Seaworthiness

The crew can have a good rest, reflect on the power of the sea, have something to eat, set broken limbs etc...

If the boat

• sank
• lost the keel or inverted
• lost the rudder

the crew will be beaten up and tired at the very least, and the boat is stuffed.

I am quite happy to concentrate on seakindliness next.
Working that into the design shouldnt conflict too much with seaworthiness.

Mainly I am thinking strength, self righting, solid flotation, and keel/rudder 'conservation' as the lowest common denominators of seaworthiness.

Hull form that is seakindly can be accommodated quite easily with the above goals.

 

As it has been repeatedly said in this thread and by a huge margin, the main situation of life risk to the crew has not do with sinking neither capsizing, but to fall overboard. Most deaths at sea, by a large factor, have to do with this.

I don’t understand your drawings and I believe you have not understood what “tripping on the keel” means. It refers not to a forward movement, but to a lateral displacement.

Putting it in the words of Eric Tabarly:

”The long keel and the deep of the underwater hull offer a big resistance to lateral displacement…then the waves that break against the boat push the superstructure and as the underwater offers a big resistance, capsizing can occur.”

he is referring to the hidrodynamics that are easily visualized in these drawings taken from one of his books: (posted drawings)


These dynamics are very well explained in another thread by Millan referring to the advantages of the center board boats over conventional keeled boats. The dynamics are the same, only that in a center-board with a keel up, they are even better (regarding stability) than in a deep keel with a bulb and a narrow fin. Worst dynamics, in what stability is concerned, will happen in big deep full keeled boats.

Posted by Milan :

"Their ultimate stability is lower but they heave some other characteristics that compensate for that. In heavy weather, boat with all inside ballast and centerboard or daggerboard, lifts the board completely out of the water. That moves center of lateral plan aft, making running very easy and eliminates danger from broaching. Also, if hit by the wave from the side, boat just skids to a side and tends to turn away from the wave. in same conditions boat with a deep keel sort of - trips over the keel, heeling much more and maybe even capsizing".

Or in the words of Jacques Mertens, a well known boat designer and experienced sailor:

“The seaworthiness may come as a surprise to beginners or to those who have never sailed a shallow draft boat but experienced seamen know that the shallow draft boat is more seaworthy than a hull with a deep keel.
Unlike the deep boat, a centerboarder will not resist the sea. Instead of tripping on it's keel, it will dodge the waves or lift over them. Properly designed, a keel centerboarder will have the same ultimate stability than a boat with a conventional keel. …
But there is more to it than safety at sea. In case of real bad weather like a hurricane, a shallow draft boat can take refuge deep inland or in very protected coves inaccessible to deep boats.“

Finally let’s not forget the wise words of Michael Kasten : “If there is one absolute truth in all of this, it is that with regard to … seaworthiness there are no absolutes, only tendencies. As a result, we may attempt to apply generalities to the problem. However generalities are necessarily prone to oversimplification, therefore they will nearly always be misleading if applied too broadly.... or too blindly...”

A Merry Christmas to everybody.

 

sometimes its a matter of seamanship,,,combined with a good boat ,,you otta know when to keep her into the wind ,,and when to run with it, longliner

 

There's a specific problem with IMOCA rule D2, and I've started a thread to discuss it at http://www.boatdesign.net/forums/showthread.php?t=15206.

Except for rule D2, I think the D (stability) section of the IMOCA rule is quite good. It (now) requires that the positive area under the righting arm curve exceed the negative area by a factor of 5, for example.

 

Paulo

Man-overboard losses can be related to sea-kindliness too, if you were catapulted of the foredeck of a lightweight vessel or washed out of the open stern cockpit, then shouldn't we also attribute these to lack of seaworthiness in boat design?

Most of the lateral "tripping " argument you have posted is contrary to the accepted principles of Naval architecture through observation and experiment over the years, and a lot of popular misconception abounds.

The trouble is that the subject is complex beyond most peoples ken and a simple misconceived belief is easier to grasp and more appealing than a complex explanation.

Consider the pictures you posted from Tabarly, these are just apologetics carefully drawn to illustrate an opinion.

In fact consider that given the same scale wave amplitude to boat size the deep keeled boat shown in lateral profile may well be leaning slightly into the shown wave at the impact (due to sub surface orbital motion). You might also want to re-draw the end view with the keel at the same proportion shown in the lateral plane. Then sketch the orbital currents and their magnitudes, this would give you an idea of what is really not shown in such a graphic.

