Sign in to follow this  
Itsabimmerthing

ORC inclining test method

Recommended Posts

My boat would need around 400 lbs of weights to be suspended if the boats spinnaker pole is used, secured to a stanchion. For me that sounds a lot..

Should the test be done by suspending the weights at the end of the boom instead? are there any downsides to using this alternative method?

Looking for the best possible rating of course but would not like to brake anything while getting there.

Share this post


Link to post
Share on other sites
4 minutes ago, Itsabimmerthing said:

My boat would need around 400 lbs of weights to be suspended if the boats spinnaker pole is used, secured to a stanchion. For me that sounds a lot..

Should the test be done by suspending the weights at the end of the boom instead? are there any downsides to using this alternative method?

Looking for the best possible rating of course but would not like to brake anything while getting there.

how big is you boat ?....the end of pole is supported by a halyard  correct?....not a lot of load really

Share this post


Link to post
Share on other sites

45' C/R

Other reason I ask is because I have slightly beefier (heavier) main boom than the standard. To account for that the boom would need to be brought off centerline to have any effect on GZ.

Share this post


Link to post
Share on other sites

400lb on a halyard pushed out by a spin pole is not much load on the pole or your stanchion base. Especially considering the set up is slowly loaded up as you add additional weight.

For the measurement it is critical to know the lever arm length. The pole length to the stanchion is easy to measure and repeatable on either side. Using the boom, the lever arm may not be as long and unlikely perpendicular to the centreline of the boat, therefore harder to get the exact lever length.

Using a pole is a standard method that works.

Share this post


Link to post
Share on other sites

My guy stuck the inboard end of the pole into his shoe against the topsides using a sail tie to hold it in place.  A halyard and a foreguy and afterguy held the outboard end in position. 500 pounds, no problem!

Share this post


Link to post
Share on other sites

different measurers do it differently.

i have always wondered about the repeatability of these measurements...

Share this post


Link to post
Share on other sites

Leave as much weight as high as possible in the boat.

Bone on runners and backstay. Run halyards with full covers and oversize. Reeflines in the boom.

Weight up high is your friend. 

Share this post


Link to post
Share on other sites
Just now, Soley said:

Leave as much weight as high as possible in the boat.

Bone on runners and backstay. Run halyards with full covers and oversize. Reeflines in the boom.

Weight up high is your friend. 

tie fenders to the bottom of the keel

  • Like 1

Share this post


Link to post
Share on other sites

Same here,

 

I wonder how to make sure that measurements are done in a way that is as fair as possible as a tiny inaccuracy in weights or freeboard etc.. would give very wrong results.

I kind of do this for a living on big ships and have some understanding about stablity.

Share this post


Link to post
Share on other sites
1 hour ago, Itsabimmerthing said:

I kind of do this for a living on big ships and have some understanding about stablity.

Wow, big ships must be a bitch to incline! :lol:

Share this post


Link to post
Share on other sites
1 hour ago, Itsabimmerthing said:

Same here,

 

I wonder how to make sure that measurements are done in a way that is as fair as possible as a tiny inaccuracy in weights or freeboard etc.. would give very wrong results.

I kind of do this for a living on big ships and have some understanding about stablity.

In a practical sense it is voodoo physics.

Had the same boat inclined 4 times with out changes and widely different results each time.

No result repeated once.

Not counting the time the Rating authority used the wrong input data for the aft freeboard.

Share this post


Link to post
Share on other sites

I do this for a living sometimes too (moderate size workboats of a few hundred tons). We often use 4 weights of a few tons each.

We get very repeatable results. Typically we use a pendulum of about 3m long to measure very small angles (~2° heel) very accurately.

 

Share this post


Link to post
Share on other sites

I’m an ORC measurer, and it’s allowed to hang the weigh from the boom in larger boats. Just check the measurement guide in the orc website.

But, to be honest, I really think that the ORC is not reliable enough, due to lack of measurement repeatability.  The software works pretty good, but the real problem is in the measurement process.

It’s very difficult to match the results of 2 inclining tests in different days, and of course the freeboards/weight are a pain in the ass. 1 single centimeter on a 40 feet cruiser racer (IMX 40, for example), means more than 200kg in the final weight. 

 

 

Share this post


Link to post
Share on other sites
1 hour ago, silex said:

I’m an ORC measurer, and it’s allowed to hang the weigh from the boom in larger boats. Just check the measurement guide in the orc website.

But, to be honest, I really think that the ORC is not reliable enough, due to lack of measurement repeatability.  The software works pretty good, but the real problem is in the measurement process.

It’s very difficult to match the results of 2 inclining tests in different days, and of course the freeboards/weight are a pain in the ass. 1 single centimeter on a 40 feet cruiser racer (IMX 40, for example), means more than 200kg in the final weight. 

 

 

Measurement is hard, and imprecise. I have a lot of sympathy for measurers, tough job. 

It's ridiculous that ORCi doesn't reference actual weights as recorded on endorsed IRC certificates when that info is available.  Actual weight can be used to validate freeboard measurements and identify potential measurement inconsistencies.  I once saw a 40 footer with a 15% (600kg) displacement difference between its ORCi and IRC certificates, and the local measurement authority didn't see a problem with that... they don't bother using this type of data comparison to assess the quality of measurements.  And they wonder why so many owners have little time for measurement-based handicap racing?  

If the actual boat weight and position of CG is known (LCG and VCG, assuming it's on the centreline) then ORCi mathematics can calculate the freeboards, overhangs, wetted surface, stability at heel and everything else to do with the boat with pinpoint accuracy, assuming ORCi has a decent hull and appendage lines file to work with.  With only weight and LCG known, measuring freeboards and overhangs is unneccesary, and an inclination is all that is required.

Then, ORCi calculated values could be fed into IRC and any other systems required locally to generate ratings.  There's long been discussion of the need for a "Universal Measurement System" but it seems not to have gained traction sadly.

Yes, inclining is a pain but there's no other obvious way to get an estimate of VCG AFAIK. 

LCG can be easily determined by two point weighing, not the single point currently commonly used.  Set up two moveable cradles with scales built in, lift the boat onto them, drip dry a few minutes, then use the readings and the positions of the cradles relative to the boat's stem to calculate LCG.  Simple math.  A small investment in hardware (mobile cradles with scales built in) by authorities in major sailing centres could go a long way to enhancing the credibility of pretty much any rating system.

  

  • Like 2

Share this post


Link to post
Share on other sites

DD

We always had about 400-600kg difference as between weight (actual) and displacement (measured)

And agree everyone just puts that is the too hard basket.

My experience was that Yachting Australia as it then was had no idea about the actual system beyond plugging in the data and charging AUD $500.

That was clearly borne out when they ran a CT with a stern freeboard 380mm higher than the bow freeboard and no one saw a problem when the stability went from 116 to 109 so no Hobart!

