Seaworthiness

Tom, I agree with what you say about the importance of maintaining a forward momentum to produce directional stability. But let me remind you that this discussion has begun centered on the seaworthiness (or not) of the Pogo 40 and as a continuation of what was being discussed at the Stix thread.

As you can see the opening post on this thread is about the Pogo 40, a boat that I had ,in previous posts, considered an oceangoing boat and a boat that was considered (in this thread) by Guillermo and Mike as a dangerous boat and a strictly coastal one.

Mike has said about it:

Guillermo has said about it:

What Guillermo considers “while at speed” is evident in this case. He very clearly specifies that the Pogo 40 is “Really Dangerous when not in planning conditions” therefore it seems clear that in this case “While at speed” means when the boat is in planning condition.

So, for Guillermo, this is a clear example of a boat that gets its “dynamic stability under speed” and also a dangerous boat, because its “frozen picture analysis” (“basically a zero speed analysis”) is a poor one, "taking into account static stability curve properties".

I have asked Guillermo to give me an example of a production oceangoing cruising sailboat. I wanted to finish with unsupported statements and run a comparison on the real” frozen” stability curves of both boats; Guillermo’s and the Pogo 40.

Guillermo never named any boat, so I will pick a traditional boat with an outstanding seaworthiness record, the Valiant 40. The boat was designed in 1973 by Bob Perry and it was a revolutionary boat in its time.

The 40 was produced till 1992 and then it was slightly modified and became the Valiant 42, a boat still in production.

Erin L. Schanen from SAILING Magazine says about the Valiant 40:

The Valiant 40…remains the benchmark.
Exactly 200 Valiant 40s were built before the boat went out of production in 1992. Today there is a permanent waiting list for used models. Would-be owners can expect a long wait. A survey done by Worstell showed that half of the 40s built are still sailed by their original owners, many of whom are live-aboards….It has been the boat of choice for a number of circumnavigators.


The 42 costs several times the price of a Pogo and even a 15 year old 40 will cost a lot more than a new Pogo.

I am going to do as Guillermo has suggested and I will compare the Pogo40 with a 40ft seaworthy traditional heavy boat (funny, in 1973 it was a light boat ).

Of course, in my opinion it will be a very unfair comparison. We are comparing the Pogo with a boat that more than doubles its cost and we are comparing a 10.2T (msc) boat with a 6T boat (msc).

To be completely fair with the comparison I am going to follow Guillermo’s criteria proposed on the Stix thread:

Following Guillermo suggestions, I will not consider the high STIX of the Pogo 40 (44.7) and I will compare AVS, the relation between the positive and negative areas of the GZ curve, the GZ at 90º and the Max GZ at 180º (in the inverted position).

I will also compare Max GZ, because if we look at Max inverted GZ as a way “ to have a better clue about the boat's ability to recover from a knockdown”, logically, the Max GZ will also give a clue about the boat’s ability to resist a knock down.

As Tom has said: “Maintaining speed, or forward momentum at or above the critical velocity that produces directional stability is critical for any vessel in heavy storm conditions”. But you don’t want to maintain that forward movement compromising a great part of boat stability. You want forward movement, but not a deeply heeled boat, you want a slightly heeled boat, say a boat with a 12º heel.
The rightening arm of the boat at moderate angles of heel is therefore also important to the boat seaworthiness. So, I will also compare the rightening arm at 12º heel.

AVS: ----------- Valiant40 – 125º ---- Pogo40 – 127º

Rightening Arm (m) at 90º: -- Valiantt40 - 0.37 --- Pogo40 – 0.85

Max positive Rightening Arm: --- Valiant40 – 0.70 --- Pogo40 – 1.10

Max negative Rightening Arm: --- Valiant40 – 0.40 --- Pogo40 – 0.52

Rightening Arm at 12º heel: --- Valiant40 – 0.27 --- Pogo40 – 0.55

Positive Stability/Negative Stb. --- Valiant40 – 4.5/1 --- Pogo40 – 6/1



The only indicator that is better on the Valiant is the negative Max GZ (inverted position) and not by a big difference. Actually that number is approximately the same for the Sabre 402 (the other boat on the picture with the Valiant stability curve.

Of course the seaworthiness of the Sabre 402 can not be compared with the outstanding one from the Valiant. But I am not saying that the Sabre is an unseaworthy boat, after all the boat was elected by “Cruising World Magazine” as the 1997 boat of the year in the categhory of: Best Full-Size Cruiser, Best Boat Overall and Bob Perry himself has said about it:

“This is a well-proportioned boat that will help elevate the standards for dual-purpose family boats”.

 

Vega,
I have had to read your post a couple of times, and let it sink in. You have drawn a good comparison between the pogo 40 and the valiant 40. This is good material for study, and quite frankly I need to study, in more detail, both examples. I am taking a hard look at all points, as I try to arrive at a good compromise as to how wide a cruising boat should be. This of course is very subjective; there are many trade offs. I will make one comment, as you have made a good point regarding seaworthiness that I don’t believe has been mentioned.

