|Topic Review (Newest First)|
|12-30-2008 06:54 PM|
First, you would probably be better served by starting a new thread regarding generators and J-40s, since that really isn't the topic of this one. Second, it is fairly unlikely that any of the posters, excepting Jeff H, will respond, since most are probably long gone from this forum.
I'd also highly recommend you read the POST in my signature to help you get the most out of your time here. It has tips on searching sailnet, writing a good post, etc..
Welcome to the Asylum...
BTW, this post was mainly written for the amusement of FarCry...
|12-30-2008 05:45 PM|
Oh Mr. Dog where art thou??????
Welcome to Sailnet Jrussell. Shortly a most esteemed SN member by the screen name of Sailingdog will come along and help you get more out of this forum. He may come across in a less than friendly manner but ignore his tone and focus on his content.
In the future please note the age of the thread you are responding to...for example this one is three years old. It might be best to start a new thread asking your generator quesiton.
|12-30-2008 05:20 PM|
J-40 for long distance Cruising
Does any body know of have completed installation of a cruising generator aboard a J-40
|01-09-2005 12:51 PM|
J-40 for Long Distance Cruising ?
excellent thread. I have no opinions on the J40 at all...
before I began the journey of boat purchase and refitting for extended cruising I had compiled a list of what I thought I would need for such a task as long distance sailing for a year or two. by the time the boat was completed, my thought process was dfrastically changed, it was no longer ''how much I need'', but was ''how little I can get away with''.
|01-06-2005 02:32 PM|
J-40 for Long Distance Cruising ?
Sorry for the long delay in responding. Was on vacation.
Anyhow, our #4 was tiny - I am hipshooting, but would say maybe 60% or so.
|12-28-2004 01:04 PM|
J-40 for Long Distance Cruising ?
In response to Paulo''s comments:
With regards to Paulo''s opening comments:
I did not say that I thought that it was a bad thing that this thread picked up a whole range of other topics, just that it had gone far beyond the original topic which was the suitability of a J-40 for distance cruising. It is the serendipitous wanderings that often make life more interesting and educational.
"This implies that it is as easy to capsize a Bavaria 40 as to capsize the BMW Oracle (America cup)….Both boats have the same beam (4m), or the famous Star and Stripes, versus the Bavaria 36 ( both 3,6m).
Of course this is ridiculous, the Bavaria 40 weighs 8,3T and has a ballast of 2.9T with a draught of 1,65 and the Oracle weights 24 T has a ballast of 20 T and a draught over 4m, and if you look at the dimension of those sails you know that the Oracle has to have a massive righting moment."
My statement does not imply that it is as easy to capsize a Bavaria 40 as it is to capsize an America''s Cup Class or a 12 meter. My comment would imply that assuming a Bavaria 40 has the same beam as an America''s Cup Class and a 12 meter, these three are equally as likely to be rolled over by a wave that is large enough to roll any one of the three. It needs to be understood that a roll over is different from a capsize and my comments were in response to a comment on roll-overs. For fully ballasted boats, only wave action can cause a roll over, and the only significantly consistent factor in determining the likelihood of a roll over is the height of the wave relative to the beam of the boat. There are some secondary factors that come into play such as the depth of the keel which means that the America''s Cup Class and the 12 Meter are more slightly likely to be rolled over than the Bavaria due to their deeper keels.
Factors like the LPS, area under the righting moment curve (with the boat inverted and its gear in its actual position as inverted), downflooding, and the like are significant in determing whether the boat is likely to re-right and survive. There major differences in the design of these three boats which would result in the likelihood of differing re-righting and survival results.
I do agree with Dave Gerr that when it comes to resistance to wind driven knock downs and the speed of re-righting, that the area under righting moment is critical. I would strongly disagree that "Displacement is responsible in equal terms with GZ in originating the force that prevents capsizing." While righting moment results from a calculation of the displacement times the righting lever arm far, in a practical sense, far and away, weight distribution relative to buoyancy distribution, is much more critical in determining the likelihood of a knocked down than a boat''s actual displacement.
I can explain this by using the example that I generally cite to debunk the ''Capsize Screening Formula'', if you visualize two identical boats, except that one has a 1000 lbs of weight at the top of its mast, the boat with the 1000 lbs of weight at the top of its mast has a greater displacement but it is significantly more prone to being knocked down and far less likely to re-right because of its weight distribution.
While I cannot imagine that anyone would intentionally place 1000 lbs at the the top of their masts, if you look at the sources of the added displacement typically found in heavier weight cruisers, the added weight is typically found in locations that do not help stability such as heavier interior appointments, heavier deck and topside structures, heavier spars and rigging, heavier mast mounted electronic components, rigid cockpit shelters, higher davit mounted- heavier weight dingies and motors, and the like. Cumulatively these can add a lot of weight while at the same time reducing stability greatly.
