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Just got off the satphone with Matt Rutherford. He is the young fellow who went around Americas alone. ( 300 day solo, 27,000 miles ) He is currently sailing non-stop from SF to Japan surveying water pollution ( OceanResearchProject.org ) He has about 60,000 blue water miles under his legs.

We had a little sidebar discussion on motion comfort at sea. Last summer he sailed 70 days non-stop in the Atlantic on a über heavy displacement old style boat. This summer he has been sailing a Über light modern style boat in the pacific now going on 38 days.

Matt thought that the lighter boat had better motion comfort because she didn't pound through waves. Instead of pounding, the light modern boat moves smoothly over the water.

This is contrary to docktalk everywhere. Docktalk consensus is a heavier boat has better motion comfort than a light boat because the heavier boat moves up and down slower.

What does the SN community think ?
Well, as with virtually every topic that is raised on sailing forums, the answer is "It Depends"... :)

There may have been reasons other than displacement alone why Matt's ride last summer might not have exhibited an exemplary Motion Comfort...



There are so many variables to the notion of 'Motion Comfort' as to render the notion almost meaningless... Point of sail, how hard the boat is being driven, the list goes on and on. I would agree that more displacement tends to ease the motion, but only in the broadest, most generic sense...

Who am I to disagree with Matt, I have nothing but admiration and respect for the guy, and his offshore experience... But I'm guessing I've probably sailed a wider variety of boats than he has over the years, and in my personal experience, lighter, modern (i.e. flat bottomed) don't ALWAYS "move smoothly over the water..." Especially, upwind... :)

For offshore sailing, I'll take a heavy dose of displacement - placed well below the waterline - every time...



Here's Matt's current ride. I'm guessing that its modest size of 29 feet might be working to his advantage on his current voyage...

Blow this hull form out to 45-50 feet, then start sailing close to the wind in a mature Force 5-6 that's been blowing for a day or more in the open ocean, and he might revise his take just a bit on "how smoothly (or 'quietly') over the water" such a boat might move... :)

 

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implosion in 3...2...1.....
I can't imagine why this should be a controversial subject :) The Either/Or approach many seem to take with regards to Heavy vs Light displacement, 'Modern vs Traditional', is just silly. In boat and sailing, as in Life, 'Moderation', or 'Balance', is generally a very good way to go. I would have thought the matter of seakindliness and 'comfort' in an offshore cruising yacht was settled decades ago with the publication of this book, but obviously not... :)



A guy named Olin Stephens seemed to know a thing or two about boats... I'll go with his take on the subject...

Rousmaniere: What are the characteristics of a wholesome cruising boat? I know most people want a simple answer, but to quote your favorite author, Claude Worth, in Yacht Cruising, "The design of a cruising yacht, like any other work of art, is a harmonious blending of conflicting interests."

Stephens: Yes, every boat represents a compromise between extremes. The deep, heavily ballasted keels and lightweight structures of today's racing types have no place in a cruising yacht. Light displacement also means small space. Room is needed for comfortable living, supplies, water, fuel, spares, tools, and all the gear needed to meet varied conditions.

...

Rousmaniere: How important are the two kinds of stability-positive stability (resistance to capsize) and initial stability (resistance to heeling in normal conditions)? And what about seakindliness? It seems to me that because the new boats are hard to slow down, there's the risk that they and the crew can be beaten to pieces. I once heard your brother, Rod, lay down a valuable rule: "There's nothing like taking two knots off a boat to make it seakindly."

Stephens: Stability is all-important in any boat, first of all in terms of positive range of stability. The boat should have the ability to always come back upright after a knockdown. I'd suggest a stability range of around 120 degrees.

Seakindliness is important, too. I have done less cruising than I might have wished, but I have felt that the rough and uncomfortable ride characteristic of sailing in a modern racing boat a few hours a day is more than enough. Getting a comfortable motion calls for judgment and a trade-off with initial stability. It is desirable that a boat not heel much in normal conditions, but only to a point. A beamy hull shape provides good initial stability but also makes for quick motion. Beam is a most important influence on seakindliness because it is the principal control on the transverse righting moment. As the righting moment increases relative to the transverse radius of gyration, the speed of transverse rotation increases. It can throw you across the deck or the cabin.

Another important influence on seakindliness is displacement. A third is speed, as Rod's point confirms.

http://www.google.com/url?sa=t&rct=...pfnX-d7WZ5x-erz4n7seJ9g&bvm=bv.68191837,d.cWc
 

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The comfort ratio doesn't even consider weight distribution. There is no consideration of moment of inertia. A boat with dinghy and engine on the back, a full enclosure, packed lazerette, and 400 ft of chain for two big anchors can have the same comfort ratio as one with the dinghy on the foredeck, a dodger, a light load, and 200 ft of chain for one big anchor.

Length to displacement ratio has the same limitations. SA/D - the same, limited by VCE and VCG being ignored. There is some residual value to ratios if you understand their limitations but they cannot be applied across the board in a meaningful way.
I've always liked Jeff H's dismissal of the utility of the 'Capsize Screening Ratio", by his pointing out that by repositioning a boat's ballast at the top of the mast, will make no difference whatsoever in its resistance to capsize according to the formula... :)
 
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