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  #16  
Old 02-05-2017, 08:56 PM
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PAR PAR is offline
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With cruisers, I've most times, the boat simply needs to be emptied of everything that isn't nailed down, then taken for a baseline run. Cruisers and liveaboards tend to acquire lots of extra gear, equipment, spares, you name it. This weight quickly adds up and you're likely just not aware of how much it might be. Start in the forepeak and empty every locker, shelf, etc., putting it all ashore. Move aft, doing the same and you'll be very surprised about the accumulation on the dock. 90% of the time, this is the only thing holding you back.
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  #17  
Old 02-06-2017, 12:11 AM
tane tane is offline
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the cheapest (but most unpopular) measure to boost upwind performance in winds >25kn would be to take off the furler & use hanked-on foresails...
(after 3 rtws changing headsails on the foredeck I myself am looking forward to tnb with a rollerfurler jib...!)
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  #18  
Old 02-07-2017, 01:10 AM
tane tane is offline
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a few more thoughts:
most cost-effective performance boost: mount a self_feathering prop
do you have a bowthruster? with a tunnel? out with it, close the holes, fair nicely
any batteries up front (for windlass & thruster)-move them aft
10mm chain? 8 high tensile is going to be plenty!
& if these proposal sound tough: no pain no gain!
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  #19  
Old 02-07-2017, 05:08 AM
Joakim Joakim is offline
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You can find polars from an ORCi certificate. This was the only one I could find: http://data.orc.org/public/WPub.dll/CC/4409.pdf

So according to it you should be able to achieve 4.56 VMG at 40 TWA at 20 knots wind. Thus 5.95 knots at 40 TWA. These values include leeway. Note that this is with 350 kg crew hiking, optimal sails at optimal sheeting (no limits by chainplates etc.), feathering propeller with minimal drag, very clean and smooth hull. The sea condition for this is average racing conditions at 20 knots wind. So not flat water but neither ocean waves.

The driving forces on a beat are small and adding drag (e.g. fixed blade propeller or rough hull finish) greatly reduces performance.

Now you can go to ORC Sailor Services http://orc.org/ and pick up that boat and run test certificates with your sail dimensions and loading.

I don't think added weight is that harmfull in heavy wind unless it is positioned very high (above deck level) or at one end of the boat.

For good performance at 20-25 knots of wind you should have a good ~100% jib. Two or three reefs + inner forstay jib sounds way too little for this boat, which is very heavy and have small sails for its size.

My boat has about the same LOA, weighs under 6 tonnes, has 20% smaller righting moment, mainsail is 41 m2, 105% jib is 31 m2. In 20-25 I would race with full main sail and 100% jib #3. In gusty wind or heavy seas I might take one reef.

Cruising with family at open sea I would have one or two reefs and 103% rolling jib fully open or a few turns rolled.

It seems you have something wrong with your sails, mast trim, sheeting or driving. AWA should be below or about 30 degrees. Nowhere near 40 on a 20-25 knots beat.

While cruising we have a lot of extra stuff onboard (dinghy, bicycles, lots of personal stuff, food etc.), but I can't really see much of a difference from instruments. Sure I wouldn't do well in a race with all that stuff, but the difference is way less than 10% in performance. A good rule of thumb is 10% added weight makes you 1% slower.
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  #20  
Old 02-07-2017, 06:48 AM
tane tane is offline
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"10% added weight makes you 1% slower" - sounds way too little for me & would vary a lot for different boattypes ("skimming dish" or S&S 70ies leadmine), conditions & weight-distribution. I'm also pretty sure it would not be "linear"...(=30% plus slow you down 3%? no way!)
I doubt that our 5,6t 37'er was only slowed down 2% by a load of 1t plus!
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  #21  
Old 02-07-2017, 08:29 AM
Joakim Joakim is offline
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Quote:
Originally Posted by tane View Post
"10% added weight makes you 1% slower" - sounds way too little for me & would vary a lot for different boattypes ("skimming dish" or S&S 70ies leadmine), conditions & weight-distribution. I'm also pretty sure it would not be "linear"...(=30% plus slow you down 3%? no way!)
I doubt that our 5,6t 37'er was only slowed down 2% by a load of 1t plus!
It depends on many things, of course. In light wind you'll always be slower with added weight. The same is true for downwind at any wind. Upwind you may benefit from the added LWL and stability (depends on where the added weight is) and actually be faster with added weight.

Typically 10% added displacement will not increase resistance by 10%. More like half of that or less, since adding weight doesn't increase wetted surface by much and also doesn't change the appandage, propeller installation drag nor windage.

