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The Art of Motorsailing

It never fails to amaze me how many jerry jugs of fuel some bluewater sailors are willing to carry on deck. Once I spotted a boat at the fuel dock in St. Georges with 16 jugs open on the quay waiting to be filled...

It never fails to amaze me how many jerryjugs of fuel some bluewater sailors are 

willing to carry on deck. Once when stopping in Bermuda on my way south to the West Indies in the fall, I spotted a boat at the fuel dock in St. Georges with 16 jugs open on the quay waiting to be filled, and another four on deck already lashed down. At five gallons a pop, that’s an extra 100 gallons of fuel this crew proposed to carry on the deck of their 40-foot sailboat. At 7.3 pounds per gallon (the average weight of diesel fuel) that’s an extra 730 pounds the boat was carrying well above its center of gravity. Or to look at it another way: that’s like sailing around with over 900 feet of quarter-inch high-test anchor chain stored on deck.

In most cases, I suspect this sort of thing really isn’t necessary. On my 39-foot boat, Lunacy, for example, I made the same passage south as the boat mentioned above and carried just one 5-gallon jerry jug on deck. I probably had about the same tank capacity (70 gallons), did lots of motoring on the way down (82 hours), and still had about 25 gallons of fuel left aboard (including what was in the jerry jug) when I arrived in St. Martin.

Over the years I’ve noticed that many cruising sailors aren’t nearly as savvy as they might be about using their engines to get where they’re going. I can’t count how many times I’ve seen sailboats bashing violently into big head seas with all their sails down and their engines running close to full out, burning scads of fuel while crawling along at ultra-slow speeds with their crews experiencing maximum discomfort.

Just because it’s time to fire up your auxiliary engine doesn’t mean it’s time to think like a stink-potter instead of like a sailor. You should almost never drop all your sails when you start motoring, except when the water is flat calm and there isn’t a breath of wind to work with. In most other cases, keeping some sail up and using it wisely will not only ease the boat’s motion, but will help the boat move faster while burning less fuel.

The most common scenario, as noted, involves motoring against headwinds. Many cruisers sailing on a schedule like to use their engines to avoid the drudgery of tacking back and forth a lot to get to a destination. Once the engine is running, it may be tempting to think you can just drop your sails and steer directly to where you want to go, but usually you’re better off keeping at least the mainsail up and motorsailing into the breeze at an angle, even if it means you still have to throw in a few tacks along the way.

If you keep your main on the boat’s centerline and flatten it as much as you can (by tightening its halyard, outhaul and boomvang and easing off its topping lift), you’ll be surprised how tight an apparent wind angle you can maintain (usually less than 20 degrees) while still keeping the sail filled. Because you’re relying on the engine to create most of the drive, you can get away with a super-flat sail shape, but the sail will still greatly stabilize the boat’s motion and also add some extra speed. In situations where you don’t have to maintain a super-tight angle, you can bear off a little, put more shape in the sail (maybe even roll out the jib, too) and enjoy even more free speed.

Just like when you’re sailing, when motorsailing you should think in terms of balancing your rig, but with the engine as another factor in the mix. Indeed, running the engine complicates things a bit, because of its vulnerabilities. For example, it is harmful for many marine auxiliary engines to heel too much while running, as this prevents the lubricating oil inside from being properly distributed about the engine block. As a rule, you should not motorsail with your boat heeled past 20 degrees, unless you know for a fact your engine can tolerate it. To verify this, check your engine’s operating manual or consult the manufacturer.

It is also not good for a diesel engine to run hard in gear while making only a small contribution to the boat’s forward motion. In an ideal motorsailing scenario, the engine is creating most of the drive, and the sail or sails are just helping out. Diesels like to run under heavy loads, as operating temperatures will otherwise stay a bit too low. When this happens, condensation may form inside the engine, which can, in turn, lead to the formation of potentially damaging carbon deposits. Also, some transmissions don’t like to have prop shafts spinning much faster than the shaft speed delivered by the engine. In most cases, therefore, if there’s enough wind for your sails to do most of the work of moving the boat, you should bite the bullet and shut down the engine.

One important tool to use when trying to figure out whether you are motorsailing productively is your GPS. Try different sailing angles, sail configurations and engine RPM settings, and compare your actual speed over the ground (SOG) to your velocity made good (VMG) toward your destination. If you are sailing off at an angle with your engine running and your sails up and your VMG is higher than it is when motoring or motorsailing toward your destination at a perfectly flat or otherwise more direct angle, the larger angle is obviously more productive. The situation is similar to one in which under sail alone you must choose between pinching to resist losing ground to leeward or footing a bit to maintain speed.

It is also a good idea to power down once in a while and put your engine in neutral so you can evaluate how hard the engine is really working compared to the sails. If you lose only a little speed with the engine in neutral, it is often a good idea to stop motorsailing and start relying on the wind alone. 

In some situations you may find there is only a small decreasein SOG, but the apparent wind angle becomes so much wider that your VMG suffers. Here you may want to keep motorsailing, but increase your engine RPMs so that the engine carries more of the propulsion load and is not underworked. Another alternative is to depower the sail by reefing or flattening it out a bit.

Motorsailing is least likely to be productive when the wind is abaft the beam. At these much wider sailing angles the increase in speed created by the engine also decreases the apparent wind speed, which makes it harder to keep the sails working properly. In many cases, if there is enough wind to really fill the sails while motorsailing downwind, you probably would be just as well off using only the sails. If the sails are not filled with air and are just slatting about most of the time (this is also and especially true going upwind) you should usually take them down and proceed under power alone.

Remember, if your sails are flogging about and not really working as sails, they are only creating resistance and will slow you down. However, if the wind (apparent or otherwise) is light and there is a lot of ground swell to cope with, you may be more comfortable with a sail up, as it will help keep your boat from rolling too much, even if it is banging around a bit. Bear in mind that this can be hard on a sail, so you must balance the value of your comfort against the health of your canvas.

One important thing to remember when motoring under bare poles is that even a relatively small reduction in speed can result in surprisingly large increases in fuel economy. In a series of independent studies he conducted as part of his EU-funded HYMAR (Hybrid marine) project, SAIL’s systems guru Nigel Calder found that on his Malö 47, Nada, he can reduce fuel consumption by as much as 50 percent by cutting back from the boat’s maximum hull speed of 8.28 knots by just one knot to 7.28 knots. A reduction in boatspeed of two knots to 6.28 knots resulted in a 75 percent reduction in fuel consumption.

Needless to say, savings like this make it much easier to keep your deck clear of jerry jugs on long windless passages.

A Mix of Sailing and Motoring

Here’s an interesting problem in how to get home quickest using both sails and motor. The direct route home at a 30 degree wind angle is 21.5 nautical miles. Assuming the speeds shown, if we pinch and sail straight home it will take 4.3 hours; if we motorsail we can be home in 3.6 hours; and if we beat home fully powered up at a 40 degree wind angle, we’ll sail 24.5 miles and take 3.5 hours, just a little faster. The fastest option, however, is to mix tactics, sailing free at 40 degrees on the heavily favored port tack legs, and motorsailing the one short starboard tack leg at a tight 15 degree wind angle: by doing so you’ll be home in just 3.44 hours after covering 23 miles. Whether the 0.06 hour saved is worth the fuel burned, is up to the individual. Every situation is different. If you log your speeds at different angles and under different sail and motoring configurations it is easier to make good decisions.

Photos by Ken Winokur/wavelengthstudios and Charles J. Doane; Illustration by Peter Bull

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