Seaworthiness

Very good to bring it up; it needs to be said though that you don't have to reach weightlessness, a reduction in weight due to vertical acceleration will have quite an impact on stability. Which is one of the reasons why analysing seaworthiness based on only the static STIX number never will give even half the truth. Look's like you know your Marchaj, seaworthiness

We need to bring in more dynamic stability into this discussion :!:

Mikey

 

About the RM ISO STIX, you know perfectly well that the ISO STIX number is mandatory to the boat certification and that the number (and all calculations) are given by the designer to be verified by an independent Notified Body that certifies the boat. As you have said:

You really think that independent Notified Bodies are to be mistrusted, or that reputable NAs like Marc Lombard, falsifies the data?

You can only be kidding…

About the POGO40 STIX, I will post the relevant part of the official document that certifies the boat. No need to make any calculations. They have already been made correctly and ISO certified.

The stability of the cruising and racing versions is the same. The deeper draft of the racing version is compensated in the cruising version by a heavier bulb.

 

Guillermo,
To answer your question “give me your opinion on the seaworthiness of what you call skimming dishes” (post 353) You make several points for comment as follows:

Extreme beam hulls are seaworthy while at speed.
Sea keeping is primarily due to damping effect of sails and underwater appendages.
(Forward velocity is implied)
There is a tendency to transfer to the cursing market this speed dependent element of seaworthiness
Sea kindliness is an important factor to crew comfort and safety for cruisers
No measure of sea kindliness is achieved with extreme beam hulls.

I hesitate to give my opinion, because I am admittedly biased towards a large blue water cruiser. Let me set aside my opinion, and draw off the opinion of those more knowledgeable than I who I have had the privilege of learning from.

My comments will be for vessels in an extremely exited sea state where alternative survival methods such as drogues or sea anchors have not been deployed. The following elements of seaworthiness are of central importance in my comments:

The boat needs to stay afloat
The boat must remain upright
If rolled the boat must return to an upright position
The boat must maintain directional stability
The boat must minimize acceleration forces on crew
The boat must be used for the purpose it was designed for

There are very few people that will argue that the boat needs to remain afloat, so I won’t comment much on that, except that it is a concern when discussing racing boats that are built with narrow safety margins in order to save on weight. Flotation is also placed in jeopardy do to down flooding if you can’t keep the boat upright. Often times discussions of seaworthiness revolves endlessly around inversion and inversion recovery, when in reality inversion is the end result of many things gone wrong. Next to remaining afloat, directional stability is the most important aspect of seaworthiness. Without it control is out of your hands and the boat will broach do to

1. hydrodynamic pressure differential between port and starboard side of the vessel in waves,
2. Dynamic forces caused by orbital motion of waves
3. The gyrostatic couple due to pitching and rolling about the center of flotation

The probability of capsize or knockdown is greatly increased once the vessel becomes broadside to the waves. Maintaining sufficient speed is important for both skimming dishes and cruising vessels. The speed which assures directional stability, of course, is dependent on many factors.

Concerning your comment about extreme beam racing boats:

Yes damping attributed to aero and hydrodynamic forces aids stability especially in the case of rolling, but the predominate factor influencing stability is the relationship between the center of gravity and the center of buoyancy in a dynamic environment.with a correlation to the distribution of displacement.

Juan Kouyoumdjian comments concerning VO 70 racing:

A similar comment made19 years earlier by Marchaj as you have pointed out in post 352:

I noticed that you left out the last sentence of the paragraph; I will include it here because I think it is the key to all matters of instability.

Marchaj continues:

You see, buoyancy is a double edge sword; it is the law of buoyancy which allows us to design a boat, but that same law imparts to the ocean the ability for the waves to use buoyancy as a handle to grab our boat and toss it however it desires. Skimming dishes have pretty wide handles that the ocean easily grabs hold of, and in the process energy is transferred from the waves to the boat in the form of heat, heaving, and rotational velocities. If you are going to own a skimming dish, you might want to review Juan’s comment above.

