Since you asked here is the pre-edited version of the article I wrote for Sailing World a few years ago.

Strategy By Bob Merrick

Switching Boats With Success

In preparation for the 470 class Olympic trials, which were going to be held in St. Petersburg, Fla., in October 1999, Paul Foerster and I wanted to do as much racing as possible on site to increase our familiarity with the local sailing conditions. But with only a few 470 regattas scheduled for Tampa Bay, racing in the Lightning Midwinters, hosted by the St. Petersburg YC, seemed like a great way to help develop our local knowledge. During the regatta I often found myself ignoring small shifts in order to sail into a bit more wind, expecting a big gain in boat speed. This strategy works great in a 470 but didn't work very well in the heavier Lightning. It had been a while since I had sailed a boat other than a 470 and throughout the course of the Lightning regatta I was repeatedly reminded that the tactics that work so well in the 470 don’t directly translate to another boat.

The Midwinters did help our Olympic effort, but what I really took away from the event was a number of lessons in how to change classes effectively.

When changing classes, the main challenge facing a skipper is finding boat speed. Every boat requires different tuning and steering techniques. For a tactician the challenge is a bit subtler, but equally as important. The variables you encounter on the racecourse—windshifts, puffs, lulls, tide—won’t change when you switch boats. What will change is how you react to these variables and the order in which you consider changes in these variables when making tactical decisions.

There are three major performance characteristics that need to be considered when you are trying to make good tactical decisions in a new boat: the boat’s top speed, how quickly it tacks and jibes, and its tacking and jibing angles.

Boat speed

The top speed of your boat will determine the importance of sailing to the puffs. In a faster boat sailing to velocity is more often important than in a slower boat. For example, imagine that you are the tactician on a keelboat that reaches its hull speed of five knots in 12 knots of breeze. You are sailing up wind in 15 knots with room to tack. The boat sails into a five-degree header but you spot more wind ahead. What do you do? In the absence of other factors, you should tack. More wind is not going to make your boat go any faster—it’s already at hull speed—so the only advantage to be gained is by taking the shift. In this situation wind shifts are a much higher priority than wind velocity. In a different boat this could be completely reversed. Imagine the exact same situation, but while sailing on a fast catamaran. The speed advantage gained from an additional three knots of breeze could easily outweigh a five-degree wind shift.

This doesn’t mean, however, that wind shifts are always more important than puffs when sailing a keelboat. In three knots of wind, this keelboat would be well below hull speed and a three-knot increase in wind-speed could easily outweigh a five-degree shift.

These examples point out the importance of being familiar with your boat and what I’ll call its speed curve. The speed cure is the graphic representation of boat speed relative to wind speed. On some bigger boats with electronic instrumentation this information may be readily available during a race. A dinghy sailor needs to develop an intuitive feel for this curve.

(This is usually achieved through experience in a specific class, but being aware of the potential changes can be a big help when switching to a new boat.)

Tacking and Jibing Speed

One of the factors that we ignored in the previous example was the boat’s tacking speed. If you, as the tactician in the previous situation, predict that the five-degree header will only last for two minutes you will then have to weigh the potential gains against the expense of two tacks.

A light, high-performance dinghy will always tack more quickly than a heavy keelboat, but this doesn’t necessarily mean you always want to tack less in heavier boats than you would in dinghy. The amount of time your boat takes to tack and get back up to speed is only part of the equation. This needs to be measured against how much distance another boat could sail in a straight line while you’re completing the tack.

In light winds a heavy keelboat may take a long time to tack and may lose a lot of distance to boats that have not tacked. In this situation the tactician must be very selective in picking which shifts to tack on. In heavy air, the same keelboat will tack more quickly, and as the wind increases and the boats approach hull speed the distance lost to boats that don’t tack will decrease, making it easier for the keelboat to play the shifts.