The highest velocity is in the jet (horizontal top part of the breaking wave) exactly where in relation to the wave shape this occurs is important (it is also variable) the danger with the type of vessel you are advocating is that she slides and gives readily enough but the high form stability means it will be following the sea surface and leaning away from the wave and accelerating sideways, the classic dangerous capsize in this type of event is due to the boat "Tripping" on the deck edge , the then further heeled hull is very prone to the breaking jet, this translates into a very high rotational moment. Consequently the beam on sea at the scale shown would be more dangerous for beamy light boats with small lateral area than the deep full keeled vessel.

Another factor to consider is that the lateral profile shown will pay off by the bow when hit broadside too. When we add roll inertia the picture is even more grim for the lightweight boat and more advantageous for the heavy boat since the dangerous high velocity breaking jet is a transient event and the high inertia vessel cannot absorb the energy as a light boat does.

It is no accident that small work vessels in rough waters developed huge and deep lateral planes relative to the exposed topsides, it produced wet boats, but boats that were almost completely immune from knockdown. These sorts of vessels were designed to go out and stay out in the worst of conditions to put pilots aboard incoming ships their safety record over many decades was testament to their incredible durability in rough weather.

For interests sake PenDuik III had a length of 57.3 feet a beam of 13.8 feet and a draft of 9 feet, not an extreme hull-form at all. If scale the beam length ratio you'll have a fairly narrow 40 footer.

There will always be a dangerous sea for any hullform and seamanship will always be the best first option wrt sea state and tactics to accommodate the same. What I think is important is to consider the events that cannot be compensated for by tactics. We cannot trivialize the complex dynamic events.

I have not read Milan's post that you refer to but I would take issue with the claims of a centre boarder with board up being a good general tactic in heavy weather.

 

Mike, you can find that post here: http://boatdesign.net/forums/showthread.php?t=9199&highlight=ovni It was thread about OVNI's and similar, (mainly French), designs sutability for offshore cruising. Taking board up is standard heavy weather tactic for this type of boats, when ever is space for running. These tactics work very well for them, as a lot's of successful voyages seem to prove.

 

Mike, about those same drawings you have said before:

So it seems that you have agreed that a “lighter dish type vessel” can dissipate the lateral push of a breaker wave with a sideways move, when a heavier full keel boat can only transform that push in a roll movement.

What happens depends on the strength of the boat and on the quality of the skipper. If he is like the one on that Oceanis 390 and has the hatches open, there is a good chance that a massive downflood can prevent the boat from returning to the upright position, but assuming it has a better skipper, the boat would not have a problem in returning to the upright position, providing it is a strong boat(heavy doesn’t mean necessarily strong)…with the mast broken.

With a light “ dish” with a long keel with a bulb, it all depends on the quality of building. As for heavy ones, the quality can be good or bad; but assuming it is a strong boat, experience shows that the accident will not turn to be more than a frightening experience.

About the “Principles of Naval architecture” that you talk about, they are old principles, applicable to old boats. The book that seems to be your bible is more than 20 years old (Seaworthiness: the forgotten factor), the work of a Polish Yachtsman (Czeslaw A. Marchaj) whose main work "Sailing Theory & Practice" was published in the mid 1960s.

Naval Architets rarely write books, they design boats, and the boats that comtemporary Naval Architechts design as Ocean going sailboats are not heavy narrow full keeled sailboats. Those are the ones that are writing the principles of Naval Architecture in what concerns sailing boats, not with words, but with seaworthy and fast boats.

About : “but I would take issue with the claims of a centre boarder with board up being a good general tactic in heavy weather”, every Cat sailor knows about that, as any centreboarder skipper.

About the seaworthiness of this kind of boats, it is not by accident that the most popular passage maker in France is the OVNI 435, an aluminium centerboarder. Based “on observation and experiment over the years”, French long distance cruisers have chosen it as the most adequate as a long range passagemaker.

The boat was (and is) built in large numbers (for a small production expensive boat), and the kind of Guys that buy them, don’t have them in the marinas. Probably it is one of the designs that has more boats doing a circumnavigation on any moment.

