Desirable and Undesirable Characteristics of Offshore Yachts

[Panoramix]

Here's a plot showing S-H boats that were  rolled ( as in fully capsized ) overlaid on the UK MCA commercial requirement for sailing craft produced by Wolfston. This is from Kim Taylors paper on the event.

Presented like this, this plot would tend to show that there is no strong relationship between being rolled and being on the safe side of this graph! But then these are just 5 data points, so statistically not very robust, it would be more interesting if boats that didn't capsize had been included. This is from a report, may be there was another graph somewhere in there showing the full picture.

Also I am a bit wary of a graph effectively saying you can't go offshore in a yacht less than 7m long as experience shows that it is possible.

 

[Fah Kiew Tu]

Also I am a bit wary of a graph effectively saying you can't go offshore in a yacht less than 7m long as experience shows that it is possible.

Of course it's possible, it's been done heaps of times.

The question for people who do this is - do you feel lucky?

Quite a few of the small boat people disappeared at sea. Christopher Ridding comes to mind - left NZ headed for Australia but never made it.

FKT

 

[Panoramix]

Of course it's possible, it's been done heaps of times.

The question for people who do this is - do you feel lucky?

Quite a few of the small boat people disappeared at sea. Christopher Ridding comes to mind - left NZ headed for Australia but never made it.

FKT

On the other hand every 2 years 80 solo skippers race across the Atlantic with overpowered 6.5m boats... I can't remember when was the last accident but that was a long time ago, probably safer than things people do daily on earth!

If you go offshore on a small boat, you need to do it right but it can be done safely...

 

[estarzinger]

Presented like this, this plot would tend to show that there is no strong relationship between being rolled and being on the safe side of this graph!

As I directly quoted in a post above - that was the exact 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 - (A) That if boat A is twice the displacement of boat B and they are both excellent examples of the NA craft, then boat A is probably more roll resistant than boat B. and (B) If Boat A and Boat B are identical designs and you take weight out of the interior of boat A and put that same weight in the keel, then boat A is probably more roll resistant than boat B. As I commented several times above - no-one is 'against' RM and AVS per say - it is just the bring compromises and for each 'mission' there is some sort of optimal compromise (which would depend on the individual owners).

With (A) the more empirical evidence suggests enough extra size does matter and can make a difference. With (B) - type gains however, they may be slight enough to be easily swamped by the power of breaking waves - and where you are related to the storm, if you end up in the most intense part of the breaking waves or not - along with how the skipper handles the situation, becomes the primary factor in whether you roll or not.

I am unable to see how his position on some of the other matters, like rudder shapes, are  likely to generate much agreement.

 But heavy boats performance are expensive to produce......

And this comment made me lol - be fun sometime to ask an owner of say a custom Persico or MULTIPLAST if they bought lightweight boats because the heavy ones were too expensive B)

 

[Panoramix]

I am unable to see how his position on some of the other matters, like rudder shapes, are  likely to generate much agreement.

I read your earlier post but didn't realise that the graph was from this report, should read more carefully! As for the bit I quoted, that would mean that a lot of people are wrong.... I've sailed old gaffers (just a bit), I love all the history behind them and as a boat geek I am just curious, Nevertheless despite all the efforts and cleverness that went into their design and construction, it is hard to ignore that boat design has progressed vastly. For a start rudder authority on these boats is - to modern standards - approximative.

 

[estarzinger]

, it is hard to ignore that boat design has progressed vastly.

The coroners report was just empirically commenting regarding the SH fleet, and this particular incident, not trying to encompass the broader full assortment of all possible sailing craft.  As shown in the graph, design distinctions did not make much differences empirically in who got into trouble. The next sentence which I did not quote was, as we have all discussed/agreed before, that the primary factor was where exactly you were regard to the worst breaking waves - there are almost always small zones which are much more intense that other places.

Ofc the naval architecture profession has progressed. They were not trying to suggest it had not.  Just that across the SH fleet the size and power of the worst waves washed away the distinctions that existed within the fleet.  It is in fact similar to the quote I posted from the fastnet testing - that sufficiently sized breaking waves could easily capsize all the models.

