Long versus short overhangs

[KeelHaulin]

Originally Posted by Jeff_H

But again, the simplest way to debunk the Comfort index is by comparing comparing two boats. If we start with a CCA era 40 footer with an LWL =30ft, Beam=12 and Displacement= 20,000lbs we come up with a MCI of 33.94. If we compare that to a 44 footer LWL =39ft, Beam=13ft and Displacement= 24,000lbs we come up with a MCI of 29.8. If weight distribution and VCG, cross sectional shapes, etc were similar, the bigger, proportionately slightly narrower, heavier boat would have a significantly more comfortable motion, yet Brewer’s MCI suggests just the opposite. And missing from the formula is such critical motion impacting elements as VCG or even Ballast to displacement ratio, dampening or even draft, and height of mast and so on. Which comes back to my central point being that the MCI produces such inaccurate results as to be worse than useless as a real comparative tool.

If you are going to "compare numbers" to "debunk" a formula; you should use scalable numbers. You can't just pull numbers out of the air and say 'yep that fools the calculator'; because it's not the case. I have a feeling that Mr. Brewer did some heavy number crunching to come up with that equation. When it comes to deriving a dimensionless number that will quantify the how comfortable the motion of a boat is; it just does not just come out of thin air and to sign your name to it as a hull designer is a pretty big commitment to it's derivation and use.

If you are going to compare numbers; start with some directly scalable values. For the 40' LOA vs 44' LOA you would have the following numbers (appx):

LOA - 40' => 44'
LWL - 30' => 33'
Beam - 13' => 13.2' (Ratio LWL/LOA = 30%)
Disp - 20,000 => 25,000 (Best guess; 25% heavier for the 4' LOA).

Those numbers give an MCR of 33.99 for the 44' LOA boat boat. If you use 24,000# the MCR is 32.63. Seems like it is directly scalable.

If you do the converse (scale the 44' boat in your example down to 40') you get an MCR of 28.17; pretty darn close to the number you calculated.

What does it mean? MCR is a measure of linear and pitch inertia forces that the boat encounters (not roll stability, not heel). The calculator on US Sailing says the following:

Motion Comfort Ratio was developed by Boat Designer Ted Brewer. The formula predicts the speed of the upward and downward motion of the boat as it encounters waves and swells. The faster the motion the more uncomfortable the passengers. Thus, the formula predicts the overall comfort of a boat when it is underway. Higher values denote a more comfortable ride. As the Displacement increases the motion comfort ratio will increase. As the length and beam increases the motion comfort ratio will decrease.
MCR = Disp / (2/3*((7/10 * LWL)+(1/3 *LOA))*Beam4/3)

Simply put it compares displacement to hull area. The more hull surface area (below waterline) the higher the tendency to pound. If the displacement relative to the area of the hull is higher the tendency to pound is reduced. That's why light displacement racers are generally a rough ride and heavy cruisers (with overhangs) are more "sea-kindly". The MCR formula does not speak to roll stability, vertical CG, etc. It was never intended to speak to those issues.

[Valiente]

Not being an NA, I can't comment on the formula, but thanks Jeff for that summary. I still would like to hear what boats you like for offshore work in each "era".

As for the CCA design rules, I can only comment that while the large majority of '70s boats around here aren't strictly CCA designs, there's a lot of influence there, and it manifests in the way C&C racer-cruisers, for instance, stiffen up at the 25 degree of heel mark and basically seem quite stable there...until they wipe out in a broach if they are on that point of sail. Anyway, a good explanation of your point of view, lunch or no lunch.

[Jeff_H]

KeelHaulin:

I don't see your point about using scalable numbers. If the formula is to have validity its needs to reliably produce numbers that reflect the relative motion comfort of the vessel being considered. I purposely chose to alter input numbers for the larger boat (which approximate the numbers that you would expect for a modern offshore cruiser) that would also, in a real life situation, be expected to improve motion comfort but which using the motion comfort index made the larger, heavier, slightly proportionately narrower boat appear to have a less comfortable motion. If a formula is to have usefulness it at least needs to get the basics right.

