Optimizing Performance

Until now, the speed of most, if not all, racing yachts has been measured using the classic paddlewheel impellor.   This technology that has been around for 50 or more years has not kept pace with other advancements being adopted in the performance sailing arena.   Prior to the Nortek/Puma DVL, sailors made do with existing technology coupled with GPS, and the expertise of individual navigators and tacticians aboard the boats. While GPS has many benefits and it has enabled much more precise navigation, information about boat speed is relative to the ground, or Earth referenced. In the presence of ocean currents, the GPS does not provide an accurate measurement of boat speed through the water.

Traditional paddlewheel technology has stood the test of time. This is probably because most sailboats are relatively slow moving and a paddlewheel’s performance characteristics have been adequate. Paddlewheel sensors also happen to be affordable. High performance yacht racing has evolved, and achieving optimum performance demands precise and rugged instruments and more sophisticated approaches to processing these new data products.  A closer look shows that paddlewheels are deficient on several levels:

• They are known to wear over time which causes performance characteristics to change.
• They protrude from hull and the delicacy of sensor leave them vulnerable to debris in the ocean and bio fouling.
• Their performance at high speeds cannot be calibrated and therefore uncertain at high speeds.
• They operate in the boundary layer of the flow against the hull.

• The boundary layer flow is highly affected on a Volvo 70 by the canting keel, twin daggerboards and shallow hull shape, leading to distorted results.

• They can become airborne when installed on planning hulls, like a Volvo 70.
• They cannot measure leeway or ambient current speed and direction.

A speed sensor that is accurate over a complete velocity range, robust in all conditions, and that measures far-field water velocity has been the “boat speed holy grail” for navigators for some time.  Obviously, knowing your true boat speed through the water is critical to optimizing overall boat performance.   There are, however, secondary benefits to accurate velocity that are worth detailing.

The performance of all racing yachts is referenced to what is called the “Wind Triangle”.  The wind triangle is the vector relationship between measured wind, boat velocity, and the true wind vector (speed and direction).  The true wind velocity is the reference for performance.

The "wind triangle" which provide the true wind speed and velocity as derived from the apparent wind velocity  and the boat velocity.

Errors in either the apparent wind or the boat velocity introduce error bias in the estimate of the true wind speed and direction.   Correct true wind velocity estimates are important as sailors try to understand wind from a tactical perspective.   It also aids in sailing a boat to its optimum capabilities.  These speeds are known as the “target speeds” and boats will have target boat speeds for true wind speed and direction programmed into their onboard computers. Knowing if you are “hitting your targets” is critical to optimizing performance – a major objective in yacht racing.

A quantity/parameter that has never been directly measured until now is leeway, or the speed at which a boat slides perpendicular to its heading.  Without going into the physics of leeway, we generally want to minimize this effect without having to compromise forward speed in the process.  It is complex because it changes with heel angle, boat speed, sea state, and the position of the canting keel and daggerboards.  Historically, it has been very difficult to accurately measure leeway since the magnitudes are generally quite small.  The manner in which leeway has typically been reported has been a guestimate based on the boat’s heel and other characteristics.  A very good navigator can estimate this on a good day, as for the remaining days it is a bit of guess work.

For the Volvo 70’s, the leeway angle (angle created by the vector components of leeway and forward boat speed) is just a few degrees.  Moreover, boats that have canting keels and adjustable daggerboards, like the Volvo 70’s, have several factors that play into the leeway.  For example, reducing leeway by lowering a daggerboard improves the gains against the wind, however this will also increase drag and therefore reduce boat speed.  Understanding this relationship is not easily discerned and having instrumentation that can gauge this relationship is an advantage.

A DVL, which can both measure forward boat speed and leeway in real time, leads to an extremely valuable data product:  real time ocean currents.  The differencing of Earth referenced boat velocity (GPS speed and course over the ground) and the through-the-water boat velocity (DVL boats heading, speed and leeway through water) provides a direct, real estimate of the currents the boat is subject to in real time. Errors with either forward speed or leeway will manifest themselves as errors in the current estimate.  If there is any doubt of the value in resolving currents then one should consider that a mild current of 0.25 knots leads to a position displacement of 450 meters in the course of an hour.

Two step procedure to estimate currents in real time;  (a) First estimate the through the water velocity -- resultant of forward speed, leeway, and heading; (b) followed by differencing with the through the water boat speed and GPS estimate (speed and course over the ground).

What do you see when you have extracted the currents?