The reality of this was borne out in the STIX research project where heavier displacement cruisers were often found to gave greater drag relative to their stability. This meant that these heavier displacement cruisers ended up needing to carry proportionately larger sail area relative to their stability, which in turn made them more prone(rather than less) prone to a knock down.
“The area under the positive part of a righting moment curve is a measure of the total energy required to roll a boat, while the area under the negative part of the curve is a measure of how much energy it will take to roll it back again.”
Here again, heavier displacement works against re-righting. With its greater moment of interia, and the fact that a heavier displacement boat is likely to float deeper in the water relative to its center of gravity requiring much greater energy to re-right, the heavier displacement boat is likely to require a longer period of time and more energy to re-right.
"In normal circumstances if, to capsize a boat, it is necessary the lateral force of a wave two times bigger than one capable of capsizing a lighter boat, then the first boat is (broadly speaking) a safer boat. If, to capsized a boat, it is necessary a wave three times bigger than the one needed to capsize the smaller boat, then this boat is safer than the one that is capsized by the wave two times bigger than the one that is capable of capsizing the lighter boat…... and so on."
The reality of wave driven knock-downs and roll-overs is that the forces are enormous, many times the comparatively small differences in displacement between one boat and another. You are correct in assuming that it is possible for one boat to have several times the righting moment of another (although again this is more a product of weight distribution and buoyancy distribution than actual overall weight of the two boats. For example, comparing a specific lighter weight boat with equal but deeper placed ballast than on heavier boat, the lighter boat could easily develop much greater righting moment and require a lot less force to re-right). But in studies of wave driven capsizes and roll-overs righting moment plays a very small roll as the over turning moments grossly exceed the righting moments involved as the weight of water and force of gravity acting on the boat are just so enormous. Using the current coefficients for wave impact, the impact loads of a breaking wave on a typical 40 footer is hundreds of thousands of pounds, easily overwhelming the comparatively minor 12,000 lb difference in weight between a very light and a very heavy 40 footer.
So, whether they are called fears or security margins, they represent pretty much the same thing; the choice of a boat depends on how much of a risk you are willing to take. But no matter what your fears or security margins, in an of itself, weight does nothing good for a boat. It does not make it more seaworthy, sturdy, comfortable, easier to handle, or stable, just heavier.
|12-28-2004 11:08 AM|
J-40 for Long Distance Cruising ?
You have said:
“This thread seems to have picked up topics like lint sticking to rolling tar ball. At the moment this thread seems to be wrestling with the relationship of displacement to strength, carrying capacity, and the relationship of displacement relative to the various aspects of seaworthiness such as initial stability, knockdown resistance, limits of positive stability, motion comfort, and the relative righting moments inverted or right side up.”
I don’t see it that way. This thread was about the qualities and eventual limitations of a “J40 for long distance cruising” and François has extended the scoop of the thread to a more global perspective, including not only the J40, but that type of boat ( light displacement 40ft cruiser racer). All the subjects you have mentioned have to do with the limitations or advantages of that type of boat, comparing with what is considered a typical passage maker (the kind of boat that is widely used for that kind of travel).
“In terms of the current understanding of knockdowns, there are a lot of factors involved, enough to fill a book, but in terms of a roll over, the only consistent factor in determining the likelihood of a roll over and rerighting has been found to be the relationship of the boat''s beam to wave height.”
This implies that it is as easy to capsize a Bavaria 40 as to capsize the BMW Oracle (America cup)….Both boats have the same beam (4m), or the famous Star and Stripes, versus the Bavaria 36 ( both 3,6m).
Of course this is ridiculous, the Bavaria 40 weighs 8,3T and has a ballast of 2.9T with a draught of 1,65 and the Oracle weights 24 T has a ballast of 20 T and a draught over 4m, and if you look at the dimension of those sails you know that the Oracle has to have a massive righting moment.
I think that it fits better here what you have said about other thing:
“I also think that this thread has suffered a problem applying broad generalities…… in a way that somewhat corrupts the validity of the points being made”.
Fact is that the forces necessary to produce a rollover in a determined boat can be measured very precisely as you can also measure precisely the force that a given boat will offer to being rolled.
Quoting what Nigel Calder (in a very good article about boat stability, published in the September issue of “Yachting Monthly”:
“The area under the positive part of a rigtning moment curve is a measure of the total energy required to roll a boat, while the area under the negative part of the curve is a measure of how much energy it will take to roll it back again.”
Quoting Dave Gerr in a good article about boat stability published in the October issue of”Sail Magazine”:
“Stability curves can also be constructed using the Rightning Moment ( Rightning Moment = Rightning Arm x Displacement).
The curve will look the same, but you should use RM when plotting different boats on the same scale of comparison. This will immediately show the difference between boats with the same Rightning Arm (GZ) but dissimilar displacements. Note that the area under the stability curve is much larger for the 26.866 pound boat than for the 19.190 pound Gerr 44, even though the boats have the same RM at all angles of heel.
