Seaworthiness

Wench
If shes anything like mine, reading a magazine called 'Ocean Racing Thrash Machine World' will put the damper on offshore sailing.

 

Picking up on Randy's earlier post on roll inertia. Some of this is relevant to what Tom Fugate is saying too.

Better to describe this as energy not force .
You can stop the roll by doing work, a deeper heavier hull is able to do this better by stirring up and shifting more water so you need to watch generalizations. As we add inertia with a heavier rig so also we get changes in other vessel factors that compensate for that heavier rig. Consequently the vessel ends up considerably safer wrt knockdown because you have shifted the response curve up in the wave height axis of the graph to where it takes bigger seas for the same knockdown susceptibility.

In reality the pitch roll heave relationships are coupled, particularly with unbalanced hulls, into this system we feed wave energy, damping and inertia… the model is complex and the math moves into complex systems of high order differential equations. The relationships are often non-linear and you cannot use a simple extrapolation to explain the relationships within a different system as you are doing.

Vessel design requires a good understanding of the compromises, also the design features that can compensate for the detriments of some of those compromises. Our understanding of what we can do and how it improves motion is better understood and researched by objective tank testing and the models have been better refined over the years.

Heavy rigs reduce stability. from what condition ? You present this as though it is a variable and you are missing an important point here, this is a design factor.
Do you understand design metacentric height and all that entails wrt motion and stability?
Putting a light rig on a vessel designed for a heavy rig is altering the design significantly and you are playing with some significant design parameters contrary to the design.

A long strut and deep ballast bulb also has a high pitch and roll inertia. The longitudinal inertia helps to reduce resistance by cutting through the wave rather than pitching over it (in some sea conditions), but this also relates to longitudinal stability and those coupled motion characteristics I mentioned before.

And this is perhaps the lack of opportunity to sail on a well designed heavier displacement vessel.

The ‘excursions’ are smaller than what? Higher acceleration = higher velocity = greater distance over time.

Racing boats compromise many desirable attributes for speed, they are also designed with attributes that suit their expected areas of operation.

I thought you could have imagined times when it was extremely beneficial, don’t mock, a vessel that cannot do this has already traded characteristics at a disadvantage.

Do Strength, Toughness, Durability, Collision resistance, Comfort, Privacy, Storage space, Security, Shelter, Dryness and warmth have different meanings?

Remember this is your house a cruiser is a live-aboard by definition not a weekend camping trip

 

This thread just won't die, will it?

All design is a compromise. If any one design was quicker, safer, more comfortable, stronger, ligher and cheaper than all the rest, then we'd all be sailing the same boat. The fact is, that to emphasis one of the factors above, one (at least) of the others will be affected. Light and beamy boats have their place in tis world, as do narrow, heavy ones. But they serve a different purpose. It is generally accepted that the the latter will be 'more comfortable' whilst the former will be quicker. Speed and comfort both play a part in seaworthiness, but you can't easily get them both. It depends on your personal preferences and priorities as to what you place greater value on. It's no co-incidence that the boat the that Sir Peter Blake sailed for the Cousteau Society did not look anyhing like the ones he raced around the world. Both were seaworthy in their own way, but the detailed requirements were different.

Anyway, carry on...

 

Of course. Especially for these ones : Comfort, Privacy, Storage space.

These notions are directly dependant of your wealth and standard of living. What you call Comfort in a 50 ft sailboat will be called Undreamable Luxury by a third world country fisherman, and Unaccepable Camping by a 200 ft megayacht owner.

 

"Do Strength, Toughness, Durability, Collision resistance, Comfort, Privacy, Storage space, Security, Shelter, Dryness and warmth have different meanings?"

Yes, very definitely.

I have known of couples who were live-aboard cruisers on Top Hats (25 footers designed as light racing boats in the '60s) including circumnavigations with babies. Such a boat may not have the attributes that many people demand in a cruiser, but that baby, born in South Africa halfway through the circumnavigation, has grown up and lived aboard his own Top Hat last time I saw him.

Many people have very different definitions of sufficient "privacy", "comfort", etc.

 

Chris
I have a very high opinion of the 2nd version of the Top Hat. It should be mentioned here that the boat was designed as a Bass Strait denizon. She is a seaworthy and capable vessel.

Does a baby make a family? I guess it does , Generally the kids are a bit older and mum and dad want some connubial privacy.

Cheers

 

Yes, as long as you don't mention another slightly incorrectly advertised boat, the Mac 26

Mikey

 

Yes, I think that a higher centre of gravity is less stable than a low centre of gravity. I think less pitch momentum is better than more pitch momentum. I think less roll momentum is better than than more roll momentum.

Can you site such a design that is more seaworthy than the Pogo?