It is impossible to escape acceleration forces at sea, but to think that the average sailor can handle the forces generated by a beamy racing design boat is just plain foolishness.(remember the context here is cruising boats, extended voyages) No other sport besides yachting can I think of where people just go missing. You don’t see airplanes crash where the crew has just decided to bail out, you don’t see Baja 500 cars parked on the side of the race course, driver missing. You don’t see campers parked on the side of the highway, former occupants lamenting -I’m never getting back in that thing. Yet this is a common occurrence for both racing and cursing vessels. The general boating public’s willingness or ability to cope with high accelerations is overestimated. (as well as their willingness to use proper safety equipment)
I believe that racing boats can be made safe and cruisers can be made fast, relatively speaking. But I also think you are on a slippery slope if you try to make a racing design a comfortable cruiser.
I know this has gone long; unfortunately I have only touch on some of the issues.

 

I suggest you go on graham radford site, to compare some options.

You have the radford 460,
http://www.radford-yacht.com/dsn064/dsn064.html
for which there is no stability information. Just that it is an upgraded version of the 450, with the COG of the ballast 300 mm lower.
The loading capacity is around 4 tonnes, for a 15T boat.
The AVS of the smaller, same serie, 415 is estimated at around 140°. But without any curve.

Then you have the radford 14m
http://www.radford-yacht.com/dsn047.html
The loading is limited to 3T for this 11T5 boat.
But the stability information is published, with the loading condition.
The boat is rerigthing, due to its pilothouse. And the max rigthing arm is around 60 cm. and GM around 1m

And then you have radford 14m racing.
http://www.radford-yacht.com/dsn036/dsn036.html
There is no information for loading capacity for this 7T7 boat, except that it meets IMS requirements for tanks and galley stawage.
But there is stability information. Rigthing arm is around 1.1m, and GM is around 2m.

Which one will be harder to capsize ?

 

MOB, you said Let me set aside my opinion, and draw off the opinion of those more knowledgeable than I who I have had the privilege of learning from.

Personally, I think that's a reasonable view. However, many people who are extremely knowledgeable, as demonstrated by round the world singlehanded passages or sailing in 45 Sydney-Hobarts, have had their views ignored here (and elsewhere). Often when people such as these are quoted for their pro skimming-dish views, we are told that they are "speed maddened racing fools". Those with experience who like heavy boats are, in contrast, held up as examples of purely disinterested, widely experienced and dispassionate experts.

"No other sport besides yachting can I think of where people just go missing."

Light aircraft, surfers; rock fishermen (and pros); bushwalkers; mountain climbers; cross country skiers; windsurfers; campers; Four wheel drive/SUVers and many other sportspeople go missing around here.

Acceleration may be a problem. The NZ-Tonga cruising rally, which saw cruising sailors rescued from Westsail 32s, Norseman 447s, Atkins double enders and a distress call from a Rhodes Offshore 40, proved that it's not just a problem that affects lightwights.

 

Dleted double post

 

CT 249

Yes, I can see that this is true, I can't comment intelligently on this, as I do not have experience in this. I am not up on all of the different racing class rules, for instance. I have specifically studied the open 60/70 class because in many ways it is similar in size as the ocean cruiser that I hope to build, with some features that I would like to incorporate into a cursing vessel. I have studied this class enough that I can say that it would be unjust to say that these boats are not seaworthy. It would not be right to conclude that these boats are dangerous just because they are skimming dishes; There are many other stringent design characteristics that have been incorporated that must be weighed. It seems only fitting that each boat is judged on its own merit as to weather or not enough safety measures are include to offset extremes in design.

I was not clear here; I was thinking more of people missing off of boats that are found floating and relatively intact. It has been reported that when a crew suffers from high G forces for to long a period of time, both physical and mental functions become distorted, leading to irrational behavior.