This is because it takes more energy to capsize the larger, heavier boat and it is why bigger boats are (broadly speaking) inherently safer offshore.”
The RM, namely the Area behind the positive part of the Righting moment curve is the measure of the resistance that the boat offers to capsize and............
RM = GZ x Displacement
This shows that Displacement is responsible in equal terms with GZ in originating the force that prevents capsizing (Rightning Moment).
The other half of the equation is GZ (Righting Arm). The GZ results of the lever created between the CG (Center of the Gravity) and the CB (Center of Buoyancy) when the boat heels.
Without wanting to be too much technical about it, I would say that the two main factors to increase GZ are lowering the center of Gravity and Increasing Beam.
But it is fair to remember that both factors together only contribute as much to the capacity of the boat in resisting rolling, as the factor weight, only by itself.
Jeff said :
“The forces involved in roll-overs … are so huge relative to the displacement of a yacht, that the weight of the boat in question has been shown to have next to no bearing at all on the likelihood of a roll over …”
I have said in a previous post:
“Safety and seaworthiness are relative parameters; the one that is not relative is the sea, that giving the right conditions is capable of sinking any small boat.”
And you seem to agree, but those are rare circumstances, the ones when size or weight don’t count.
In normal circumstances if, to capsize a boat, it is necessary the lateral force of a wave two times bigger than one capable of capsizing a lighter boat, then the first boat is (broadly speaking) a safer boat. If, to capsized a boat, it is necessary a wave three times bigger than the one needed to capsize the smaller boat, then this boat is safer than the one that is capsized by the wave two times bigger than the one that is capable of capsizing the lighter boat…... and so on.
Jeff said :
“In the end all boats are compromises, and we each chose our specific compromises based on our needs, tastes and fears”.
I agree with that, if you change FEARS to SECURITY MARGINS.
And those margins depend on a lot of conditions: people involved (family), your obligations to others ( again family) etc.
I Think that even with a well prepared light (27ft) typical small coastal boat you have very good probabilities to accomplish an ocean passage. After all last year a guy with a 18 foot Hobbie Cat crossed the Atlantic, and the first man to cross solo the Atlantic (East-west) has done that in a 5m open sailing Dory….But I also believe, quoting an old captain,” that the oceans are paved with the bones of the optimistic sailors”.
|12-27-2004 11:22 AM|
J-40 for Long Distance Cruising ?
This seems to be the ''Energizer bunny'' of discussions. This thread seems to have picked up topics like lint sticking to rolling tar ball. At the moment this thread seems to be wrestling with the relationship of displacement to strength, carrying capacity, and the relationship of displacement relative to the various aspects of seaworthiness such as initial stability, knockdown resistance, limits of positive stability, motion comfort, and the relative righting moments inverted or right side up.
In a very broad general sense there is absolutely no inherent direct relationship between a boat''s length to displacement ratio and any of these items. It is true that in many cases, a boat that is long for its weight (or light for its length) may be inferior in all of these categories to an equal length boat of heavier displacement, but it is equally likely to be superior to an equal displacement boat of a shorter length in most if not all of these categories.
In all of the major studies on major storms and seaworthiness, displacement relative to length, or even simple relative displacement, has not proven to be a factor. The only factor consistently shown to affect seaworthiness is length, with longer waterline length consistently showing up as the single consistent factor in determining the likely survival in extreme conditions.
To me this discussion once again comes back to how we define a boat''s size. If we size the boat by its displacement, then the longer boat of equal displacement will generally be the superior boat in almost all ways but will require more careful engineering to achieve equal strength.
I also think that this thread has suffered a problem applying broad generalities to an otherwise specific set of issues in a way that somewhat corrupts the validity of the points being made. I think that talking about boat like the Malo vs the X-boat in such broad general ways really does not address the specifics of the individual boats capabilities and liabilities. Similarly the information contained on sites such as Dick Koopman''s site does not really apply because he is making very generalized observations that may or may not apply to the specific vessels in question.
In terms of the current undertsanding of knockdowns, there are a lot of factors involved, enough to fill a book, but in terms of a roll over, the only constistent factor in determining the likelihood of a roll over and rerighting has been found to be the relationship of the boat''s beam to wave height. Similarly the only consistent factor in determining the likelihood of a pitchpole is length in relationship to wave height. The forces involved in roll-overs and pitchpoles are so huge relative to the displacement of a yacht, that the weight of the boat in question has been shown to have next to no bearing at all on the likelihood of a roll over or pitchpole. (In fact in the Sidney Hobart Race disaster, the boats that suffered the worst roll overs, pitchpoles, and knock downs were at the heavier end of the displacment to length ratio spectrum.)
