There’s no better way to get the feel from a boat you’re not used to than to sail with only the mainsail. It minimizes the touch points you need to pay attention to and keeps your speed at a manageable level.
Your mainsail is important in the dynamics of the boat’s forward drive and sailing with it alone will help you isolate nuances of its effect. Different boat designs have an impact on what part the mainsail plays in the handling of the boat. For instance, fractional rigs rely more heavily on the mainsail than the headsail and its sailing characteristics are considerably different.
It’s important to understand that without a headsail, the boat will not “point” or sail well into the wind. So keep this in mind when choosing your course and area of sail. Don’t get into trouble in shallow water or areas where you need to head higher into the wind because the boat probably won’t help you out of that jam.
You will notice however that the boat sails downwind well. With the sheet eased well out the yacht will move considerably well. Again, reaching and closed hauled sailing will be slow and you may experience considerable weather helm.
Try different maneuvers like tacking and jibing. The boat may or may not tack well through the wind. Note its performance and characteristics when doing so. Practice turning in as tight a circle as possible. Know the radius will be beneficial in real life experiences like racing or maneuvering in a tight space with only the main.
The boat will luff up easily and it usually takes more momentum to tack than with both sails up. You may even stall the boat and that’s ok. Again, your purpose here is to learn.
Especially if you plan on racing, the skill of sailing under mainsail alone is extremely important. Pre-race maneuvering demands knowing how your mainsail will react in all points of sail and in confined areas. Its also just good seamanship to take in your headsail first when coming into dock or starting the engine so take some time to learn your boat under mainsail only – you won’t regret the effort.