In survival conditions the skipper reefs down, to keep the boat within a safe heel angle. The lighter boat has an advantage, in that it doesn’t need to carry as much sail to keep it moving. This is of course a generalization; if the boat is heavier because it caries more ballast, then it may very well be able to carry sufficient sail. Maybe I will comment more on weight in a later post.

 

All,
Please read my post 198 at the STIX thread. Thanks.

 

Or not sails at all, which is the worst case.
Cheers.

 

The designer has more contribution to this than you appear to acknowledge. You also seem to be a little unaware of just how much people suffer in heavy weather. Perhaps you one of the lucky few who are largely immune from the debilitating illness and stress from boat motion. A light boat can be uncomfortable at low speed too.

You introduced the aircraft I was flying with your analogy. My point is that an aircraft can be designed to be operated in the most violent weather systems on earth with confidence. Try and do the same for a boat and you will find why the sea is a harsher less predictable environment. The ocean surface stores wind energy for days and this makes the surface so unpredictable. The sea surface conditions can only be generalized, no one can forecast the combination of several wave trains at your precise location, combinations of leftover swells tides and wind ... suddenly and without any warning is a trait of the sea.....a reasonable safety factor stops you disappearing without trace.

Blue water boats must be designed for severe heavy weather. You should design for the worst, not design light and trust to strain gauges. It might work for monitored and accompanied racing fleets but for lonely cruisers they tend to be on their own.

We have designed many rigs and masts over the years ( to engineering principles not scantling rules) . At times the scantling rules are a bit light. The limitation with Aluminium masts as I said is buckling, wooden masts are better at this with the big wall thickness, so overall it can even out. It is also why steel masts are not as heavy as some people presume. we have designed some interesting masts.

Yes a dye test only shows fractures.

Principally because we have to design for all eventualities and as I said the loads are so much harder to predict than with aircraft.

Often you want a rest from the motion. However heaving to is perhaps part of the lost seamanship, perhaps you'll be able to think of some convincing scenarios as to why you may want to do this or lie to a drogue.

As for the propensity of the vessels to surf sideways and get into a violent inversion. This is a very well documented and studied phenomina. A good start for you once again would be to read Marchaj. This is one reason that the skimming dishes are much safer while making way in heavy weather .

So lets stay objective Perhaps we should leave aircraft and cars out of the discussion.

Cheers

 

Stupid Pogo...

• No one is going to be waiting in a queue for a Pogo 40 in thirty years time.
  Especially as a round the world cruiser.
• Run those numbers on the Pogo again before it casts off on a round the world trip.
  While the crew of the Valiant can tip stores down through the hatches, you will be cutting the handle off your toothbrush, and ripping the covers off books to save weight.
• Stability in regard to seaworthiness, on a real boat sailed by real people with a real chance of dying is not measured at 12 deg. Performance is 12 deg.
• The difference between 10.2T and 6T is elementary if you have sailed in rough weather.
  6T is a smaller boat and will be active, for better(yehaa lets go fast ) and for worse(the sea is throwing my boat around ).

And by the way Vega, those righting arm numbers should be resolved with the respective displacements to produce comparable moment/torque Kgfm, Nm or ft/lb.
Displacement and righting arm go hand in hand and one without the other is only half the story.
How you came across the righting arm data in your post,I dont know, but combining it with your displacement numbers says something a bit more helpful.
eg
Righting Moment (Kgfm) at 12º heel: --- Valiant40 – 2754 --- Pogo40 – 3300

Righting Moment (Kgfm) at 90º: -- Valiant40 - 3774 --- Pogo40 – 5100

Max Righting Moment (Kgfm): --- Valiant40 – 7140 --- Pogo40 – 6600

Max Inversion Moment (Kgfm): --- Valiant40 – 4080 --- Pogo40 – 3120
(max inversion arm numbers seemed a bit out to me....)

 

Yes I've been rumbled I make it all up for fun. Or maybe I don't. Working on the basis that one example will illustrate this lets look up one vessel from my earlier examples in this thread then we can play damn the boat ok?

Vessel name TAKA
47 foot ULDB with LPS of 108 deg.
Osaka Guam race 1992 (Dec 29) crew 7.
B2 Knocked down while running under number 3 jib and furled main in a quartering sea of 15 to 22 feet. Wind was 32-48 knots. Vessel remained inverted for 45 minutes. Three men were trapped in the cockpit and four were inside . The four in the cabin fearing the vessel would sink escaped the inverted hull to the life raft. One of the men in the cockpit drowned after being unable to unclip his harness 3 removed or cut their tethers and escaped. After 45 minutes, the vessel righted itself. By this time the remaining 6 men had taken to the life raft which became separated from the vessel subsequently 5 of these 6 died.


There is enough info for you to verify this I am taking it from a paper with other examples from other races which is where we find ULDB craft in heavy weather. There have been enough incidents to refine the prediction accurately enough.