I also want to touch on your point, "MCR is a measure of linear and pitch inertia forces that the boat encounters (not roll stability, not heel). "

First of all, the formula is called Motion Comfort Ratio and not heave and pitch ratio. It has been promoted as being a broad based indicator of of motion comfort in all six degrees of movement. In Ted Brewer's own words "The ratio was dreamed up by your author for an article in Cruising World magazine. The article was tongue-in-cheek but the comfort ratio has been accepted by many as a measure of motion comfort, and it does provide a reasonable comparason." That may have been true when that was written in 1985 but we know a lot more about motion than was known when that was written and we know that almost none of the major factors that impact motion comfort are present in that formula.

But for the sake of discussion, lets say that you are right that the MCR is only intended to evaluate pitch and heave, then it does not even do that well. Starting with heave, lets use the CCA era 40 footer from above, 40 footer with an LWL =30ft, Beam=12 and Displacement= 20,000lbs with a MCR of 33.94 and compare that to a boat that is identical except that the second boat has a waterline length of 35 feet and a significantly lower (seemingly less comfortable) MCR of 30.68. But as explained above, the longer waterline length would produce a boat that pitched through a smaller angle, and because of better and more progressive dampening, would pitch more gently as well.

Heave is a much more complex motion. As an aside, although varying with boat speed, asa a general rule, pitch is the more critical motion until you achieve wave lengths approaching and exceeding two times the boat's length. At that point, heave starts to be felt more significantly.

To explain why the Motion Comfort Ratio fails to predict motion even for pitch lets compare two boats with an equal area water plane and short overhangs, but one has substantially heavier displacement. In a seaway where heave has become significant, the lighter boat will move upward almost with the surface of the wave, changing directions vertically at the same rate that the wave face changes direction vertically and rise and fall just a little more than the wave rises and falls as its small inertia carries it only a little bit higher than the wave surface and causes it to sink at the bottom just a small amout.

If we look at the heavier boat, because of its bigger inertia there is a lag before it starts to rise, but as it rises it stores more kinetic energy which pushes it further out of the top of the wave and causes it to get out of phase with the wave falling further with less dampening from the wave surface so that again its heavier displacement pushes it further into the wave trough. It is just what you would expect; greater inertia resulting in a slower motion through a longer range of travel. In this example the Motion Comfort Ratio conceivably picked the more comfortable boat, at least for one third of the population for whom the rate of speed change in more uncomfortable than amount of motion.

But if we look at the example above and add a third boat, this one with an equal waterplane and displacement to the ehavier boat but with long ends, we would have the worst of all worlds, because the inertia caused delay between the time that the boat feels the upward force of the wave, and starts to move allows the wave to collide with the underside of the bow overhang and the rapid increase in buoyancy causes the bow to snap upward, or in a really big wave to also catch the counter and cause the whole boat to snap upward, and so the motion becomes disproportionately snappy, and more energy is being imparted into the hull so that the boat is thrown higher above the wave top, and comes down out of phase with the wave, but again rapidly increases buoyancy and crashes to a stop harder at the bottom. But if we look at the less comfortable heave characteristics of the longer overhang boat and compare that to more comfortable motion of the the lighter shorter overhang boat the prediction of the Motion Comfort ratio once again gets it backwards. (33.94 to 30.68)

Just to clean up a few other items from your post, "The more hull surface area (below waterline) the higher the tendency to pound." Strictly speaking you have that backwards. Pounding occurs at the interface between water and air, so the more that you have out of the water but near the waterline the more likely you are to pound. But of course the intensity of how hard you pound is also controlled by the shape of the part that is hitting the water, so the relatively flat and shallow surfaces of an IOR forefoot, or the fullness above the waterline typical of Alberg's work, is more likely to pound harder than the more knifelike, finer, short ended bows currently favored for offshore cruisers.