If we take a look at the RM curve of the boat, it will not look very good. Fact is that it would look worse than the one from the Oceanis 390, particularly in what concerns the AVS and the inverted stability. But fact is that I have no notice of an OVNI 435 being capsized, and they are out there, in large numbers, used mainly as passagemakers, for many years.

Finally, I would like to say that I am not defending a particular type of boat (I even like oldies, with narrow hulls and long keels) and I agree with you when you say :” The trouble is that the subject is complex beyond most peoples ken and a simple misconceived belief is easier to grasp and more appealing than a complex explanation”.

The difference seems to be that you believe that nothing has chanced since the 60’s in what concerns seaworthiness and boat design and I believe that the boats designed by all the contemporary Naval Architects as fast passagemakers are also seaworthy sailboats, never mind being lighter and faster. They are just better sailing boats that behave differently in what regards heavy seas, and are a lot faster and less expensive to build.

I agree with you on one thing, they won’t last so long, but I believe that everybody knows that, when they buy a new boat, and probably the boat will out-last the owner anyway.

Best Regards and happy new year.

 

Milan

I understand, however as I am sure you are aware this hull-form is a compromise, it could be improved upon in most aspects other than its shoal draft. I have no doubt that these vessels have made long and successful voyages.

The problem is that people use successful voyages as benchmarks for seaworthiness. A boat can have even severely dangerous tendencies compensated for by an experienced and knowledgeable crew. This is then extrapolated to be a recommendation for the ideal hull-form of a cruising boat.

I think designers should be more honest in that the discussions should be over just how far the compromises can go without the risk outweighing the benefits and designers should be aware of the inherent risks in the hull-form they advocate, not believe that it is superior in all aspects.

Interesting hull though, thanks for that.

 

Instead of keeping on with your kind of nonsense, you should study those books first and then think twice before posting.

To all:
Maybe the attached document is of general interest.
Cheers.

 

It's an interesting document, Guillermo.

One thing I find interesting is the following excerpt;

"Initially during the following seas experiments a trimming moment was added to the model, usually utilised in model yacht tank testing to simulate the trimming moment induced by the sail force, however this led to nose diving problems. Therefore no trimming moment was applied to the model during either the wave or calm water experiments. From a practical standpoint, this may be
assumed to be acceptable since when sailing downwind in large waves the crew weight would be moved aft and there may be considerable vertical lift from the spinnaker, therefore providing only a small net trimming moment."

Now, those words "there may be considerable vertical lift from the spinnaker" are interesting. Elsewhere, there's a been a reference to the well known work of Mikko Brummel of WB Sails, whose work on a programme created by the Universityt of Technology in Helsinki has indicated that the spinnaker provides a bow-down pitching moment - the opposite of what Renilson et al believe.

Renilson's work after the '98 Hobart also indicated that coachroof design has no significant effect in recovery from inversion - the opposite of the Southampton studies after the '79 Fastnet, I'm fairly sure.

So here we have one well-reputed scientist, whose findings are opposite to those of a recognised expert sailmaker using university-designed programmes (in one case) and another well-reputed scientist in another. Given such contradictions in the expert studies, what are we to do? If we try to work out whether (say) coachroof design is important in inversion recovery, aren't we just left to work out whether the study that said it is irrelevant is better than the study that said it was relevant?

The Renilson study deals with downwind performance. Marchaj's conclusions on downwind control indicate that a Finn should be steered downwind standing up, with the tiller between the skipper's legs. In that case, Marchaj (who IIRC never won anything significant in years of Finn sailing) is right and Elvstrom, Ainslie, etc are wrong. Given than Elvstrom and Ainslie would normally finish a long way ahead of Marchaj in practise, can we not doubt his theory?

Yes, I know that in the final case there are extra factors. However given that the contradictions in some of the few available studies seem to indicate that the theoretical knowledge is still scanty and contradictory, can't we put a lot of emphasis on what happens in the real world? Compared to the number of studies that go into areas of science like the one my girlfriend works in, the number of studies on seaworthiness seem to be negligible - yet their proponents seem to be dogmatic about them. As Claughton said himself, a small change in wave height can render a higher LPS (IIRC) all but irrelevant, so why the concentration on LPS to the apparent neglect of other factors?????

