MEASURING MULTIHULL PERFORMANCE
Ullman Sails squeezing every last drop from light and shifty breeze while racing in the Whitsunday Islands
"…as a species hard wired for optimism, we're prone to magical thinking, especially in the hands of marketing pros and advocacy scientists delivering theoretical solutions to complex (environmental) problems."
I didn't write the above. But I could have, although maybe not quite so eloquently. It's about the adoption of electrical power in marine craft and it was written by Aaron S Porter, editor of Boat Builder Magazine. The full article appears on page 3 of the just released issue No.195 February/March 2022.
Why do I care? I care about the environmental crisis as much as anyone on this planet. But there's a heck of a lot of hype out there about the degree to which the adoption of electric motors in cars, boats, and planes is going to save the environment, and there's a lot of avoiding the real damage that is being caused by the manufacture and eventual disposal of redundant solar panels and turbine blades, not to mention the massive cost and environmental damage being done by the infrastructure required to get the electrical current ashore and into the grid.
But this article is not about electric power or even the environment. It's about sail boat performance, and it has parallels with the "electric transformation" issue because it is a very complex issue that seems to have been boiled down into a simplistic solution, namely the production of polar diagrams that pop up in sailing magazines, brochures and other marketing material, with zero attempt to either qualify or verify the information that is being published.
That is not to say that all polar diagrams are intentionally misleading or exaggerated in their claims. What it does say is that many people in the boating community are being led to to believe there is a simple and reasonably accurate method for predicting sailing yacht performance that is represented in the polar diagrams we see published.
How do I know this? Quite simply because of the number of requests I get from potential customers wanting to see the polars before they purchase the plans for a particular design.
Prediction and Observation
Prediction and Observation are two are two very different things. Let's take prediction first.
Computer science has come a long way in the time I have been living on this planet, but there is no magic to the incredible advances in understanding of complex technologies we see evolving in front of eyes on an almost daily basis.
These advances are made by highly skilled, highly funded researchers and scientists working hard with the backing of universities, research institutions and large corporations.
With the exception of the America's Cup and a number of other well funded professional level yachting events the resources to accurately predict the performance of a sailing yacht in a range of sea states, a range of sail combinations, and a range of sailing angles, simply does not exist.
Not only does this not exist for any one class of yacht, but it certainly does not exist to allow for the variations in performance of individual boats in any given class.
If you doubt that this is true simply go and watch a fleet of one design yachts racing on any given Saturday afternoon and observe the variation in performance within the fleet.
The variation may be less in a fleet of one design dinghies where the design parameters are strictly governed. In a developmental class like A Class cats, the variations will be significantly greater, and for one off or custom designed yachts much greater again, especially when they are not all produced by the same boatyard.
Let's just take one very significant but rarely mentioned design feature that has a significant effect on performance, and that is wind resistance, a drag factor that can only be calculated with a high level of computational power and a thorough knowledge of fluid mechanics.
Its contribution to the overall drag varies with speed of boat and wind, frontal area, the drag profile, the sailing angle and disturbed flow resulting from pitching. But how often do you see it taken into account when making performance predictions?
2. Observation
If you have a significant number of boats of a given design and very little weight variation it is realistic to draw reliable Polar Performance Diagrams that apply to the whole class based on the known data collected from GPS or the instrumentation on the boats. This information is not all that useful for predicting the performance of a new design or an existing design where the exact weight can only be estimated.
Finding Solutions
Is there a way to make accurate polar diagrams? Yes, most certainly. It can be done with observation and recording of actual sailing performance on the water, preferably using GPS and professional or highly experienced sailors so that the results can be produced consistently. The more miles you sail, and the greater the variety of conditions you sail in, the greater will be the accuracy of the data produced.
So how much credibility to give to Polar Diagrams?
I'm not sure where this came from but I think it's fitting to the topic
"As the saying goes, if you have integrity, nothing else matters, and if you do not have integrity, nothing else matters."
If you are publishing polars and they are reasonably accurate and verifiable, then why not state how those polars were produced? If from computer software what input data was used and has it been verified on the water? If from real time data on the water then just how much data was collected and how? In particular was the boat weighed and if so under what loaded state and what was the weight?
These should not be difficult questions to answer if the data presented is real and authentic. These are the questions I suggest you should be asking if you see a set of polar diagrams published and you want to make a valid comparison with other vessels of that type.
See also this article Multihull Performance Factors