(This is a long one. If you are interested in it, it might be easier to read if you print it out.)

Very often rounding up is a matter of sail trim more than mast rake. You did not mention the size or type of your boat. That kind of information is helpful in being more specific in giving advice.

I am assuming that you are sailing a small sloop. On small performance boats 15 knots often approaches the maximum wind speed that you can use a full mainsail and genoa. In wind conditions varying between 10 and 15 knots it becomes important to be able to ''shift gears'' quickly, in other words going from powered up to depowered quickly. Depowering is not reefing but reducing the heeling moment produced by the sail.

At ten knots, most small boats still can be quite powered up. Halyards and outhaul can be moderately tight up wind and slightly eased for reaching. The traveler might be on or above the centerline. The outhaul might be slightly eased. You might have a little backstay tension on up wind and be eased as you come off the wind. Jib sheet leads are set so that the telltales break evenly.

At fifteen knots, things change dramatically. Most small boats can no longer be powered up. To depower, halyards and outhaul are really tightened up wind and for reaching and only eased on a broad reach or run. The traveler car is dropped well below the centerline and the mainsheet really tightened upwind and the vang set tight as you ease off to blade out the mainsail. The outhaul is set up tightly except down wind. You carry a lot of backstay tension on upwind and close reaching and only ease when the wind is well past the aft of beam. (Backstay tension is especially effective in depowering a fractional rigged boat as the increased backstay tension tensions the luff of the jib and opens its leech as the induced mast bend flattens the leading edge of the mainsail and opens its leech up high as well.) Jib sheet leads are moved slightly aft so that the head of the jib spills off a little. In 15 knots of wind, you also need to be sure that you are not over trimming the sails (especially the mainsail).

Because you want to have your sails flat in a breeze older sails can be a problem. First of all, all sails stretch out in a breeze. In other words they try to get fuller. This stretching does a lot of things, but essentially it powers the sails up and increases heeling moments (the force trying to make the boat heel). Also as the sails stretch, the camber (curvature) in the sails moves aft and creates greater drag and more heeling moment. As sails age, the become more stretchy and they have a permanent increased camber depth (roundness) and the center of the camber moves aft. This makes blading out (depowering) all the harder.

Roller furling genoas are particularly a problem in this regard. First of all the sail cloth on smaller jibs want to be slightly heavier than the sail cloth used on genoas. So it is more stretchy than smaller sails. With a roller furling sail, the cloth is often selected for lighter wind conditions and so on a gusty day, even partially furled the jib tends to be a bit stretchier and therefore more prone to powering up. Even without the increased stretch, the geometry of a roller furled sail tends to increase the depth of camber of the sail and migrate the center of camber aft. This can be offset a bit by luff foams and the like BUT even with these prosthetics the sail tends to creep down from the head and up from the foot so that over time the partially furled sail powers itself up.

In shifty conditions, you need to set up somewhere between the two windspeeds- not overly bladed out in the light stuff or you will go painfully slow and not too powered up in the breeze.

When as you sail into the gust you need to do a number of things. First of all most gusts, simply because of their greater speed, will move the apparent wind more abeam of the boat. So the first thing you can do is allow the boat to turn slightly toward the wind (Feather up). This is often a little tricky on a gusty day because often, just before the gust hits, there is a small lull. This lull looks like a header (the apparent wind moves forward making you want to turn down to avoid luffing) so the impact of the gust is even greater. So let the boat head up slightly before trying to pull the boat back to course. If you are broad reaching turn down slightly as the wind first starts to build to "burn off" the gust by letting the boat accelerate down wind.

If you are sailing in to bigger patch of higher winds then you definitely need to shift gears. First step upwind is to lower the traveler to leeward and apply more mainsheet. Then on a fractional rigged boat apply more backstay tension. If you are reaching, apply a lot more vang and ease the mainsheet some. This is called vang sheeting) Make sure that your sails are not over trimmed. Check that the telltales on the leech of the mainsail and jib are flowing aft and that the luff telltales on the jib are both flowing aft or that the inside (weather) jib telltale is slightly lifted or jumping.

On some small boats you may need to ease the mainsail out so that it carries a slight bubble. This is not as slow as rounding up or dragging your turned rudder through the water. When you have the rudder far over, many small rudders will develop so much low pressure on the low-pressure side of the blade that the rudder will literally suck air down the blade. (This is often called cavitation but it is really not cavitation in the true sense. It is actually aeration but that is a fine point.) Once the rudder has aerated, it produces less turning force and more drag. It is often helpful to push the helm toward the center of the boat for a second until it hooks up and then to slowly pull the rudder to windward loading up the blade again. This is particularly effective in flat water.

I cannot emphasize enough that the key single thing here is to make sure that your sails are not over trimmed (too far) and too powered up (Too much curvature in the sails) as the wind builds.

Have fun out there playing with this stuff.
Regards
Jeff