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  #1  
Old 06-25-2009
Philippe
 
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Comfort Ratio - How to read 5 to 10 pts differences?

Hi,

I am making a short list of blue water boats (for a potential purchase and sabbatical) by looking at their ratios, and one of the criteria my girlfriend and I are trying to understand is the comfort ratio.

While I understand how it is calculated, and what is the overall meaning, I have no clue (due to being an inexperienced sailor, still taking sailing classes) what 5 to 10 points difference really make once blue water sailing.

Case in point (from online database):
  • Tayana 37 - CR: 43.8 -> Reference point (RP)
  • Pacific Seacraft 37 - CR: 38.0 -> -5.8 of RP
  • Cabo Rico 36 - CR: 35.6 -> -8.2 of RP
  • Shannon 37 - CR:33.5 -> -10.3 of RP

My question isn't which boat is better, I am not there yet. What I'd like to understand is whether the differences between these boats, in term of motion comfort, can truly be perceived at sea? Is a Shannon 37 really less comfortable than a Tayana 37? Tayana 37 vs. PS 37? PS 37 vs. CR 36? etc... or am I paying too close attention to what is just a number?

None of the boats I am considering seems to be apart of more than 10-12 points on the comfort ratio. Should I be happy their comfort are in the 30-40 range and move on to care about other numbers / other issues?

Thank you for your help,

Cheers
Philippe
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  #2  
Old 06-25-2009
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Quote:
MCR = DISP / (.65*BEAM4/3(.7*LWL+.3*LOA)) This ratio was invented by Ted Brewer who say's he dreamed it up "tongue in cheek" as a measure of the motion comfort of a boat. A boat that has a more corky motion is considered less comfortable then one less affected by wave action. A higher value is better (if you like comfort). Smaller and beamier boats tend to have a lower ratio. This is best used to compare boats of similar size. A 26 footer should probably not be compared to a 40 footer using this ratio. The ratio is a factor of LOA and LWL and it may assume that boats with long overhangs tend to have wineglass shaped cross sections which provide more gradual buoyancy as they are immersed. However, a boat like a Valiant 42 has a long LWL for it's LOA and possesses this more wineglass shaped cross section. The ratio also favors displacement (higher gives larger result) and there is no accounting for distribution of weight. It also takes no account of waterline beam, a value that can be quite informative but is rarely available on stat sheets.
My interpretation is it relates how "corky" a sailboat feels, based on ratio of overhangs to waterline. But it does not take into account how "cow-like" or sluggish a non-corky boat can feel. My own boat only rates a 24.0, but I have no open ocean plans.

Based on these measurements the sailboats of the 60's through 80's will have a great advantage over those designed in the last 20 years with the more "European" hull shapes (beamy with lwl = loa). And maybe that is so.
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Old 06-26-2009
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Keep in mind that any center cockpit or pilothouse boat is likely to be more comfortable by virtue of the fact that you are not at one end of a boat that is moving like a slow see-saw through the waves.

Comfort is best appreciated on the boats themselves. I have found that I don't notice the "snappy" motion of a race boat if I am having fun on a race, just as I don't resent the stately tack of a heavy displacement full-keeler.

I certainly notice a change in motion, however, when I go up from the pilothouse of that full keeler onto the aft deck and the "outside helm".
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Old 06-26-2009
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According to the formula by Delerious, the comfort ration is based on three values: LOA, LWL, DISP, and BEAM. The four boat you list are very close in LOA and Beam. That means the main cause of the different values are Displacement and LWL. All else being equal, a boat with a higher displacement is slower, but has a better motion. All else being equal, a boat with a shorter water line is slower. I think the better motion in this case is probably more controversial.

In any case, all of these are quite heavy boats that have moderate numbers in other areas. They will all be comfortable. I would focus far less on the motion comfort ratio than other factors when comparing these boats. Other factors that I think are more important when comparing these four boats are build quality and function of deck, cockpit, and cabin layout (you will have to tour all four do decide this).

Even more important: which of these boats makes your heart sing when you look at it? They are all great boats, but the one that takes your breath away is the right one for you.
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Old 06-27-2009
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Thanks all for the answers. I appreciate it.

I'll definitely ask more information when I am further in my research (and reading) of boats. For now, the cruising plan is still very fuzzy, just at the beginning, when dreams tend to be bigger than reality

Cheers
Philippe
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Old 04-09-2010
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5 to 50 points difference is totally useless in telling you about a boat characteristics. Its seems that as soon as someone posts a question about the seaworthiness of some particular boat, that a well meaning responder sends them to Carl's Sail Calculator to look at the Capsize Screen Formula and the Motion Comfort Index. And no sooner than poster questions the seaworthiness of some boat, that someone cites the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.


Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.


I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.


And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin structure, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.


There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: ffice:smarttags" />lace w:st="on">Hulllace> Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and lace w:st="on">Hulllace> Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )


There is a convenient calculator at http://www.sailingusa.info/cal__avs.htm


It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return.

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Last edited by Jeff_H; 04-09-2010 at 08:11 AM.
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Old 04-09-2010
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LOL...Hey Jeff, you do know you're replying to a thread from last summer, right??
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Old 04-09-2010
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Yes I was aware of that. I had seen Phillipe's post about boats not to take voyaging and in forming an answer I had taken a couple minites to look at his earlier posts trying to see if I could get a sense of where he was coming from and how experienced he was since his short list and criteria were so strange.

At the time I noticed this thread and thought I would return to it rather than highjack his other post when he had specifically said that he did not want to discuss his criteria on that post. Since he had posted the other thread I figured he was actively on the board and so might see this.

Jeff
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Old 04-09-2010
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Something very odd happened above. At least, on my screen anyway.

I see Jeff's post #8, with Sailingdog's signature line at the bottom of his post. Then in post # 9, Jeff appears to be replying to a post (by SD?) that doesn't show up on my screen. Anyone else seeing this?
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Old 04-09-2010
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Quote:
Originally Posted by JohnRPollard View Post
Something very odd happened above. At least, on my screen anyway.

I see Jeff's post #8, with Sailingdog's signature line at the bottom of his post. Then in post # 9, Jeff appears to be replying to a post (by SD?) that doesn't show up on my screen. Anyone else seeing this?
You must have slipped in between the cracks in the posts/web/universe because now I'm seeing normal stuff and in fact your post is #9 Time warp........................ Danger ...... Danger...........
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