My quick scanning of "Seaworthiness - the forgotten factor" also seems to indicate that it was created as an indictment of IOR boats, which are as Vega says, old fashioned. I seem to recall that it dealt a lot with the famous case of the Nicholson half tonner versus the Contessa 32 and their inversion characteristics. While I prefer a boat of teh Contessa's beam-to-length ratio, the fact that there was so much emphasis on the inversion of the Nicholson in the '79 Fastnet seems odd, considering that it killed half the number of people as the Ohlson 35, which was almost exactly the sort of boat that Marchaj et al held up as a superior style.

Exactly why so much study and abuse has been put on the Nicho, considering it killed many fewer people than more conservative boats, remains something of a mystery to me.

Dogmatism on either side seems odd in such an area. Vega's quote from Jacques Mertens - “The seaworthiness may come as a surprise to beginners or to those who have never sailed a shallow draft boat but experienced seamen know that the shallow draft boat is more seaworthy than a hull with a deep keel" also seems odd. Isn't it implying that all experienced seamen know a shallow boat is more seaworthy? I have some sympathy with Merten's concept, but to imply that all experienced seamen "know" shoal draft is better is simple rubbish. Many experienced seamen - like Mike here etc - have different views which should be respected.

 

Paulo (Vega)

You are a passionate stubborn advocate for a certain style of boat but I really think you are wrong in the application of much of your observation and in the logic of your arguments.

As for yielding to the sea…
Yes these boats you advocate give to the sea there is no doubt about this at all and no-one has said this is not so, but in accelerating sideways they gain energy, by remaining in and accompanying the wave they gain even more momentum since the time the wave has to act on the vessel is increased.
Where does the energy go? It can damn the vessel, accelerating (sideways out of control ) down the face of a large wave into the trough and sometimes a crashing halt in the trough. Common damage here with large coastal waves, is the splitting of hull deck joins, damaging or even imploding the lee ports and tearing out the weather chain-plates. Such damage is commonplace enough, in hand with this is the overstressing of rudder and keel attachments, damage that is often hidden for later.

Following on with your chosen illustration ………
It is not a corollary that deep keel boats overturn while skimming dishes give to the sea….. If you compared a shallow long keel vessel with high topsides to a skimming dish with minimal transverse immersed area then; yes………. the full keel may appear a poorer hull-form in comparison.
However; the proclivity of a moderate boat to overturn in a beam-on breaking wave is directly proportional to the ratio of topside to immersed lateral area. That is the bigger the immersed area and the smaller the topside are the more immune it is to knockdown in a beam on breaking sea. Within a range the characteristics will go from highly unlikely to highly likely. Eventually as the immersed area decreases (along with displacement) we end up with a boat that will generally tend to slide off sideways with little bite on the water. There are no hard and fast boundary conditions (i.e. below this this happens and above this this happens) but there are trends. A narrow deeply immersed hull with shallow topsides will be very hard to capsize, the beam sea required would need to be a very big steep faced breaker, in the same condition (beam on on the face of that wave) the beamy dish will suffer terribly with a high likelihood of severe damage. The crews of either vessel would be in a poor state to let such a scenario eventuate but this is your chosen scenario.

If you remain unconvinced I can refer you to several modern papers and perhaps you could try and illustrate this with some experiments you could perform given some very basic polystyrene models or even weighted planks which you could take to the beach to illustrate the extremes. There are many other important factors like roll inertia that I am not mentioning now but, have in the past.

Your disapproval of Marchaj is unfounded and a little worrying; Marchaj put the science of small sailing boat characteristics on the table and others have supported his work with a few small changes. “Sailing Theory And Practice” has been re-written and is current. Aero-hydrodynamics remains definitive and Seaworthiness from the historical depths of 1986 also remains definitive. Marchaj also published many papers continuing his research actively and vigorously up until his retirement. High tech facilities and designers around the world draw on his work and have supported it in nearly all aspects. Our own local facility the “Australian Maritime college” have also conducted vigorous research, the papers they have written are all in support of prior studies with a few minor changes.

It seems that you feel that this science should be ignored or vilified when it disagrees with your views? Without awareness of any research to the contrary you are unfortunately on the rotten limb of a belief system gathering opinions and often poor logic arguments that suit your outlook.

Modern social driven trends don’t alter the fundamental science whether the science stems from 1860 or 1986 and socially driven design trends will be quite different in 50 years time as they were 50 years back. It doesn’t make a 100 year old hull form built to be seaworthy un-seaworthy, and in “severe” conditions you would happily get onto a Le Havre Pilot cutter from 100 years ago from a Pogo 40 and increase your chance of survival massively into the bargain.