I don't have my boats measured anymore!

 

  • Like 1

Share this post


Link to post
Share on other sites
14 minutes ago, lydia said:

DD

We always had about 400-600kg difference as between weight (actual) and displacement (measured)

And agree everyone just puts that is the too hard basket.

My experience was that Yachting Australia as it then was had no idea about the actual system beyond plugging in the data and charging AUD $500.

That was clearly borne out when they ran a CT with a stern freeboard 380mm higher than the bow freeboard and no one saw a problem when the stability went from 116 to 109 so no Hobart!

I don't have my boats measured anymore!

 

Plenty of owners have taken your course of action.

Our ex 40 footer has 30kg difference between weight and ORCi displacement and we were very satisfied with that.   400kg on a 40 footer is around a 20mm error.  It is possible to get it fairly right but indeed it's hard work.

I once saw another 40 footer rated ORCi with a carbon mast that was listed as weighing 336kg.  Correct answer is half that at most.  No-one noticed.  Needless to say ORCi rated it as pretty slow upwind when the pitch moment calcs kicked into the VPP.

Share this post


Link to post
Share on other sites

My experience is that the two main error sources in an ORCi inclining and freeboard measurement is unsuitable weather conditons and errors (or misunderstanding) in the offset files especially with regard to the position of the freeboard measurement points. Both the measurer and rating officer needs to vigilant in order to get that right. 

Share this post


Link to post
Share on other sites

Thanks for all the replies,

I was looking at the offset files and there are several for my boat model, how to know what to use (or know that the right one is used?)

Another question i have is regarding "heavy items" such as underdeck furler, anchor winch, etc at the extremities of the hull, these should be specified to account for the effect they have for pitching? Stuff in the middle of the boat and low has less effect and can go straight to CG/displacement without having too much effect. How about the heavier than standard boom I have?

If not specified, these will just go to "unknown DWT" as we call it on ships, and will be added to CG i.e wrong place. On ships we try to keep the unknown DWT under 1,5% of the disp. 

Looking to get things right this time...

Share this post


Link to post
Share on other sites
14 hours ago, Itsabimmerthing said:

My boat would need around 400 lbs of weights to be suspended if the boats spinnaker pole is used, secured to a stanchion. For me that sounds a lot..

Should the test be done by suspending the weights at the end of the boom instead? are there any downsides to using this alternative method?

Looking for the best possible rating of course but would not like to brake anything while getting there.

The boom might not be able to extend perpendicular to centerline and it might not be able to be positioed at  LCF

a spi pole solves this 

what the measures want is a technique that is repeatable and easy to perform 

any method could be used...a pile of lead pigs on the rail....but it would cause grief for the poor guy measuring the boat .

Share this post


Link to post
Share on other sites

Dumb question, but how does ISO attain the stability numbers then, as compared to ORCI? 

I found the whole ORCI measurement process pretty demotivating as we didn’t even get close to our ISO/STIX numbers. Enough to rule me out of Cat2 races, let alone Cat1!

When I went back with the results to the vendor, they came back with:

-the boat should have been aggressively heeled and measured rather than the 1.5 odd degrees we achieved, or;

-if minimal heel is used, the boat should have been measured in millpond conditions with a very high degree of accuracy .

Share this post


Link to post
Share on other sites

Now Shaggy has come out about this issue I can comment.

The Pogo is a perfect example of the voodoo physics and problems with measurement.

So the boat has ISO Cat A Ocean at least in Europe so trans-ocean so north of 120.

But was also inclined in Australia for ORCi and AVS came up at just under 110.

But ISO is otherwise acceptable in Australia now but because Shaggy tried to get a ORCi ct and got a adverse result this excludes application ISO by race committee at least for one ocean race.

So under 110 sticks.

Then this gets better because Australian inclination give a result nowhere near the European sisterships.

Like not even close.

Then there is the issue whether an extreme beam chine boat like the POGO has an issue with the ORCi calculation as to stability in any event which is my bet at the end of the day.

I say this having steered the boat upwind in the ocean in 48 true. (and accepting stiffness is different to stability)

I also think the  unusual hull/deck join on the POGO has caused problems when measuring the freeboards. (this also needs to be looked at)

So in Australia at least this boat comes up nowhere it's European sisterships and can't presently rely on it's ISO classification and is deemed unsafe to sail offshore in coastal ocean races.

Got to be wrong at a number of levels.

For the Gladstone race the organisers have recognised the bullshit and opened up the way in which stability can be demonstrated.

So ISO for the boat works for the race.

It should be noted that by telling SA to stick it on these types of issues the Gladstone fleet has gone from 22 boats to 70 boats in 2 years!

 

Share this post


Link to post
Share on other sites
2 hours ago, Itsabimmerthing said:

Thanks for all the replies,

I was looking at the offset files and there are several for my boat model, how to know what to use (or know that the right one is used?)

Another question i have is regarding "heavy items" such as underdeck furler, anchor winch, etc at the extremities of the hull, these should be specified to account for the effect they have for pitching? Stuff in the middle of the boat and low has less effect and can go straight to CG/displacement without having too much effect. How about the heavier than standard boom I have?

If not specified, these will just go to "unknown DWT" as we call it on ships, and will be added to CG i.e wrong place. On ships we try to keep the unknown DWT under 1,5% of the disp. 

Looking to get things right this time...

The choice (and verification) of offset file is at the discretion of the rating officer and as an owner I would ask questions until I was satisfied it is the right one. That is basicaly your only recourse. Also ask the measurer to double check the position of the freeboard points against some known position on the hull such as the height between the bottom of the transom and the aft freebord point.
When it comes to heavy items I think that today only internal ballast, batteries, tanks and internal stuff such as boilers aircon units etc are recorded on boats with LOA<24 m (IMS rule B4.4)

 

Share this post


Link to post
Share on other sites
1 hour ago, shaggybaxter said:

Dumb question, but how does ISO attain the stability numbers then, as compared to ORCI? 

I found the whole ORCI measurement process pretty demotivating as we didn’t even get close to our ISO/STIX numbers. Enough to rule me out of Cat2 races, let alone Cat1!

When I went back with the results to the vendor, they came back with:

-the boat should have been aggressively heeled and measured rather than the 1.5 odd degrees we achieved, or;

-if minimal heel is used, the boat should have been measured in millpond conditions with a very high degree of accuracy .

The obvious differenc between the stability calcs in ORC and ISO is that ORCi discards the effect of any volume above the sheer line. This effect should not give a huge difference in AVS but might be a degree or two. I looked at the two Pogo 12.50's with ORCi certs in Europe, one with fixed keel and one with the folding and the folding keel boat has AVS just under 110 deg  ( http://data.orc.org/public/WPub.dll/CC/65019.pdf  ) so yours seem to be about the same.  The fixed keel has a litte higher at 114 ( http://data.orc.org/public/WPub.dll/CC/91322.pdf )

Getting just 1.5 deg heel angle in the inclining test is definitely too litte IMHO and you are at the mercy of the weather conditions if you want any kind of accuracy.  