Also increasing resistance by 10% will not slow you down by 10%, since resistance increases at least to the second power of speed. So again the decrease of speed will be less than half of the increase of resistance at constant speed.

1% from each 10% is typical value for rating change in most rating systems.

Just to check what ORC VPP thinks about this boat I run the boat linked earlier (http://data.orc.org/public/WPub.dll/CC/4409.pdf) with 2017 VPP and got 760.0 sec/m GPH and 4.56 knots VMG at 20 knots (now with 43.8 TWA).

Then I decreased the freeboards 40 mm and the displacement went from 8634 kg to 9418 kg (9.1% increase). GPH became 765.9 sec/m, but no change in VMG at 20 knots. So GPH and thus average speed decreased 0.77% from 9.1% displacement increase.

I didn't change the RM measurement, which meant that the VCG became 8 cm higher (very close to water line). This equals adding the 784 kg at about 1 m above the water line thus at the deck level.

Then I decreased the freeboards addionally by 80 mm and the displacement became 11 059 kg (28% increase). Now GPH is 778.5 and VMG at 20 knots 4.55 knots. So 2.4% slower from 28% more weight.

VCG is now 22 cm higher which means the added at 1 m above the waterline.

If in the latter case the RM measurement is also changed so that the extra weight is added to the water line level instead of the deck level, GPH is 777.4 and 20 knots VMG 4.67 knots. So 2.3% slower in average, but 2.4% faster 20 knots upwind.

This all is from VPP, but it can't be way off. If it was all the top boats in the ORC Worlds would add or take out much weight to gain a better rating. Neither is happening.
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  #22  
Old 02-07-2017, 08:36 AM
Joakim Joakim is offline
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Here are the wetted surfaces for these cases: original 28.77 m2, +9.1% 29.76 m2 (+3.4%) and +28% 31.83 m2 (+10.6%).

Also the Reynolds number of the hull is larger than for the appendages, thus frictional resistance increases a bit less than the added wetted surface.

LWL increased from 9.021 m to 9.131 m and 9.472 m.
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  #23  
Old 02-07-2017, 09:41 AM
tane tane is offline
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another thought:
heeling angle upwind: too much? or not enough (sailarea)?
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  #24  
Old 02-08-2017, 12:42 PM
Alby1714 Alby1714 is offline
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Joakim,
first of all, thank you very much for fishing out the ORC certificate for a sister ship of my own. It is all very interesting.
I am not sure that I fully understand the assumptions of the VPP but the “Beat VMG” line is clear and it seems to incorporate some leeway. To evaluate it, I have compared the given VMG of 4.56 kt at 20kt of wind and a TWA of 40 degrees, with the corresponding value from the designer polar diagram which does not account for leeway. From the comparison I have derived a leeway angle of around 3.6 degrees. That is the prediction of the VPP. Fairly optimistic, I think.
The 52° line in the table, where 52° is a true wind angle, I believe, indicates a speed of 6.65 kt, again for a wind speed of 20kt. This speed matches rather well the designer’s polar. That would seem to suggest that the VPP assumes that the boat can sail at 52° to the true wind with zero leeway. If my interpretation is correct, the VPP is not only optimistic, it is unrealistic.

Clearly the VPP provides a common reference for assessing target speeds and speed potential for racing yachts and this is very useful for racing crews but it seems to me that the VPP falls short of being a practical tool to model the performance of a (cruising) boat in a seaway. This is particularly true in my case where the focus in on leeway and furthermore in a suspected non-optimal (my boat has got chainplates !) and possibly unbalanced configuration.

The way you have used the VPP to evaluate how different displacements would affect boat speed is interesting but it still applies to the “unreal” optimal boat of the VPP, furthermore assumed to be sailing in an unspecified sea state.
I remember you said that although the VPP does not assumes “ocean waves” it does not assume a flat sea either. Very well, however I could not find anywhere a specification of the sea state according to the VPP. Does it exist ?
Yet your results are useful because they hint that the excess weight that I carry is not likely to be the origin of my perceived problem. At the same time it should be said that my excess weight makes the boat stern heavy but the effect of this unbalance is invisible to the VPP.

So thanks again for trying to help, but it seems to me that there would be a need for a much more powerful and comprehensive computer model and this, to the best of my knowledge, may not even exist and for certain is not available to me.

