Purists might not approve, but there’s no doubt that motor sailing – using both the engine and sails – can make a very useful combination.

Sometimes you just have to push on and get there.

We might be boosting our sail-power with a few revs from the engine to punch through a head sea – or perhaps there’s no wind, so we’re motoring with the mainsail up to reduce rolling.

Whichever is providing most of the propulsion, it’s the combination of sails and engine that often produces the results when either alone simply wouldn’t.

The trick is knowing how to make motor sailing work most effectively; whether it’s worth hoisting the mainsail or unrolling the headsail if you’ve just been using just the engine, for example, or whether it would pay to point a few degrees off your course to get the mainsail filling so it adds some extra drive.

Would the increased speed offset the greater distance – and which way would be the most comfortable?

Motor sailing: Trial and error

To answer some of these questions, I headed out into the bay with Graham and Judith Davies and their Marcon 34, Tomboy.

The Marcon was designed by the late David Thomas and sails better than many boats that are unashamedly motor sailers.

Nonetheless, she arguably nods in that direction with her heavy, full-sectioned hull, relatively modest rig and 35hp of Nanni diesel beneath the sole of a cockpit that separates the main living accommodation from the two-berth cabin in the stern.

We chose a day when a moderate onshore wind was kicking up enough of a chop for the angle of the boat to the waves to make a difference, inducing a fair amount of pitching at times and some rolling at others.

Motor sailing: making upwind

Sailing to windward

This is the base measurement with which the results of everything else are compared. If the engine failed, it’s how you would get home sooner or later.

Propulsion: mainsail and genoa
Speed through water: 4 knots (average) True wind angle (TWA): 42° approx.
True wind speed: 11 knots
Speed made good to windward: 2.97 knots
Motion: A slightly ‘wallowy’ feel because the boat was under-powered and took a while to rebuild speed after being stopped by a wave When to do this: When you’re making sufficiently good progress not to need any help from the engine!

Motoring dead into the wind (no sails)

Propulsion: engine only (2,000rpm)
Speed through water: 4.3 knots
True wind angle (TWA): 0°
True wind speed (TWS): 12 knots
Speed made good to windward: 4.3 knots
Motion: Plenty of pitching, some rolling and a few speed-killing plunges into troughs
When to do this: If your destination lies directly upwind and you have plenty of horsepower at your disposal, there might be little point in hoisting any sail.

Motoring dead into the wind (with mainsail)

Propulsion: engine (2,000rpm) plus mainsail
Speed through water: 4.2 knots
True wind angle (TWA): 0°
True wind speed (TWS): 12 knots
Speed made good to windward: 4.2 knots
Motion: As before, but less rolling when any waves came in from off the bow
When to do this: If the wind is on the nose but the seas are from a slight angle, the mainsail will act as a steadying sail to reduce rolling. It makes the boat more visible and, if the wind will be coming off the bow at some point, it would make sense to have the mainsail up and ready as long as it’s not flogging in the meantime. Flogging is bad for the sail and will slow you down. Fully- battened sails tend to flog much less than short-battened alternatives.

Wind off the bow (engine and mainsail)

Propulsion: engine (1,900rpm) and mainsail sheeted amidships
Speed through water: 4.6-4.7 knots
True wind angle (TWA): 15–20°
True wind speed (TWS): 12 knots
Speed made good to windward: 4.3-4.4 knots
Motion: More comfortable than dead into the wind. The slight heel induced by the mainsail will often reduce any tendency to slam and the extra drive from the mainsail will help the boat punch through the seas and maintain a more constant speed
When to do this: Depending on the state of the sea, the type of boat and power of the engine, you might make better and more comfortable progress if you bring the wind off the bow to get the mainsail driving.

Motor sailing with both sails

Propulsion: mainsail, genoa and engine (1,600rpm and 2,000rpm)
Speed through water: 5 knots (1,600rpm) 6 knots (2,000rpm)
True wind angle (TWA): 37°
True wind speed (TWS): 11 knots
Speed made good to windward: 4 knots (1,600rpm) 4.8 knots (2,000rpm)
Motion: Similar to when motoring with the wind off the bow. More abrupt in an awkward sea with higher revs and greater boat-speed
When to do this: When you want to make upwind and don’t mind tacking the headsail from time to time – or if you can lay your course directly. With this boat in these conditions, this approach produced the best speed made good to windward. Using the engine gave another 5° or so of pointing compared with just sailing.

Speed (velocity)_ made good to windward: how to work it out

If, in these examples, pointing 20° off the wind gives you an extra half-knot compared with motoring directly into it, how do you know which will get you upwind fastest?

If you have instrumentation that gives you VMG (velocity made good) towards a destination, that makes it easy.

But if your electronics are more basic, just multiply your SOG (speed over ground) by the cosine of the angle you’re steering away from the wind or your destination.

We used boat-speed in our calculations because the tide was negligible.

In the fourth example (wind just off the bow) we recorded a speed of 4.6-4.7 knots at an angle of 15-20° to the wind.

Using the more conservative figures (4.6 knots and 20°), that’s 4.6 x cos 20°, which is 4.6 x 0.9397, giving us 4.32 knots as opposed to 4.2 knots heading straight into the wind with the mainsail up.