 

Rayk

I really think very few people would select a Pogo 40 as a round the world family cruiser now or ever. If they did I'd wager the majority of them would change boats fairly smartly. The comfort and security and privacy is abysmal, for the same money they could buy a very nice used boat and enjoy far more comfort. Long distance cruisers quickly find their values changing. Life at anchor and the livability of the vessel puts performance very low on the list particulalry for the non-speed oriented family members.

cheers

 

Here's an article of interest by John Vigor I'll copy the article as an upload since the link will perish.

"Planning for an unplanned inversion"

 

Mike, this is not the issue. I never said that the boat is an ideal Family cruiser for circumnavigate.

The difference of opinion is (or was) that I have said that this boat is an Oceangoing boat and that you and Guillermo have said that this boat “Pogo40…is a boat for limited coastal cruising” or “Really dangerous when not in planning conditions” .

About the abyssal security I don’t real understand what you mean, but obviously this is a very seaworthy boat.

 

Guillermo raised the issue of wether the Pogo was oceangoing seaworthy.
It is safe in a regulated environment, ie a race.
The Pogo and its ilk were not intended for an independent crew.
Rules, regulations, external monitoring and assistance compensate for its unseaworthiness.

Oceangoing seaworthiness is a measure of endurance (think endure) that many modern yachts dont meet.

Endurance.

 

• 
  
  Yes of course, because normally an heavier boat has a bigger RM moment at all heeling points.
  
  As you have discovered by your own calculations (that I have not checked but I am assuming correct) that is not the case with the Pogo.
  
  The Pogo has, relatively to the Vailant, a superior RM on most of heeling points.
  
  If the Vailant as a slightly better Max RM moment ( 7140 to 6600), the Pogo was a much better RM where it is more important to safety and I mean at 90º of heel. As Guilermo says (and I agree): ” GZ at 90º value. A high value here is to ensure that the boat is able to right herself from a knockdown to an angle of around 90º. ..” .
  
  At this particular point of heeling the Vailant has a 3774 RM and the Pogo a 5100 RM. This means that at a limit situation, with the boat lying down, the Pogo is making 35% more force to rightening up, compared with the Vailant.
  
  
  • 
    
    I have posted the GZ curves. The one on the Valiant is on ft. To compare it with the Pogo one, I have converted ft in meters.
    
    
    • 
      
      The importance of this moment to the boat safety is very well explained by Guillermo:
      
      “Also GM at 180º or GZ max in the inverted position should be considered to have a better clue about the boat's ability to recover from a knockdown”
      I believe Guillermo is referring to a total inversion.
      
      Here the Pogo has also a big advantage over the Vailant. It will be necessary less energy to recover the Pogo from an inverted position, if compared with the one necessary to the Valiant. The Rm moment at that position is better for the Pogo (smaller) by 31%.
      
      
      
      • 
        
        I fully agree to you. I was not utilising my criteria, but Guillermo’s criteria. He thinks that GZ curve (Rightening Arm) is the important criteria to judge a boat stability.
        
        I do believe that for accessing a boat stability, RM values (that take into account the boat displacement are the important ones).
        
        Take a look:
        
        
        
        
        
        http://www.boatdesign.net/forums/showthread.php?t=13569&page=2

 

Hi Vega
Security as in security of person aboard, the bulwarks of the vessel and the secure cockpit protecting you from the sea and the spray and keeping your person aboard and comfortable. These are Seakeeping issues rather than stability but they are part of the seaworthiness of a vessel too.

That wide open shallow stern cockpit will be a wet and insecure place at times. The low deckhouse is scant protection.

I don't know enough about the planing characteristics wrt heavy weather stability to comment.
I am not sure that the vessel needs to plane but it does need to make enough headway to generate a decent flow around the surfaces and provide some dynamic stability. The faster it goes the more dynamic stability it develops but then it is not prudent to go too fast the vertical accelerations will be high and you will add some dynamic instability as well. Options will be dictated by the sea state.
In heavy weather I see no option with this boat but to helm it manually and sail it well or lie to a drogue, I think an Autopilot would be overpowered on some courses as the vessel surfed and would easily broach-to unless helmed with experience.

As I said before skimming dishes are not safe in heavy weather to lie-a-hull or heave-to due to the small (and stalled) foils (rudder keel) and the low immersed lateral area. They are prone to vigorous sideways translations which is not so good for either the dynamics of knockdown or the crew aboard. So you have to keep going…limited options reduce the safety.

Despite some opinions heaving–to is a very useful tactic and I would not like to sail a vessel blue-water that was not so capable.

Its a young mans boat, a racing boat.......... That's my opinion . I want to put the marketing person aboard in heavy weather who wrote the "easy cruising miles enjoying a cool beer" line. I have a sneaking suspicion that person has not sailed an ocean before or targets those who have only sailed coastally.

 

You would have been more honest if you had posted all what was said at the STIX thread on the matter, as, per example:

You're stubbornly annoying

 

Thank you for finally stating what seemed obvious from the start. That the Pogo 40 is unseaworthy ... in your opinion. It is not unseaworthy by any objective measure.

Since you don't like the idea of going to sea in a boat that you think is unsafe, you shouldn't sail a Pogo. People should stay within their comfort range.

Fair Winds,

Randy