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Motion Comfort Ratio - how useful is it?

sgwright67

Member III
I've noticed the E29 has the highest MC ratio (~28) of any Ericson under 36', with the exception of the 31i and 35-2, with MC values of 30 and 29 respectively.

Although the 32-2 appears to have a similar hull design, it gains little in displacement over the 29, but the LWL and LOA of course increase, to the MC drops to 25.

I have read and understand how the MC ratio is calculated, but what I am not sure about is how comparable they are between boats of differing LOA. Is the 29 actually more "comfortable" in rough seas that the 32-2, or is the comparison not valid, or perhaps more likely, is a difference between MC=29 and 25 not significant?

Having read several accounts where larger, lighter boats have been less comfortable in ocean crossings than smaller, heavier boats, resulting in the larger boat deciding to reef sooner and actually sail slower, this got me thinking. (I am currently reading "Confessions of a Long-Distance sailor" - https://arachnoid.com/sailbook/index.html, and the author speaks of his Crealock 31 outpacing a 40' light displacement boat)

It will be a while before we try crossing an ocean, and I doubt it will be with an E29, but I just want to know how crucial the MC ratio is.
 
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Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
I don't even belief in WCT--the famous Wind Chill Factor of the evening news.

Because, see, you could always stand behind a tree, if chilly. Or reef. And the day a Pacific Seacraft beats anybody I will stop laughing.

So I guess I'm not going to have anything useful to say about subjective scales of comfort. :)
 

sgwright67

Member III
I don't even belief in WCT--the famous Wind Chill Factor of the evening news.

Because, see, you could always stand behind a tree, if chilly. Or reef. And the day a Pacific Seacraft beats anybody I will stop laughing.

So I guess I'm not going to have anything useful to say about subjective scales of comfort. :)

I know what you mean... it was -6C here last night, and hasn't been above freezing for several days, but I still hiked to the top of mountain today, unzipped my jacket in the sun, and read a book for a few hours. It's all in your perception and what you're prepared for.

But boats aren't like audio systems -- where I can ignore the specs and listen for the qualities I like -- I can't try them out all that easily, so I'm hoping to get a sense for which specs I should pay attention to, and which I shouldn't bother myself with. Maybe the MC is in that category, since most of the boats I am considering are fairly close, with a low around 22, and a high around 32.

Thanks
 

toddster

Curator of Broken Parts
Blogs Author
I don’t know how much to believe in these formulae - even their inventors offer them with a large grain of salt.
If you like, I have a spreadsheet I can share that shows how the magic numbers change with changes in lading and sail area. (Not sure that it properly accounts for changes in waterline length.) You can find on-line calculators and make your own spreadsheets, which might be a good exercise, especially if you are comparing two or three different boats. If you believe that the formulae at least sort of represent the things they are supposed to, you can sort of see how the trade-offs work. I think the bottom line is that the E29 is a relatively heavy and narrow boat. If you load it up with a modest amount of cruising gear (or six well-distributed friends) it becomes very stable and solid. (As long as everyone is cool with a fun degree of heel.) But it quickly slows down and you have to throw up more sail area to compensate, unless you live in a wind tunnel, as I do. I.e. MC goes up and compares very favorably with well-known cruising boats, but SA/D goes down quickly into “slug” range. A larger boat would not be so sensitive to the same amount of cargo.

I think, after sailing the boat for six years, I am beginning to get a feel for what this all means. If you strip it down and sail with only one or two crew, it seems almost like a different boat - lively and a bit squirelly. But still twice as heavy as your neighbors sport boats of similar length. Probably would need to sail those other boats too, to get the complete picture.

Also, the formulae do not address the distribution of weight - gotta get it below the waterline, and in the middle, as much as possible. Still, people have crossed oceans with it, for better or worse.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Theory and Reality

It is a short cruise from calculation ashore to experience afloat, and the route seems to be strewn with opinions.
Firmly in the anecdotal evidence area I can only say that I have also wondered aboutt this. Wondered.. and still have only guesses to go by. :confused:

Fastest deliveries have been on more modern fin-keel boats, and as predicted their motion seems somewhat more abrupt.
Slowest has been a number of trips on a Crealock-designed Willard 30 "nine ton" cutter. That Willard is also the only boat I have barfed on in rough-ish cross seas off the WA coast. Ever.

Given a choice I prefer speed. My trip down the West coast on a Cascade 36 was fast... running down 19 foot seas in gale force winds at a bit over ten kts, with only a reefed stay sail for power.
I am thankful not to have done that in a double ender. :0

I have had deliveries in an 80's designed BK Ericson 38 and a 32-3. Both VERY comfortable and I ascribe much of that to the gentle "V" in the forefoot that gives it a soft landing coming down off a wave face.

Having no education in boat design, I also believe that there is, after all the calculations are in, there is some real "art" involved in hull shapes and that BK has that artistic talent !

Now back to your regularly scheduled informative discourse.... !
 
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