I am not advocating that we should all be sailing antiquated Le Havre pilot cutters either, these sorts of vessel have other issues that we can improve upon significantly but it illustrates one extreme against another and are useful for that purpose; to aid understanding of the principles .


No matter how attractive the French centre-boarders may look, if they are as bad in the AVS as you say then we can predict that there will be deaths in time when they are caught in severe conditions in the ocean. There is a price for such a shoal draft capability.
I would like to see the cruising trim static stability curve for these vessels. It would be better to see a ballasted keel in this configuration, then we could have a safer boat if you were caught (and a higher AVS). As I have said before Cruising boats should not be presumed seaworthy when built to the paradigm of racing boats for the reasons I and others have outlined before.

Finally do you understand that the 390 may have had its righting ability improved by the free surface of the flood water inside when it was inverted ? (I think I mentioned this before).

I would like you to read Seaworthiness and then perhaps we can have a more meaningful exchange on precisely which parts you disagree with and why. I understand why Guillermo gets frustrated with you, I do too


Thanks for the happy new year. And the same to you and yours.

 

I know, I know you are the guardian of the sacred old and forgoten holy truth about boat seaworthiness.

 

I don’t disapprove Marchaj; what I am saying is that he is talking about 25 year old boats and about different realities.

As CT said:

.. "Where does the energy go? "

The energy, of the breaking wave is dissipated in a roll movement in a boat with a long keel, but on a “Dish” boat it is dissipated in the lateral fast movement.

Mike, you are defending a certain type of sailing boat as if it was the only seaworthy sailing boat.

I am not defending a particular type of sailboat, but the several tendencies that modern boat designers have adopted in designing modern sailing oceangoing sailingboats.

From the French centerboard passagemakers to the bulbed keels of Farr or Finot, to the French tendency of big transoms or the Nordic more narrow approach, they have only a common dominator, they are LIGHT and FAST. I like them all and find them all seaworthy. It would be very strange that the Oceangoing boats designed by the best contemporary NA were unseaworthy.
Saying that their approach is unscientific compared to Marchaj approach is absurd. The increased knowledge and the use of computer simulations, programs and tank testing have increased the quality of design and the behavior prediction of the boats to levels only dreamed in Marchaj time.

I find very odd that you think that modern tendencies in design produce unseaworthy boats. I find it even odder, because statistics show quite the opposite. Never there has been so many boats in the water and never the proportion of accidents was so small.

This does not mean that I don’t think that all modern boats are safe or that specific points should not be discussed, it means that your defense of “good old heavy boats” as the only viable option to seaworthiness has limited the scope of this thread. More important things, like specific points regarding the seaworthiness of modern designs has not been discussed, because according to your very particular point of view, they are all unseawotthy.

Again quoting Karsten:

“If there is one absolute truth in all of this, it is that with regard to … seaworthiness there are no absolutes, only tendencies.

And the tendencies we should be looking here should be contemporary tendencies, the ones that are followed by modern NAs, not old tendencies that have only relevance under a historical perspective.

Regards

 

Ct, sorry if I give you the impression of being defending a particular kind of boat. That quote regards the opinion of Jacques Mertens. I believe that he is right about some particular dynamic qualities of a centerboarder, but I don’t share the opinion that a centerboarder is more seaworthy than any other kind of sailboat.

I even like heavy long keel saiboats and find them seaworthy. Any kind of well proven type of Oceangoing designs has its strong and weak points. I can see the strong points of heavy long keelers, but it seems that there are some that have difficulty in seeing the advantages of other type of designs (that’s why I have posted Mertens post).

In my personal opinion ( that is shared by the generality of modern boat designers), it is not acceptable to trade speed in a sailboat by seaworthiness, if you have other ways to accomplish that objective. Contemporary Ocean boat design is about that, the ways of having good and fast sail boats without compromising seaworthiness, and facts and statistics show they have managed that and not only in one way, but in multiple tendencies.

 

Then why the displacement hull 36 MLB ( 100 hp, 9 kts) has been replaced by the semi displacement 44 MLB (2 * 200 hp, 15 kts), with round stern. And again superceeded by the planning hull, transom stern 47 MLB (2 * 435 hp, 25 kts)?

In survival conditions, I will quickly and happily exchange a place in a 36 MLB for a place in a 47 MLB.