How about Higher acceleration = higher velocity = same distance over less time? If the energy is high enough that it would heel the boat to X deg at equilibrium, but does not act long enough for the boat to reach the equilibrium state, the heavy rig will not heel as far *during the event* but will continue to roll after the event has passed. The light rig will heel farther during the event but not continue to roll as far as the heavy rig. In this thread, the light rig is on a boat that has a larger righting arm, hence there is greater force to resist smaller momentum. In general, we don't like to see mass concentrated in the ends of the boat because a 100kg anchor and chain on the bow and a 400kg dinghy on the stern davits do not improve stability. The same holds true for rigs, lighter is better. Unless you want to tell me that a sailboat is more stable with a man at the masthead than with him on deck ...

Absolutely right, for short handed passages, what boats are designed for two crew? The racer's expected area of operation is ocean in any weather on autopilot much of the time. How many cruisers share this design goal?

So you keep saying. That's your opinion, I don't share it.

Yes, they do. Apparently you don't think so. As is your habit, you provide a list of subjective criteria and apparently cannot see that others might have different standards. 30-40 foot sailboats are not ocean liners, those terms mean different things in different contexts.

I have lived aboard. One year on a 26 foot boat with 55 inches of headroom, and 5 years on my C30. Thus I know for a fact exactly what I need to be happy living on a boat. I don't see that I'm giving up anything when I compare the C30 to the Pogo.

 

Tom, the area behind the positive area of the RM curve represents the energy needed to capsize the boat.

I have learned about it from many reputable sources:

Some are in the net and I can post them.

From the small PDF from the RYA, announcing its book RYA book G23 - Stability and Buoyancy available:

“For ocean-going and offshore yachts one of the most easily seen and meaningful aspects of a GZ curve is the AVS.
But a GZ curve and its AVS are by no means the whole transverse static stability story. A boats mass (displacement) is also very important.
A lever, when multiplied by the force pushing it, becomes a moment. With a boat the lever is the GZ and the force is the boat’s mass. So by multiplying GZ by the boat’s mass gives a righting moment (RM) curve. As the area under this curve represents the energy needed to heel the boat, then for the same GZ curve, a boat of double the mass will need twice the energy to capsize (and twice the energy to re-right after capsize)”.

http://www.rya.org.uk/NR/rdonlyres/43D83922-ED3E-4D5D-BBCF-6A14AA352EFA/0/StabilityIntro.pdf

or Peter Nielsen:

“the negative area inside the curve is a measure of the wave energy that would be required to right the boat”.

http://www.sailbuyersguide.com/articles/boatpages/StabilityAndTheGZCurve.cfm


But of course, these are only the basics, we are only talking about static stability, and dynamic stability is the other side of the story.

“Monohull ballasted sailing yacht stability considerations come in two types – static and dynamic. Both are normally expressed in terms of transverse or longitudinal stability but mainly transverse. A stability curve is generated by plotting a stationary (static) boat’s righting lever against its angle of heel.”

http://www.rya.org.uk/NR/rdonlyres/43D83922-ED3E-4D5D-BBCF-6A14AA352EFA/0/StabilityIntro.pdf


You are talking about dynamic stability, and I agree when you say about static stability:


"It should be regarded as a starting point that leads into deeper analysis of dynamic forces that the vessel is expected to handle. It may be able to be used a probability indicator to capsize and re-righting, but if so, it certainly should not be used as a final analysis on stability".


But unfortunately it is much more difficult to analyze dynamic stability than static stability. I don’t believe that this thread has the potential to permit a meaningful discussion about it. Look at the way things went when we were only discussing basic things like static stability.


Old designs and new designs take dynamic stability by different angles, and talking particularly of the kind of boats like Pogo, their strong points are completely different from the strong points of old heavy designs. For example, the water ballast, in what refers increased inertia and longer roll periods, the capacity to transform the lateral push of a breaking wave (energy) in kinetic energy, through a lateral skid (instead of transforming it in a roll movement), the capacity to increase dynamic stability by the ability to maintain a steady course (because the boat continues to sail even in a storm, instead of being thrown around) are some of the strong points of this kind of boat.

Of course, heavy boats also have good points, different points, but do you think we are going to be able to have a productive and fair discussion about this issue?

 

I asked
[Do you understand design metacentric height and all that entails wrt motion and stability?]

You are confusing matacentric height and COG. We all know about COG, this wasn’t what I asked you.
Momentum needs velocity not just mass and higher mass vessels gives higher damping due to various factors. Consequently you need considerably more energy for the same movement. You get less momentum through greater inertia coupled with good damping. You are (perhaps willfully) misunderstanding this.