Share this post


Link to post
Share on other sites
3 hours ago, Pelle said:

The obvious differenc between the stability calcs in ORC and ISO is that ORCi discards the effect of any volume above the sheer line. This effect should not give a huge difference in AVS but might be a degree or two. I looked at the two Pogo 12.50's with ORCi certs in Europe, one with fixed keel and one with the folding and the folding keel boat has AVS just under 110 deg  ( http://data.orc.org/public/WPub.dll/CC/65019.pdf  ) so yours seem to be about the same.  The fixed keel has a litte higher at 114 ( http://data.orc.org/public/WPub.dll/CC/91322.pdf )

Getting just 1.5 deg heel angle in the inclining test is definitely too litte IMHO and you are at the mercy of the weather conditions if you want any kind of accuracy.  

And then just think about the inclination measurement on lightweight sportboats where measuring inclination on a boat without crew means a value that has no relation to anything (unless you sail the boat as an RC boat without crew).  The same goes for bigger boats where the difference between empty and crewed is naturally far less severe but still significant.

Measurement rules always claim to be very scientific but when making some glaring mistakes in measuring parameters that are far from reality all it amounts to is just another kind of bullshit marketed as science.

Share this post


Link to post
Share on other sites
59 minutes ago, Christian said:

And then just think about the inclination measurement on lightweight sportboats where measuring inclination on a boat without crew means a value that has no relation to anything (unless you sail the boat as an RC boat without crew).  The same goes for bigger boats where the difference between empty and crewed is naturally far less severe but still significant.

Measurement rules always claim to be very scientific but when making some glaring mistakes in measuring parameters that are far from reality all it amounts to is just another kind of bullshit marketed as science.

+1

I remember having freeboards measured  on a 700kg boat with the measurer sitting on the boat. Ridiculous.

Share this post


Link to post
Share on other sites
5 hours ago, Pelle said:

The choice (and verification) of offset file is at the discretion of the rating officer and as an owner I would ask questions until I was satisfied it is the right one. That is basicaly your only recourse. Also ask the measurer to double check the position of the freeboard points against some known position on the hull such as the height between the bottom of the transom and the aft freebord point.
When it comes to heavy items I think that today only internal ballast, batteries, tanks and internal stuff such as boilers aircon units etc are recorded on boats with LOA<24 m (IMS rule B4.4)

 

B4.4
Measurement inventory shall be recorded as follows:
a)Interior Ballast: description, weight, distance from stem, height from the waterline
B) Batteries: description, weight, distance from stem, height from the waterline
c) Engine: manufacturer, model
d) Tanks: Use, type, capacity, distance from stem, height from the waterline, condition at measurement
e) Miscellaneous: description, weight, distance from stem, height from the waterline (boiler, aircon,heating etc)
 
I have a heater so that will get to the list for sure, windlass would kind of fit in the same category? 30-40 lbs at the bow sure has an effect on performance? Does anyone know for sure if it cannot be accounted for? 
 
We say if I would place a battery at the bow, that would be the same thing performance wise.
 

Share this post


Link to post
Share on other sites
5 hours ago, Christian said:

And then just think about the inclination measurement on lightweight sportboats where measuring inclination on a boat without crew means a value that has no relation to anything (unless you sail the boat as an RC boat without crew).  The same goes for bigger boats where the difference between empty and crewed is naturally far less severe but still significant.

Measurement rules always claim to be very scientific but when making some glaring mistakes in measuring parameters that are far from reality all it amounts to is just another kind of bullshit marketed as science.

The purpose of the inclination is to determine the VCG of the boat without crew. The effect of the crew both on displacement and righting moment is taken into account by the ORC VPP so it is actually not that bad.

Share this post


Link to post
Share on other sites
12 hours ago, lydia said:

Now Shaggy has come out about this issue I can comment.

The Pogo is a perfect example of the voodoo physics and problems with measurement.

So the boat has ISO Cat A Ocean at least in Europe so trans-ocean so north of 120.

But was also inclined in Australia for ORCi and AVS came up at just under 110.

But ISO is otherwise acceptable in Australia now but because Shaggy tried to get a ORCi ct and got a adverse result this excludes application ISO by race committee at least for one ocean race.

So under 110 sticks.

Then this gets better because Australian inclination give a result nowhere near the European sisterships.

Like not even close.

Then there is the issue whether an extreme beam chine boat like the POGO has an issue with the ORCi calculation as to stability in any event which is my bet at the end of the day.

I say this having steered the boat upwind in the ocean in 48 true. (and accepting stiffness is different to stability)

I also think the  unusual hull/deck join on the POGO has caused problems when measuring the freeboards. (this also needs to be looked at)

So in Australia at least this boat comes up nowhere it's European sisterships and can't presently rely on it's ISO classification and is deemed unsafe to sail offshore in coastal ocean races.

Got to be wrong at a number of levels.

For the Gladstone race the organisers have recognised the bullshit and opened up the way in which stability can be demonstrated.

So ISO for the boat works for the race.

It should be noted that by telling SA to stick it on these types of issues the Gladstone fleet has gone from 22 boats to 70 boats in 2 years!

 

I've spoken to the wonks behind ORCi in the past and they've been very helpful in identifying obvious measurement hoolies and limitations - they see it as in their interests to enhance the facility and credibility of measurement for the good of the system.  Seems in the Pogo case a detailed study of inclining results for the class worldwide would be helpful in resolving what seems like an obvious impasse and dare I say injustice in Shaggy's case.    The ORCi guys have all the data behind the certificates which they can no doubt make available if there's a good case to do so.

Plus if it were my boat I'd get it reinclined at a range of larger heel angles.  Say, start with 1.5 degrees (do it again) in good conditions, then in the same session crank it up to 5 and even 10 degrees if possible and then compare the RM and AVS results when these three sets of values are fed through the ORCi maths.  I'd imagine the ORCi wonks could advise from a theoretical perspective which set of results would be most representative all else being equal.

Share this post


Link to post
Share on other sites
18 hours ago, Pelle said:

The purpose of the inclination is to determine the VCG of the boat without crew. The effect of the crew both on displacement and righting moment is taken into account by the ORC VPP so it is actually not that bad.

Well if it were measured in the right way I would agree with you - BUT - the fact is that hull shape has a significant influence on that measurement (as in how much COB is shifted due to even small degrees of heel) so the measurement can vary from fine to fucked

Share this post


Link to post
Share on other sites

Maybe silly question about chasing greater measurement accuracy for ORCi certification, but why not allow the option of conducting the test with greater inclination angles to accommodate hull shape oddities etc?  For example the 90 degree test for box rule boats like the IMOCA 60's and Mini's seems to work. 

