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Discussion Starter · #1 ·
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 ?
 

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THIS will open up a can of worms! and explode a thread

I would suggest researching it in other places beside forums

have you ever asked what oil is best for your engines? same thing happens

the THREAD IMPLODES!

jajaja
 

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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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Discussion Starter · #4 · (Edited)
Jon,

Matt did mention something about the deep keel increasing motion comfort, but we had other more pressing subjects to review( ie balancing dragging the net for science for the next 2,000 miles versus a rapidly approaching Typhoon season ) , so the comfort subject was passed over.

He did spend a few days in 30-40 knots nose on in some serious seas off the coast of California on the boat.

What I understand from your comment is that stability is a key variable rather than pure displacement.

Certainly agree that those extreme wide flat bottom sterns are likely horrid upwind, but just considering moderate form hulls.
 

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Weight/displacement is just one factor related to the motion of a sailboat. But don't take my word for it. Here's what Ted Brewer says about his "Comfort Ratio":

"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 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 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 comfort ratio formula is as follows: Displacement in pounds / (.65 x (0.7 LWL + 0.3 LOA) x B^1.333). Brewer says ratios vary from 5.0 for a light displacement daysailer to the high 60.0's for a super heavy ocean cruiser.

More here if you're interested: Comfort, Capsizing, and SailCalc

And hopefully Bob will chime in too...
 

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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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That is lovely boat Jon (Ignoring the thread's purpose)
 

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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...
have you ever read an OIL THREAD?

the simple reason is people are too damn biased on the subject and despite facts and knowleadge settling the matter no matter what people will not understand or respect the other point of view...

of course there are tremendous amounts of facts on the matter and yes it has been settled but the point will always be lambasted from each side

I love that book btw...

Im sure you know what Im taking about...:)
 

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oh btw...we should all know this, basing all you info or facts from one designer would be foolish no matter how good he or she was...

I once got into a deep conversation with a bunch of cruisers out on the hook from around the world

there were french, canadian, german, belgian, spanish and italian

and they were talking designers both old and new and I said I like many a S and S design and I got a few chuckles here and there...nothing bad...but just saying...

one needs to be openminded on certain things...whats comfortable to one person might be hellish to someone else etc..etc...
 
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Here is a quite good description what it is like to sail on an open 60... ;)

"What conditions are the most comfortable?** Generally, any time the water is flat, life on board is pleasant, no matter how strong the wind. As the wind and waves build, it is progressively more comfortable on board the farther aft the wind goes. Upwind and reaching are a tedium of slamming into waves, ranging from gunshot sounds upwind to banging together two trash can lids while reaching. Until about 22 knots that is. After that, life becomes increasingly violent, and the boat's always heeling at least 15 degrees. It becomes necessary to pay attention to your movements, especially down below, where there are plenty of things to fall on. Outside, above 22 knots, there are always waves and water on deck and in the cockpit, meaning foul-weather gear no matter how hot, unless you are planning on just staying wet, in which case the apparent wind quickly has you quite cold. Upwind is bang bang bang like a war, and as soon as you're reaching, you're going so fast that it's like being on a runaway train-one that's constantly falling over cliffs but never crashing. It is super stressful on the nerves, wondering if you will get into the trough of a particularly big wave and nosedive.

Storm conditions on a boat like this are brutal. In winds over 40 knots, it's more or less impossible to go upwind, certainly not advisable. The risk of breaking the boat or your nerves is quite high, and the accelerations and decelerations in waves are like a car accident. Even downwind, over 38 knots, you've reached terminal velocity, and the boat buries into most of the waves. It's almost impossible to slow down. Even with three reefs and a storm jib, in 40 to 45 knots of breeze, it's possible to be pushed to 25-28 knots of speed quite easily."
From: How to Sail an Open 60 | Sailing World
Light and stiff means fast... And with speed comes the really violent movements...
Just my 2 cents and i still prefer speedy crafts over a fat truck... :)
 

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Discussion Starter · #14 ·
Cap't,

Great description of racing a skimming dish across the ocean. Wondering what the motion comfort would be if he only plodded along at 5 Knts like most blue water cruisers ?
 