Again, I wont deny this, any ship can end up in wave patterns that stress the ship beyond its design envelope. There is to much emphases placed on displacement, and not enough consideration to how the displacement is distributed. Nor is there enough emphases placed on the hight of the center of gravity. The farther you spread the available displacement out and away from the center of gravity, the more energy that is going to be absorbed by the various wave patterns encountered while under way.( is it more correct to say the center of flotation? I think it maybe should be the center of flotation as we are discussing more specifically displacement of a fluid)
The subject is complex, because the mass distribution within the vessel interacts with displacement/buoyancy principals. Span wise displacement is fixed by hull design, but total mass, and mass distribution can be altered. Movable ballast including water ballast allows for flexibility; Transverse and longitudinal moments of inertia can be altered too change rolling and pitching characteristics in survival weather conditions. I would not be too quick to discount water ballast especially for wider beam cruisers. A beamyer boat in synchronous rolling conditions will roll through greater angles, and begin rolling in comparatively smaller waves than a boat with less beam (similar displacement)

 

Tom

Thanks for your posts, I'd like to add that rotational "damping" is from both eddy making and the inertia of the water entrained by the hull.

Light displacement entrains very little water and deep narrow keels at low velocities give poor damping from eddy making since they operate in turbulent water of their own making. This is another of the reasons they are uncomfortable when hove-to or windless in a sloppy sea.

In my opinion the ability of an ocean cruising boat to comfortably and safely stop is an very under-rated feature.

 

Welcome back Mike

I think there is a word that describes a boat that stops comfortably and safely ... the word is barge.

Not much fun to sail though ...

Just teasing ...

Randy

 

Yes I fully agree with you and I think that it is one of the few points that is unquestionable (but of course I can be wrong) .

About this, I would say that you make your personal opinion look like an unquestionable opinion, and I don’t think it is the case.

In this thread (and elsewhere) people have expressed their preferences about the kind of motion they prefer in a sailing boat (heavy or planning type) and they don’t coincide. It is a question of personal preference.

About the accelerations, let’s put aside the ones generated by a more powerful rig, and let’s concentrate on hull shape, what you call a beamy “skimming dish” boat, kind of Open 60’s hull.

You seem to imply (I can have misunderstood) that this type of hull is not adequate to a cruising boat. You make it sound like it is not only your personal dislike, but that it is a rule.

Here I have to disagree, not because I know much about boat design, but because a lot of Designers and NAs think otherwise and that would make your opinion, just one more opinion, not a rule that proves the unfitness of that type of hull, for a cruising boat.

All these designers and NAs have designed cruising boats with that kind of hull, and those boats are not old designs, but contemporary cruising designs:

Jean Marie Finot, Pascal Conq, Merf Owen, Allen Clarke, Guillaume Verdier, Marc Lombard, Martin Defline, Peter Gallinelli, Luc Bouvet, François Lucas, Albert Nazarov, Julien Marin, Bakewell-White yacht Design, Guy Design Group, Dieter Blank, Yann Dilasser, Gildas Plexis, Xavier Fay, JJ Design, Martin Billoch, Julian Bethwaite, Simonis and Voogd, CDE Design (Lars T. Olsen , Jacob Vierø, Søren Flening), Pierre Roland ….

This is not an exhaustive list, I am sure that many more have designed, in the last years, cruising boats following the lines that give what you call “skimming dishes”. These are only an example of some; among them, some of the most reputable in the field.

The first posted picture is of an Open 60 (Finot), the others are some of the cruising boats designed by the NAs referred above, not all, even if I could post examples of cruising boats designed by all of them.


About the picture of the boat, before the last (ADP52), Jean Marie Finot (the Designer) says:

"In the last around the world races, the evolution of craftmanship of the boats and the evolution of materials (in particular carbon masts, water ballasts ..) lead us to design a new generation of boats, simpler, spacier, with less fitting and less useless items.

These boats, compared to those of the previous generation, see their speed increased by 10 to 20%.

With low heeling, easier handling, they are more stable, more solid, thus much safer. Their CWing© mast, watertight in the aft part and their deep keel make them self-righting.

At the same time, navigators are looking for sailing pleasure: these boats, simple and easy to maintain, give their life aboard easier and increase their time spent at sea.

This stability makes the boat very safe, the number of manoeuvres is reduced, the boat sails only slightly heeled and the handling is safer.
The relatively symmetric hull shape is studied to have a good stability under sail. This balance under heel gives the boat the capability to follow a straight course , even under auto-pilot.

She is a fast offshore cruising boat. We designed her as we would have done for our personal use in family cruising.”