On a different matter, when I mentioned two or three reefs + inner forstay jib, I was spanning the whole range from 20 knots up to 30 kt and beyond with a variety of sea states, but indeed if you sail at 25 kt with full main and 100% jib on your lighter boat then you carry more sail than I normally do.
As you know, when you beat in 20kt and above the wind angle indicator wobbles about an average value. This is because, like I do in those conditions, you may luff up a bit on climing a wave and bearing a way a little when descending in the trough and yes my average is 40° AWA. When the AWA goes below 35 and touches 30 I can feel the boat pinching and losing speed, it is time to bear away. Your boat is different and obviously points higher than mine if you can steadily keep your apparent wind angle at 30°.
As a matter of curiosity I would like to ask you what is your leeway in those conditions ?

Turning again to the VPP speed table for my boat, I would like to draw your attention to the column for 20 kt of wind. The speed difference when you luff up from 60° TWA to 52° TWA is a mere 0.31 kt , on the other hand when you luff up from 52° to “Beat VMG (40°)” the speed drops by a huge 2.09 kt, a rather sharp decrease.
I understand that you do not know my boat as you have not sailed on her, but even these theoretical tables by themselves, exhibiting that sharp drop in speed between 50° and 40°, suggest that at 40° TWA the boat is very close to pinching. My opinion is that for a cruising boat that would not be bad at all if the leeway were truly only 3.6° as the VPP likes to think. What is in fact the point of slamming against steep waves when they are steep, or smashing into wave crests when they tumble against you if, by bearing away a few degrees, one can have more speed and a more comfortable ride ? After all I am not racing; but my concern remains because I dread that treacherous lee shore that might trap one day sailors like myself and push them to beat hard into the wind to get away from disaster !

I have recently come across an excerpt from “Aero-Hydrodynamics and the Performance of Sailing Yachts: The Science Behind Sailboats and Their Design” by Fabio Fossati where mention was made of hull polar diagrams where the total resistance and lift developed by the immersed part of the hull is given for various boat speeds and leeway angles. Presumably these diagrams are the result of a computer program modelling the underwater lines of a yacht. Has anyone got information about it ?
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  #25  
Old 02-08-2017, 03:57 PM
Joakim Joakim is offline
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You can find what ORC VPP does from its documentation: http://orc.org/rules/ORC%20VPP%20Doc...ion%202016.pdf

You can also find some VPP results that output leeway, but for a different boat. Anyway you can see how leeway varies with wind speed and TWA. http://www.blur.se/polar/first367_pe...prediction.pdf

You can't derive leeway by comparing two VPP outputs. They are not the same. They most likely have different input data (sails, crew, displacement, sea state etc) and the different prediction come also from different models. The one by Frers is probably a very old one dating from the design year of HR36. The accuracy of VPP programes has improved a lot since then. The polar by Frers may well include leeway. It's impossible to say from it.

VPP can "see", if the boat is stern heavy. ORC VPP uses the crew weight to balance that out, but it can't do all. I didn't make inputs for stern heavy boat. You can do it by your self, if you are interested.

If your AWA is around 40 degrees at around 20 knots TWS you are sailing
55 TWA + leeway, thus around 60 TWA. That's way too low. Leeway is quite hard to measure and I don't know mine, but I can make 80 degree tacks in 20 knots, if the waves aren't especially bad, and always well below 100 degrees.

You don't seem to understan the concept of VMG. It's Velocity Made Good, thus velocity straight toward wind direction in the case of beat. To get the speed of the boat you have to divide it by cos(TWA). So 4.56 knots VMG at 40 deg TWA equals to 5.95 knots boat speed. The current (2017) VPP thinks that the optimum angle is 43.8 degrees, but VMG is still the same. So the boat speed is 6.32 knots.

All boats have chainplates. Some make sheeting more difficult than others. But this is much more of a problem in light and mid wind, when optimum AWA would be much lower than in 20 knots. E.g. for my boat optimum AWA is around 25 degrees in light/mid and around 30 dregrees at 20 knots. Also chainplates are not a limit for a small jib.

I haven't sailed HR 36, but having sailed quite a few boats at that size range, I'm confident that you should sail around 6 knots in 20 knots wind. Also I'm confident that you should not have to sail to 40 degrees AWA to reach that speed, if you have decent sails, mast trim, sail trim etc.

A friend of mine has owned a HR 36 since early 90's. I can ask him sometime.

