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

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.

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

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

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.

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.

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

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.

Roll Pitch & Comfort
 

Brewers' designs are known to be sea kindly. Maybe he is on to something?
Gordon

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