PRB.jpg

Share this post


Link to post
Share on other sites
14 hours ago, Christian said:

Well if it were measured in the right way I would agree with you - BUT - the fact is that hull shape has a significant influence on that measurement (as in how much COB is shifted due to even small degrees of heel) so the measurement can vary from fine to fucked

I've had my boat inclined so as to obtain a stability declaration for Cat 2 races.  And I would tend to agree, that inclining the boat only a few degrees must be mainly testing hull form stability.  The 90 degree test as indicated by JS above would be more indicative - but i believe there are reasons they wont do it

Share this post


Link to post
Share on other sites
18 hours ago, Christian said:

Well if it were measured in the right way I would agree with you - BUT - the fact is that hull shape has a significant influence on that measurement (as in how much COB is shifted due to even small degrees of heel) so the measurement can vary from fine to fucked

Not so.  ORCi knows the hull shape so calculates how the COB moves at any angle of heel.

Share this post


Link to post
Share on other sites
On 09/03/2018 at 9:33 AM, DickDastardly said:

I've spoken to the wonks behind ORCi in the past and they've been very helpful in identifying obvious measurement hoolies and limitations - they see it as in their interests to enhance the facility and credibility of measurement for the good of the system.  Seems in the Pogo case a detailed study of inclining results for the class worldwide would be helpful in resolving what seems like an obvious impasse and dare I say injustice in Shaggy's case.    The ORCi guys have all the data behind the certificates which they can no doubt make available if there's a good case to do so.

Plus if it were my boat I'd get it reinclined at a range of larger heel angles.  Say, start with 1.5 degrees (do it again) in good conditions, then in the same session crank it up to 5 and even 10 degrees if possible and then compare the RM and AVS results when these three sets of values are fed through the ORCi maths.  I'd imagine the ORCi wonks could advise from a theoretical perspective which set of results would be most representative all else being equal.

Dd,

Thanks for the advice, I didn’t think that ORCI would listen to a single owner. I will contact them, it’s worth giving it a try. It interests me,  I confess to not being any near as educated as most of you guys on the intracies, so as a minimum it will be great to obtain a deeper understanding of the formulas behind the numbers. 

Appreciate it,

SB

Share this post


Link to post
Share on other sites
43 minutes ago, shaggybaxter said:

Dd,

Thanks for the advice, I didn’t think that ORCI would listen to a single owner. I will contact them, it’s worth giving it a try. It interests me,  I confess to not being any near as educated as most of you guys on the intracies, so as a minimum it will be great to obtain a deeper understanding of the formulas behind the numbers. 

Appreciate it,

SB

Welcome!  The ORCi guys are science geeks and teh rule is all about science.  If you're stability is so different from sister boats (assuming they're similarly configured) around the world there simply has to be a measurement problem ... sounds like you have a good case.  

Share this post


Link to post
Share on other sites
On 08/03/2018 at 10:05 PM, lydia said:

Now Shaggy has come out about this issue I can comment.

The Pogo is a perfect example of the voodoo physics and problems with measurement.

So the boat has ISO Cat A Ocean at least in Europe so trans-ocean so north of 120.

But was also inclined in Australia for ORCi and AVS came up at just under 110.

But ISO is otherwise acceptable in Australia now but because Shaggy tried to get a ORCi ct and got a adverse result this excludes application ISO by race committee at least for one ocean race.

So under 110 sticks.

Then this gets better because Australian inclination give a result nowhere near the European sisterships.

Like not even close.

Then there is the issue whether an extreme beam chine boat like the POGO has an issue with the ORCi calculation as to stability in any event which is my bet at the end of the day.

I say this having steered the boat upwind in the ocean in 48 true. (and accepting stiffness is different to stability)

I also think the  unusual hull/deck join on the POGO has caused problems when measuring the freeboards. (this also needs to be looked at)

So in Australia at least this boat comes up nowhere it's European sisterships and can't presently rely on it's ISO classification and is deemed unsafe to sail offshore in coastal ocean races.

Got to be wrong at a number of levels.

For the Gladstone race the organisers have recognised the bullshit and opened up the way in which stability can be demonstrated.

So ISO for the boat works for the race.

It should be noted that by telling SA to stick it on these types of issues the Gladstone fleet has gone from 22 boats to 70 boats in 2 years!

 

Is this boat much different to other Class 40s?  There are 3 racing to Osaka - all comply with Cat 1 stability...

 

Share this post


Link to post
Share on other sites

^^^ Shaggy's cruiser racer has same hull as Pogo Class 40 Series 2. The S2 was designed to Class Rules (max RM at 90 degrees, max beam, min ballast etc) and so to be self-righting.

While his version has I think slightly different rig, deck, layout, 3m pivot not fixed keel, some veneer, upholstry and no water ballast, the fact that it has a ORCi stability issue seems to be fuckin ridiculous.

Seems to me as I noted above an actual 90 degree test would put it all beyond doubt. In fact do a 40' White Boat like a Benny40 at the same time to make that test really interesting.

Share this post


Link to post
Share on other sites
3 minutes ago, jack_sparrow said:

the fact that it has a ORCi stability issue seems to be fuckin ridiculous.

yes it does

Share this post


Link to post
Share on other sites

The mesuring system for sure has its problems on accuracy. 

Question about my gph:

Few identical boats have been measured for 2018, so I kind of have an idea what to expect. I have about 5% larger spin, 5% longer pole and 5% larger main. Any educated guess how much lower rating I would get? 

Share this post


Link to post
Share on other sites

People suggesting that it should be heeled 10 degrees or "large angles of heel" are simply incorrect. 

On 3/8/2018 at 2:02 AM, shaggybaxter said:

-The boat should have been aggressively heeled and measured rather than the 1.5 odd degrees we achieved, or;

-if minimal heel is used, the boat should have been measured in millpond conditions with a very high degree of accuracy

It absolutely should be done on as calm a day as possible, mooring lines slack, no current, with a long pendulum. We typically use a 3m pendulum and get very repeatable results. 

How was the angle actually measured? Digital inclinometer? If so, when was it last calibrated? 

Was the density of the water measured in a couple places with a certified hydrometer? If you're near Brisbane, in brackish waters that can really affect results.

Was the position outboard of the weights measured accurately as far as you can tell?

2 weeks ago I did a measurement on a 24m tugboat. It was flat calm when we started. Later in the process, at the 6th movement of the weights, we noticed a change in the heel angle. (you move the weights in a repeating manner). A beam wind of about 10 knots caused the tug to heel about 0.1 degrees more. That's the accuracy you can easily get with a 3m pendulum.