It’s a calculation you can do on a mobile phone in about 30 seconds.

Motor sailing: wind on the beam

Engine only

Propulsion: engine only (1,900rpm)
Speed through water: 6 knots
True wind angle (TWA): 90°
Motion: Uncomfortably rolly
When to do this: Only if you have to. Motoring under bare poles in a beam sea can be uncomfortable. It’s not something you would normally choose to do for any distance in a monohull.

Sailing only

Propulsion: mainsail and genoa
Speed through water: 4.2 knots
True wind angle (TWA): 90°
True wind speed (TWS): 10 knots
Motion: A little rolling, but reasonably comfortable
When to do this: When the sails provide enough drive and you don’t need any help from the engine. Many cruisers of this nature will slip along nicely on a beam reach if there’s enough wind, though rolling can be a problem with any monohull in marginal conditions.

Engine and mainsail

Propulsion: engine (1,900rpm) and mainsail
Speed through water: 6.5 knots
True wind angle (TWA): 90°
True wind speed (TWS): 12 knots
Motion: Much more comfortable than with no sail
When to do this: When the sea state would induce rolling without any sail but there’s not enough wind to keep the headsail filling as well.

Engine and genoa

Propulsion: engine (1,900rpm) and genoa
Speed through water: 6.8 knots
True wind angle (TWA): 90°
True wind speed (TWS): 10 knots
Motion: A little more rolling than with mainsail only, but much better than under bare poles
When to do this: Whenever you want the extra drive and steadying effect of a sail and are on a suitable course, but would prefer not to hoist the mainsail. A headsail won’t fill as close to the wind as a mainsail. With the wind astern it will backwind if you exceed wind-speed and will offer less of a steadying effect than the mainsail. Between close-hauled and a broad reach are the best angles for motor-sailing with the genoa.

Motor sailing with both sails

Propulsion: mainsail, genoa and engine (1,600rpm)
Speed through water: 5.5 knots
True wind angle (TWA): 90°
True wind speed (TWS): 10 knots
Motion: Even at low revs, the extra speed from the engine reduced the rolling
When to do this: When the wind is light enough for rolling to be uncomfortable. A few revs from the engine will add a good deal
of speed (nearly 1.5 knots in this instance), increasing the apparent wind and moving it forward. This means the sails can be sheeted in closer, reducing the rolling. A monohull will always tend to roll in a beam sea, but this helps to control it.

Motor sailing: sailing downwind

Engine only

Propulsion: engine only
Speed through water: 6 knots (2,000rpm)
True wind angle (TWA): 180°
True wind speed (TWS): 12 knots
Motion: A fair amount of rolling becomes tiring after a while and makes the helmsman/autopilot work harder
When to do this: Motoring downwind in a seaway with no sails up is rarely something you would choose to do for any distance. If you’re exceeding wind-speed downwind in flat water, it’s probably not worth having the mainsail up – but things are different when there’s a sea running.

Engine and mainsail (sail eased)

Propulsion: engine plus eased mainsail
Speed through water: 5.8 knots (1,900rpm)
True wind angle (TWA): 180°
True wind speed (TWS): 12 knots
Motion: Lots of rolling
When to do this: You would ease the mainsail if it provided extra drive and ensured better control than when pinned in (sheeted centrally). With enough wind to induce a broach or make the boat hard to steer, you probably wouldn’t be using the engine. In
our trials, the wind-speed was roughly double the boat-speed: we had about 6 knots of apparent wind from astern. Even so, the boat’s rolling frequently shook the wind out of the sail. The boom swung in and out and across the boat in a series of unplanned gybes – not great for crew safety.

Engine and mainsail (sheeted centrally)

Propulsion: engine plus sheeted-in mainsail
Speed through water: 5.8-6 knots (1,900rpm)
True wind angle (TWA): 180°
True wind speed (TWS): 12 knots
Motion: A lot less rolling
When to do this: Even when you’re not exceeding the wind-speed downwind, there are times when it’s best to sheet the boom into the middle. If the sail is otherwise swinging around, as in the previous example, you would want to pin it in to avoid frequent and unpredictable gybes. The motion was vastly improved and boat-speed increased slightly. Particularly in light airs the sail will tend to bang as it fills each side, which won’t do it much good. You will have to make a judgement about comfort versus the life of the sail.

Don’t forget the cone!

If you’re motor sailing, strictly speaking you should display an inverted cone.

We didn’t in our trials because we were continually switching between motoring, sailing and motor sailing.

Neither do many other yachts in motor-sailing mode, but you should if you want to do things properly.

PBO conclusion

There are few hard-and-fast rules when it comes to motor sailing.

What works best for any particular boat will depend on its size and type, its hull shape and propensity to slam, its sail plan, its weight, the size of the engine and a multitude of other factors – including the maximum angle of heel at which you should run the engine.

Our trials showed that it’s worth experimenting and doing the VMG calculations.

Continues below…

They also showed that a few revs from the engine to supplement your sails can help your speed on a passage, greatly improve the boat’s motion, add punch through the waves and make steering easier.

Hoisting sail(s) when you’re motoring will often reduce rolling even if not adding to the speed.

Sail when you can, by all means – but there are times when using sail and power together really does make a lot of sense.

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Enjoyed reading Motor sailing: how to add power to your sails?

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