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#11
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 27, 4:08 pm, " wrote:
On Feb 27, 5:29 am, wrote: " said: IMO, Ted Brewer's motion comfort ratio is BS. ... No, it's defensible on several grounds. It was invented by a knowledgeable NA, it does model that "bigger + heavier = more comfort" which is true, and it is widely quoted. No, the ratio does not say that "bigger" is more comfortable. Yes it does. Look at the math. Increase displacement, "Comfort Ratio" goes up. Increase LOA but not LWL, the ratio goes up. The funny thing is that if you take the same LOA and Disp, and reduce LWL, the ratio goes up. If you keep everything the same and decrease beam, the ratio goes up. The assumption is that the heavier a footprint a boat has, the higher load per waterplane area, will give a boat more comfortable motion in a seaway. There is some real-life justification for that, but to assume that narrow heavy boats with long overhangs are the ultimate in seagoing comfort is absurd. .... Just the opposite. It penalizes length (70% waterline + 30% overall) and beam to the 1/3. The results of this can be silly. The idea that an Alberg 30 is vastly more "comfortable" than a Transpac 52 is absurd. In this case, "comfort" is defined only by the boat's motion in a seaway. I want to be clear that I'm not disparaging Brewer. He has drawn some beautiful boats and written at least one very good book. Many moons ago, my family owned a Brewer design. It was a good boat. It's just his "CR" that's bogus. Brewer claims that heave and pitch response will be slower on a boat with a heavily loaded water plane and low ppi all else being equal. That's true for heave and probably reasonably indicative for pitch. He also asserts that roll response will be slower on heavy narrow boats. That may be true, but misses some major factors. The problem is that he goes on to assert that boats with slow initial pitch and roll and heave responses are "more comfortable". This completely ignores major factors like period, amplitude and damping. Agreed. But those can't be reasonably quantified with the very simple dimensions commonly available. .... And, of course, begs the questions of what is comfort is and what conditions we are talking about. Down wind boats with high "CR" will be slow and have a tendency to roll uncontrollably. Upwind they will be slow but wet; if you don't rise to meet a wave you must go through it. Indeed, a high "CR" pretty much assures slowness. It is true that slower is generally more comfortable. It is also true that you can sail a fast boat slowly but you can't make a slow boat fast... Agreed Theres more, but my work is calling. I'd say your work here is done DSK |
#12
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 27, 2:51 pm, wrote:
... No, the ratio does not say that "bigger" is more comfortable. Yes it does. Look at the math. Increase displacement, "Comfort Ratio" goes up. Increase LOA but not LWL, the ratio goes up. The funny thing is that if you take the same LOA and Disp, and reduce LWL, the ratio goes up. If you keep everything the same and decrease beam, the ratio goes up. ... Help me out Doug. The formula is: C = D/(.65(.7Lwl+.3Loa)*B^1.33 Where C is the "Comfort Ratio", D is displacement, Lwl is the length on the load waterline, Loa is the overall length and B is the beam. Now, if you fix the displacement but make the boat bigger in any dimension the ratio goes down. All of the dimensions are in the denominator. Thus, it seems to me that making the boat "bigger" makes it score less well on the CR. I don't follow how you can say that if you increase the LOA but not LWL the ratio goes up. Are we looking at the same formula? -- Tom. |
#13
posted to rec.boats.cruising
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Roll Pitch & Comfort
tsmw wrote:
C = D/(.65(.7Lwl+.3Loa)*B^1.33 Where C is the "Comfort Ratio", D is displacement, Lwl is the length on the load waterline, Loa is the overall length and B is the beam. Now, if you fix the displacement but make the boat bigger in any dimension the ratio goes down. All of the dimensions are in the denominator. Thus, it seems to me that making the boat "bigger" makes it score less well on the CR. I don't follow how you can say that if you increase the LOA but not LWL the ratio goes up. Are we looking at the same formula? -- Tom. Try it in excel. I think the multiplier of less than one has an effect. If you keep everything else constant and increase the LOA, the "Motion Comfort Ratio" goes up slightly. That's making the boat bigger, nyet? If you keep everything else constant and decrease the LWL, the "Motion Comfort Ratio" goes up by a greater amount than above. Ditto for decrease in Beam. This isn't increasing size, but decreasing LWL while holding everything else constant is increasing the overhangs, which does not really improve seakindliness. The greatest increase in the ratio is of course had by increasing Displacement, which is again increasing size. Disp & LOA are the largest factors in a boats size IMHO DSK |
#14
posted to rec.boats.cruising
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Roll Pitch & Comfort
Tom wrote:
IMO, Ted Brewer's motion comfort ratio is BS. Brewer is an interested party and he admits that the "ratio" was made up as a kind of joke. There is absolutely no evidence that it works. As a general theory is is indefensible. On any given day I'd bet you'll be a lot more comfortable in a Catalina 30 than you will be in a 5.5 meter both in terms of motion comfort and amenities but you certainly will not learn that from the "comfort" ratio. Any comfort motion rating system that says that the Colin Archer ketch is the pinnacle of comfortable is just plain cr@p. I'd rate the "comfort ratio" as significantly less important than the color of the mast step in my boat comparison list. Of course, TB has a different view. You can see his rational he http://www.tedbrewer.com/yachtdesign.html. -- Tom. Hi Tom, While I have no experience in yacht design whatsoever, I can see what Brewer was trying to accomplish. The following is a partial copy of his description from his website: "COMFORT RATIO (CR): This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state. ...The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch." So while I agree that there may, or may not be, hard "science" behind it (and I admit I wouldn't know), it seems useful as a comparison when looking at similar type boats for a given usage. And it seems to agree with the descriptions and discussions that were put forth in that earlier thread by Roger Long and others. Red |