Valiente: That's a good question that will take some time to answer, but I need to turn in for the night. Just to start the ball rolling and whet your appetite, we often forget about the Midget Ocean Racing Rule from the same era as the CCA. MORC tended to produce much more wholesome sea boats with nicer motion comforts.

A good comparason might be to compare the Tartan 27 which was an S&S designed MORC boat to something like the Bristol 27, an Alberg designed CCA rule boat. (LWL 21.42 vs 19.75). Another good CCA era comparason might be to compare the Halsey Herreshoff Bristol 33 (LWL 25.75) to the Hood designed Bristol 32 (LWL 22.1). From my perspective both the Tartan 27 and the Bristol 33 offered superior motion comfort to the similar sized (and priced) boats in the comparason.

Good night,
Jeff

[seabreeze_97]

Jeff, you can be sooooo snotty sometimes (and the nerve..closing your dissertation with "Respectfully"), it's almost like you're two different people. CCA boats heel, and lengthen their waterlines. Short overhang boats, i.e., non-CCA boats, tend to be newer designs, right? They also tend to be wider, right? They sail faster when heeled less, right? So why would whether they lengthen waterlines when heeled matter, since it would be avoided? Understand, my statement was comparing both designs in optimum position. And I hate to break it to you, but there's plenty newer boats out there that don't pick up anything in waterline when heeled because they have an ass like a doublewide!
Also, I didn't "object" to the 500lb weight example. Granted, that would be too much for smaller boats for obvious reasons, but what I said was your 500lb weight actually improved motion comfort, not worsened it, as you seem to be saying. My recent research into mast design backs up the "slightly heavier mast isn't necessarily a bad thing" mantra, which is a more reasonable version of your 500lb mast weight.
Your two examples of the Comfort Index are not appropriate. The CI was never intended to compare any two boats. It was penned to compare similar boats to each other. Mr. Brewer explains this. You consider the data inaccurate, but you are looking at it as a side-by-side comparison, which is not correct. Comparing boats with 4,000lb difference in weight and 9 FT!!! difference in waterline is not a reasonable comparison. The 44 footer may well have a lower number (implying less comfort), but it would be more comfortable (or at least AS comfortable) as measured by the butt meter. The CI is much more accurate when you, for example, compare something like a 32ft CCA boat with 22ft waterline to something like a new 25 footer with 22ft waterline. Typically, said CCA boat would rank higher because it would be heavier, but the comparison is much more appropriate, mostly due to the waterline of the hulls.
Now, while you believe the CCA rule to be the "antithesis" of what a boat should be, in all my digging, so far, you're the only one I've seen so far who claims such a breadth of knowledge, but has such a distaste for them. We see you vs the likes of Alberg, Luder, Hood, Brewer, etc. Hmmmm. Who should I believe? Now, I'm not saying it's wrong to have an opinion, but me thinks you protest too much.

[sailaway21]

I believe Jeff to be mistaken in some of his basic assumptions. That being said, I'll confess to following his posts closely as he is one of the more well versed members of sailnet. If you're not reading him, you're missing out.

Jeff's illustration of the two similar boats and their statical stability curves being similar is patently wrong. Hull form itself effects two things only in those curves; location of center of buoyancy and form stability. Jeff refers to it further down, but inadequately, that the greater weight and the resultant placement of G, center of gravity have a large effect on stability. where he misleads is in implying that G is not part of the statical stability curve. G and B's relative postions are part and parcel of the statical stability curve and it is the reliance on form stability, by definition occuring at lower angles of inclination, that has gotten us into this mess.