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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...

So is that the boat he was on last year, or an amusement prop? More like something from the Pirates of the Caribbean amusement park than a real blue water boat. That thing looks like it would be very uncomfortable with any kind of following seas with all that flat overhang. No wonder he was uncomfortable and that is not "heavy displacement" more like "TALL displacement!" I bet it rolls like a soda bottle in rough seas.
 

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It seems about time for me to weigh in on this. In and of itself, weight does nothing good for a boat. In and of itself, weight does not make a boat stronger, it does not make it more seaworthy, it does not add carrying capacity and it certainly does not make a boat have a more comfortable motion. These virtues or liabilities emerge on from how the boat is engineered, how it is constructed, how it is shaped and how weight is distributed.

The public perception that a 'so-called' heavier boat has a better motion comes from the assymetrical argument in how we define a heavy boat. It is possible, and perhaps more typical that a 20,000 lb 38 footer will have a better motion than a 11,000 lb 38 footer and that is where the public opinion originates.

But where this thought process falls short is that a 20,000 lb 44 footer will typically have an extremely improved motion over a 20,000 lb 38 footer. Yet folks would call the 20,000 lb 44 footer very light and therefore expect it to have an uncomfortable motion.

The other issue is waterline length and the distortions implied by that. A 33 foot 1960's era racer-cruiser might have a 22.5 foot water line and a displacement around 11,500 lbs resulting in a D/L in the 430 range. By the same token, a reasonably modern 33 footer could easily have a waterline length around 29 feet, and so while not much lighter, weighing around 10,800 lbs for a D/L down below 200. Based on a comparison of 430 to 200, most people assume the short waterline boat to be much heavier and therefore offer a better motion, which is just not the case. Anyway, what follows below is a more in depth description that I wrote for another purpose probably 10 years ago.

Much of the 'common knowledge' concepts about motion and light boats came out Marchaj's book, "Seaworthiness, the forgotten factor" but that book was written at a time when our understanding of motion and weight was at a very primitive state of study and does not reflect the 20 years of research and 20 years of design evolution that has occurred sincep.

Marchaj's book clearly explained most of the dynamics of motion but many if not most of his conclusions were based on studies of light boats of the IOR type form, which tended to have beamy hull forms with pinched ends. small ballast to displacement ratios and high vertical centers of gravity. His conclusions about the causes of problems with this type form were right on target, but the IOR type form was not a very good pbasis for designing light weight boats. Since then a much better understanding of how to design a light weight boat has emerged and so have light weight boats that offer exceptional seakeeping and seakindliness while advancing the speed of these boats as well.

To explain further, in the late 1980's and into the early 1990's, designers of IMS and Volvo type performance boats came to understand that motion was a major Un-rated factor in the performance equation because large roll angles and sharp accellerations disrupt the flow over the sails, keel and rudder, creating drag and limiting the production of lift. There was a huge amount of study that went into developing an understanding of motion control and being able to computer model motion. Full sized boats were instrumented and that data was used to calibrate, validate or discredit the various theories floating around.

In the end, the real predominant factors that control the faster motions of a boat, (roll and pitch), were found to be weight and bouyancy distribution (both static and dynamic), with dampening being a much bigger determinant than previously understood. The overall weight of the vessel has less than zero to do with the roll or pitch speed or angles.

So it is that many of the better designed 'lighter weight' boats, which have dramatically lower and more concentrated vertical centers of gravity, progressive dampening, and hulls modelled to minimize sudden increases in buoyancy, tend to have gentler rotational motions (roll, sway, and pitch) through smaller angles than more traditional heavier weight boats.

Heave, which generally tends to be one of the slower of the six degrees of motion, is the only direction that modern light boats generally do poorer in, but even with heave, this is true only in some conditions. And even in this case the greater rate of motion is partially the result of modern bouyancy distribution rather than being simply weight driven.

