Desirable and Undesirable Characteristics of Offshore Yachts

Diarmuid

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cross waves and/or current (oh yea, and/or odd bottom contour - even in reasonably deep water) are the typical situation . . . but (going a bit far afield) there is also some quantum mechanics theory suggesting that even without those factors individual waves can steal wave energy from the surrounding wave field and become 'rogue size' (eg more than twice significant height).
Transients are an inescapable feature of wave train interference. They can be caused by two trains of comparable frequency intersecting at angles, or by parallel trains of differing frequency. The transient can be a one-off event at a single location -- like right under your boat -- or it can 'travel' as the constructive interference pattern is reproduced thru the matrix.

 

MikeJohns

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But it isn't rudder angle that steers a boat, it is rudder lift. And the rudder lift of the high AR rudder is higher. From experience steering both types downwind in a seaway, I can assure you that the more efficient, high AR rudder makes control easier on both helmsman and autopilot. It has more lift, a steeper lift slope, and lower hinge moments, all of which are beneficial. To generate the same steering forces, the lower AR rudder will require much greater movement with higher forces, and ultimately cannot generate as much. 

As a single example, when I rebuilt my rudder, it went from AR 1.82 to AR 2.47, and the area went down from 12.8 to 10.8 sq ft. Everything about the steering improved, even with a 16% reduction in area. 
But what about this:

.........................But if we consider Robust control in rough seas, then a lower AR rudder is going to be better in a seaway at preventing for example a wave  induced broach.  The significantly higher inflow stall angle is the factor...........


Forget lift slope and force for given area for the moment. I think that's getting sidetracked.

It’s not the maximum lift that’s important its retaining enough steering moment to keep the boat reliably on course.

Consider conditions where it's prudent to put the waves on the quarter and run when running slower than waves that are breaking.

Wave impact from astern and especially taken at any angle on the quarter result in the boat traveling at an angle relative to its orientation ( moving sideways to some extent.)

What happens when the rudder Inflow changes suddenly from say 6 degrees to 30 degrees in seconds without moving the helm ?

 As soon as the angle  between  heading and actual motion exceeds say 18 degrees for the AR 6 rudder, then it’s already stalled turning the rudder further doesn't help. But the boat requires effective rudder to prevent broaching and keep it on course down-wave while it gets up to surfing speed.

The wave is acting very quickly to slew the boat even further around, the rudder has no chance of reattaching flow and loses effectiveness.  The boat quickly assumes a position beam onto the breaker.

Lower AR rudders have a much larger inflow angle where they remain able to give effective directional control and  also have better stall characteristics. So even if they end up in the stalled condition they can still provide enough steering moment to retain control while being accelerated and then surfed by a breaker traveling faster than the boat.

It's not about the max force or the lift slope, just the stall angle and the stall characteristics of the foil in this case.

 

DDW

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But what about this:

What happens when the rudder Inflow changes suddenly from say 6 degrees to 30 degrees in seconds without moving the helm ?
While I get your point, you'll have to go out and get some data on how often this happens. I haven't seen it happen, but I haven't been in all that many days of large breaking waves either. Yeah the wave pushes the stern, but it isn't instant. At 30 degrees even a low AR rudder will be stalled (the keel as well), the difference in lift between them will be small, and the difference in resultant force (lift vector plus drag vector) very small. At 30 deg, 50% of drag acts as a steering force while 85% of lift does. At 45 degrees inflow, the drag and lift have an equal contribution to steering. 

A good helmsman will anticipate the wave and input steering prior to it hitting. A good autopilot does this as well, and tirelessly. 

Also, in normal cruising boats we aren't talking about AR 6 rudders, rather 3 or less. Even on something as extreme as a Pogo, the rudder AR is less than 4. On all out race boats, you will get 6 or a little higher - but the AR of the rudder will have inconsequential affect on the seaworthiness of that type of boat, no matter what it is. 

 

MFH125

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Thanks, DDW.