For purposes of illustration, let us eliminate the effects of sails. We cannot in practise, but the point is easier made without them and, if necessary, we can discuss their effect afterwards. The stiff, light boat that Jeff prefers has a deficiency. While it has a large GZ, or righting arm, it does not necessarily have a large righting moment, GZ x displacement. This boat will have a short rolling period. This may seem desirable, and it is in certain types of boats, but it will lead to a very quick and violent motion in a seaway. Boats of this ilk, commonly with a bulbed fin keel, are highly dependant on headway to dampen their roll by making use of the flow over their keel. Reducing GZ, or righting arm, by moving weight upwards results in a slower period. But this is rarely done in a sailboat on it's own without a concomittant increase in displacement. The heavier boat can carry a smaller GZ and still have a good righting moment. She will roll much more gently, if deeply, and will exhibit more inertia. The reason I have disallowed the consideration of sails is that we are really talking about seakeeping abilities here and the conditions encountered will be ones where minimal, if any, sail is carried.

This is important because, unlike ships, the boat rides over the waves. the stiff boat is much more likely to get into a pattern of synchronous rolling, where the period of the roll and the wave period coincide. That's dangerous.

In mine and Jeff's defense, as well as yacht designers everywhere, these are difficult matters to divine accurately. The yacht architect does not have the resources and capability to test these things in the way that the ship's naval architect does. And the yacht hull form is much more complex.

Larsson and Eliasson in Principles of Yacht Design,3rd ed., 2007 discuss these matters relating to "STIX".

Regarding length they state:
The size of the yacht is the single most important parameter, when assessing safety at sea, since it defines a scale with which to measure the waves. The larger the yacht the smaller the relative size of the waves. In this approach the size is simply taken as a weighted average of the length overall and the waterline length. Whether or not this is a valid assumption can be discussed. To consider the waterline length twice as important as the overall length , as done in the formula, is reasonable when it comes to the sailing performance of a yacht. But in this context, with the yacht often heeled to 90 degrees or more, the overall length should be more important than the waterline length. As the formula stands now it penalizes yachts with overhangs (old ones) and encourages yachts with square ends (modern ones).

In discussing displacement:

...Penalizing light yachts may seem unfair considering the experiences from races under very rough conditions, such as the Fastnet race in 1979 and the Sydney-Hobart race in 1998, where light yachts indeed survived well. However, these yachts were handled by full racing crews. Yachts covered by the ISO standard are supposed to travel the same waters with a minimum crew, perhaps rather inexperienced. Then a very fast responsive yacht is not a good asset......based on research carried out both in England (Wolfson Unit, Southhampton) and the USA (SNAME) after the Fastnet disaster, it has been concluded that large beam in combination with light displacement accentuates the risk of wave capsize.


The authors go on to discuss hull form and keels with a preference for a traditional long keel for seakeeping abilities. They describe quite well the inhetrent efficiency of the long keel versus the fin keel at resistence to rolling. Much of the fin keeled boats resistence to rolling is produced by her making way. That's a bit off topic other than being a common characteristic of boats with long overhangs.

To overhangs themselves the author's state:
A large forward overhang is likely to increase pitching, since large pitching moments are created when a wave hits this part of the hull far from the center of gravity. Aft overhangs may, of course, have a similar effect in following seas, but the frequency of encounter is then much lower so the problem is small. On the contrary, the stern overhang may be beneficial since it may damp the pitching motion in head seas. A high freeboard forward, and a flared one in particular, prevents green water on deck, and spray hitting the cockpit is effectively avoided also. The hull thus gets much dryer......

....Most modern designers strive for small gyradii, a light hull, large stability and a small keel. All these features tend to increase the accellerations on board the yacht, thus making it less seakind. For a cruising yacht this is unlikely to be the optimum solution.


The author's conclusions seem to mirror those of CA Marchaj in Seaworthiness: the forgotten factor regarding offshore hull design. Both of these books have been quite interesting to read but I'd encourage anyone not familiar with basic styability and trim to read LaDage's Stability and trim for the Ship's Officer which will not make you a naval architect, but is an excellent two hundred page primer on stability, profusely illustrated, and extremely well written given it's professional nature. (that is to say, anyone from any background can read it and readily understand in short order what keeps a ship from tipping over!)