An inclining experiment is used to determine the VCG of a vessel. Nothing to do with form stability. You're trying to determine the height of the "metacenter", a virtual point above the CG that the boat pivots about - BUT ONLY for small amounts of heel. Typically in commercial shipping 1 degrees is the minimum, 2 degrees is typical and 4 is about the most that is even allowed by the authorities. Above these amounts the different hull shape starts coming into place and you cannot accurately determine VCG because you are gaining RM though the hull shape.

Having said all that, if the boat was measured with keel lifted in the up position, an AVS of around 110 would not be unlikely. Any chance you could convince the authorities to re-measure for long offshore races with the keel pinned in the down position? (i.e. have 2 certificates; one for keel pinned down, one for keel able to be lifted)

 

 

Share this post


Link to post
Share on other sites
3 hours ago, Zonker said:

People suggesting that it should be heeled 10 degrees or "large angles of heel" are simply incorrect. 

It absolutely should be done on as calm a day as possible, mooring lines slack, no current, with a long pendulum. We typically use a 3m pendulum and get very repeatable results. 

How was the angle actually measured? Digital inclinometer? If so, when was it last calibrated? 

Was the density of the water measured in a couple places with a certified hydrometer? If you're near Brisbane, in brackish waters that can really affect results.

Was the position outboard of the weights measured accurately as far as you can tell?

2 weeks ago I did a measurement on a 24m tugboat. It was flat calm when we started. Later in the process, at the 6th movement of the weights, we noticed a change in the heel angle. (you move the weights in a repeating manner). A beam wind of about 10 knots caused the tug to heel about 0.1 degrees more. That's the accuracy you can easily get with a 3m pendulum.

An inclining experiment is used to determine the VCG of a vessel. Nothing to do with form stability. You're trying to determine the height of the "metacenter", a virtual point above the CG that the boat pivots about - BUT ONLY for small amounts of heel. Typically in commercial shipping 1 degrees is the minimum, 2 degrees is typical and 4 is about the most that is even allowed by the authorities. Above these amounts the different hull shape starts coming into place and you cannot accurately determine VCG because you are gaining RM though the hull shape.

Having said all that, if the boat was measured with keel lifted in the up position, an AVS of around 110 would not be unlikely. Any chance you could convince the authorities to re-measure for long offshore races with the keel pinned in the down position? (i.e. have 2 certificates; one for keel pinned down, one for keel able to be lifted)

 

 

Thanks for the insight, but I'm wondering whether there's a disctinction between the porcess you use to find VCG and that used in ORCi?  I'm not schooled in ship stability, pardon my ignorance.  ORCi has the lines of the boat - usually measured with a laser want these days. With freeboards and overhangs also measured it knows the position of the LCG and can calculate the position of the , and hence it has an exact 3D picture of the size and geometry of the immersed volume at any angle of heel and hence the poisition of the CoB and hence the positin of the VCG at any angle of heel.  is this also the case with the types of stability measurement you do?

Share this post


Link to post
Share on other sites
12 minutes ago, Presuming Ed said:

Inclining is probably the wedge that keeps ORC and IRC apart.

This gap has closed almost completely since IRC is recording bulb weights and keel types while ORC has lowered it's rating cost of stability.

Share this post


Link to post
Share on other sites
On 3/10/2018 at 5:18 AM, DickDastardly said:

Not so.  ORCi knows the hull shape so calculates how the COB moves at any angle of heel.

:huh:^_^:o:lol::lol::lol:

  • Downvote 1

Share this post


Link to post
Share on other sites

Yes we use 3D hull files (since we define the hull shape for the builder, we know the shape of the hull). But a 3D hull file only tells you the shape of the hull and thus the immersed volume (displacement) when you measure freeboards or drafts. Yes it can give you an CoB at any angle of heel.

But the VCG can only be determined by heeling the boat a small amount with known weights and distances. 

Put these things together (displacement, hull shape, and VCG) and you can calculate the AVS.

Share this post


Link to post
Share on other sites
3 hours ago, lydia said:

Z

Where are getting your displacement from!

Same as ORCi.  If you have a 3D representation of the hull in the computer then measure freeboards (accuracy being important...)  you can mark where the waterplane is, so the volume below that plane in the 3D hull file is immersed, multiply by the density of water and you have displacement (and of course boat weight).

Share this post


Link to post
Share on other sites
6 hours ago, Zonker said:

Yes we use 3D hull files (since we define the hull shape for the builder, we know the shape of the hull). But a 3D hull file only tells you the shape of the hull and thus the immersed volume (displacement) when you measure freeboards or drafts. Yes it can give you an CoB at any angle of heel.

But the VCG can only be determined by heeling the boat a small amount with known weights and distances. 

Put these things together (displacement, hull shape, and VCG) and you can calculate the AVS.

Many thanks, but humour me ... why only small angles and weights?  Surely if the boat is at equilibrium at any angle of heel the CG and CB are in a vertical line... assuming the VCG is on the centreline of the boat (as it floats level and assume away fluid moving around in tanks for simplicity) then can't you simply work out where it is?

Share this post


Link to post
Share on other sites
1 hour ago, DickDastardly said:

Same as ORCi.  If you have a 3D representation of the hull in the computer then measure freeboards (accuracy being important...)  you can mark where the waterplane is, so the volume below that plane in the 3D hull file is immersed, multiply by the density of water and you have displacement (and of course boat weight).

Dd that was my understanding but I wanted to see if Zonker had a different view.

Share this post


Link to post
Share on other sites

The logic is also why there must be a unmeasured but calculated waterline beam measurement in IRC worked out in reverse so to speak.

Share this post


Link to post
Share on other sites
19 minutes ago, lydia said:

Dd that was my understanding but I wanted to see if Zonker had a different view.

No other way to do it, been that way since Archimedes day...

Share this post


Link to post
Share on other sites
Quote

Many thanks, but humour me ... why only small angles and weights?  Surely if the boat is at equilibrium at any angle of heel the CG and CB are in a vertical line... assuming the VCG is on the centreline of the boat (as it floats level and assume away fluid moving around in tanks for simplicity) then can't you simply work out where it is?

I suppose you could in theory. But for small angles of heel, where sin (angle) is approximately = tan (angle), the math to determine the VCG is simple algebra. You don't need the shape of the hull to find VCG, just do the math knowing the weights, heel angle, and weight movements. Google "inclining experiment" if you want to get into the gritty details and the relatively simple math.

Say you have a hull heeled at 30 degrees; it's a lot harder to determine exactly its position in how it is floating. Taking freeboards at small angles of heel is very accurate and simple. It's not simple when your hull is also trimming bow down because the waterplane is assymmetric. 

(You don't want to get into shifting free surfaces of partly full tanks, though modern stability software can easily deal with it. Don't know if ORC stab software deals with it.)

Which reminds me:  Shaggy - were all your tanks empty or totally full? Does your Pogo have water ballast and were the interconnection valves closed?