As in anything in boat design, the unstayed rig forces compromises, the main one being a jib is inefficient. The cascade of decisions starting there leads to a non traditional looking rig, and anything non traditional is a hard sell in yachting. In small truly open development classes, jibs have been gone for 50 years. Larger boat racing rules explicitly or practically require a sloop rig, cruising boats follow racing trends, I think that is the short answer. 
Is this about "small" or about "truly open" (or both)?  There are surely some scaling issues here.  Stability increases up by the 4th power of length.  Deflections of a cantilever beam are cubic, and buckling of a column (which I presume is the limiting factor in stayed mast size and weight) is quadratic. I haven't given it a lot of deep thought, but it strikes me that the laws of relativity and similitude might give free standing rigs a weight advantage for a ~15 ft. boat that they don't have at ~65 ft.  That doesn't take into account aerodynamic considerations.

Sorry, this is thread drift...

 

estarzinger

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scaling issues here. 
Related to this topic, I am struck by who the various recent wild and wacky Americas cup editions have all kept jibs and stays.

Even Larry's big tri - a completely 'open' unconstrained design by rules and money.  During the lead up there was 'rumor' that it was faster with just mainsail, but when it came down to the race they (mostly) used a jib (and always stays).

I have not even stayed at this holiday inn for this level of aero and structural engineering, so I'm just left with questions based on empirical observations.

Sorry, this is thread drift...
Well, it is really not . . .if there is a possibility that some variation of un-stayed rigs are both better sailing and more likely to survive a roll-over... then it is dead smack on topic.  I guess so far I'm listening but have not really heard a compelling case - admittedly I might not recognize the compelling case if it bit me in the ass  :huh:

 
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DDW

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Thanks, DDW.

Is this about "small" or about "truly open" (or both)?  There are surely some scaling issues here.  Stability increases up by the 4th power of length.  Deflections of a cantilever beam are cubic, and buckling of a column (which I presume is the limiting factor in stayed mast size and weight) is quadratic. I haven't given it a lot of deep thought, but it strikes me that the laws of relativity and similitude might give free standing rigs a weight advantage for a ~15 ft. boat that they don't have at ~65 ft.  That doesn't take into account aerodynamic considerations.

Sorry, this is thread drift...
Unfortunately not nearly that simple. Euler column buckling is dependent primarily on stiffness, carbon is only a bit stiffer than aluminum (though specific stiffness is higher). Cantilever mast is dependent on flexural modulus, many times what aluminum is. There are so many differences that you have to compare real examples. It is hard to optimize a carbon compression rig because there are too many nodes and too much asymmetry, while a cantilever is pretty simple. Beyond pure weight is VCG, my 65' rig VCG is about 20' off the waterline (because both diameter and wall thickness are tapered), a marconi would be 32' +/-, that is a pretty big difference, about the same as adding a ton to the keel, or making the rig 40% lighter. Speaking of capsize resistance. 

Related to this topic, I am struck by who the various recent wild and wacky Americas cup editions have all kept jibs and stays.

Even Larry's big tri - a completely 'open' unconstrained design by rules and money.  During the lead up there was 'rumor' that it was faster with just mainsail, but when it came down to the race they (mostly) used a jib (and always stays).

I have not even stayed at this holiday inn for this level of aero and structural engineering, so I'm just left with questions based on empirical observations.

Well, it is really not . . .if there is a possibility that some variation of un-stayed rigs are both better sailing and more likely to survive a roll-over... then it is dead smack on topic.  I guess so far I'm listening but have not really heard a compelling case - admittedly I might not recognize the compelling case if it bit me in the ass  :huh:
In racing, allowed sail area is king, and if the pressure was insufficient for the wing to fully power the boat, the jib was flown. I can guarantee that any winged boat would much rather have the right size wing for any condition, rather than a wing and jib, but the rules don't allow it. It's also hard to swap wings during a race when the pressure drops. Where sail area is strictly limited (C class cats for example) the last jibs were seen in the '60s (~20 years before wings). And that is, actually, one of the problematic areas for a bald headed rig, changing jibs to adjust area is not an option. Reefing gear takes care of most of that, but it's just another one of those compromises. We do carry a 1000 sq ft asym for broad reaching, but it is much less useful that the asym on a sloop because the main is so much more efficient off the wind than a sloop rig generally. 

