Enjoy :
I didn't follow your example, but as I do the math for an comparison of two catamarans, one of which has twice the beam and displacement of the other, I come up with 3x the stability for 2x the increase in beam and weight. This makes me think that his rule of thumb must be meant to factor in the forces with tend to overturn a boat from wind force, sail center height and sail area, and to compare those forces with the increased stability.
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Sorry, this does not make any sense to guess what they mean by formulating that 'rule of thumb' (Chris should specify it clearly or we should guess?). In terms of RM it does not work - see my posts.
In general, I suggest to study more professional books that are peer-reviewed by naval architects; at least if we are talking about boat design books.
Albatross, I haven't been able to find any professional books directed towards designing sailing catamarans, just disconnected hints and comments widely scattered. There are several textbooks about designing sailing monohulls.
As I look at the sail area and mast height of a FP 36 compared to a Lagoon 440, and multiply the mast height x .4 to give some idea of the CE, I come up with the Lagoon 440 having twice as much force (SA x CE height, assumed,) as the FP 36. I compare these two because the one weighs (allegedly!) twice as much as the other. If the larger boat had twice the beam (it doesn't) as the smaller, you would get 2x the force working to overcome 3x the stability. I am sure my various estimates are off somewhat, but not hugely, I think.
I'm beginning to think I misinterpreted Chris' rule of thumb. Since you have a copy of 'The Cruising Multihull,' perhaps you can review page 198 (part of 'Countering Wave cCapsize by Design,') and see if I have misinterpreted him in some way. He doesn't really draw a picture of what he means by his rule of thumb - perhaps he is talking about the roll moment of inertia? Do you think that could increase by 16 x if the boat size is doubled in every dimension?
Currently concentrating on http://earthnurture.com .
Let us then calculate the multihull size factor:
SF = 1.75 * mMOC * (LH * BCB)^0.5, where
mMOC is the minimum operating mass (kg)
LH is the hull length (m) and
BCB it the beam between centerlines (m)
In stability standard ISO 12217-2, the multihull size factor is "a degree of protection against being inverted by breaking waves".
Just put the numbers in and compare different catamarans. When LH/BCB ratio is between 2.2 and 3.2 the SF over 40 000 shows an ocean capable catamaran. For wider and narrower catamarans, see the standard.
Last edited by terhohalme; 10th October 2009 at 10:30 AM.
I think this thread shows how hard it is to write the "truth" for all people. Chris White is making some general comments for the lay reader, not ones to be considered the complete and absolute truth by naval architects.
Unfortunately yacht design is too complicated for generalizations. One part of the yacht designers "art" is deciding what compromises to make, knowing full well what the disadvantages are.
For example, I said I'd always go for the wider beam boat, but in the same thread I also said that wide boats are heavier, thus slower, thus need more sail to compensate, thus need to be wider to regain the stability etc
So design spirals can be quite complicated to follow.
You certainly must not think that a boat twice as long is 8 or even 16 times more stable. That could only be true if you assumed that the rig stayed the same size - highly unlikely.
Some things are fixed "facts of life" and I guess can be considered the naval architecture bit of designing a yacht. Others are dependant on ones own philosophy and are the "yacht design" bit.
As many know, I have been the "multihull expert" on the ISO stability working group for around 15 years. You may like to know that we are currently revising the ISO standards with several meetings planned for this year.
It is unlikely that the MSF will stay in its current form. Instead we are trying to include both pitchpoling and roll factors as well as a "Bare Poles" factor. We are also seriously exploring the possiblity of have a class of "uncapsizable multihulls"
Hope this helps the discussion
Richard Woods of Woods Designs
www.sailingcatamarans.com
Size factor formula is well known to anyone who had ever designed sailing catamaran to CE certification. Besides it does not prove any of BigCat's statements on stability
It could seem that I am trying to be too pedantic, but this is attitude developed for years of teaching the subject and developing stability simulators for navy and merchant ships. Those days I tried to explain complicated things in simple but correct words not allowing interpretations. Let's take it right even if we are talking about simplified rules of thumb; no need to replicate aberrations.
I will try to have a look at the book, but only on Monday because it is in office. We had good sea trials today, and tomorrow we go fishing on the islands
I don't think that I was writing here to prove any of BigCats statements, it is his job.
Still I can't see writing something totally untrue. In maximun righting moment the rule of thumb is only about 5 % too high and very much used in catamaran estimations. Unfortunately there isn't that easy way with merchant ships.
Well, this started out as a quote, but now that I examine it, I can't substantiate the quote from Chris White's book as either a correct statement of righting moment, righting moment compared to sail area times sail center height, or as compared to the EU standard for seaworthiness offshore. Working through some samples, it seems that the EU formula declares a boat twice as big in beam, length and weight produces a claim of roughly twice as seaworthy offshore. Incidentally, the EU formula, per TerhoHalme above, worked out for a typical 35' catamaran requires a least displacement of about 3,600 kilos (@8,000#) to be considered seaworthy offshore.