Now you have that jerky motion you were so keen to deny. However in the same conditions this is not the case, the low inertia boat moves a lot, the high inertia boat moves a little.

This might sound intuitive but what’s happened to damping in this model?

….Eh ?

Again we want to “tune” the system with a knowledge of the vessel characteristics pitching is a complex characteristic too and as I explained it’s coupled to other characteristics and is not as simple as weight in the ends. If the vessel is too light in the ends or has too low a fore-n-aft inertia it will be slower and less comfortable. Shifting mass in and out of the ends even ballast is not uncommon.

Dynamic stability yes, and the vessel may be safer and more comfortable because of it (as much as it goes against your intuition) .

You keep confusing subjective and objective a list of criteria is not subjective the discussion of the values you put on those criteria is subjective.


By now you should have a good idea how mass damping and motion relate even if it were from your experience on motor vehicles. You need to think about the effect of the dampers in the suspension perhaps.

 

No Vega, I can absolutely not agree with that. Of course we can discuss dynamic stability and frankly, the discussion would be quite useless if we didn't discuss it as a dynamic event, because that is what stability of a sailing yacht is. But we will never be able to agree

Mikey

 

I did not grossly miscalculated RM1200 STIX. The 32,9 figure was calculated as STIX(100) and not as her final figure STIX, and thas was clearly stated in my post. The more accurate figure of 33,521 is not that far away from 32,9 for STIX (100). What was grossly wrong was "Increasing Dfl to 116º (=AVS) only brings STIX up to 33,531", but you didn't even noticed.

No, I'm not saying that. I'm saying I do not trust the data you post about these kind of matters, as you have demonstrated to me you either fallaciously manipulate such data or, on the other hand, you post it in good faith but not understanding it. All this in my humble opinion, of course.

Always.

No. You said 44.7 is her official STIX, and you still do not know if that is true. I have given you some clues to think it may very well be not so.

At least I apologize. And I try to understand by myself, by working numbers, the reason of things even if I make mistakes, what you do not do. I've realized through your posts along time in these forums, you blindly believe whatever you want to believe at any given moment, as your opinion varies randomly because of your lack of knowledge and scarce ability to deeply analize things.

And please do not be so condescendent to me. If I make mistakes I'm grown enough to asume them. But probably that's not the point, because the reason of protecting me or these forums is ridiculous. What I still think is that most probably at that document there's some info you do not want others to know.

That may be precisely your problem. Somebody gives you that info, and you do not have the concern of asking yourself why the MTL condition is the one chosen to calculate STIX, specially for a boat with water ballast. I think this is because you know very little about STIX calculations and the meaning of concepts there. I recommend you yo write the designer and ask him to confirm you in written the thing, before asuming what you have been given is the correct figure.

Absolutely.

 

Mikey, I agree with you. But first of all we have to define what we understand as dynamic stability and what our approach to it. Traditional dynamic stability calculations and analysis based on the static stability curves, which is the usual way to look at the thing, can be a powerful tool to understand many aspects of a vessel's behaviour, but it is of little use to analyze other effects i.e. hydrodynamic damping or course keeping ability.

And posters should know at least the basics on dynamic stability as well as on static, and also to be prepared to work by themselves the dynamic stability curves, the energy-work analysis, the hydro lifts, etc, etc, and have good books to consult, as there is little information on these matters available at internet or from boats' designers-manufactures.

Somebody posted a while ago: "Old designs and new designs take dynamic stability by different angles,..... their strong points are completely different from the strong points of old heavy designs. For example, the water ballast, in what refers increased inertia and longer roll periods...."

This kind of mental confusion is precisely the one we should avoid not to go into stupidly circular discussions anymore.

Cheers.

 

The relevant parts to me are bold italic, the red herrings are ... well ... red

Metacentric height is not relevant when discussing angles greater than about 10 degrees. You are you are (perhaps willfully) misunderstanding this.

Please note that if increasing metacentric height is done by lowering the centre of gravity, or increasing the moment of inertia of the water line, lowering it by raising the center of gravity with a heavy rig has an effect on the motion at small angles, while at large angles (90 degrees) the heavy rig reduces stability.

I'm starting to think that your background is in comercial power vessles. While metacentric height plays a large role in such designs, "where the steadying effect of sail is absent", in sailing craft, increasing rig weight is shear folly. That may explain why no one recommends hoisting a 20 gallon barrel of water up the mast to "stabilize" a sail boat.