#15
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 28, 4:50 pm, wrote:
Try it in excel. I think the multiplier of less than one has an effect. ... I don't do excel but since I have it I put in: =D2/ (0.65*((0.7*B2)+(0.3*A2))*(C2^1.33)) and if you increase any dimension the CR goes down. Remember the mathematical operators have an order of precedence that not all programming languages enforce so you might need the extra parans to get the formula to work as Brewer intended. -- Tom. |
#16
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 28, 5:05 pm, Red wrote:
Hi Tom, ...So while I agree that there may, or may not be, hard "science" behind it (and I admit I wouldn't know), it seems useful as a comparison when looking at similar type boats for a given usage. And it seems to agree with the descriptions and discussions that were put forth in that earlier thread by Roger Long and others. ... Hi Red, Sorry I went ballistic over all this. It is great to be talking boats again in the group and I appreciate your post. Obviously, I don't think the CR has any value at all, but I'm certainly not an expert either. I'll Let Roger speak for himself, but he's selling boats that would rate miserably on the Brewer CR (because they have lightly loaded water planes) and has written a paper on their seakindliness. Do keep in mind that Brewer is selling boats designs that are more expensive to build, slower and have less usable interior space than they typical mass market boats. It happens that the only generally used metric that they rate well on is the one he made up... And, while I actually like many of his designs a lot I'm waiting to see some empirical support for his CR before I give it any weight at all... -- Tom. |
#17
posted to rec.boats.cruising
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Roll Pitch & Comfort
Tom wrote:
Sorry I went ballistic over all this. Ha! Going ballistic seems to be something most of us here do at least once in awhile (I'm guilty), so no prob. It is great to be talking boats again in the group and I appreciate your post. Obviously, I don't think the CR has any value at all, but I'm certainly not an expert either. I'll Let Roger speak for himself, but he's selling boats that would rate miserably on the Brewer CR I would welcome Roger's further enlightenment on this subject. Do keep in mind that Brewer is selling boats designs that are more expensive to build, slower and have less usable interior space than they typical mass market boats. It happens that the only generally used metric that they rate well on is the one he made up... And, while I actually like many of his designs a lot I'm waiting to see some empirical support for his CR before I give it any weight at all... -- Tom. I understand. What I have seen though, are ratings that Brwewer himself has done on numerous boat designs other than his own, and he still finds the CR to be useful for comparison in those cases - not comparisons *against his own designs*, but against similar boats for similar usage regardless of who designed either of them. Red |
#18
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 28, 6:29 pm, Red wrote:
...What I have seen though, are ratings that Brwewer himself has done on numerous boat designs other than his own... I have not seen these. Do you have a link? However, what I want to see is experimental evidence that people actually experience less discomfort on boats with high CRs. I think the physics of the CR is so simplified that there is little reason to believe there will be much correlation between high CRs and more comfortable boats or even boats with slower rates or smaller magnitudes of heave, pitch and roll. The argument that it only works for very similar boats seems to suggest it is broken, too. Very similar boats will behave so similarly that it may be the only way to tell if you're more comfortable is to calculate the CR... Sigh, since the horse is dead it won't mind me beating on it. I think Brewer's basic premise is wrong. One more reason I think the CR is bogus is that it assumes that the sails can always provide roll damping. This is often not the case underway and is seldom if ever the case at anchor. A boat with a high CR will almost certainly be unlivable at anchor if there is even a suggestion of a ground swell. It is axiomatic that cruisers spend 90% of their time at anchor. So, even if Brewer is, by some ineffable chance, right about the CR underway you're still likely to gain a whole bunch more "comforts" on a low CR boat at anchor than you will lose underway. -- Tom. |
#19
posted to rec.boats.cruising
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Roll Pitch & Comfort
Brewers' designs are known to be sea kindly. Maybe he is on to something? Gordon |
#20
posted to rec.boats.cruising
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Roll Pitch & Comfort
On Feb 28, 6:29 pm, Red wrote:
...What I have seen though, are ratings that Brwewer himself has done on numerous boat designs other than his own... Tom replied: I have not seen these. Do you have a link? No Tom, I have seen these comparisons in magazines he writes articles for. If I can dig one up I'll send you the info. Red |
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