About the boat in the last picture (Owen and Clarke 65), the Architects say:

"This futuristic sailing machine was designed for an experienced sailor whose remit was for a fast long distance blue water luxury cruising yacht. In particular the vessel has to be capable of being sailed by a small crew, and occasionally single-handed.
As with open 60 race boats, the design is geared towards performance for fast passage making. Our single-handed race design pedigree ensures that this yacht is at the same time easy to handle, stable and safe to sail for prolonged periods under autopilot in any conditions."

Of course, the Designers could be wrong and Guillermo could be right when he says: “No measure of sea kindliness is achieved with extreme beam hulls”...And if it were like that, the boat could have proved to be very uncomfortable and for that reason, not at all adapted to cruising.

The Owners, after cruising more than 2483 miles, in12 days, have said about it, in a letter to the Designers:

.. "We have had wind till 45 kn and the ship was sailing very safe .. The ship was always easy to control and to handle.
Also the interior is fine designed. On the whole trip we have had always a good lunch easy to prepare and to work in the galley. The idea to adjust the beds in the aft cabins is also super. When the ship was heeling, we adjusted the angle and it was fantastic to sleep well…

So I will confirm you once again this ship is really my dream. Thank you".

But it doesn’t seem the case

Regards

 

Vega,
I would like to thank you for your response, and for the time you took to provide evidence to counter my post.

Keep in mind that at the beginning of my post I made a statement that was intended to have us focus on the extreme situation that tends to separate out the truly seaworthy vessel from the occasional cruiser, or the day sailor. It is not that this is necessarily the only way of looking at seaworthiness, but it seems to have validity in that a serious cruiser is more likely to end up in very harsh weather that it cannot escape from.

I reiterate:

You are correct that a portion of my statement is opinion; it mixes fact with opinion, so lets separate out the fact and as far as my opinion goes, well, that can be changed through sound argument, and scientific evidence that substantiates your view. Let me separate out the facts:

When taking a candid look at my statement, this is the only portion of the paragraph that is truly fact. Let me expound on this:

'For any given displacement, the wider a boat is the more energy in the form of motion that is imparted to the vessel do to wave action.'

Without drawing any conclusions based on preconceived notions, the real question that I propose is-
How much motion, and for how long can the average person take before the following two events occur:
1. The individual says, I want off this boat
2. The individual can no longer function rationally, and or physically.
I know it is easy to fall back on the adage ‘it depends on the individual’ to some degree this is true, but it would be nice to see a study that gives real evidence on this question.

Your examples provide a more subjective look at what is accepted as a seaworthy, and sea kindly vessel. For those reading along, let us not be polarized in our views, but rather, look more carefully to see if there is evidence that might justify the personal testimonies of the designers that Vega has chosen as examples. In fact let us focus at least for this post on the Finot Open 60 that Vega has mentioned, as it has reference to my post earlier, and also to Guillermo’s original post.

The designer states:

This statement says much, let me point out that water ballast has been especially highlighted by the designer, along with carbon masts. Vega has been kind to me; while he questions my opinion, he has not completely ruled it out. I think in light of what Jean Marie Finot has stated about their 60’ open cruiser, my opinion becomes more clear, and so I will restate it

This is not all that is claimed by the designer, there are a number of other sea kindlinesses features that it appears they have focused on.

This is just what Vega has posted about this particular design; I have noticed other desirable features, which I won’t go into here, Like I stated before, an interesting designer to keep tabs on; I don't think we should discount the design just because it is based on a wider platform. There are many other aspects of sea worthiness/sea kindliness to consider. I am keeping an open mind so I might learn as much as possible; even so, do you think it is wise to follow the crowd?

 

I think we should separate concepts.

Aero and hydrodinamic forces influence damping to a great extent depending on rig, hull forms and speed, thus being relevant to what we could differentiate here as the 'dynamic stability under speed', not to mislead it with what is usually understood in naval architecture as 'dynamic stabilty'. This last analyzes the various energies acting on the vessel and its response, taking into account static stability curves properties, basically a zero speed analysis (or, to be more precise, a 'frozen picture' analysis)

Relative positions of CB and COG are most relevant to the low (initial) end of such 'frozen picture' curves (both static and dynamic), but its effect on this 'dynamic stability under speed', when at planning speeds, is not as relevant as the relative positions of the hull's Center of Dynamic Pressure and COG.