Share this post


Link to post
Share on other sites
4 hours ago, Zonker said:

I suppose you could in theory. But for small angles of heel, where sin (angle) is approximately = tan (angle), the math to determine the VCG is simple algebra. You don't need the shape of the hull to find VCG, just do the math knowing the weights, heel angle, and weight movements. Google "inclining experiment" if you want to get into the gritty details and the relatively simple math.

Say you have a hull heeled at 30 degrees; it's a lot harder to determine exactly its position in how it is floating. Taking freeboards at small angles of heel is very accurate and simple. It's not simple when your hull is also trimming bow down because the waterplane is assymmetric. 

(You don't want to get into shifting free surfaces of partly full tanks, though modern stability software can easily deal with it. Don't know if ORC stab software deals with it.)

Which reminds me:  Shaggy - were all your tanks empty or totally full? Does your Pogo have water ballast and were the interconnection valves closed?

Hi Zonker,

No water ballast, but the 12.50 has 2 x 200ltr water tanks under the cabin settees closest to the centreline, and have an interconnection valve. They were empty for the inclination test 

The diesel tank had approx 30 litres in it from memory, the tank is located on the centreline under the cockpit.

Im googling inclining experiment as we speak! This is all really cool, but I’m struggling with keeping up with you guys, so I’m going back to basics , then I’ll reread the thread here. This really interests me, so appreciate all the info.

SB

 

 

Share this post


Link to post
Share on other sites
On 8 March 2018 at 5:05 AM, Itsabimmerthing said:

Thanks for the replies gents. Just wanted to get a second opinion on the method...

Any tips for easy "optimizations" that should be done/checked?

Wet your halyards and run them forward before they measure your water line length. 

Fill your water tanks before weighing the boat and lie that they are empty. 

Use your motor when no one is looking. 

Share this post


Link to post
Share on other sites

Does measurement take salinity into account? Ie: measured in a river marina at low tide and the boat will float lower in the fresh water than when she races on the sea.

Share this post


Link to post
Share on other sites
3 hours ago, HFC Hunter said:

Does measurement take salinity into account? Ie: measured in a river marina at low tide and the boat will float lower in the fresh water than when she races on the sea.

yes, the measurer is supposed to measure the density of the water on the day.

Share this post


Link to post
Share on other sites

Ask the factory for the stability curves for your boat !

If you have a furler, ask the factory if the curves include a furler (this matters because foils are relatively heavy up high)

Share this post


Link to post
Share on other sites

I just noticed this thread. I have designed, manufactured and calibrated most of the digital inclinometers used for ORC measurement around the world. Here is a simple datasheet. The same inclinometer with a different software is also used by ship yards, design offices, notified bodies (e.g. DNV/GL) and  measurers for measuring from small boats (CE marking etc.) to big ships.

As you can see the accuracy is 0.5% + 0.005 degrees and the resolution is 0.001 degrees. These are about on par with a 3 m pendulum. The actual accuracy is even better, about 0.1% + 0.005 degrees in most cases. 0.5% takes into account a wide temperature range and also wide range of gravity (the inclinometer is based on measuring gravity).

I have made these since 2007. I have tested several old ones and never found one out of specs. It is quite easy to test the accuracy with the help of a long bar or a laser pointer.

The inclining test is meant to be done in the linear range, thus at relatively small angles. ORCi measurement uses only one angle (3-4 degrees to both sides for small boats and less for bigger ones), but the standard 8 weight shift system uses several ones leading to a line, which should pass through (or close to) all 9 data points. This makes it easy to see possible errors in the measurement.

The boom inclining system for ORC uses four angles (boom on STB with and without weights + boom on PORT).

One important benefit of an electronic inclinometer is the ability to average for a long period of time. In ORCi measurement each angle is measured 4 times and each measurement is an average of 600 measurements taken during one minute. For ships longer periods are used due to longer roll period. An experienced measurer can average a reading of pendulum or a water scale, but most of my clients have found that the digital inclinometer pruduces better linearity and repeatability.

The measurement should be done with boat moored only from the bow and thus free to align with the wind, since not much wind is needed to heel a boat a few hundreds of a degree. When the boat turns freely the times having wind from STB will average out the times from PORT.

As it has already been said the purpose of the inclining test is to find VCG, which then can be used to calculate the whole stability curve when 3D hull form is know. ORCi measures hull only to freeboard and assumes the deck to be flat. This typically lowers LPS/AVS. Depending on the superstructure this can be more than 10 degrees. Also ISO/STIX LPS/AVS can be clearly wrong or valid only for a specific model (keel, mast interior, hull construction).

ORCi depends on the 3D hull form and can't thus use weight from the scale to replace weight from the freeboards. If freeboards give a wrong displacement, the offset file or the measurement is false. If both are well made, the displacement is within 50 kg from scale for a 40' boat. While a 40' boat has a sink of about 20 kg/mm, the freeboards are measured from 4 positions. To get an 200 kg error one would need to measure all four of the 10 mm wrongly to the same direction. If the conditions are even decent, 10 mm is a big error.

E.g. for an X-41 a 10 mm error in all the four freeboard measurements in the same direction would cause a 225 kg difference in displacement and a 0.33% difference in rating. In a measurement protest an over 0.1% rating change would lead to rescoring all the races, but no penalty. An over 0.25% error would lead to a penalty. There have not been problems with these limits despite quite a few during the recent Worlds and Europeans. Most boat are within 0.1%  and some 0.1-0.25%. Over 0.25% is a clear sign of a mesurement error (or cheating). Thus repeatability is good enough for this purpose. For comparison IRC has a limit of 0.005 in TCC thus about 0.5% and twice the upper limit of ORC.

  • Like 2

Share this post


Link to post
Share on other sites

Thx Joakim!

Thats great info, and the references you make to the measurement offsets needed for an incorrect weight are very helpful to get an understanding of the accuracy.

My measurer used a water scale out of interest, to me this would not provide the same accuracy in the amount of averages in comparison to a digital setup, ie: it could only provide a right-now measurement? I assume that the overall test would accomodate these types of aberrations, say for example taking  a singular freeboard measurement when a gust came through, but a squillion averages would negate this type of issue completely?

Dumb question if I may: the purpose of the test is to find the VCG of the boat. In my simplistic head, that’s the variable, so the other critical parts of the formula must be known values. In determining VCG, I’m assuming that hence it is M (metacentre) that is already known, is this correct? If so, I assume it is gleaned from an information source, I’m guessing the offset file? (Or is it B?)

I thought maybe the MH (metacentric height) may be the known value, but that doesn’t make any sense, as MH is the distance between M and VCG, true?

This is all starting to make sense, but I’m floundering on some of the detail, pls excuse any error in the questions!

Cheers,

SB

 

 

 

Share this post


Link to post
Share on other sites
On 11/03/2018 at 9:51 AM, Dark Cloud said:

Is this boat much different to other Class 40s?  There are 3 racing to Osaka - all comply with Cat 1 stability...