A planning dinghy will experience an opposite scenario. A light dinghy will tack quickly and get back up to speed in a hurry. In light air the distance lost to tacking is minimal. In heavier air the boats will start to plane and while you may be able to tack and get up to speed more quickly, the boats that don’t tack will be moving even faster in a straight line and the loss in each tack will increase as it gets windier. A 49er may tack as fast as a Laser but the 49er will loose much more distance to its competitors because the boats are sailing faster. As a result the 49er will need a bigger wind shift to make tacking worthwhile.

Tacking and Jibing Angles

The third thing for a tactician to consider when switching boats is the tacking and jibing angles. A boat with broad tacking angles is more affected by wind shifts than a boat that sails closer to the wind. This concept is also true on the down wind legs. A boat that can sail closer to dead down wind is less affected by wind-shifts than a boat that must sail hotter angles. This principal is especially important downwind because there is a much greater variation in jibing angles from one boat to the next.

Consider this diagram. The circle represents one quarter of the compass. A and B are two different boats sailing up wind at different angles but the same speed. By breaking each vector into two components (one perpendicular to the wind, one parallel to the wind) we get a graphic representation of each boat’s speed toward the mark, or VMG in this case.

After a wind shift of fifteen degrees the two boats sail on courses represented by a and b. You can see by the diagram that B’s VMG towards the mark has increased more than A's. The theoretical explanation of why this happens is a little complicated, but it boils down to this; a lift will benefit a lower pointing boat more than it will a higher pointing boat. If you’re sailing a boat with a very broad tacking angle—or one that doesn’t point as well as the boat you usually sail on—it’s especially important to always sail on the lifted tack.

To consider how this principal applies to a downwind leg just change the wind direction 180 degrees. The boat than can sail closer to dead down wind is less affected by wind shifts than the higher sailing boat.
In addition boats that sail lower upwind angles and higher downwind angles will also build more leverage across the course compounding the benefits of staying on the lifted tack.

Jumping into a new boat is always a challenge. Paul and I found this out when we finished tenth at the Lightning Midwinters at a time when we were one of the best 470 teams in the world. Fortunately switching to a new boat does not mean that you will be starting from scratch. Everything that you have learned in your old boat will apply to the new one, only in a slightly different way. By comparing the performance characteristics of the new boat with the design you’re used to sailing you can anticipate some of the differences and speed up the learning process.

How to assess you boats performance characteristics.
-Figuring out the tacking angle of a new boat is fairly easy, simply note your upwind heading, tack and note your heading again. The tacking angle is the angle between the two headings. Notice that the angle may change with different wind velocities.
-A good measure of your boats tacking speed in a particular condition can be measured by its ability to lee-bow a similar boat. If you need to be far ahead of another boat to lee-bow without getting rolled your tacking speed is relatively slow. If you can be almost even with another boat and lee-bow your tacking speed is fast.
-By being aware of the puffs as they move down the course you can gain a sense of where your boat is on the speed curve. While sailing upwind and close to a similar boat pay attention to how close together you are. As the two boats sail into a puff note whether or not one boat makes a gain. Most often when two boats sail into a puff one boat will hit the puff first. If this boat makes a measurable gain chances are you are still on the steep side of the speed curve.

As an example, when switching from a 470 to a Lightning I would consider the following before racing.

In light air both boats will be well below top speed. The Lightning, like the 470, will respond well to puffs. Looking for the best wind will be a priority. Because the Lightning is about twice the weight of a 470 it will accelerate much slower in light wind causing it to have a slow tacking speed. I will have to be more conservative with tacks and jibes in the Lightning when the wind is light.
As it gets windier the Lightning's will become less responsive to puffs than the faster 470. At the same time the Lightning's tacking speed will improve. In windier conditions the focus in the Lightning will move towards taking advantage of smaller wind shifts. On the runs the Lightning's sailing angles are much lower than the 470's. Wind shifts on the runs will be less important in the Lightning. Other factors like fleet positioning should be my main focus.