How do I Achieve my VPP Speeds?

[barnakiel]

Originally Posted by Paul L

(...) I believe that the wind speed is spec'd at 33ft height. (...)

In wx / meteo aspect yes. But a VPP may simply take an 'input wind force'.

b.

[Jim Cate]

Originally Posted by barnakiel

I would add that the gradient factor may be grossly overrated in the cruising lore:


Wind instruments are assumed accurate out of the box and as long as the sensor is in good condition. Hence the wind data is mostly influenced by quality of COG and SOG input.

Cheers,
b.

G'Day Barnie,

IIRC, Practical Sailor did an evaluation of sailing instruments some years ago, and I was interested then to note that they said that Standard Horizon wind instruments (and knotmeters) were set to read 20% high of reality. I had had two different SH wind instruments, and had noted that they seemed to read high compared to my amateurish Beaufort estimates, and compared to other nearby anemometers. I could not find a useful way to calibrate that instrument in situ then, nor have I since -- hence my query.

The info about gradient is interesting and somewhat in contrast to old ideas, but certainly plausible. But my point about turbulence is, I think, still valid. One needs but to watch the masthead fly (with the boat at rest) for a few minutes to become convinced that the wind direction at that altitude is constantly varying. Similarly, the velocity changes rapidly and often.

To me, this says that any VPP that assumes a constant wind vector across a sail is not reflecting a realistic situation, and that a failure to meet its predicted boat speeds is not surprising.

All that said, in a non-racing environment it seems fairly unimportant! But interesting, none the less!

Cheers,

Jim

[poiu]

Originally Posted by Paul L

I think ExCalif is on the right mark here. Very few of us can drive or trim sails to the 100% mark that a top driver and top sailmaker can. I believe that the wind speed is spec'd at 33ft height. Also the instruments need to be carefully calibrated as they tend to be off the most at low speeds.
Are your less than stellar results the same on both tacks?

No, one tack is better slightly.

If what you are saying is the VPP reference wind speeds are the speeds measured at 33ft mast height - I wonder how much difference that would make. My masthead unit is at 89ft; I would have thought if it is measured like that then as most of the wind gradient is low down it can't add more than a kt or so, which will account for just a small part of the speed difference.

I wonder if a way to go in trying to improve things might be to do away with antifoul and to fit an ultrasonic hull cleaner. It is really hard to keep a super clean hull. A month away and the bottom is like a coral reef again. A whole new set of issues there though.

[barnakiel]

BTW

I am not sure it was Panbo or elsewhere but there is a market-wide wind instrument review somewhere on the web (unless I read it in one of the magazines). I think the spread was maybe max 10% hence the instruments are pretty single-minded.

A very neat way to calibrate one is to place on a car on a calm day then go for a ride.

There is a newish sensor (the no-moving-parts type) that corrects the wind data for roll pitch and other such nuisances. It is fine for VP games as well as for downwind sailing under AP.

b.

[Ex-Calif]

Originally Posted by poiu

No, one tack is better slightly.

If what you are saying is the VPP reference wind speeds are the speeds measured at 33ft mast height - I wonder how much difference that would make. My masthead unit is at 89ft; I would have thought if it is measured like that then as most of the wind gradient is low down it can't add more than a kt or so, which will account for just a small part of the speed difference.

I wonder if a way to go in trying to improve things might be to do away with antifoul and to fit an ultrasonic hull cleaner. It is really hard to keep a super clean hull. A month away and the bottom is like a coral reef again. A whole new set of issues there though.

My boat sails faster on Starboard. It's annoying and something I need to chase down next haul out. The worst thing is it is influencing my course management, I tend to go right, and do short port tacks. Sometimes left is where ya gotta go...

Great conversation but it sounds like you have a big cruising boat with a fouled bottom and a theoretical VPP plot. Get the bottom squared away before you start messing with rigging. If you have a 89 foot stick you have a lot of bottom area to induce drag. Easily a knot and a half.

