fer­ences due to small scale. Un­less they change pitch like the Su­per­wind (more on this later), they are de­signed to pro­duce op­ti­mum out­put at one given rpm only. If the tur­bine goes too fast, it loses ef­fi­ciency (like a car in too high a gear); same if it ro­tates too slowly.

It is usu­ally the blades that pro­duce the noise and vi­bra­tion as­so­ci­ated with a tur­bine. Air flow be­comes un­sta­ble at very high tip speeds, caus­ing blades to flut­ter. There was a well­doc­u­mented prob­lem with the orig­i­nal Air-X from Primus Wind­power, where the blades be­gan to flut­ter at high speeds, cre­at­ing a shriek that would drive other boats from an an­chor­age. The com­pany has ad­dressed this—first in the Air Breeze, with its wider blades, and later in the Air Silent-X, which uses an even qui­eter blue, car­bon-fiber blades (made by Si­len­twind).

“Cus­tomers have noted an in­crease in power per­for­mance, es­pe­cially in the lower wind speeds,” says Primus Wind­power’s Paul Landino. “The pre­ci­sion car­bon-fiber blue blades are bal­anced and there­fore when in­stalled have less of a ten­dency to cre­ate vi­bra­tion again adding to im­prove­ment of life on board.”

The “tip speed ra­tio” (TSR) of a tur­bine can be a help­ful guide here. TSR de­scribes how many times faster than the true wind speed the blade tips are mov­ing. The orig­i­nal Air-X tur­bine, for in­stance, had a TSR of 16—so in a 20-knot wind, the tips would be mov­ing at 320 knots. In only a mod­est gale, they’d ap­proach the speed of sound. The D400, on the other hand, quotes a TSR of just 3.9, in­di­cat­ing that it is de­signed to ro­tate more slowly than the other tur­bines. It is heav­ily built, with more than 2lb of pure cop­per in its wind­ings alone, but it has the ad­van­tage of its rel­a­tively low rpm.

Some tur­bine man­u­fac­tur­ers quote a top “sur­vival” speed for their ma­chines, but this should be taken with a grain of salt as Peter An­der­son at Eclec­tic, ex­plains: “It is the tur­bu­lence level within a wind stream that is most dam­ag­ing, and that is a vari­able you can nei­ther pre­dict nor eas­ily quan­tify. We have had tur­bines run through sus­tained winds of 90-plus-knots with no dam­age. Equally we have had tails torn off at a re­ported 55 knots.”

Unique among major man­u­fac­tur­ers, Su­per­wind has de­vel­oped a me­chan­i­cal ap­proach to brak­ing its sys­tems. When in op­er­a­tion, it ex­ploits the physics of ro­ta­tion to al­ter the pitch of the blades me­chan­i­cally, so that the faster they spin, the more the blades feather, slow­ing the tur­bine down again. Su­per­wind says this sys­tem re­sponds very quickly, and can pro­tect the sys­tem in the event of fail­ure of the elec­tronic brak­ing. s

Wind gen­er­a­tors aren’t at their best off the wind

Rut­land 914i Su­per­wind

Silent-X Si­len­twind Air Breeze

Rut­land 1200 D400

Nature Power

In brisk reach­ing con­di­tions, wind gen­er­a­tors pump out plenty of cur­rent

Any cruis­ing boat can ben­e­fit from a wind gen­er­a­tor

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