 

estarzinger

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. I can guarantee that any winged boat would much rather have the right size wing for any condition, rather than a wing and jib,
DDW, my understanding is that this is in fact not true.  Across various iterations of AC wings, they have had more than enough breeze for just the wing but preferred wing plus jib. . . . I understand that this is at least in part because they can control/twist the top of the wind actually create positive righting moment while the bottom of the wing and the jib create drive.  But empirically, many times with the wing, they have quite obviously had a bunch breeze and still used a jib.

 

Zonker

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Even Larry's big tri - a completely 'open' unconstrained design by rules and money.  During the lead up there was 'rumor' that it was faster with just mainsail, but when it came down to the race they (mostly) used a jib (and always stays).
Jib trimmers Union. Didn't want to put Robby out of a job.

 

DDW

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DDW, my understanding is that this is in fact not true.  Across various iterations of AC wings, they have had more than enough breeze for just the wing but preferred wing plus jib. . . . I understand that this is at least in part because they can control/twist the top of the wind actually create positive righting moment while the bottom of the wing and the jib create drive.  But empirically, many times with the wing, they have quite obviously had a bunch breeze and still used a jib.
Could be? Also the wing alone offers little directional control for prestart or crash recovery maneuvering. And, you do not know how conditions will change during the race, there is no opportunity at all to put one up should it become advantageous. The wing can be feathered if too much pressure, but if too little you will just slow down. If it were more efficient over a wide range of conditions, you would see them on class A and C cats where they are legal (but not used for decades). I watched some of the AC races on SF Bay, as I recall in some of them no jib was carried. 

If the wing twist were controllable to a greater extent, you could use the top to create righting force and the bottom of a bare wing to create drive, with an improved L/D (this was suggested even back in Marchaj's books). We know for a fact that a wing with a jib in front of it is aerodynamically inefficient, things like this have been tested in wind tunnels for 100 years with no ambiguity. Racing sailplanes aren't designed that way for a reason. I suppose there could be some operational advantages for this peculiar type of racing that outweigh the aerodynamic inefficiency.

 
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estarzinger

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'gentleman's boat' ........ Hanuman?
I spent some time around Royal Huisman when she was being built.

This would all be a matter of definition and personal opinion . . . while a fabulous boat - she would not be a 'gentleman's yacht' in my book. More in the superyacht/billionaires toy category - no disrespect to the boat or the man just a (at least in my mind) clearly different category - and a guy (or gal) could have one (or two) of each :)

In my definition/mind, the gentleman's yacht will certainly have a pro maintenance shore team, but can and is sailed by the family and/or Corinthian friends.  For racing, might or might not (depending on seriousness) need to bring the gardener, who just happens to be a past Olympic sailor.

 

CapDave

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who are skilled enough to sail a j-class . . . 
Easier to sail, by far, than his other yacht

DSC07814.jpeg

90m Athena in January off Antigua

 

estarzinger

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Easier to sail, by far, than his other yacht
When your 'easier to sail boat' is a j-class . . . . you do rather need an entourage of 'pros and help'.

Again, not knocking it. But there is rather something nice about a super elegant gem of a boat which you can cruise with your wife and race with a few friends.  Even when, perhaps especially when, you feel like you need to mostly live with an entourage around you.

 

[email protected]

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There are several instances of boats with unstayed rigs doing a barrel roll and the rig surviving. In fact I am not aware of one that has failed that way, though I imagine there must be an example somewhere. With robust cruising specs, I believe a carbon unstayed rig is lighter, and has a lower CG, than a stayed rig. The components critical to failure are reduced from perhaps 100 to only one. 
IIRC Bill King's "Galway Blazer" lost her freestanding rigs in the (original) Golden Globe to a rolling... had they been carbon in may've been a different story...

 http://wavetrain.net/wp-content/uploads/2012/09/king.2.jpg

king.2.jpg

 
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estarzinger

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 freestanding rigs Golden Globe
I believe there was one freestanding rig in the BOC (1982  Tony Lush's cat ketch Lady Pepperell - pitchpoled and abandoned in the Indian ocean) and one in the around alone (1998 cat schooner Project Amazon - probably a better design by Spondberg, but the two owners were always underfunded - ended up snapping its foremast and being abandoned on the hard I believe).