For knowing the basics of VPP "Principles of Yacht Design" by Larsson and Eliasson is a good read.
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  #26  
Old 02-08-2017, 11:31 PM
tane tane is offline
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btw:roller furler main?
what I presume about those polars: they represent the best-case scenarios, right? best sails, best rigging-tune, etc...
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  #27  
Old 02-09-2017, 12:41 AM
Joakim Joakim is offline
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Quote:
Originally Posted by tane View Post
btw:roller furler main?
what I presume about those polars: they represent the best-case scenarios, right? best sails, best rigging-tune, etc...
Of course it is. Tacking through 110 degrees at 6 knots gives beat VMG of 3.44 knots, which is 25% below this. Almost any 24' boat would be faster than that at 20 knots upwind, if sailed well. But it's easy to find sailors that can't reach that upwind speed with a 36' boat.

It is quite easy to get within 10% of the VPP prediction. No need for racing sails etc. for that. But can be hard with e.g. 150% genua rolled to do the job of a 100% jib. Or with a large 3 blade fixed propeller and fouled hull, but that would show even worse in light/med wind.
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  #28  
Old 02-09-2017, 03:24 PM
Alby1714 Alby1714 is offline
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Joakim,
there are many things about my boat that I still do not understand and which puzzle me and that is obviously why I have turned to this knowledgeable forum and appreciate very much your contribution and willingness to help; however let me point out that the concept of VMG has always been very clear to me and equally clear is the simple trigonometry on which it is based.

In principle you are correct when you say that one cannot derive leeway from the comparison between the two VPP outputs. Indeed, for want of better data, I have tried nevertheless to gain a rough estimate of leeway based on assumptions that may well prove to be wrong.
I came up with a value of 3.6°; certainly it is only a coincidence that the ORC VPP 2017, has increased by 3.8 ° to 43.8° the optimal angle at which the boat VMG is 4.56 kt.

Far more interesting than the previous point is the exhaustive set of data that you kindly sent me regarding the First 36.7. That modeling is not recent because it dates back to 2001, yet it provides an estimate of leeway for each TWA and TWS. That is exactly what I would like to see for my boat.
Is the VPP used a proprietory program of Farr Yacht Design ?

Does one have to own a measured boat or, for a subscription, anyone can gain access to the ORC VPP ?

If you have the chance, please do ask your friend, owner of an HR36, about his experience with the boat.
I would be very interested to know what he thinks.  Thank you.
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  #29  
Old 02-10-2017, 12:59 AM
Joakim Joakim is offline
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Anyone can buy the designer ORC designer VPP or some other VPP, but you need to get access to hull data as well. You may get that from the designer.

Anyone can register to ORC Sailor Services and run test certificates (10 € each) online for any boat ever measured in the history of ORC and IMS. You can also change most of the measured data. No need to own a boat.

I don't know which VPP Farr and Frers were using. There are several commercial ones and many design offices have their own. Even I have my own, although I'm not a designer. Some VPP's calculate and output leeway others do not. Leeway is not a must in a VPP. It depends on how the models are developed.

I dig some tests from the Yacht (Germany) magazine. They tested HR36 in 18-20 knots wind and up to 1 m waves. They sailed with one reef in main and "normal fock", which looks like a 110% jib in the pictures. They reached 6.5 knots at 40 degree TWA. They always measure without leeway thus you need to add leeway to that so maybe 45 TWA. They said that they could get the average 80 degree tack in several tack they made and they regarded that as "absolutely normal value". In gusts the freeboard was pushed under water.

Also HR 34 was tested in 20 knots and 1 m waves. They sailed with one reef and 20% rolled genua (looks like to be 110-120% when rolled). They also reached 6.5 knots at 40 degree TWA. They said that the boat heeled 20-24 degrees on a beat.

HR 39 was tested in lighter wind. It reached 6.1 knots at 44 TWA in 14 knots and 0.5 m waves using full main and about 110% jib.

I also looked at test of Najad 361, which is quite similar to your HR. Unfortunately they did not give any speed values, but they did say it sailed very well. The test was done in 7 Beafort (28-33 knots) with short and steep waves. They sailed with two reefs and 110% jib (not a rolled genua).

I don't know your background, but it may help to take some one with racing knowledge on board. They may notice right away something that could improve the performance a lot.
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  #30  
Old 02-10-2017, 01:12 AM
Joakim Joakim is offline
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The change in optimum beat TWA in ORC VPP can be caused by a very minor adjustment of the model. At the optimum the VMG vs. TWA curve is flat and there is always a rather large region of almost the same VMG. E.g. if you look at the VPP output for First 36.7, you'll notice that at 20 knots upwind VMG is about 4.9 knots from 36 to 45 TWA and the absolute best value is from 39 to 42 TWA.

When you are pointing higher the boat slows down and the leeway increases (due to slower speed and perhaps more side force) very gradually.
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