The current theories on heave is that the relationship between the heave acceleration to the wave configuration is proportional to the weight of the vessel per waterplane area. In other words, the force of a rising wave acts on the overall area of the waterplane of the boat. The more weight per square foot of waterplane, the slower the boat will accellerate vertically. Since modern designs tend to have a lot of waterplane for their displacement, they tend to be affected more quickly by heave; pretty much following the contour and speed with which the wave surface is rising.

Heavier boats per square foot tend to expeience a kind of delay. It takes longer for them to feel the upward force and change direction, but once they do they store more energy and so momentum takes them higher than the wave surface at the top of the wave. This delay can be very helpful in a short seaway but can be a real liability in steep seas where being out of phase means a pretty hard landing sometimes. (been there, done that, have the broken toes to prove it)

I also think that it is very much a mistake to say "the heavier boat will have a much more favourable sail area to displacement ratio, sail area to internal volume ratio and will usually outrun a light displacement boat of similar length in light weather. Long range cruisers encounter far more calm or light weather than heavy weather, and a heavy, stable boat with large sail area tends to turn in a high average speed in relation to her waterline length"

That simply is not the case and hasn't been for well over a decade. It may have been true of early ULDB's but modern light weight boats have enormous stability relative to their drag or displacement and carry sail plans that are proportionately larger than any heavy displacement boat that I know of. SA/D's in the 22 to 24 range are extremely common on newer light weight designs and when coupled with the very low drag foils and high efficiency rigs these new boats are much faster in an absolute sense at both the light and heavy ends of the wind speed range.

I also want to bring up this issue how light vs. heavy boats seem to be defined, and particularly as it seems to be done on this forum. This goes back to a point that I have raised here before. If we size boats by length then we seem get into an endlessly circular discussion of the merits of light vs. heavy boats. I agree with Mr. Welsford when he says, “a boat of a given construction type costs pretty much by the pound or kilogramme. “ But I disagee with him when he goes on to say that heavy for their length are scarce on the market because the are more expensive in ‘a price sensitive market’.

I think that he is mistaken because of how he is choosing to ‘size’ the boats in question. In other words, I think that displacement is a much more accurate indicator of the ‘size’ of a boat. This is especially true when you talk about a boat intended for long distance voyaging. In other words, while it is tempting think of a boats size boat solely on a length basis and the need for specific accommodations, the displacement of a particular boat says a lot more about its 'real' size.

And it is here that I have the problem with the apparent implication that light boats are inferior for offshore voyaging. If we size the boat by its displacement and compare two equal displacement boats, one being longer for its weight than the other, all other things being approximately equal, the longer boat will offer better motion comfort, be more seaworthy, be easier to handle, have an ability to carry more supplies, and be quite a bit faster. In most cases and for most costs, if the boats are of equal weight they will have a similar cost to buy, and maintain.

I believe that the popularity of longer boats for their weight (or lighter boats for their length) is not about people wanting to buy boats of equal length for less money as much as it that people want to buy longer boats for their weight. Most sailing magazines that I get have been talking about the trend in new boats where people are buying longer boats than they used to. I contend that they are buying the same sized boats that they used to (by weight) and that these longer boats are just plain superior designs on all counts to earlier, shorter boats for their weights.

I hope this proves to be food for thought rather than the source of a food fight.

Respectfully,
Jeff
 

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Discussion Starter · #19 ·
Jeff,

Fascinating - definitely agreed that LOA is a misleading measure of size of a boat.

A good example of this might be the Columbia 50 which is in today's terms really a 38 ftr.

And your comments on heave illuminate what Matt was describing
 

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What you are saying is very interesting, can you please explain to me the difference underwater profiles makes with regards to motion? I see a lot of posts criticising the 'soap dish' shapes of modern cruisers.

I would like to see more boats with more 'V' shaped underwater profiles but that isn't from understanding of physics, more intuition.

I read about a boat recently called a riptide 55 which I thought was very interesting.
 
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