I have sat on a few of these sorts on incident/accident investigations, and they tend to try to stick at least initially reasonably narrowly to their brief.  It is hard enough to draw conclusions and agreement when doing that.  Trying to encompass the whole possible world of sailing would tend to make it impossible.  Sometimes you get follow up activities that are intended to be broader - like the investigation into Cheeki Rafiki was extended into a look at the whole school of plexus pan liner structural construction (that ball was then just totally ignored/dropped dropped by the industry). These sorts of incident reports, including the SH corner's one, are typically quite worthwhile reading. Usually the team writing them has a quite good knowledge base and worked quite hard to tease out the best conclusions (yea, sometimes they fail at that, but usually they are a worthwhile read anyways).

(edit: these things are difficult to discuss in twitter size bites. I suspect we would all have more agreement if we were face to face with some nice red wine).

 

[DDW]

I'd like to come back to your claim that  "although  low aspect rudder stalls at high angles of attack. It does not necessarily generate higher loads....." what  AR.e are you considering there ?

Also Vacanti and his code are all about ideal free stream flow.  The problem I have is that in a seaway the inflow is nothing like ideal.

Aspect ratios in rudders on normal sailboats range from about 1 to about 8 or so. The rapidly varying region is in the lower numbers. I'm talking here about real AR, not a doubled version that some speak of. The mirror effect requires gap sealing rarely seen on boats. Some of the data from tests that are published is polluted by rake, causing spanwise flow. In classic wind tunnel testing going back to about the Wright Bros., very low AR foils have a lower lift slope and stall at (at best) the same or (usually) lower CL. This has been shown to be because the tip vortex wraps to the low pressure side, having two effects: it delays separated flow on the low side and releases pressure from the high side producing the measured effect. Load or lift is proportional to CL, area, and dynamic pressure; lower CL means lower load.

Now, when operated well beyond stall angles, you may find the lower AR foil producing more load because the flow separation is soft, and filled in by the vortex. At 35 deg angle of attack, an AR 1 rudder may produce more lift than an AR 5 rudder. This is how lifting strakes on jet fighters, crab claw rigs, and hang gliders can fly at very high angles of attack. If you are frequently operating your boat's rudder like that, you have a serious rig balance problem or are under ruddered. 

I'll agree that in a seaway the flow is not an ideal free stream but it isn't that far off either. Most normal rudders behave in practice just as aerodynamic theory suggests they should. 

 

[Panoramix]

(edit: these things are difficult to discuss in twitter size bites. I suspect we would all have more agreement if we were face to face with some nice red wine).

I was actually agreeing with you, I don't take time to make long paragraphs with qualified sentences but what I just meant is that there is no reason to be nostalgic on stuff like barn door rudders. They existed at some point for good reasons but now that design knowledge and material science are completely different, saying it works because it was done like this in 1950 has little value. Chances are that there is a better way to do it nowadays. When I go down a wave that is much steeper than I would like, I definitely want to choose where I bury my bow!

 

[Panope]

Can the following generalizations be made about heavy weather survival?

For running downwind and actively steered - Light boat/high AR spade rudder is better.

For points above beam (hove to, fore reach, lie ahull) - Heavy boat/low AR barn door rudder is better.

Disclaimer: I don't know anything about this subject.

Steve

 

[Whinging Pom]

Please explain the acronyms for the unwashed! Thanks!

 

[Panope]

Please explain the acronyms for the unwashed! Thanks!

AR = Aspect Ratio.

 

[estarzinger]

Can the following generalizations be made about heavy weather survival?

it is an interesting question - the reverse of how this is usually asked - usually you have a boat of a particular design and the question is what is the best tactic for this boat.  You are asking for a particular tactic, what is the best boat. 

And at least for me idk the general purpose answers to that off my head. I would guess that the answer - to for instance what in the best design for running in breaking waves - would be niche boats that would not be great at general purpose cruising.

I personally felt a successful cruising boat needed to be able to go both directions (up and down) in harsh weather.  But I know some which were not able (just for example, the Pardey boat could fore reach and heave to but did not run well).

I did build a custom boat specifically to be good in a wide range of hard conditions and it was more in a middle ground that the edge choices you mention - a mid-weight displacement, mid size-fin/bulb, very large not super high AR spade rudder.  We were near the outer achievable limits of RM and AVS (given some constrains on practical draft and construction strength and materials).  This boat both ran well and forereached well.  It forereached much better than it hove-to, so we generally did not heave-to. And we did not lie ahull ever in hard conditions.  It did benefit from a drogue in some running conditions.

Another way to answer your question, from various empirical findings we have already discussed, is perhaps to say that rather more important than the parameters you mention is raw boat size (get the biggest one), and skipper skill and experience (duck those intense small hot zones and know what tactic is best for each condition in the particular boat and make sure everything on the boat is maintained)  . . . and then some amount of raw luck.

