VMG targets for race yacht using VPP

Hi,

Have been browsing the forums for a while and found much usefull information.

My question concerns a boat I sail on. Wanting the extra knot, we recon a nice set of polar diagrams and a table of Up and Downwind VMG targets would help. Having done a bit of research I can't find the relevant data anywhere on the net (if anybody knows of such a database that would be good). So I've been looking at various software packages to produce them and have come across the PCSail spreadsheet. The boat is not high performance so a high degree of accuracy is not required but more a 'ballpark' figure. I understand that the spreadsheet calcs are based on the Delft algorithms. How accurate are these compared to the comercial VPPs outthere such as WinDesign, Span or TMSail (which looks like its based on the IMS VPP)? Can I trust the figures to a degree of certainty?

The second part of the question is related to the PCSail spreadsheet. The ideal would be to generate tables with a range of true wind speeds with the corresponding best apparent wind angles and VMGs for those winds. I can't figure out how to do this, is it possible or should I use a different program? Is it worth getting one of those doorstops by Claughton et al out of the library to help me understand? I have a basic knowledge of Nav Arch.

Cheers,

 

A VPP is no more than a program the calculates the forces on a boat and equates them.

The Delft Series is certainly sufficiently accurate for most things. The bigger problem is what to use for the sail forces. Hazen's method is probably sufficient, as presented in "Principles of Yacht Design".

The first thing I'd do is to contact whoever designed your boat and ask them for the polar. It will save you a lot of time finding information that is often very hard to get from a hull.

Tim B.

 

Creating Your Own Polars

Even before a grand prix racer hits the water, the engineers have plotted the polars. But then a great deal of time and effort is spent by the crew validating and calibrating those polars. Is that not correct? In the last Volvo Ocean Race, there was mention that some boats were still working to tweak their polars well into the ride.

So, what are the steps required to either modify an existing set of polars or to create a new set of polars?

With instrumentation capable of showing true wind angle, true wind speed and boatspeed, any boat should be able to establish a baseline set of polars. What is your experience?

 

Excellent article can be found here which was published in Seahorse International Sailing Magazine, Article p37-p39 Issue 306, August 2005.

I have been on a number of boats where the laminated VMG table in the cockpit has permant marker over it as some of the targets have been off. Most of this is due to applying generic targets for the class to a specific boat.

It's possible to collect data using software packages - I think our instrument package allows this. However, you then need to go through the data with a Stats package like SPSS to sort the good data from the bad.

Tim - thanks, have contacted the designer. Though I am doing this so as to learn a little about the process as well.

Anybody had any experience with PCSail then?

 

The University of Michigan developed PCSail. They were very helpful when I was doing the polars for my boat. Might want to ask them. I did it the hard way, ran PCSail to get the polar based on true wind speed then manually converted the chart to show the polars based on apparent wind. I used the apparent wind tool found at: http://www.sailingusa.info/true_wind_calculator.htm