Sailing and motoring are often viewed as two different and mutually exclusive aspects of boating. The truth is, sometimes, they can work together beautifully! 

The idea behind sailing is that the wind will rush through your sails at an appropriate angle, allowing your sails to generate lift and pull your yacht through the water. The idea behind motoring is that the rotational energy from the motor will be transmitted to a propeller used to push your yacht through the water. 

On days with plenty of wind, sailing can exist as the sole method of powering your vessel. On days with no wind, there is no way for the sails to power you and the trusty motor will provide mechanical propulsion to keep you moving. But what about days where the wind is a little light?  

If there doesn't seem to be enough wind to power your yacht, you could always supplement your speed with a bit of throttle from the motor. The idea is simple, as your motor pushes your yacht through the water, it will also push you through the air and create apparent wind over your vessel. This apparent wind can then be used to power your sails and provide a nice balance of power, between sails and motor, working synergistically to motorsail you along at a comfortable speed. 

While motorsailing might sound like the solution to low wind days, there are a few important factors that need to be present for it to work well: 

There needs to be some wind present. 

The true wind can't be coming from directly ahead or directly behind you. 

If there is no wind at all, meaning you are completely becalmed, then motorsailing will be futile. As you move in any direction, the apparent wind will always be coming from directly ahead and the sails will luff just like if you were caught in irons. Having the sails up might make you feel like you are motorsailing, but the energy wasted on making the sails flap and slat is going to be robbed from your potential forward speed. 

The true wind needs to be coming from any direction other than directly ahead or directly behind you. If the wind is from directly ahead, motor sailing will only increase the amount of apparent wind coming directly at the vessel and you will actually suffer in speed because of air drag. If the wind is directly behind you, as you move forward, the apparent wind will become even less. 

Having the wind off to an angle, just like if you were sailing, is the trick. The apparent wind will always move forward as you motor sail, making it appear that you are on a close reach, no matter where the wind is coming from. As you begin to move forward, simply trim your sails to the apparent wind and you will see your speed go up a little bit as the sails begin to help provide a bit of forward power as well. 

We were once becalmed in a naval firing range testing area (while not in use, obviously) and had no chance of getting out of there. It was getting late and we wanted to anchor somewhere for the night, but anchoring was prohibited there as well! We motored along at 2 knots for a while until a slight zephyr came over us. We raised the mainsail and staysail and found that our speed increased to 3.5 knots! I then cut the power from our electric motor and our speed dropped down to 0.3 knots.  

As you can see, the sails alone provided almost no speed in the light breeze, but with the apparent wind from the motor, we were able to scoot along at an appreciable speed for having no worthy wind around us. 

One last scenario where motorsailing can pay huge dividends is when pinching. If you are sailing along and there isn't really enough breeze to make the keel as effective as it could be, a touch of throttle will mend all these ailments.  

As you sail, the wind is actually pushing your yacht to leeward but the keel offers some resistance to this motion. As you move forward through the water, the water passing over the keel provides lift, just like the sails do, and pulls your yacht to windward. This cancels out the leeward slip and allows your yacht to move to windward. 

This system only works if you have enough speed of water passing over your keel. As you move slower, the keel is less effective as a hydrofoil and you begin to slip to leeward. If you are trying to pinch, then you will begin to loose speed and will slip to leeward. Obviously, if you are pinching, it is because there is something to leeward that you are trying to avoid and slipping further to leeward would be deleterious to your navigational plan. Eventually, you will be forced to tack and sail away from this obstacle, only to tack back and clear the obstruction. All of this takes time, and if you are trying to get someplace as quickly as possible, this could be viewed as time wasted.

The alternative in this situation would be to motorsail just a bit as you pinch. The propeller will give you the speed you need to maintain the functionality of the keel as a hydrofoil while the sails pinch with the close apparent wind. Once you round your mark, you can then turn off the motor and fall to leeward as you resume sailing on a normal and relaxed pace. 

We have used this trick multiple times, when tacking would cost us around an hour of additional sailing time. This has saved us hours in our arrival and meant the difference between reaching our anchorage with sunlight versus having to anchor in unprotected waters for a night because we didn't make it in time.