[daddle]

For accurate wind speed and angle the sensor needs to be far upwind and above the sails. Perhaps a meter on large yachts. The wind is accelerated and twisted near the sails.

Instruments can be calibrated on an automobile on a windless day. Again, at least a meter above the auto body.

Speed sensors must be on centerline. That is rare. Race boats often have P and S sensors and a healing switch. The difference tack to tack can be considerable on a performance hull. Again the water is accelerated near the hull.

[Jim Cate]

Using an automotive speedometer to calibrate one's anemometer may be an option prior to installation, but a bit of a nuisance if the instrument is there in place in your boat. But yeah, if you had thought of that in a timely fashion, it would work ok!

And Daddle is right about the sensor needing to be well away from the upwash flow from your sails... something seldom seen on cruising boats.

Cheers,

Jim

[denverd0n]

Originally Posted by sailorF54

There is no such thing as 'A' VPP. Some VP programs are geared towards certain kinds of boats, some use newer coeffs to estimate sail forces or wave resistance...

Exactly! VPPs are just computer models. There are a bunch of them, written by different programmers, designed by different designers. None are any better than they guys who wrote the code (probably summer interns), or the guys who designed the parameters and algorithms.

As someone who spent ten years working in the development of computer models, trust me, you don't want to put too much faith in them!

[Paul L]

Originally Posted by denverd0n

Exactly! VPPs are just computer models. There are a bunch of them, written by different programmers, designed by different designers. None are any better than they guys who wrote the code (probably summer interns), or the guys who designed the parameters and algorithms.

As someone who spent ten years working in the development of computer models, trust me, you don't want to put too much faith in them!

In this case I don't think the OP is using VPPs generated by some model he found. But using the VPPs as delivered from the designer/builder. They should be the gold standard o what to shoot for. As noted above, very few of us will be able to sail to our VPPs or, for that matter, to a boats racing rating.

[barnakiel]

If the bottom is like a coral reef then I would start just there. Before the boat is fully powered up hull drag has paramount influence on speed.

b.

[denverd0n]

Originally Posted by Paul L

In this case I don't think the OP is using VPPs generated by some model he found. But using the VPPs as delivered from the designer/builder.

Yeah, and the builder almost certainly just bought that software from someplace, not at all unlike you would go and buy some software off the shelf at Wal-Mart.

Again, don't put too much faith in computer models. They are just someone's "best guess" about how things might work.

[barnakiel]

Originally Posted by denverd0n

(...) They are just someone's "best guess" about how things might work.

They are educated guesses too: that someone who designed the algorithms was most often someone with in-depth understanding of boat's dynamics!

b.

[Paul L]

Originally Posted by denverd0n

Yeah, and the builder almost certainly just bought that software from someplace, not at all unlike you would go and buy some software off the shelf at Wal-Mart.

Again, don't put too much faith in computer models. They are just someone's "best guess" about how things might work.

This is a large boat. The designer didn't just pull numbers out of his a**.

[denverd0n]

Originally Posted by Paul L

This is a large boat. The designer didn't just pull numbers out of his a**.

Interesting comment, but since no one said that he did--or anything even remotely like that--one has to wonder what the point is supposed to be.

All I have said is, don't read too much into the numbers that come out of a computer model. If, however, you have a burning desire to read too much into them, by all means, be my guest.

[Paul L]

Originally Posted by denverd0n

Interesting comment, but since no one said that he did--or anything even remotely like that--one has to wonder what the point is supposed to be.

All I have said is, don't read too much into the numbers that come out of a computer model. If, however, you have a burning desire to read too much into them, by all means, be my guest.

while I agree about not reading too much into the numbers that come out of a computer model, I don't believe that is the case here. If the numbers came from the designer then the designer used the model and made adjustments to get to where he felt the vpp is very close to his design. The point is that it is very difficult to set a boat up and sail it to its rating or its vpp. But blaming bad numbers as your the reason you can't is probably not constructive. I know I can't sail my boat to its vpp or ratings, but I'm just not a good enough driver or trimmer.