I certainly like to think I do. Weight in the ends of any performance vehicle is to be avoided if possible. Weight in the ends is one of the reasons that Porsche's are so difficult to drive quickly. If we want a sail boat to perform well, the rig must be stable. Mass far from the center of rotation requires large amounts of energy to control. Every yaw excursion requires greater rudder energy to correct and slows the boat. Every roll excursion requires more energy to correct, boats with low roll inertia and high RM roll less, but roll that small amount quickly. The high RM acts to return the boat to trim and the low roll inertia requires less dampening. At low speed the combination of sail area and lateral area of the hull provide the dampening, at higher speeds a small efficient keel sees a change in AoA and lift acts counter to the roll. Every pitch excursion requires greater dampening when the ends are heavy. Dampening comes from the hull displacing water (in general, sails have less damping effect in pitch than they do in roll). Displacing water requires energy that could be used to drive the boat. Hence fine entries with little flare to minimise the effect of sailing into waves (at he expense of a wetter boat) and wide sterns with flat runs to promote planing and keep speed up. Weight in the ends is a performance reducing feature. Hell, weight is a performance reducing feature.

It is certainly possible to "tune" a boat to have less motion in certain conditions by moving weight around. Some boats deal with the chop in San Pablo Bay due to the ebb flowing against a Force 4 - 5 breeze by putting a larger anchor on the bow roller, the same tactic makes other boats worse. And in other sea states and boat speeds the positive effects become negative.

I think the point you are trying to make is that the Pogo will have higher accelerations than a heavier boat in the same sea state. At some point the motion in both boats will have a negative effect on the crew. That point might well come in less extreme conditions for the Pogo. For the sake of argument, lets say the Pogo has motion = bad at 25 knots of breeze and Big Bertha has motion = bad at 35 knots of breeze. In 20 knots of breeze the Pogo can cover between 150 and 310 miles in 24 hours (that 150 is hard on the wind at VMG = 6.25). I say there is a better than even chance that the Pogo will be able to avoid having to endure 25 knot breeze if the skipper wishes. Of course motion = bad is defined by the sailor on the boat. Apparently in 50+ knots, motion = "I think I'll take a nap and let Auto drive" for the Racing Pogo. The Cruising boat is more moderate.

The more I think about it, I think the Class40 Cruisers are the Cal40's of a new generation of fast cruisers. If we enter the way-back machine to 1965 or so, the Anti-Pogo "experts" would be grousing about how "unseaworthy" the Cal 40 is ... Fin Keels and Spade Rudders being totally unfit for offshore work and all. All the same arguments ... 40 years later and the Cal has proved to be a very cabable and seaworthy boat.

Come on, I can be convinced ... show me the beef.

Be objective, use numbers. Come up with data that shows something along the lines of: At 8 knots cruising speed in 15 knots and typical 6-8 foot waves, the Pogo 40 will have a Byflspick Motion of 7.49 furlongs per furlong. Byflspick Motions of 7-8 furlongs^2 for more than 7 hours have been proved to cause cold sores in 66.79 percent of male smokers over the age of 45. Something? Anything?

No matter that none of the criticisms from earlier in the thread have gotten even a "I guess my assumptions were wrong" response when data that contradicts the assumption is presented. Now we get Metacenter smoke and mirrors from someone that should know that it is not relevant and thinks that heavy rigs make boats stable.

Two words: Top Hamper (it is not your friend)

I'm having fun ... I wonder what they will come up with next?

 

I seem to be getting dragged into this thread. Arghh!
GM is only an indication of initial (small angle) stability. Boats with a high GM will be stiff, those with a low GM will be tender. All the stability criteria I know of specify a minimum value, but none specify a maximum value, although it is certainly possible to have too high a GM for comfort (fast accelerations inducing sea sickness). A high GM is a useful feature of a racing yacht, but a lower (but not low) GM is advantageous for cruising. If your cruising style is to sail your boat pretty hard then a higher GM can be okay, but all blue water cruising boats must be designed for that 'once in a lifetime' situation where the sailors are incapacitated either through sickness, fatigue or boat damage.
It is worth noting that the old RNLI lifeboats were designed to be self recovering (ie extremely high AVS and very small negative area). They were nicknamed 'self-capsizing' by the crew as they barrel rolled so easily. The point being that if a boat is designed to recover easily from a knock down, it will probably be easy to knock down in the first place.
Dynamic stability is an interesting topic, but boats must be designed to static stability criteria because there are no guarantees that a boat will always be moving quickly enough to generate significant dynamic effects. Dynamic effects are also much harder to quantify.
The one area I would like to see covered in more detail in stability rules is the effect of heaving on reducing the weight (and hence stabilty) of the vessel.

Just my two pennys. I know I've added nothing new, but I do not know enough about STIX or POGO's to be able to contribute anything to that part of the discussion. At any rate, it is hard to say anything new on the subject! At the risk of sounding patronising though, I must say I find the standard of English to be quite superb.

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