At high speeds, relative positions of the hull CDP and COG, in conjuction with Cp, moments of inertia, as well as lift and dynamic CLR from the keel, are some of the relevant factors affecting a boat's behaviour; while at zero or low speeds 'frozen CB' and COG relative positions, CLR (hull + apendages), waterplane characteristics, displacement and ultimate stability, are the factors mainly in charge, also in conjuction with moments of inertia.

Zero speed (or 'frozen pictures') characteristics are the ones more relevant to survival conditions when boat speed has come to very low figures, in my opinion, and designers should take them into careful consideration when designing boats intended for extensive cruising or commercial use. This approximation to the problem is the one considered, for good reasons, in all regulatory bodies, be them old or the very last (see http://boatdesign.net/forums/showpost.php?p=122199&postcount=4). In spite of all the actual fashion trend, I still think and maintain (and no serious and independent sources have been quoted here to make me change my opinion) light displacement and beamy hull is a dangerous recipe from this point of view.

Paradigmatic to low seaworthiness characteristics are the cases of some modern 'said' bluewater-cruising boats, with big beams and volumes, bulb-keeled but with high B/T and low ballast/displacement ratios (this last even under 0.3), D/L's as low as 120 and SA/D ratios over 20, to exemplify what I'm saying. Additionally to stability, seakindliness and seakeeping considerations, the pursuit of mass-produced-cruising-boats' performance got through inadequately applied racing boats characteristics, is bringing into scene structural failures (read february 2007 issue of Yachting World, just to not go any further). Too many examples of this, lately.

By the way: 'Skimming dishes' are not a new concept. Herreshof already studied and designed such boats back in 1892 ( See his 'Wenonah' type). This discussion is pretty old indeed....

Cheers.

 

It is just an undocumented and unscientific rant.

If you take any car, and drive it at full speed againts a wall. You will be dead. Whatever "safe" is the car. For planes also. Any plane that will crash is likely to make deads. How safe it is.

Why do you want boats go against Force 15 storm, and survive. Are not sailors clever enough to know they should avoid hurricanes and typhoons ?
Are they dumber than average pilots and drivers ?


And for the arc, if there was no mass-produced-cruising-boats' performance, The style of cruising barge you promote would NEVER have allowed 150 boats to cross Atlantic in 2006 in less than 24 days. Because your style of boats would be too EXPENSIVE to gather 150 boats and too SLOW to cross within 24 days.

You seem to forget the reality of the world you are living in.

 

Tom, your attitude is a very refreshing one. I am not of the “kind” type, I say what I think and many times that is not the wise thing to do.

I didn’t rule out your opinion because it is also true. The only thing I am against, is the idea that there is only a one-way view on seaworthiness and boat stability, a dogmatic view that states, no matter the evidence, that only a certain kind of boat is seaworthy and adapted to long distance cruising.

I have posted all those designers and boats to show that there are a lot of reputable NAs who, by their successeful work, prove that there are several ways to obtain seaworthy safe long distance cruisers, including the options for light boats with large transoms, water ballast , fin rudders and narrow bulbed keels.

I am not saying that it is the only way (quite the opositte), or that those boats have only advantages. Any design is a compromise, with advantages and disadvantages; obviously for those designers, the advantages of that kind of boat, for long distance offshore cruising, are bigger than the disadvantages.

I am also not saying that I subscribe all those boats as seaworthy oceangoing cruisers. What I am saying is that I don’t rule them out, just because they have large transoms and are light. I have to see the stability data (and have more information on the boat) to make a personal evaluation.

About: “do you think it is wise to follow the crowd?”

The answer is NO. In my life I have never followed the crowd. Following the crowd in this thread would be to agree that nothing new has been improved in the last 20 years regarding the relation between hydrodynamics and boat hull shape and that a good oceangoing cruiser sailboat designed 20 years ago would still be the state of the art today.

I don’t believe in that, but I am very cautious in the evaluation of new developments.

