Seaworthiness. Do the maths tell the true story?

[John Willis]

I have the privilege of being the custodian of a Frances 34 Pilothouse. The maths of AVS, Capsize Formula, Comfort Factor etc suit her conventional shape perfectly and she gets straight 'As' in every subject, and I happily set sail on long trips knowing her results. Now I am thinking of downsizing and perhaps sailing a boat with a modern surf board hull shape I wonder if the maths still tells the true story?

Take a Pogo 3 or Django 770 and run their data through the same maths and they probably won't even score a pass. Put simply, one look at their results and you would be wary of tackling the Channel, never mind Biscay . Yet Pogos fly across oceans with spinnakers flying in 40 knots (if your bonkers) and Djangos aren't shy about dicing with the rough stuff either.

This makes me wonder if the maths needs a redesign to better suit modern hull shapes by people with brains far bigger than mine, or whether the results simply need to be realigned to tell the real story?

Perhaps it should include skippers in some way, for I, a pensioner, cannot imagine how anyone can fly in a Pogo through a gale and stay sane or safe!

 

[Chae_73]

I have been pondering a similar question, in anticipation of helping out with or contributing to my father's plans to acquire another boat. I have put together this spreadsheet using various metrics sourced from SailBoatData.com, so not guaranteed to by 100% accurate. The colour coding is based somewhat on the advice provided by Charles Doane in his book "The Modern Cruising Sailboat". Clearly, "modern" is a relative term...

 

[Chae_73]

here's how a POGO 30 stacks up on those metrics (swing keel version):

 

[TernVI]

People try to read way too much into ratios and magic numbers.

 

[Ingwe]

I don't think that a simple formula will tell you what a boat is like to sail any longer as the more modern designs use hull shape to give them much more form stability than a traditional design of a similar displacement. Unfortunately hull form is much more difficult to quantify than something like sail area to displacement ratio so I doubt there will ever be a simple way to assess and compare modern boats "comfort factor".

From a seaworthyness point of view though I am quite sure that most of these boats will manage more than the crew will ever want to push them to.

 

[John Willis]

I agree with Ingwe and Tern VI, whatever the ratio (Comfort Factor, AVS, Capsize Factor etc etc). But I would love to know what the designers of these modern hull forms consider makes one an ocean goer and another not, assuming equal skipper and safety equipment. Whatever, I guess a Pogo 30 or its baby sister Pogo3 would get wherever way quicker than say a Rival 32, assuming you can hang on.

 

[Chae_73]

It is hard to imagine that a very light, flat bottomed boat would be a comfortable place to spend long periods of time. Certainly exciting to sail, but I guess it would be tiring.

It would be interesting to hear from anyone who has made extended passages in such a boat.

 

[Laminar Flow]

I suggest strongly to get yourself a copy of Marchaj's book, "Seaworthiness, the Forgotten Factor". It makes for interesting reading and he also delivers on the math and scientific background if you have a bent for that sort of thing. It also delves into relevant ratios and investigates certain aspects one might not have considered relevant, or perhaps relevant in this context.

You also need to consider what the boat is to be used for.
Cruising, IMHO, is as specialized an activity as racing with an emphasis on living.
If you are interested in the sort of minimalist existence defined by the parametres of weight saving and performance then, by all means, choose something that offers the culinary versatility of a single burner hob and a sink so tiny it makes a persuasive argument for doing the dishes in the cockpit and the rain.

In regards to seaworthiness, size does matter, as does the comfort of what has to be the weakest link, the crew.
The accelerations on a lighter boat are necessarily quicker in a seaway than on one with greater weight and consequently more inertia to resist sudden movement.

 

[John Willis]

Hello Laminar Flow,

lovely monika!

I have Marchaj's book (well thumbed) and totally take your point about cruising being something to enjoy in comfort and indeed about size and accelerations. My curiosity however leads me to Chae 73's comment about hearing from someone who has made extended passages in such a boat, for I am pretty certain I couldn't do it. I am however, seriously considering such a vessel for shorter trips for excitement (in safety) and relative comfort in port - close to a decent restaurant and perhaps hotel too of course.

 

[Birdseye]

Following the Fastnet disaster, the power that be asked some tech institute to formulate a safety screen for sailing boat in exposed waters. Its called STIX. You cant get stix numbers for most French mass produced boats though they have to generate them to get the class A etc. and thats because many of them are marginal. That said the modes of failure for boats at sea are very extensive - you could make a plausible argument for example that the level of protection offered to the crew is a key factor in seaworthyness, as is the comfort factor.

The US coastguard did some research work on this issue and the conclusion they came to was that the key factor in seaworthyness was size.

 

[KompetentKrew]

I have been pondering a similar question, in anticipation of helping out with or contributing to my father's plans to acquire another boat. I have put together this spreadsheet using various metrics sourced from SailBoatData.com, so not guaranteed to by 100% accurate. The colour coding is based somewhat on the advice provided by Charles Doane in his book "The Modern Cruising Sailboat". Clearly, "modern" is a relative term...

View attachment 97267

If you'd care to put my Van De Stadt Caribbean into your spreadsheet I'd be interested to see the results, Chae.

