- Apparent Wind and How to Figure It Out ( the answer to the bunny saga)
Ok, so it’s time to review Mr. and Mrs. Bunny’s sailing adventure. If you remember Mr. Bunny wanted to take Mrs. Bunny for a day of sailing. He didn’t want to have to do any work, if you recall. The reality is that he really just wanted to get Mrs. Bunny on the bow of the boat in her new daring naughty bunny bikini. He just had Bunny tail on his mind. He needed our input to see if he could leave the dock, set up for a beam reach in perfect conditions (no current and no leeway), sail for a couple of hours, tack and return back to the dock on another beam reach and not have to sail any other point of sail other than a beam reach.
Apparent wind is more important than you might think
He got tired of waiting for my answer and told Mrs. Bunny that I said that he was correct and the day was going to unfold just as he said. Then he gave her $75.00 for a bunny wax. Well, had he called Tousey, a close friend and sailing buddy of mine, he would have gotten a timely answer to his question and been able to come up with a less risky scheme.
As Tousey figured out, the answer is NO. He cannot sail out on a beam reach, tack, sail back on a beam reach, AND wind up back where he started.
So here’s why. To illustrate the reason for his miserable failure let’s make up a realistic example. Here’s the facts:
- They are sailing on a Catalina 350
- When they clear the Vinoy Harbor they set sail on a beam reach
- They have the auto-pilot steering the boat to avoid any steering errors
- They sailed outbound for 1 ½ hours
- The apparent wind is 12 knots
- The boat is traveling at 6 knots
- There is no current
- The waves are one foot and not a factor
- They were sailing on a course of 090 degrees
- The apparent wind was from 000 degrees
- They needed to return to the dock on a course of 270 degrees
- Bunny was looking really hot
Now, I’m not a mathematician so, my explanations may not be perfect. But, a boat’s speed and direction can be represented by something called a vector. A vector is a line segment that’s length represents the velocity (speed) of an object or force and that’s direction represents the direction of travel of the object or force. An arrowhead is placed on the end of the line segment indicating which direction the travel is in. For example, if the boat is traveling in the direction of 090 or 270 degrees the line segment would be the same. It is the arrowhead that indicates which direction the boat is going.
There is another critical concept we need to understand. We are discussing wind and its impact on the boat. If the boat was moving and there was absolutely no wind, we would still feel wind as the boat moved through the water. That wind is the apparent wind made by the boat’s movement. We would feel that wind in the exact opposite direction and velocity than the boat is moving. It is that wind force that we are using along with the true wind to make our analysis. So, the vector representing that wind velocity and direction is drawn in the direction that it is felt—180 degrees from the boat’s velocity and direction. In the Bunny’s case we will use a vector to represent the wind created by the boat (BAWD & BAWS), independent of the true wind (TWD & TWS). (see figure 1)
Planning for apparent wind vector
It is possible to add vectors together. The sum of which is the combined velocity and direction of the two vectors that were added. So, if we add the boat’s wind vector and the true wind’s vector we can obtain the apparent wind’s vector (AWD & AWS). In fact, as long as any two of the vectors are know the third vector can be determined.
Vectors are added by drawing them with both sharing a common starting point. From that common point each vector is drawn utilizing the information known about the vector (length and direction.) With the two vectors drawn a parallelogram (a four sided object with opposite side parallel) can be completed by redrawing each vector from the other’s arrowhead. (see figure 2) Once the parallelogram is completed the sum of the vectors is determined by drawing a final vector from the initial common starting point to the opposite corner.
In the Bunny’s case we already know the apparent wind speed and direction (090 and 12 knots), so the missing link is the true wind speed and direction. To find the true wind vector, draw the two vectors that we know—Apparent wind and Boat’s apparent wind. Next, draw another vector from each of the existing vectors arrowhead, the first of which will connect the two existing arrowheads and the second of which will be parallel to its opposite and extend in the opposite direction. Finally complete the parallelogram by adding a final vector on the remaining open side. The final vector is the true wind vector, representing the true wind speed and direction (TWS & TWD.) (figure 2)
So, back to the problem at hand. Mr. Bunny got his bride, Mrs. Bunny, to go on the trip by telling her that it would be beam reaching the whole way, that she wouldn’t have to do any work, and it would be a great way to spend a frisky bunny hopping day. And, so they set out. All was going as planned. Mr. Bunny got the boat setup on a beam reach and they sailed at six knots for an hour and a half. The apparent wind was a perfect 12 knots.
After about fifteen minutes Mrs. Bunny came up from below exhibiting her new naught bunny bikini and Mr. Bunny thought the 75 bucks he spent for the bunny wax job was the best money he ever spent. The naughty bunny bikini certainly showed it off and there was no question that there was going to be a nearly all bunny tan hopping around the den later that night. In what seemed like no time they were at the shallow water off Apollo Beach, due East of the Pier in St Pete. Mr. Bunny called to the bow for Mrs. Bunny to lay flat while he gybed around to head back to the Vinoy. Mrs. Bunny asked if she should come back to the cockpit to which Mr. Bunny replied, “No, baby it will be a smooth beam reach back, just stay put.”
And, so she did. When the gybe was complete Mr. Bunny was a little rattled when he had to trim in the sails so much. Before he could say “Holy carrots” the boat was heeled and starting to pound in the waves now striking the bow. He watched in horror as Mrs. Bunny started sliding off the bow toward the leeward lifelines. Thank goodness for the stanchions, her tail had just caught on one and it saved her from falling off the boat.
Acting quickly and trying to save the day he fell off the wind and back onto a beam reach. He quickly realized that he was now heading, at six knots, directly at the shoals of Ruskin Inlet. That wouldn’t work and so he turned back to the original plan of 270 degrees. Once there he found he was schlepping along at 5.2 knots and on a very close reach into 16.4 knots of wind. Mrs. Bunny had made her way back to the cockpit, put on a cover-up, and was visibly bunny hopping mad! Nearly two hours later they pulled back into the Vinoy where Mrs. Bunny hopped off the boat and caught a cab.
Apparent wind can get any-bunny hopping mad
So, what happened and why? Well by taking what we know from figure 2 we can complete a new parallelogram showing the return trip (see figure 3).
We can see that Mr. Bunny was doomed from the start. Looking at figure 4 we see that if there had been a clear sailing path to sail the course that figure 4
prescribes they would have been heading to the Manatee River and may never have gotten back.
By the way, I heard that Mrs. Bunny has since divorced Mr. Bunny and has been hanging out with some guy named Bugs. I hear that this guy, Bugs, has a big powerboat and is always playing hippty-hoppity music onboard. And, that Mrs. Bunny’s near all bunny tan is now an all bunny tan. Can you say high maintenance?
I created these drawings using a CAD program, that’s why this article took so long to get printed; I had to learn the damn program. But, you can work these problems out just as well with about $20.00 worth of tools. It doesn’t have to fancy, it just has to work.
Planning is everything and planning takes understanding. Understanding requires knowledge and knowledge takes learning. Come learn with me, On the Water With Captain Frank.