In the end, the validity of whether a specific lighter weight boat makes sense as a distance cruiser is dependent on the specifics of the boat involved, as well as being very dependent on the needs and abilities of individual''s who are cruising the boat in question, and the venues in which they are sailing. In and of themself, the numbers are of little bearing. [For example, looking at the numbers for my boat, which is a 38 footer with an IMS design weight of 10,600 lbs, and SA/D around 22, and an IMS LPS of 112 or so (sometimes listed as 108 but which equates to roughly 125 degrees fully loaded, including the displacement of cabin and cockpit in the calculation) you would never consider these to be offshore capable boats. Yet sisterships of my boat have made all kinds of long distant passages in all kinds of rugged sailing venues. They are routinely sailed out of Capetown and as of a couple years ago, as 20 year old boats, were still being raced in the replacement race for the brutal Capetown to Rio race]
Once again, the reality comes down to the specifics of how the boats are engineered, configured, and constructed as compared to the compromises that an owner is willing to put up with. In the end all boats are compromises, and we each chose our specific compromises based on our needs, tastes and fears.
|12-26-2004 08:37 AM|
J-40 for Long Distance Cruising ?
About the importance of displacement regarding reserve stability in a 40ft boat
look at this:
Those are a comparison of stability curves between 40ft boats designed by Dick Koopmans. Boats with different kind of keels and weights.
Dick Koopmans his a Dutch designer responsible for a lot of boat plans, some used by know boat builders, like Victoire yachts or Hutting yachts. His boats are well known by its seaworthiness.
Comparing those graphs you can see that the cruiser racer has a much more inclined curve and a bigger righting arm and that at 25º, his righting moment (the force the boat makes resisting capsizing), is the biggest of them all.
You can see also that it is by far the one with worst curve regarding inverted stability. This means, the one that giving a capsizing will remain more time inverted.
That racer/cruiser curve, even if it shows the typical enhancement of initial stability versus reserve stability is exceptionally good in the amount of the reserve stability and AVS point and you have to considerer that the high value of the RM at 25º (5,5tm) is not only the result of that good curve but also of the relatively big displacement of that boat , for a 40ft cruiser/racer - 10T.
If we consider the displacement of the X-412 (7,4 ton) and consider the curve of the cruiser racer (that is actually a little worst than the one of the x-412 in initial stability and a lot better on reserve stability) we will see that the force that the X-412 can makes at 25º resisting capsizing (RM) will be only 4,1tm and that value is worse than the majority of the other boats considered in that example.
But the real difference between the cruiser/racer stability curve and the others concerns its reserve stability (the capacity to resist a knock down) and in the inverted stability, that is much bigger. About the inverted stability the curve is self explanatory. About the capacity to resist a severe knock down, let’s consider an angle of 100º:
At that angle, considering a boat with 7,4T and a stability curve like the one of the considered cruiser racer, the boat will have a RM (the force preventing capsizing) at 100º of 3,7tm. The heavy displacement 40ft long keel considered in the comparison will have at that angle a RM of 8.0tm. That is more than the double. Putting it in another way, I would say that the force necessary for capsizing a Cabo Rico 40 would be at least the double of the force needed to capsize a 40ft cruiser/racer like the x-412.
Because it is not necessary much to capsize a 7,4T 40ft cruiser/racer, I think that an oceangoing boat of that size and that displacement should have a relatively small negative stability (simplifying, an AVS of at least 135º). With that (if it is a strong boat) you will know that you don’t stay upside down for a long time (less than a minute) and you know that even if you are in the cockpit, providing you are clipped to the boat, you will survive it.
Safety and seaworthiness are relative parameters; the one that is not relative is the sea, that giving the right conditions is capable of sinking any small boat.
The choosing of a boat and the safety margin one wants for himself are a personal matter, and I am not talking about those (normal) conditions when you can control your boat but about those when your boat does all the work. Conditions so severe that you have no choice but to stay inside in a bare polled closed boat (may those never happen to us).
|12-23-2004 03:58 PM|
J-40 for Long Distance Cruising ?
The ballast ratio is sometimes misleading.
What really counts is the position of the GC and the weight. Off course, with a lower CG you need less weight to have the same RM, but with identical GSs, the weight makes the difference.
You know, on a 40ft, when it is really bad, the crew doesn''t count. You take the sails out, drag a lot of ropes, close the boat and stay inside.
The risk of pitchpoling is a lot smaller than the risk of rolling.
About breaking things, the Malo is a lot stronger comparatively with the X-412.
The X-412 is a very good boat, but is a cruiser/racer, has to be light. The Malo is a Passage Maker, is built extra strongly for that.
About Nigel Calder, the guy knows a lot about boats, he had plans to build the perfect boat...Guess what? He bought a Malo 45. Now he says the Malo 45 is the perfect Passage Maker.
Good Christmas to you
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