Mine is built in woodcore-epoxy, which I assume is the "wood" weight and displacement listed on De Stadt's site: Van de Stadt Design - Caribbean 40

 

[TernVI]

A lot of people bang on about how certain older designs are 'seaworthy'.
Unfortunately, a lot of these old boats have a lot fo old gear on them which tends to break.
Seaworthy can be a wide-ranging term.
When apologists for MABs are forced to admit their boats are no more capable than newer ones, they tend to start talking about 'seakindly' or somesuch instead.

There is some science to all this, if you look at a serious small fishing vessel, it's not designed to be 'as stable as possible' it's designed to roll at a frequency where the crew can survive and keep working.
A big and interesting subject where a lot of people will be talking at cross purposes.

 

[Chae_73]

If you'd care to put my Van De Stadt Caribbean into your spreadsheet I'd be interested to see the results, Chae.

Mine is built in woodcore-epoxy, which I assume is the "wood" weight and displacement listed on De Stadt's site: Van de Stadt Design - Caribbean 40

Looks like a lovely boat. Fairly light for a 40' boat (compared to the designs I've been looking at) but I'm sure that is reflected in good performance.

Also, the sail plan area was taken from what was stated on the website which probably includes an overlapping foresail, whereas the standard is for 100% of the area between the mast and the fore-stay, but those data are not provided on the website.

This is what came out; I hope I haven't made a conversion error somewhere along the line:

 

[Laminar Flow]

There is some science to all this, if you look at a serious small fishing vessel, it's not designed to be 'as stable as possible' it's designed to roll at a frequency where the crew can survive and keep working.
A big and interesting subject where a lot of people will be talking at cross purposes.

There is a large chapter in the book I referred to earlier on the increasing number of capsizes among small modern fishing vessels due to to a lack of inherent roll resistance, a feature that apparently was less of a problem with older designs. Consequently one could argue that modern fishing craft, designed with the benefits of mathematical insight and formulas, are less seaworthy than their ancestors whose builders managed quite well with very modest mathematical imput and by a gradual and slow implementation of an evolutionary process, shaped by the demands and the rigorous selection process of the sea.

No matter, sea-kindliness is a factor and the irrefutable fact that many "modern" designs have it to a lesser degree does not mean its bunk. It is simply the result of a deliberate trade-off. You just can't have it all.

 

[KompetentKrew]

Looks like a lovely boat. Fairly light for a 40' boat (compared to the designs I've been looking at) but I'm sure that is reflected in good performance.

Also, the sail plan area was taken from what was stated on the website which probably includes an overlapping foresail, whereas the standard is for 100% of the area between the mast and the fore-stay, but those data are not provided on the website.

This is what came out; I hope I haven't made a conversion error somewhere along the line:View attachment 97325

Thanks very much, Chae. Very interesting.

 

[Aquaboy]

If a Pogo or similar turned turtle would it right it self? I wouldn't think so........

 

[michael_w]

Youv'e got the Pogo with the keel raised. I'm certain they draw more than 3'.

A few years ago I had a chat with a Dutchman who owned a very light 40' single handed special and a Frers Swan 46. He much preferred the light boat in heavy weather as it was less tiring to sail albeit noisy. The principle being it was better to be a ping-pong ball rather than a golf ball.

 

[Chae_73]

Yes, should have been clear that the Pogo 30 number was for swing keel up, as my purpose on that column was to compare drafts Vs water available.

 

[[178529]]

If a Pogo or similar turned turtle would it right it self? I wouldn't think so........

I think Pogo' s have done some incredible journeys including the very small ones for the single handed transatlantic races. I would think in the conditions where a yacht gets knocked down the wave height would be sufficient to go past the inverted avs pretty quickly .

My guess is that the maths excludes the additional form stability from sailing at speed in difficult conditions that a yacht like a pogo would have. Presumably in storm conditions it would be going with the wind and waves with just a storm jib at pretty much at wave speed. Which is what you see with ocean racers. I think a lot of damage and knock-downs happen when a fast moving breaking wave hits a slower moving yacht and the kinetic energy just overcomes the yacht. If you are going much the same speed as the waves, as long as you are in control, then there must be much less energy transfer....

I've not sailed a pogo but would love one. I did sail a lot in Malbec 29s which just about plane. They were great fun down wind, or with the wind on the quarter , in strong winds. Horrible and slamming heavily upwind in a heavy sea though.

 

[Laminar Flow]

For any boat such as a Pogo to perform as specced it needs to be and remain light. That is not a likely scenario in a cruising boat and the smaller the boat is, the more improbable.

Sea state significantly reduces stability as does speed in a displacement hull. It should be noted that all the common ratios including the AVS curves are based on hydrostatics which, as the the name says, are just that, static. Dynamic stability is quite a different matter and much more difficult to assess.

In an AVS curve the steepness of the transition at zero and the relative size of the negative stability area below, indicate the inverted stability. The greater this inverted stability is, the more likely the boat will remain upside down and the longer it will take for a wave of sufficient energy to come along to right it. Statistically this is usually expressed in minutes; I guess it depends how long you can hold your breath in freezing cold water. A boat with an AVS of 150 degr. or more is considered to be pretty much immune to remaining upside down.
Various organizations that have concerned themselves with the safety of yachts, racing and otherwise have found that the minimum AVS for boats under 10m should be 130 degr.

I always love to hear storm tactic recommendations from people who have never experienced such conditions at sea and whose profound insight is based on the successful viewing of a Youtube video.