 

Sorry Dark, I missed your question. The 12.50 uses the same hull mould as the Class 40, so no difference there, but it does have a higher freeboard, ie: they bolt on an assembly on top of the Class 40 mould. I’m unsure how this impacts the stability by adding a few inches in freeboard, but there is a difference in that regard. 

Thats why the question to Joakim, I’m trying to nail down what “value” changes with higher freeboard. I’m assuming “M”, the metacentre, as my uneducated understanding is this is the point in vertical height that the hull would rotate around. 

It can’t be “B”, ballast/centre of buoyanc, as not only does that reference point move depending upon the angle of heel, but I can’t see B changing if I’ve added  freeboard.

I keep thinking you need a fixed dataset of some description to reference all the measurements against, whether that is M or B or something else I don’t know.

Cheers,

SB

Share this post


Link to post
Share on other sites
1 hour ago, shaggybaxter said:

The 12.50 uses the same hull mould as the Class 40, so no difference there, but it does have a higher freeboard, ie: they bolt on an assembly on top of the Class 40 mould. I’m unsure how this impacts the stability by adding a few inches in freeboard, but there is a difference in that regard. 

Shaggy the mould assembly you speak of I think refers to the original Pogo S1 which predated introduction of the class 40 and was retrofitted to make the S1 fit the rule. Your boat uses the Class 40 specific S2 hull where I would have thought no bolt on mould bit is necessary?

Share this post


Link to post
Share on other sites

G'day Jack,

You could be right you know. I just had a look at a Class 40 S2, and the freeboard at the transom looks the same. I just had a look back through my photos of the build, and I cant see the outside of the mould (S2). This is the mould from the inside...

5aadc3f61a1e6_photo3.thumb.JPG.de0916c24af2ddbc7fb7ef0834ed2f8d.JPG

I'll bang a question into Pogo and find out for sure, thanks mate.

 

Share this post


Link to post
Share on other sites
10 hours ago, shaggybaxter said:

Thx Joakim!

Thats great info, and the references you make to the measurement offsets needed for an incorrect weight are very helpful to get an understanding of the accuracy.

My measurer used a water scale out of interest, to me this would not provide the same accuracy in the amount of averages in comparison to a digital setup, ie: it could only provide a right-now measurement? I assume that the overall test would accomodate these types of aberrations, say for example taking  a singular freeboard measurement when a gust came through, but a squillion averages would negate this type of issue completely?

Dumb question if I may: the purpose of the test is to find the VCG of the boat. In my simplistic head, that’s the variable, so the other critical parts of the formula must be known values. In determining VCG, I’m assuming that hence it is M (metacentre) that is already known, is this correct? If so, I assume it is gleaned from an information source, I’m guessing the offset file? (Or is it B?)

I thought maybe the MH (metacentric height) may be the known value, but that doesn’t make any sense, as MH is the distance between M and VCG, true?

This is all starting to make sense, but I’m floundering on some of the detail, pls excuse any error in the questions!

Cheers,

SB

When I got the first inclinometers ready it was thoroughly compared to water scales, which is the other option in ORCi measurement. There was no difference in the results. A water scale should have a dampening screw, which limits the water flow from the "tank" to the tube. This stabilizes the reading and takes on average of some unknown period. Skill is needed to make the adjustment correctly and for reading the scale at the correct time. All this is very easy in totally calm conditions, but more demanding with waves or wind.

E.g. the Finnish Navy made their own tests to compare with a pendulum and found the inclinometer to be more linear. For the pendulum a tank of water, oil or some other liquid is used to dampen its motion. It is often not possible to get a stable reading and one needs to take the average in the middle of min and max or do some other type of averaging.

It is not uncommon to see rather much of rolling during the inclinining test. Often the range (max-min) is more than one degree and can even be over two degrees while averaging one angle. Still good repeatability can be achieved for inclining part, but measuring the freeboards becomes more demanding. Below are some real examples of normal, excellent and bad measuring conditions. Unfortunately it is not always possible to choose good conditions for the inclining test due to location and schedule.

When the hull 3D form is know and freeboards have been measured the ORC software knows how the boat floats during the measurements and can calculate the shifting of center of buoyancy with heel angle. Thus it can calculate the location of the metacenter referenced to the 3D hull form. When inclinning test is done the software knows the moment needed to heel the boat to a known rather small angle. From this it can calculate the distance from M to VCG. With all this information the boat can be heeled in the software to any angle and full stability curve can be calculated. As said earlier the deck is assumed to be flat and accuracy of the stability curve starts to worsen when superstructure or cockpit touches water.

normal.png

excellent.png

bad.png

Share this post


Link to post
Share on other sites

ORC just posted some pics on FB from boat measuring at Annapolis. 

Using the boom for inclining on a relatively small boat. Wonder if the result would be the same on two identical boats, one measured with poles, one with boom. 

My guess is that the one measured/inclined with boom would get more favorable rating. 

Share this post


Link to post
Share on other sites

In both cases you have to consider the effects of moving the weight of the boom or poles outboard. So best practice would be to weigh them and find out their C.G. If they ignore that, then a heavy boom swung outboard will heel the boat more, making it seem less stable. I bet ORC just estimates the weight of a boom/pole from known similar ones which is probably close enough unless you have an abnormally heavy or light boom.

 

Share this post


Link to post
Share on other sites
 

In both cases you have to consider the effects of moving the weight of the boom or poles outboard. So best practice would be to weigh them and find out their C.G. If they ignore that, then a heavy boom swung outboard will heel the boat more, making it seem less stable. I bet ORC just estimates the weight of a boom/pole from known similar ones which is probably close enough unless you have an abnormally heavy or light boom.

 

Fill the boom with lead shot?

When done, drill a hole in the bottom of the boom and give it a good shake.  Just trying to keep the ol' IOR spirit alive :P

Share this post


Link to post
Share on other sites
On 23.3.2018 at 10:36 PM, Zonker said:

In both cases you have to consider the effects of moving the weight of the boom or poles outboard. So best practice would be to weigh them and find out their C.G. If they ignore that, then a heavy boom swung outboard will heel the boat more, making it seem less stable. I bet ORC just estimates the weight of a boom/pole from known similar ones which is probably close enough unless you have an abnormally heavy or light boom.

 

In boom inclining the measurement is done with the boom out on one side empty and with a weight. It is the difference of these two angles that count, not the difference of angles when the boom is swung to the other side. Thus the weight of the boom does not need to be known and it only affects the starting "zero" angle of the measurement, which is not big in normal cases, but can be several degrees for special cases like an old 12MR.

Several boats have been measured with both spinnaker poles and main boom. They do give the same results. Both measurement use a fixed setting of boom/poles with only weights moved (on/off for boom and side to side for poles).

