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Old 17-05-2012, 09:40   #1
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How do I Achieve my VPP Speeds?

Try as I might I can't achieve my VPP speeds. The VPP calculation was done by the designer for my boat as made and equipped with an in-mast furler. I have new sails, a clean bottom, a feathering prop and am not overweight. Looking at other VPP figures for production boats I have the impression they are universally optimistic - is this the problem. Has anyone any suggestions as to where I need to look to try to achieve design performance. I don't think it is my trimming to blame as I have had others in charge who have also failed.

Thanks,
John
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Old 18-05-2012, 06:23   #2
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Re: How do I Achieve my VPP Speeds?

VPPs are just theoretical models. There are several, so which one you are using makes a difference. Then, most assume certain "ideal" situations--a perfectly fair hull, perfectly trimmed sails, perfectly flat water, and so on--which are always impossible to achieve in the real world. Different models are also geared towards different hull and keel shapes, so if the model you are using was written with a deep fin keel in mind, and your boat has a wing keel, the numbers will be off some.

Bottom line, you cannot expect to exactly match the numbers that come out of whatever VPP you use unless you can also exactly match the conditions that the designer programmed for (which you can't). Like comfort motion rating, and capsize screening, these are just theoretical numbers that you really shouldn't read too much into, nor take as gospel. They're somewhat useful for relative comparisons between similarly designed boats, but they mean fairly little out in the real world.
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Old 18-05-2012, 07:03   #3
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Re: How do I Achieve my VPP Speeds?

The numbers as far as I can tell are exactly for my boat's spec and even in flat water I can't get the speeds, so the VPP theoretical assumptions are actually very close if not exactly the same as the real world, yet it doesn't match. I wonder if it ever does match?
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Old 18-05-2012, 07:48   #4
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Your displacement must be the same as the VPP data. And the location of that mass. You bottom is probably assumed to be a hard smooth race bottom, not antifoul. The sails will need to be molded. No deck crap, dodgers, anchor rollers, etc. and then sailed to the numbers. You should be able to reach an honest VPP prediction.
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Old 18-05-2012, 16:33   #5
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Re: How do I Achieve my VPP Speeds?

What Daddle said, plus what sort of prop are you fitted with? The VPP may even assume no prop at all...

IMO achieving the full VPP speeds in a real world boat is unlikely.

Cheers,

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Old 18-05-2012, 19:36   #6
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Quote:
Originally Posted by poiu
The numbers as far as I can tell are exactly for my boat's spec and even in flat water I can't get the speeds, so the VPP theoretical assumptions are actually very close if not exactly the same as the real world, yet it doesn't match. I wonder if it ever does match?
How far off are your numbers?

The most enlightening thing I have done sailing was to go out with our top J24 sailor in two well prepared boats with full crews.

Our first test was a ddw spinnaker run with spinnaker only. This allowed us to remove as much variation as possible to test the boat speeds against each other. The boat speeds were close enough to identical.

Then we did some beats. And he beat us everytime.
Then I moved to his boat and his genny trimmer moved to our boat. With me trimming his boat we were still faster.
Then my skipper moved to his boat. So our boat had him and his trimmer. He beat us in our boat.

We had to conclude it is the skipper/main trimmer that is losing speed. Not much but enough to lose races.

Finally my skipper went on his boat, he trimmed his boat. His trtimmer helmed our boat and I trimmed our boat.

It was closest at that point, my skipper learned a lot about boat handling but we still never beat him in a regatta.

This guy has sailed the J24 worlds and is an excellent J24 driver.

The point being, I think I know how to sail and trim. But if I was really good I would probably be on the match racing circuit somewhere. I doubt I could really sail consistently to most polars. I am just not that good.
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Old 18-05-2012, 19:39   #7
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Re: How do I Achieve my VPP Speeds?

You must know the input conditions: e.g. sea state plays a great role - few cruising boats' VPP predictions are made for sea state relevant to input wind force, MOST are for flat water. Also, load counts: many VPP tables are made for light load. Etc.

I have found that boats tend to match their VPP data when they are sailed in conditions as declared in the input data. There may be differences but they tend to be minor and can vary either side of the mean. Once in the open water, you can make your own VPP for all your sail plans / loads / sea states, but I am not sure how important it is in the cruising aspect.

I have supported two Oysters on extended ocean passages (model 46 and model 72) and these delivered whatever they were supposed to deliver. They are not the fastest boats around but still produce respectable daily runs.

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Old 19-05-2012, 04:28   #8
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Re: How do I Achieve my VPP Speeds?

Thanks for the suggestions.

Jim Cate: I have a feathering max-prop and I'm pretty sure the VPP was done for cruising trim as I have also seen the VPP done off the drawing board and it was even more out.
Daddle. Yes, I do have a few of these go-slower things. Big dodger/bimini, a little flora/fauna on the hull and the finish is not hard and smooth as there is antifoul, some of it not perfect. I can't remember if my through hulls are flush or not. Certainly some are. I just looked at a picture of my hull and I see a few anodes on the shaft. A rope stripper and a few of the through hulls are not flush.
Ex-Calif. I would say I am about 1 to 1.5kts off across the board. At lower wind speeds it is worse. For example with 8kts at 90TWA I should get a log of 8.4kts, but I get maybe 6.5. At 14kts at 36TWA I should get 7.4kt, but I get 6.3kt at this and higher wind speeds I am 1 to 1.5kts off across the board. I am sure my trimming and helming is not perfect, but should only account for a small bit of the difference.
Barnakiel. Yes waves do slow me down even further, but not too much as she is a big heavy thing and cuts through the chop without much effect. and I expect the VPP was done for flat water
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Old 19-05-2012, 04:40   #9
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Re: How do I Achieve my VPP Speeds?

