Ted Brewer's 'Comfort Ratio'

Kelpie

Kelpie

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I've been comparing a few different boat designs and seeing how they fare on this 'comfort ratio'. A bit surprised by some of the results- especially that my current 33ft boat scores really very well (31) compared to some much larger boats that I have looked at, which have been in the low to mid 20s.

I've been using this website to work out the ratio: http://www.sloopit.co.uk/sailing/br_calc.asp
(I'm sure other online calculators are available).

Just curious about how useful people think this measurement is- and does it reflect your own experiences?.
 
Entered the following metric numbers for my Westerly Fulmar.

Sloopit for fin Westerly Fulmar.png

A Motion Comfort Ration of 158.85

Changing to imperial units.

Sloopit for fin Westerly Fulmar Imperial.png

A Motion Comfort Ration of 21.94

Interesting concept.
BUT just a load of rubbish with the current calculations.
Back to the drawing board I think.
 
Concerto said:
Entered the following metric numbers for my Westerly Fulmar.

View attachment 68843

A Motion Comfort Ration of 158.85

Changing to imperial units.

View attachment 68844

A Motion Comfort Ration of 21.94

Interesting concept.
BUT just a load of rubbish with the current calculations.
Back to the drawing board I think.
Click to expand...

Why does the displacement to ballast ratio change from approx. 4:1 to 2:1 when you go from metric to imperial? :confused:

Richard
 
The conversion for metric to imperial isn't working. I input the boat length in metres, and quite clearly it thinks it is feet for the max hull speed!

Still, 168.75 ain't bad for the "motion comfort ratio".

Nice idea in principle. But only if the calculations are working properly. Rubbish if not.
 
RichardS said:
Why does the displacement to ballast ratio change from approx. 4:1 to 2:1 when you go from metric to imperial? :confused:

Richard
Click to expand...

You tell me. That's why I posted the figures. The figures are those published for the Fulmar in both imperial and metric. Something very screwy in the calculations.

May be someone else can find similar errors for their boat.
 
Quote from Ted Brewer's web site:
http://www.tedbrewer.com/yachtdesign.html
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.
 
Concerto said:
You tell me. That's why I posted the figures. The figures are those published for the Fulmar in both imperial and metric. Something very screwy in the calculations.

May be someone else can find similar errors for their boat.
Click to expand...

But I thought that you had filled in the figures for metric and then filled in the figures for imperial which would mean that the change in ratio which I'm reading from the Displacement and Ballast boxes results from your input. If that's not how you do it then I'm as confused as everyone else. :confused:

Richard
 
The Comfort Ratio is Mass over Area.

That is : Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333)

Imperial: lb/ft^2 Metric: kg/m^2

But mass: 2.2 lb = 1 kg and area 3.28084 ft = 1 m

So the ratio will not be the same because the difference in relative magnitude of the mass and area units.

Jumbo will be along to correct me.
 
I was a bit puzzled by what units to use, e.g. kg or tonnes, so I selected one of the 'popular yachts'. That showed me it was kg but the sail area was crazy - 722 sq m on 32.5m LWL?

It didn't take kindly to my LWL of 11m and beam of 7m ;)
 
RichardS said:
Might I suggest that post #2 kinda nailed that one? ;)

Richard
Click to expand...

Exactly. Look at Brewer's boat designs and that will explain the discrepancies. He developed the idea before "modern" boats became popular and the lists in #18 supports his preferences.

Question about whether he writes for government is also pertinent as illustrated by yesterday's long term forecasts of growth. First determine the answer you want, then select a way of producing figures to confirm that answer and explain it in a language that tries to reassure people that it is all well though out.
 
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