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CruisingWiki

Routing 4 of 7 : Boat characteristics & learning to Wally!

This is part 4 of the quest for the best optimal routing calculations within ECS/ECDIS systems today. In this posting I wanted to consider the use of polar diagrams and data that represents the performance of the boat in a variety of wind conditions.

Catalina30-Polar Of course wind cannot be taken in isolation when comparing your actual boat performance against any given polar diagram. The manufacturers polar diagram will tend to be an idealised version of what your boat will do at its design weight and under good sea conditions. The most likely source of polar table information is a velocity prediction program, or VPP: a computer program that uses information about the boat’s design—the hull lines, rig plan and so forth—to predict the yacht’s performance.

The image to the left for example, shows a pretty good polar diagram supplied for a Catalina 30 – and here you can see it notes that this was with a 150% genoa and the boat had a folding propeller.

Software designers sometimes incorporate other factors at this stage that will either automatically or manually take into account wave height, and other boat performance characteristics such as surfing down large waves for example. For a glimpse of the maths involved in calculating polar diagrams have a glance at this presentation from Pau Herrero, Luc Jaulin, Josep Vehi, & Miguel A. Sainz


Every aspect of the boats set up will influence the polar diagram, including the number if people on board the laden weight of the boat and the condition and configuration and the set of the sails.

SeaPro-polar For all these reasons it is essential to attempt to create your own polar diagram – it will still be an approximation in any given situation when you come to use routing software but at least it is your boat not the ideal boat as the designer intended.

The polar diagram contains all of the target performance data for your boat – how fast you should be going wind strengths and angles, as well as the optimum true wind angle and boat speed when beating and running. Good ECS/ECDIS software will give you VMG (Velocity Made Good) when beating or running, along with VMC (Velocity made towards your waypoint), which is very helpful when reaching.

Good routing systems will not only plot the polar diagram for you but may also give you trend analysis for the wing direction and speed. See the SeaPro screen to the right doing just that.

MaxSea - performancedatadisplay

The MaxSea documentation has this to say …”The Performance data display gives you constant updates on target wind angles and target boat speeds. Your performance efficiency is indicated graphically and numerically, so you can see how you are doing relative to your boat’s theoretical potential. This new feature gives you an easy-to-follow graphic display of wind and performance data, and barometric pressure. You can input this data manually, or have MaxSea log it automatically. ..”

Instead of showing just the route or CTS, the software should be able to draw laylines. Upwind and downwind laylines should be displayed on your chart, and constantly updated as the true wind direction shifts, letting you know graphically when it is time to tack or jibe.

Packages such as Advantage go a step further and try to show how the laylines drawn cannot or should not be straight lines since they are subject to the strength of tidal streams and therefore a starboard or port tack cannot be of equal duration or distance from the rumb line.

MaxSea - laylines1 Layline
MaxSea displaying straight laylines Advantage displaying curved laylines
According to the designers of Advantage Software …At shorter range, or where currents are important, our competitors don’t help you. They either ignore current, or use average values which make it impossible to determine the tactical advantage of one sequence of tacks over another. Using averages is equivalent to assuming uniform (unvarying) current, which implies that any sequence of tacks to the mark takes the same time. But when current varies, the optimal sequence can be minutes, sometimes many minutes, faster than another choice of tacks. This is critical information if you want to have the tactical advantage, and only Advantage can supply it. …In fact, without a proper treatment of current variations, it is impossible to compute a realistic, current-corrected layline, or estimate the time to either layline…”
Many sites – such as Sailmath resources and Deckman have ready made boat polars available for download – a good starting point.
The whole area of understanding what your instruments are really telling you and then how your boat will react, and how you may take a racing advantage is probably beyond the ken of most cruising sailors – who are happy just to see the tell tales flying nicely!…
But, if you want to delve a bit more closely and you do have one of the software packages listed in TableA or Table B above then after reading the manual you may also like to have a look at the OckamU web page…where you will learn that “Wallying gains distance when you can’t or shouldn’t tack….” …so now you know 🙂
…back to school…happy reading ….:-)

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6 comments to Routing 4 of 7 : Boat characteristics & learning to Wally!

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