Join Date: Jan 2000
Thanked 0 Times in 0 Posts
Rep Power: 15
Performance Basics for Routing
Many years ago, it actually seems like another life, I was preparing to sail in the BOC Challenge (now called the Around Alone). This single-handed, around-the-world race demands a boat that is built with the strength of a Sherman tank, but has the speed and agility of a Formula-1 race car. Fulfilling both these needs in a sailboat is a true challenge, and in the process of integrating these two demands, there is also the overriding requirement of having to have the boat be fast on all points of sail.
I was able to design and build such a boat, though in the end the Sherman tank component, i.e. strength, fell short of our expectations. What our boat did do extremely well was go fast. In conditions where winds were abaft the beam we could sail at the speed of the true wind. Yes, in 10 knots of true wind our boat speed was 10 knots, and in 30 knots of wind we sailed at 30 knots!Sailing at 30 knots is a feeling that is almost beyond my ability to describe. First there is an underlying "hummmm" as the boat's hull skims across the water, and there is no wake, just a trail of flat, white water streaming toward the horizon. Standing on the transom and looking astern you see water peeling off the hull and swooshing past. There is no relative wind and thus no sound other than water moving at high speed. This feeling is almost like an out-of-body experience, and it leads me to believe that there is an ingrained desire and need in humans to have motion, and to go fast.
So, how do you achieve a fast boat? Well you keep wetted surface, i.e. the amount of the hull touching the water, at an absolute minimum. Why? Because every inch of a hull that touches the water induces friction and friction means drag, a loss of speed. Minimum wetted surface, as naval architects say, is our goal. Next, we must keep the boat vertical at all times when sailing. Why? Because when a boat heels its projected sail area is reduced. Less sail area means that less force being applied to the sails and so less power results and uiltimately less speed. Ask yourself why multihulls go so fast? They have little wetted surface and they stay more upright, so there's less heeling and consquently less loss of power from the sails.
Most large monohulls stay upright by virtue of their beam and ballast. Given identical ballast, a beamy boat stays more upright while a narrow boat will heel over easily. Additionally, we can use movable ballast, such as water, to keep a boat upright. By adding port and starboard water-ballast tanks we can shift water as we tack. A little aside is needed here: we can also add fore and aft water-ballast tanks, which will allow trimming by the bow and stern. Why do this? Well if you are sailing downwind in light air and want to reduce wetted surface, and thus reduce friction, you can trim the bow down, which raises the stern. And since on most boats there is more hull in the water aft of amidships, you will now see an increase in speed. If you watch small-boat sailors racing, you'll see that they always attempt to reduce immersed hull area when sailing in light winds, where friction significantly reduces speed.Now, in contrast to light-wind conditions, where friction slows a boat, in strong winds it is a monohull's displacement that limits its speed. As speed increases on a monohull it immersed hull pushes an increasing amount of water. When hull speed is reached (approximately 1.2 x the square root of the waterline length) the wall of water in front of the boat prevents any additional gain in speed. So how do we increase speed? The solution is to move the boat up onto a plane, in the same way a powerboat does. Sailboats planing? Yes. Most modern sailboat hull designs incorporate a V-shaped forefoot (area of the hull forward of the keel) to part the water and reduce pounding, and a flat underbody aft to promote planing.
An excellent example of this design concept is the J/105, which easily moves from displacement sailing to planing. I sailed back from Bermuda to New England on a J/105 a few years ago and we spent most the four-day voyage surfing along under spinnaker and main, establishing an average speed far above normal displacement hull speed. An advantage of planing, in addition to the pure emotional satisfaction of making 12 knots on a 32-foot boat, is that you are moving faster than many waves. So what? Well, by sailing faster than waves your vessel is more stable, your comfort is improved, and it is a safer condition as waves are not catching up to you and breaking over your stern. (For calculation purposes, waves move at three times their period, so waves with a six-second period are moving at 18 knots).So how do you quantify boat performance? Well numerical presentations of a boat's capability, based upon its hull and keel design, sail inventory, ability to plane, and its weights are called a boat's polars. This term, polars, is used because data is displayed in a polar format, i.e. using a 360-degree true wind graph with boat speed displayed as a function of wind speed and wind angle, often shown in increments of one knot. Polars are viewed both in printed format and used within weather and routing software programs. A premier performance program called OCEAN, which is used for many offshore, long-distance events uses wind values from 0 to 80 knots, in one-knot increments. This precision in measuring a boat's performance allows the determination of highly accurate routes.
How do you determine your boat's designed performance? US Sailing (PO Box 1260, Portsmouth RI 02871, 401-683-0800, www.ussailing.org) has approximately 700 measured boats on record from which they are able to produce Performance Predictions. These boats are drawn primarily from the North American IMS and AMERICAP Fleets. US SAILING also publishes a Performance Package that includes polars in addition to stability characteristics, instructions, and explanations on the content. These can be custom ordered to ensure that the data is matched with each individual boat. The cost of each Performance Package is $170 (for members) and may be ordered online at www.ussailing.org, or by calling 1-800-US-SAIL-1. For more information contact Daniel DeWindt at US Sailings Offshore Tech. Assistant at 401-683-0800.
So, as you hone your sailing skills in either cruising or racing, always know your boat's polars, and gauge your performance and your abilities, against these values!
Suggested Reading List
- Using a Weather Service on Your Next Passage by Michael Carr
- A Navigation System for Your Notebook Computer by Jim Sexton
- SailNet Buying Guide - Autopilots