I'd like to thank everyone for their postsm in these design threads; they're not your typical two sentence subject and surrounded with controversy, some passionate. That the discourse is so civil and friendly is refreshing and a delight. And it's a lot of typing for no tangible gain either!

Remember Giu's adage, different boats for different jobs.

[KeelHaulin]

Originally Posted by sailaway21

To overhangs themselves the author's state:
A large forward overhang is likely to increase pitching, since large pitching moments are created when a wave hits this part of the hull far from the center of gravity. Aft overhangs may, of course, have a similar effect in following seas, but the frequency of encounter is then much lower so the problem is small. On the contrary, the stern overhang may be beneficial since it may damp the pitching motion in head seas. A high freeboard forward, and a flared one in particular, prevents green water on deck, and spray hitting the cockpit is effectively avoided also. The hull thus gets much dryer......

While I understand the comment that "overhang will increase pitching"; I can't say I agree. If the same boat were built with a plumb bow the pitching moment due to bouyant force would be equidistant from the CG. Now if you shorten the boat's hull down to the LWL you are going to reduce pitching moment; but now you also have a much smaller boat; and a much different hull form. How do you achieve a high forward freeboard; with flare, and little overhang? Seems like you would have one strange looking bow; but that's just my way of thinking

....Most modern designers strive for small gyradii, a light hull, large stability and a small keel. All these features tend to increase the accellerations on board the yacht, thus making it less seakind. For a cruising yacht this is unlikely to be the optimum solution.

This is what I was trying to point out earlier; many of the hull design factors that would make a racer quicker on the race course are the same that will make a boat less seakind/stable. That's why it is possible that many of the new era racer/cruisers are less well suited to cruising than they are for racing. That's fine if you want a fast boat that will pound your guts out but you'll finish ahead of the fleet; but that same boat might be dangerously fatuiging on an offshore passage.

In regards to the issue of LOA having a negative effect on MCR; well that is only true if you increase the LOA (and LWL) without an increase in displacement. Meaning, if the weight per unit length is reduced the boat will have a lower motion comfort (the chop and waves will have more effect on the motion of the boat). If that is not true I would like to see some data or calculation that disproves it.

[Jeff_H]

Seabreeze: I want to apologize to you and to anyone else who was offended by the tone of my posts above. I apparently have a writing style that comes off as snotty or brusk, especially when I am writing quickly as I was yesterday. I in no way intended to put anyone down or to distort anyone's position.

In this post, as in most of my posts, all I trying to do was to put out ideas as I understand them which hopefully would allow for a meaningful discussion. My goal is not to put anyone down or club any idea to death but to allow a discourse on complex issues that are difficult to explain. My hope in posting is that even if we don't end up with a complete agreement, we at least end up understanding where the other party is coming from. So if my post seemed snotty to you, I am deeply sorry, I meant no offense and did literally intend the salutation, 'respectfully'. Respectfully was is truly how I meant my discussion.

But just for the record, I want to touch on your comment "Also, I didn't "object" to the 500lb weight example." I did not say that you objected but that you raised a valid objection. Here are the two things that I said about your comments on that particular example."Both Seabreeze and Plumper question my choice of the example of putting 500 lbs at the top of the mast to illustrate why the Capsize Screen and Motion Comfort Index is being dismissed as misleading. "

And went on to agree with your objection in the following portion of the post, "Which brings us full circle back to Seabreeze 97’s objection to my earlier example, a boat with a 500 lb weight up its mast would have a tendency to roll further in the wave, and would have less of a tendency to right itself. I have to agree that the impact on motion comfort of a 500 lb weight up the mast is more complex than that example suggests which is precisely what Seabreeze 97 is pointing out. "