You are right, but In my opinion, only when the boat is not sailing.

When the boat is sailing the kind of movement of a powerful, stable, fast and light boat, even in bad weather is completely different from the movement of a heavy boat. The light boat keeps sailing (even on autopilot) and its sails maintain it stable, while the heavy boat bounces around.

I have experience with heavy boats and light boats and even if my somewhat small experience (10 000 hours) doesn’t make me an expert, others with an incomparable bigger experience have said the same. Look for instance at this description, taken from another forum, of a Guy that has sailed his light modern fast bulbed 40ft boat, from California to Australia :

"To make it short, I loved the boat. .. She will outsail most anything you come up against, handles BIG seas fine, goes to weather very well (which we did much more than I ever expected). ..

…. When we were making landfall in Australia … we were in our 3rd front of the passage, and doing 7+ knots beating into the seas (~55 degrees apparent) in the high 20''s/low30''s windwise. The boat was well balanced (reefed, #4 jib), and I was having fun as we rode the waves. Now this other boat we passed very quickly would climb up a wave, then the bow would smash down the backside, submerge, the next wave washing over bow and deck, and pretty much completely stopping the boat. Then slowly, he''d gain speed again, just to climb up the next wave, crash/submerge/and stop again. This all under power, because with sails alone he didn''t have the strength to fight the seas".

Take also a look to these videos. Two boats about the same size, one narrow and heavy, the other an Open 60, both in heavy weather with big waves. See what I mean about the different kind of movement?

http://video.yahoo.com/video/play?&...44e5d429833c3f66&rurl=www.biology.ualberta.ca

http://www.courseaularge.com/upload/video/1213170011.wmv

 

You offer an extreme (lumbering tub) as the antithesis to a skimming dish or a ULDB displacement hull, this is very misleading and seems to be a ruse used commonly in this thread (and others).

Consider this;
When sailing ships were designed for speed (the clippers) considerable changes were made, the bluff bows went, vertical stems went, harder turns to the bilge were introduced, a gentler run aft was introduced and a large press of sail was able to be flown on large heavy steel masts and yards.
These massive, heavy "antiquated" steel vessels carrying 100's of tons of cargo still hold records that are hard to beat. The clipper ships frequently overhauled steamers traveling at 15 knots.
What I am trying to say here is that it is not weight that determines whether a sailing vessel performs, it is the waterline length and the ratios SA/D , SA/WSA the vessels Cp and above all her power to carry sail. The displacement is just one variable.

No displacement hull can exceed her waterline length factor and the difference between a considered "Fast" boat and a slow boat of the same LWL is often of the order of 10%.

Even were a cruising hull built for speed It is very rare for a cruising boat to be pushed to her max hull-speed for several reasons, comfort being one of them safety another.

Also consider that speed from lightweight scantlings comes with sacrifices as we have said before such as durability, comfort, reserve strength and compromised stability from loading. Yes the vessel may be very comfortable in some conditions but also very uncomfortable in others.

As for crossing the Atlantic leg of the ARC with trade winds and favorable currents even the old converted fishing boats and pilot vessels (lumbering bluff bowed tubs) of 100 years ago used to often sail that route in 20 comfortable days.

When it comes to cost; do we base that on the value of the vessel or on its new replacement cost? Most buyers buy used boats, with the current marketing trend there are far more opportunities to get real value when buying heavier comfortable well found cruising boats rather than race-capable (or look-alike) boats, at least that would be my judgment looking at sites like yachtworld.com.

The reality of the world we are living in is that in some European countries there is a large popular culture swing to open 60 look-alikes scaled down to sizes where the seaworthiness is questionable, ( and it is very questionable in certain circumstances and conditions). With the Pogo 40 types If all the designers marketers are so passionate about the safety of these boats why can't we get detailed technical info for them ? It seems the designers feel vindicated by the STIX rating .

Open 60's boats may well be capable sailing machines and their safety record has improved markedly but they are still a pure go-fast hull with little thought to the compromises of the cruisng world. That people think that they can be scaled to 40 feet and make safe blue water capable cruising boats has yet to be demonstrated. IMHO

End of rant