With boom inclining the measurememt is done twice, once on each side, and an average is used. With spinnaker poles two poles are used fixed on both sides.

Share this post


Link to post
Share on other sites

I had a quick look at different AVS/LPS found for Pogo 12.50. The yard seems to claim 124 degrees for the 3 m swing keel: http://www.fastsailing.gr/the-yachts/high-performance-line/pogo-12-50/

RORC list of STIX gives 118 degrees for the same keel: https://www.ircrating.org/images/stories/pdf/stix/stix_web_latest.xls

The only ORCi certificate states 109.4 degrees: http://data.orc.org/public/WPub.dll/CC/65019.pdf

The reguirement for design category A is 130-0.002*m. For 5500 kg that becomes 119 degrees. Thus both ORC and RORC values are too low for A and the one given by the yard is just above.

It's not at all uncommon to have ~110 degrees AVS/LPS in ORCi for boats in design category A. My own boat is one of them. But these boats are just at the limit of A and I would think twice before entering a race like SH.

Pogo 40S2 is quite a different boat, with at least 1000 kg lighter displacement and different fixed keel with probably much lower VCG and thus much higher AVS/LPS.

Share this post


Link to post
Share on other sites

In any event Shaggy needs 115 and no more and we want to hire one of your machines (for good money) to get a proper inclination so all your help is greatly appreciated.

Shaggy needs to prove stability while I want the tippiest boat I can get for rating purposes as I am over 115 and can lose a bit.

 

Share this post


Link to post
Share on other sites

OK I'm now sick of this theoretical fuckin nonsense as enlightening as it is..ORCI should rejig the measurement to include an Inversion Test...that will sort the men out from the boys and their machines ..Shaggy's Pogo 40 rating will plummet and fuck the pussycat Cat 1 Hobart ..he is off to do this year's  Route du Rhum....aren't we Shaggy??

 

unnamed (20).jpg

  • Like 1

Share this post


Link to post
Share on other sites
29 minutes ago, lydia said:

In any event Shaggy needs 115 and no more and we want to hire one of your machines (for good money) to get a proper inclination so all your help is greatly appreciated.

Shaggy needs to prove stability while I want the tippiest boat I can get for rating purposes as I am over 115 and can lose a bit.

 

I have sold 5 units to Australia, some to ORCi measurers and some to design officies. I hope you can use one of them. It's not very practical to ship one from Finland for one measurement due to shipment costs and all the paperwork/cost required for customs. Send me a PM, if you want to have contact details.

Share this post


Link to post
Share on other sites
18 minutes ago, jack_sparrow said:

OK I'm now sick of this theoretical fuckin nonsense as enlightening as it is..ORCI should rejig the measurement to include an Inversion Test...that will sort the men out from the boys and their machines

The purpose of the inclining test in ORCi is to find out the righting moment for normal sailing heel angles in order to get as accurate velocity predictions as possible. The stability index and AVS/LPS are just side products of that. While ORCi doesn't include the coachroof etc. and thus underestimates AVS/LPS it's a very well defined process producing repeatable results.

I don't think STIX and AVS/LPS done for EU/ISO design category are as well defined and repeatable processes. Not all keel, mast, engine, interior etc. variations are measured. Typically just one version is measured (with a very similar inclining test to ORCi). Then someone (e.g. the designer) does the AVS/LPS and STIX calculations based on that measurement + correction for a different keel etc. using a variety of different software. E.g. it's not uncommon for the designer CAD file to have clearly different freeboards than the boats actually built.

Share this post


Link to post
Share on other sites

Thanks Joakim..you have forgotton more than what I know about this subject.. however I have had enough and just topped myself. 

Heads up ..I left a suicide note indicating my estate was in a position to sue you for sending me over the edge so my advice from the  other side is you put everything in the wife's name...providing she doesn't have a straying tendancy.

Good luck.

Share this post


Link to post
Share on other sites
On ‎08‎/‎03‎/‎2018 at 8:37 AM, slug zitski said:

The boom might not be able to extend perpendicular to centerline and it might not be able to be positioed at  LCF

a spi pole solves this 

what the measures want is a technique that is repeatable and easy to perform 

any method could be used...a pile of lead pigs on the rail....but it would cause grief for the poor guy measuring the boat .

Problem with that is you are trying not to increase the displacement.  2 tonnes of lead on the rail might equate to 500kg on the tip of the spinnaker pole.

Share this post


Link to post
Share on other sites
On ‎12‎/‎03‎/‎2018 at 2:21 AM, Zonker said:

Yes we use 3D hull files (since we define the hull shape for the builder, we know the shape of the hull). But a 3D hull file only tells you the shape of the hull and thus the immersed volume (displacement) when you measure freeboards or drafts. Yes it can give you an CoB at any angle of heel.

But the VCG can only be determined by heeling the boat a small amount with known weights and distances. 

Put these things together (displacement, hull shape, and VCG) and you can calculate the AVS.

Calculate???  Are you sure?  Do you now allow for deck camber, cockpits and deck houses? 

Share this post


Link to post
Share on other sites
On ‎23‎/‎03‎/‎2018 at 8:36 PM, Zonker said:

In both cases you have to consider the effects of moving the weight of the boom or poles outboard. So best practice would be to weigh them and find out their C.G. If they ignore that, then a heavy boom swung outboard will heel the boat more, making it seem less stable. I bet ORC just estimates the weight of a boom/pole from known similar ones which is probably close enough unless you have an abnormally heavy or light boom.

 

I inclined an 80 odd footer a few years ago, using a dive bag hanging off the boom, which was slung under a small loadcell, and would be filled with water.  By setting the boom / loadcell / bag arrangement up, and then putting the boom outboard to port, and then outboard to starboard, and measuring how far the boat heels under the weight of the boom alone, you can negate this from the "loaded" heel angles.

By the way, I used one of these https://www.amazon.co.uk/DNM-60-Digital-Spirit-Inclinometer/dp/B00407R5MU to measure the angles.  I wasn't aware that ORC were using the digital inclinometers at the time (this is one of the very few times I have measured anything for ORC) so the digital spirit level seemed like a good bet.  Calibration is straight forward - put it on a surface, take a reading, turn it through 180 degrees, see if it says the same - if it doesn't, run the self calibration!

Share this post


Link to post
Share on other sites

Back in the early 70's IOR days boats would be moved up the Messina river in Cowes IOW for incline measurements to take advantage of more dense brackish water.  Whatever it takes to get an edge! 

Share this post


Link to post
Share on other sites
48 minutes ago, 2savage said:

Back in the early 70's IOR days boats would be moved up the Messina river in Cowes IOW for incline measurements to take advantage of more dense brackish water.  Whatever it takes to get an edge! 

You could have done the ORC incline test a few years ago when your boat was lying up against the City Is Bridge:o

Share this post


Link to post
Share on other sites