I also want to touch briefly on Sailaway21's point, " Jeff's illustration of the two similar boats and their statical stability curves being similar is patently wrong. Hull form itself effects two things only in those curves; location of center of buoyancy and form stability." Sailaway, you are correct that the shape of the boat only affects the position of the center of buoyancy (in 3 axis), which of course is impacted by amount of form stability present at any given heel angle. I purposely chose an example where every thing was the same, shape of the hull and displacement, but where the vertical center of gravity was significantly higher in the boat. Having done this exersize on smaller simplier boats (to look at what happens with a ballasted dagger board raised vs in lowered position), assuming that the boats do not change logitudinal trim with heel angle (which I know is a big assumption), I think what you would find is that the shape of the two curves are similar except that the boat with the higher vertical center of gravity will have a plot that is compressed both vertically and horizontally.By that I mean that the profile of the curves will have similar humps and shelves, but that the righting moment will be smaller on the boat with the higher VCG and that the maximum righting moment, and the limit of positive stability will occur at a smaller angle of heel.

I beleive that this is true because in both cases, the center of gravity remains static and in the same longitudinal and lateral positions, while the center of buoyancy changes in the same manner with heel angle both vertically and laterally, again assuming that trim does not change. Does that make sense?

And to touch on Sailaways comment "How do you achieve a high forward freeboard; with flare, and little overhang? Seems like you would have one strange looking bow; but that's just my way of thinking "

If you want to see what that bow looks like, you can look at both new and older designs. Here are a few examples; the Hylas 46, Tayana Annapolis 43, Bristol 33/34, Farr 11.6 or Farr 44 (des#89), almost any post 1990 Hallberg Rassy but especially the Hallberg Rassy 43 mk II, Almost any Brewer cruising boat but especially the Brewer 12.9/Whitby 42, J-120, Herreshoff Alerion, and so on.

And at the risk of beating a dead horse, if the Capsize Screen Formula and the Motion Comfort Ratio are only valid for comparing boats of nearly equal beam, waterline and displacement but even then can't distinguish between boats with differing VCG what good are these formulas? If you have to go through a separate and detailed thought process to validate the results of thier numbers, (as in lets see the CSR, and MCI says the boat with the longer waterline, length on deck, lower VCG, narrower B/L, and heavier displacement is more prone to capsize and will have a poorer motion, but of course we all knolw better than that) what good are they? And that is my point.

Respectfully, (and I do mean with all due respect)
Jeff

[seabreeze_97]

"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."
Gotta love that last sentence. Can we say CCA? Anyway, I digress.

"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."-Ted Brewer.

Nowhere does it say this formulas is intended to be dead-on balls accurate down to the molecule. It is not an all-in-one measure of any two boats on the ocean. Doing so is to misuse the formula.

[Jeff_H]

I still say what is comfort ratio good for if it can't be used as a comparative measure or if it totally misses the boat on basic gross indicators (as illustrated in the examples that I mention above)?

If I look at the basics of the formula, it appears to employ the waterline length backwards when it sees increasing waterline length as decreasing comfort. If you look at motion studies, increasing slenderness should result in a more comfortable motion from a pitch, yaw, and surge perspective, and to a lesser extent from a roll perspective.

At the heart of it, I understand that it is tough to develop surrogate formulas that actually provide a reasonable degree of accuracy with readily available information. At one time the motion comfort ratio may have had some usefulness, but as it stands it lacks so many critical factors that I can't see how anyone can argue that it has any utility at all. It would be nice if someone sat down and wrote a useful motion comfort formula that took into account such factors as ballast weight, draft, perhaps spar height, some kind of corrector for how the boat carries its beam (a narrower boat that carries more of its beam towards the ends can have more form stability than a beamier boat that carries its beam at a single spot and which has a lot of flare- visualize a Seidleman 25 for example.) and so on.

Jeff

[chucklesR]

87.5 percent of all statistics are made up on the spot. Mathmatical formulas to prove that are of course available.

CR is related to how many drops of rum are spilt between the galley and the cockpit - which is of course why Catamarans are comfortable indeed.

in plain english - comfort is relative - some of the white knuckle guys on this forum are just getting started at the high end of my low end.