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

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.

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

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.