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Old 23-08-2017, 18:37   #16
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Re: Ideal displacement for a certain length?

Gtom,

Here is what I was really looking for, from Ted Brewer:

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.
Beam does enter into it as as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. 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.
The CR is : Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). Ratios will vary from 5.0 for a light daysailer to the high 60s for a super heavy vessel, such as a Colin Archer ketch. Moderate and successful ocean cruisers, such as the Valiant 40 and Whitby 42, will fall into the low-middle 30s range.
Do consider, though, that a sailing yacht heeled by a good breeze will have a much steadier motion than one bobbing up and down in light airs on left over swells from yesterday's blow; also that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet. Nor will one human stomach keep down what another stomach will handle with relish, or with mustard and pickles for that matter! It is all relative.
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Old 24-08-2017, 05:53   #17
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Re: Ideal displacement for a certain length?

CR, the classical measure of seaworthiness puts displacement in the nominator, length in the divisor while the modern STIX prefer long vessels, interesting.

Anyway, I prefer the classical long, encapsulated keel design - not just for its high CR, but for its sturdiness.
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Old 24-08-2017, 22:37   #18
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Re: Ideal displacement for a certain length?

Stu'n's'ls! Rig in light air. Stow in a blow.
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Old 25-08-2017, 02:31   #19
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Re: Ideal displacement for a certain length?

The Displacement/Length Categories are:

Under 90 - Ultralight;
90 to 180 - Light;
180 to 270 - Moderate;
270 to 360 - Heavy;
360 and over - Ultraheavy;

For my tastes, a cruising boat in the moderate-to-light range is the way to go, but the heavy displacement ocean plodders would strongly disagree with me.

For more on this, have a look at www.sailboat-cruising.com/boat-displacement.
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Old 25-08-2017, 05:14   #20
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Re: Ideal displacement for a certain length?

It's not my phrase/concept, but 3 days at sea in moderate comfort on a fast boat, along with 2 days in port at anchor, tends to trump 5 days at sea in a slightly more comfortable but slower boat.

That, & when harbor hopping up & down coasts, being able to count on covering an extra 20nm while the sun's still up can make a big difference in one's boating experience & enjoyment. Especially if timing arrivals around the sun's location in the sky with regard to spotting coral & the like is a factor.

Doing 60-90nm port to port, in one day, vs. turning the trip into an overnighter can make a big difference. As on trips where only 1 or 2 nights are spent at sea, for the most part, few if any of the crew get much quality rest to speak of. Off watch or not.

And yes, in order to count on getting to the next port that's 80nm away while it's still daytime may mean leaving at 0430. But typically once the anchor's up, then most of the crew's back in their bunks for a bit. Getting a few more quality hours of sleep, owing to circadian rythyms. Especially if it's still dark.

It's something worth pondering, the concept that is. Especially if you'll be doing a lot of coastal cruising. Which actually constitutes the majority of cruising, except for the occassional ocean passage. Where while miles per day sound impressive, once you're in the routine of being at sea, usually adding a day or two to a passage is of little consequence. And many find long stretches at sea to be some of the most pleasant saiing there is.

So that other than speed giving you more options for dancing with weather cells while on passages, it's not always a giant game changer on longer stints that some might have you believe.
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Old 25-08-2017, 08:25   #21
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Re: Ideal displacement for a certain length?

I'd choose the "best of both worlds" is possible: upgrading the rig/sails on e.g. a 40'/10tons heavy classic.
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Old 10-01-2019, 12:46   #22
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Re: Ideal displacement for a certain length?

With heavier displacement boats there are solutions to having a high SA/D ratio. My boat, 42' LOD, displaces 35,000 lbs fully laden for cruising. She has a topsail gaff rig which sets a large amount of sail on short masts. The lower center of effort of the sail plan allows for greater sail area for a given righting moment. SA/D is 20. The topsail is a light air sail only and puts a lot of sail area up where the wind is. Striking it is the first reef.
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Old 10-01-2019, 13:55   #23
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Re: Ideal displacement for a certain length?

Quote:
Originally Posted by StuM View Post

DLR = (Displacement in long tons) / (LWL in feet *0.01) ^ 3

Commonly accepted DLR numbers are:
Ultralight: less than 90
Light: 90-180
Moderate: 180-270
Heavy: 270-360
Ultraheavy: 360+
Fixed
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