We are liveaboards,with all the weight and easy-on that entails and have found that on ocean passages 87% is right for us. Sure that we could do better!
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Old 19-05-2012, 09:27   #10
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Re: How do I Achieve my VPP Speeds?

Quote:
Originally Posted by poiu View Post

(...) I would say I am about 1 to 1.5kts off across the board. At lower wind speeds it is worse. For example with 8kts at 90TWA I should get a log of 8.4kts, but I get maybe 6.5. At 14kts at 36TWA I should get 7.4kt, but I get 6.3kt at this and higher wind speeds I am 1 to 1.5kts off across the board. I am sure my trimming and helming is not perfect, but should only account for a small bit of the difference.(...)
If the offset is larger in light winds it may indicate the boat is underpowered: not enough SA carried or the sails/boat not trimmed optimally. The other end of the same stick is the boat is sailed fine but she is too heavy.

Single factors (sails condition / trim, bottom condition, boat's weight, etc..) will add-up, hence you can see speed loss accumulate.

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Old 19-05-2012, 09:58   #11
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Re: How do I Achieve my VPP Speeds?

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...
The ORC VPP for instance keeps tuning scores of coefficients year after year... Seems like black magic at times...
http://www.orc.org/rules/ORC%20VPP%2...ion%202011.pdf
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Old 19-05-2012, 20:14   #12
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Quote:
Originally Posted by poiu
Thanks for the suggestions.

Jim Cate: I have a feathering max-prop and I'm pretty sure the VPP was done for cruising trim as I have also seen the VPP done off the drawing board and it was even more out.
Daddle. Yes, I do have a few of these go-slower things. Big dodger/bimini, a little flora/fauna on the hull and the finish is not hard and smooth as there is antifoul, some of it not perfect. I can't remember if my through hulls are flush or not. Certainly some are. I just looked at a picture of my hull and I see a few anodes on the shaft. A rope stripper and a few of the through hulls are not flush.
Ex-Calif. I would say I am about 1 to 1.5kts off across the board. At lower wind speeds it is worse. For example with 8kts at 90TWA I should get a log of 8.4kts, but I get maybe 6.5. At 14kts at 36TWA I should get 7.4kt, but I get 6.3kt at this and higher wind speeds I am 1 to 1.5kts off across the board. I am sure my trimming and helming is not perfect, but should only account for a small bit of the difference.
Barnakiel. Yes waves do slow me down even further, but not too much as she is a big heavy thing and cuts through the chop without much effect. and I expect the VPP was done for flat water
Faster than the wind reaching? Wow!

You could easily be giving up half a knot or more due to the bottom conditions you describe and the bimini drag. I easily see up to a knot with a dirty hull. I can "feel" on my face the wind, know that the boat should be going faster and sure enough when I dive the hull she is fouled.

As a racer I would say you have to eliminate the known stuff before you can continue the science project.

Also, those that say they know, say a wet sailed boat will be slower vs. a dry sailed boat. The J24 guy I described has never left his boat in the water.

And I am sure you have considered this but everything needs to be calibrated, log, anemometer etc.
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Old 19-05-2012, 21:05   #13
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Re: How do I Achieve my VPP Speeds?

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Originally Posted by Ex-Calif View Post
And I am sure you have considered this but everything needs to be calibrated, log, anemometer etc.
Yep, that's another question: just how does one calibrate one's anemometer? We just sorta assume that they are correct when taken out of the box, but I have my doubts.

And for that matter, does the VPP take into account the typical wind shear from masthead (where your anemometer sits 70 or 80 feet above the sea), or does it assume uniform laminar flow across the sails? And does it account for the real world turbulence that exists in air flow above a rough sea and perhaps not all that far from land? These factors will make a significant difference in the available energy to be collected by the sails.

Finally, if your hull is at all foul (the flora and fauna to which you refer) it will very definitely reduce your speed. While it is possible (though IMO unlikely) that the published VPP speeds are calculated in "cruising trim" (whatever that is), you can be sure that they don't include drag from any fouling whatsoever.

Again, I think it unlikely that you will achieve the VPP speeds in the real world.

Cheers,

Jim
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Old 20-05-2012, 07:21   #14
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Re: How do I Achieve my VPP Speeds?

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Originally Posted by Jim Cate View Post
Yep, that's another question: just how does one calibrate one's anemometer? We just sorta assume that they are correct when taken out of the box, but I have my doubts.

And for that matter, does the VPP take into account the typical wind shear from masthead (where your anemometer sits 70 or 80 feet above the sea), or does it assume uniform laminar flow across the sails? And does it account for the real world turbulence that exists in air flow above a rough sea and perhaps not all that far from land? These factors will make a significant difference in the available energy to be collected by the sails.

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

1) most cruising boats today have boom at or above 2 meters above the sea level and
2) most cruising boats use engine in light winds,

more details here: (source Wiki, wind gradient):

"...According to one source,[38] the wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface.[39] This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters[40] and with a statement by the Australian Government Bureau of Meterology[41] according to which differences can be as little as 5% in unstable air.[42]..."

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.

VP database can be easily built onboard any adequately instrumented boat that carries a laptop and a piece of software. I am not sure why a cruiser would care to build such a database but sure and why not this is one of the many ways to entertain oneself in a boat.

Cheers,
b.
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Old 20-05-2012, 07:54   #15
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Re: How do I Achieve my VPP Speeds?

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?
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