CW states that the roll moment of inertia is a product of mass x distance squared. If we assume that this can be reduced to (displacement times .5 x hcl to hcl beam) squared, (which neglects the contribution of the rig,) examples I work out come out to exactly 16x, so Chris' rule of thumb refers not to RM, or RM compared to wind leverage on the rig, but rather as a statement regarding roll moment of inertia. So, unless Chris has mis-stated the factors affecting the roll moment of inertia, this is the meaning of his statement, and it is accurate.
Last edited by BigCat; 10th October 2009 at 10:38 PM.
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Moment of inertia has no meaning for STATIC stability which is common representation. Yes, moment of inertia effects the roll but in this case we should also consider a) added masses and b) damping factors, both effected by underwater geometry and roll accelerations. Then, everything becomes complicated and it is definitely not CW's level
This sample is for terhohalme:
I have done study of catamarans stability with variation of BCB (BCB=BCL if hulls are symmetrical). Just some results here, this is data for 44' displacement power catamaran, for real catamaran BCB=3.33m. For this boat VCG is 0.6m above WL (for reference - vertical clearance of bridgedeck is 0.9m). So we should expect similar results for sailing cats as well.
Note that GZmax is not BCB/2 as You state in Your post, that is specially true for narrower boats where this assumtion can be dangerous.
Last edited by Albatross; 11th October 2009 at 02:34 PM.
OK, now I see the light. Modern sailing catamarans have LH/BCB ratio between 2-3. Very seldom more. My experience is in sailing catamarans where LH/BCB is near 2. Ever designed one?
Power catamarans seem to be much narrower, your example have this ratio more than 4. If this ratio is 5 or more, the boat have to consider as a monohull in stability standard.
We are both right, just looking the beast at the other edge.
If we take our 44' powercat example, then BCB=5m gives LWL/BCB=2.48; BCB=4m gives LWL/BCB=3.10. This well complies with numbers You specify for sailing cats, so it should work for sailing cats. The only reason why I show example for powercat is becasue I have this sample ready made (for one publication).
Sure, pls look at our web www.amdesign.co.th or my blog www.albertnazarov.blog.ruEver designed one?
Yes and no, some of them are quite wide. Our 50' trawler cat is 7.6m wide...Power catamarans seem to be much narrower, your example have this ratio more than 4.
Yes, but this clause is applied only for sailing boats. In ISO12217-1 and 3 there is no catamaran definition for non-sailing cats, they are treated as monohulls with high initial stability anywayIf this ratio is 5 or more, the boat have to consider as a monohull in stability standard.
I am just trying to emphasize that every simplified formula or rule of thumb works in certain range that should be specified. I have met few 'simplifed' formulas and factors that are just dangerous if used without knowledge of its limits.We are both right, just looking the beast at the other edge.
Last edited by Albatross; 11th October 2009 at 06:00 PM.
2BigCat:
OK, finally I have opened CW's book at p.198 and have no idea on what he is takling about 'doubling size increases its stability 16 times'. Again, stability is complex a property of boat that is characterised by many parameters. Anyway in terms of RM it does not work. Why don't You ask this question to the author of book?
CW is clearly talking about stability, not roll moment of inertia!
Again, roll moment of inertia has no meaning here. Look at equation of free roll motions on flat water:
(Ixx+Jxx)*dw/dt + Kxx*w + DISPL*GM0*sin(H) = 0
Ixx - moment of inertia of boat around longitudinal axis;
Jxx - added moment of inertia for underwater body;
w - angular velocity around longitudinal axis;
Kxx - roll damping coefficient;
GM0 - initial gravity metacenter;
DISPL - displacement
H - heel angle
So the first component is inertial, second is damping, third is hydrostatic.
Moment of inertia is only a part of one component here; other components should be considered before saying that 'stability increased 16 times'.
I still recommend to ask question to CW on his '16 times' statement, just send a link to this thread. We do not need to guess what he means
Last edited by Albatross; 12th October 2009 at 06:32 PM.
I don't think flat water is what he had in mind. He is referring to the famous Fastnet race where lots of (monohull) boats rolled over more easily after losing their masts, and points out that their decreased roll moment of inertia due to mast loss made them more vulnerable to rolling again. Whether you agree with him, I will leave between him and you - I am satisfied to feel sure that I have correctly understood and reported what he said and meant.
I can't make anything out of your formula, because it references other formulas (or measurements,) without giving them; Kxx, for example.
Currently concentrating on http://earthnurture.com .