Zirilli’s analysis confirms that the TP52 fleet (left/above) is even more refined and optimised than the very well-optimised Maxi72 fleet; given the TPs race boat for boat while the 72s race in a narrow IRC band this is to be expected. Rated against each other, the TP52 wins below 9kt TWS and generally above 15kt; in heavy air this superiority is built on the back of a lower displacement/sail area ratio that gives the smaller boat its planing ability downwind
and geographical characteristics as well as the wind speed itself).
VMG), the target numbers for TWS above 10kt are realistic, with errors of 0.15kt at worst. In general the boat is also now sailing in a slightly higher mode at the same VMG.
In Figure 4 there are also heel and rudder displays for the TP52 upwind. We can see here that real heel is above pre- dicted heel, with a maximum difference of 2° at around 10-12kt TWS. Rudder angle follows the trend of the VPP with a slightly larger peak at 10kt.
The right-hand graph in Figure 3 displays the downwind data. Again we find the largest difference between the real and VPP data is in the light 6kt TWS range, above which transitioning through 8-12kt TWS the numbers converge to within 5 seconds/mile. Finally, above 12kt our TP52 generally has a faster VMG than predicted – with a gap up to 20 sec/mile. This is the result of a faster/higher sailing mode where real boatspeed can exceed target speed by as much as 1kt. Summarising, it seems that our VPP and the actual data can be very different from
each other especially in light conditions. Moreover, our predicted VMGs are more accurate upwind than downwind. The next issue to look at is the different mast height of the two types of boat. RW: Yes, they sail in different breeze if you ask the ‘three scoops’? BZ: Correct, the TP52 has the anemo - meter at 25.2m above sea level while on the Maxi72 it is at 34.7m (wand height included). Indeed, the TWS reading at the top of a TP52 rig is nearly always different from the reading on the Maxi72, and so for an appropriate performance compari- son we must take into account the wind gradient – for which there are now some well-proven vertical profile models. The law commonly used is the ‘Power Law’ which relates windspeed and height and incorporates a coefficient known as the Hellman Exponent. (Many recent studies have focused on the Power Law exponent especially in the modern wind power industry; the coefficient mostly varies between 0.06 and 0.27 for offshore sites depending on various environmental
Fundamentally, therefore, the only way to compare performance of boats for which data has been measured by anemometers installed at different heights is to choose a reference altitude and use the correspondent TWS reference calculated by means of the Power Law for this elevation. Normally boat designers refer to ‘10m TWS’ – however, the reference alti- tude used here is 23m; we arrive at this number based upon the minimum height of a TP52 anemometer sailing upwind at the boat’s maximum normal heel angle. RW: Time to get to the core of the matter, Bruno. If I were a betting man, where do I put my money in the Maxi72 vs TP52 competition under IRC? BZ: The maximum 2015 IRC TCC for a TP52 is 1.412. As all TP52s are optimised to the class rule upper limits, I use that TCC for my calculations. In the Maxi72 class the TCCs vary slightly with each boat having its own characteristics. Selecting a TCC of 1.607 corresponds to the Maxi72 we will use as our model for data analysis. The reference race is a windward-leeward course, two laps, equally split 50-50 upwind and downwind for a total length of 9.6nm (results do not change for longer or shorter courses because our data is only a function of the upwind/downwind ratio). In order to equalise the data from the two different boats, the wind vertical profile is modelled by the Power Law already mentioned with TWS calculated for a height of 23m.
SEAHORSE 41
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INGRID ABERY
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