I feel like I'm missing one further one, vague memories of a cat schooner getting into trouble off s Africa in one of these races, but my memory refuses to dredge up details.

 
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MikeJohns

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........................conclusion of the coroner report.  

"There is no evidence that any particular style or design of boat fared better or worse in the conditions. The age of yacht, age of design, construction method, construction material, high or low stability, heavy or light displacement or rig type were not determining factors."  

Reading various posts and considering his own boats - Mike's 'theoretical' position on RM and AVS may actually be pretty simple and agreeable -............
I missed replying to this and it's important for you.

The coroner made no such finding.

You are actually getting confused with the statement of the race review committee of the CYCA. That report was at odds with specialist professional evidence tendered by both Barry Deakin from Southampton/Wolfston and Martin Renilson from the AMC.

The race review committee's report was based on statistics that included both knockdowns and fully rolled boats as stability casualties. That's not the professional path which only considers full inversion as a stability casualty. 

In Fact, in the coroners report the relevant and significant finding was :

 [SIZE=11.5pt] "That the lower a vessel's limit of Positive Stability the more susceptible it is to being knocked down and being inverted;[/SIZE][SIZE=11.5pt][/SIZE]

"


That actually directly contradicts Dovell who as a technical advisor to the CYCA on this was suggesting that LPS made no difference.

 

DDW

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I believe there was one freestanding rig in the BOC (1982  Tony Lush's cat ketch Lady Pepperell - pitchpoled and abandoned in the Indian ocean) and one in the around alone (1998 cat schooner Project Amazon - probably a better design by Spondberg, but the two owners were always underfunded - ended up snapping its foremast and being abandoned on the hard I believe).

I feel like I'm missing one further one, vague memories of a cat schooner getting into trouble off s Africa in one of these races, but my memory refuses to dredge up details.
Lady Pepperell was a modified Hunter, she did pitchpole and was abandon because the keel began to come off. The rig was still standing when abandon after the pitchpole. I did not think Project Amazon snapped a mast, rather was stored on the hard for 10 years looking for a buyer before being cut up for scrap - but could be wrong, hard to find much info on the boat's later years. Dismasting is much more of an issue with a marconi rig than a carbon free standing one. There are very, very few dismastings of Nonsuch or Freedom and it is a pretty large pool of boats. A few of the very early Nonsuch had problem because the mast builder did not understand engineering (drilled a big hole thru the aluminum mast right at the partners).

 

estarzinger

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 I did not think Project Amazon snapped a mast, 
my memory, which also could well be faulty, was that with the second owner, and named Tin Can, he was trying to sail with just a jib on the foremast and it snapped and then it went on the hard because he did not have money to replace the mast.

Pete Goss's Team Philips was another interesting attempt.  It seemed like their analytical team did not do high quality work, which surprised me give Pete's funding and background.

It is too bad that none of these boats really ever had chance.  The French dont seemed to have dabbled in this - I always presumed they modeled all this out and decided it was not worthwhile but idk.

 

estarzinger

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I missed replying to this and it's important for you.
No, actually - its all pretty much what we have already agreed.

 [SIZE=11.5pt] "That the lower a vessel's limit of Positive Stability the more susceptible it is to being knocked down and being inverted;[/SIZE]
As a general statement, we all agree that more RM and higher AVS is beneficial, all else being equal. I have stated this repeatedly. we all agree on this.  However, very rarely is all else equal.  You end up with design compromises and trade-offs and where the optimum point for those trade-offs will depend on the specific vessel/owners mission/purposes.

That actually directly contradicts Dovell who as a technical advisor to the CYCA on this was suggesting that LPS made no difference.
No, they are considering different things, and when you properly read/understand them, they both can/are true.  In theory and in generality, we agree more RM and AVS are beneficial.  BUT when faced with the power of actual 60 breaking waves, in practice the force of the waves can simply wash out the comparatively little differences between the RM/AVS's.  Both of these things are true, both of these things were also findings from the post fastnet studies.

 
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