 

[estarzinger]

just as an aside. . . . I understand the focus on storms . . . but light air is really the cruising challenge - there is (generally) a lot of it. It is both highly desirable and seaworthy to be able to move well in light air under sail.

 

[DDW]

Also CL = coefficient of lift, a primary characteristic of sections and planforms. 

When hove to classically, you are backing down with both keel and rudder stalled. You could argue that a low aspect ratio rudder would be better under these circumstances, because it has a greater and more predictable slope of lift vs. angle of attack after separation (stall). I have the feeling all of that is greatly overwhelmed by other factors in the hull, keel, and rig. Perhaps the biggest impediment to heaving to properly is a sloop rig, it lacks options to vary the CP (= center of pressure) of the rig. Pretty easy on a ketch or yawl, impossible on a una rig, sloop is in the middle but tending toward the una in that there is no way to get the CP aft very far. 

 

[Bryanjb]

Sometimes you get follow up activities that are intended to be broader - like the investigation into Cheeki Rafiki was extended into a look at the whole school of plexus pan liner structural construction (that ball was then just totally ignored/dropped dropped by the industry).

This is a surprise?  The majority of industry build boats with pans, it's quick and cost effective.  They aren't really repairable and not especially strong but does anyone believe it's going to change.  Look at the problem the Lagoon 45 owners (+1,000 owners) are facing, the builder just walks away and blames the sailor.

 

[estarzinger]

 problem the Lagoon 45

yea, that is a bit of a mess.

 

[olaf hart]

Interesting discussion on capsize, especially the rubbery relationship between design characteristics and capsize history.

My take is capsize is a rare event, determined by multiple variables, and what matters is the boat doesn’t sink under you, you are not seriously injured, and the boat retains some degree of function.

So the details of construction are more important to me than the overall design. Things like fixed batteries and floorboards, effective restraints, a mast and boom that remain standing, hatches and windows that don’t pop open, and keels that stay attached to the boat.

I grew up racing dinghies and skiffs, capsize was inevitable, what mattered was the recovery and getting back into the race.

As far as offshore design characteristics go, I am more interested in comfort and maintaining a good passage speed.

 

[estarzinger]

So the details of construction are more important to me than the overall design.  . . . .  a mast and boom that remain standing

100% agree.

I dont know much about what it would take for a rig to survive a roll over (say beyond 120 degrees) - a decent size one for good light air performance? 

Is it even possible to engineer a rig with good/competitive performance and a high likelihood of coming back up intact?  If so, what would be the key factors?

I always figured the odds were quite high I would need to do a jury rig after  a roll.

 

[olaf hart]

100% agree.

I dont know much about what it would take for a rig to survive a roll over (say beyond 120 degrees) - a decent size one for good light air performance? 

Is it even possible to engineer a rig with good/competitive performance and a high likelihood of coming back up intact?  If so, what would be the key factors?

I always figured the odds were quite high I would need to do a jury rig after  a roll.

I have a case study of one, our V40 no 101 has done three 360’s in its long and busy life, still has the original rig.

The first in the 1983 BOC  RTW race east of Cape Town, some boom damage, sailed back upwind to the Cape and then finished the race in fifth place.

The other two were off New Caledonia on the way from Brisbane in 1988, lying a hull, broke a spreader on the second one. The crew had enough and hopped onto a passing freighter and the boat was salvaged and towed to Noumea by the French Navy.

The boat then went RTW again from 1998 to 2008 without incident.

Its the first V40 built, spars are massive, single spreaders, keel stepped mast.

The rig has what BP called “ soldier stays” which are like fixed running stays. Bob told me they were common in the seventies, they were dropped because they increased rig loading, but they give the boat two complete levels of rigging, one triangulating at the head of the inner forestay and the other at the masthead.

When we replaced the standing rigging, the riggers were happy to do it with the mast standing, they replaced the masthead stays first then the lowers.

Not sure how important this is to the mast staying up, but our Adams 36 has a cutter rig and running stays and I would definitely tension them both up in survival conditions.

 

[Panoramix]

Is it even possible to engineer a rig with good/competitive performance and a high likelihood of coming back up intact?  If so, what would be the key factors?

That's a bit of a left field option but I would consider an unstayed rig :

You get rid of lot of potential failure points by removing the stays!