Multi vs monohulls in Beaufort 12

We know some monohulls are designed for rough seas, but what about the workboat catamarans that we see servicing windfarms etc. way up around Norway and far north places.
I never see anything about how they perform in sever conditions. Anybody out there know?

 

Good question . Capsizing.

 

@Ad Hoc replied in another thread that those vessels are not designed for those conditions.

I’ll venture VCG is too high and the l/b too low to make sure they don’t ever get abeam.

But the thread title is all wrong because you are not comparing two capable vessels against each other.

And, probably a self righting vessel is safer, and not really something multihulls generally offer.

 

When Lock Crowther paid for a testing study on capsizing of power multihull's the general conclusion of the study was a power multihull would only be under threat of capsize if the wave size was greater than the beam of the power cat. That was assuming the power cat was running parallel with the wave faces. Any deviation of course to run from or point into waves improved your safety. This study was done as Lock started to design larger power cats for charter and ferry work. Translation a power cat with a 26 foot beam should be able to handle 26 foot seas.

The following is a generalization as there are many variables like water depth and fetch to build the waves but EG in 30 knot winds you will get 16 foot waves, in 50 knot winds you will get 32 foot waves. Finally a well designed power cat with good hull freeboard and underwing clearance as well as a low centre of gravity will be better than a multistory beer barn.

 

I'mmmm... not sure it works that way. Maybe in pure rigid body kinematics... I'd love to see the analysis, but all I can find is hearsay on the said study. So I'm with fallguy and Ad Hoc on this, it is never that cut and dried in survival conditions.
In my experience cats or tri's, and even SWATHs fail structurally long before they flip (EDIT to add...due to waves). It is the nature of the beast...a well designed cat/tri/SWATH has to be built long and light with good deck clearance, leading to structural issues with the demi-hull to cross deck connection. And head on into the wave face also has it's issues with submarining and wave slap.
Remember, in typhoon Iniki a 40' cat was picked up and speared through the bridge of the 82' monohull USCGC Point Harris and made her a constructive total loss.
A time to prepare: Lessons from Hurricane Iniki https://content.govdelivery.com/accounts/USDHSCG/bulletins/2560eb0

 

Suggest you look at some of the papers here: Resources – Wolfson Unit https://www.wumtia.soton.ac.uk/published-papers-and-articles/?redirect=true
Various studies done Wolfson Unit over time. They do know a little about the topic and it is not hearsay. All things are shades of grey but tank testing work gives a strong hint. If you want perfect safety don't go to sea otherwise accept there is some risk. All I am aware of is that of the multihulls from 10 foot to 85 foot I have sailed on, the one that came close to a capsize was a 55 foot sailing trimaran that buried a float. Every other cat or tri sail or power were sailed or powered sensibly to suit the conditions.

As for structural failure, any boat can fail. It may be designed very well but if the builder does not use the specified materials, the worker does not weld or mix the materials together properly, if the vessel is not maintained correctly, if the boat hits a sandbar or sharp object on a pier and weakens a portion of the build there can be from minor to major failures. EG a cat of the USA had its crew rescued, why? A hose fitting letting water in and the cat was sinking. AD HOC has to prove in engineering terms a vessel will withstand the forces on it and in reputational and contractual terms within the constraints of the design specifications, the vessel will meet the sea keeping requirements. PS size of the vessel helps but it is not a guarantee. Have you seen jpegs of large cargo ships with their bow crushed or broken by large seas? Nothing is bullet proof.

 

I am firmly convinced that when you you utter the phrase "bullet proof" Mother Nature and Murphy sit down to create some new ordinance.

 

James,

The best boat I know of when sea conditions are severe is the one tied up to the dock.
Or a submarine at depth.
Have you considered either in your quest?

 

An underdog of yachting world shaped like a brick and named Hoghfish Maximus fared quite well in ocean gales.
The combination of high freeboard, massive internal ballast, shoal draft with no appendages to trip on when the daggerboard is up and high initial stability from flat bottom has a lot to offer when going gets tough.

 

After looking quickly over the papers that are present in the referenced site, I notice that almost all are most are pure kinematic, never addressing structural strength. Indeed, some show photos of small models in situations that a full size vessel could not structurally survive (especially the rig). The Cube-Square Law is a real bug-a-boo with such model tests.

Even a better argument about about not thinking cats are the absolute best in a survival situation. They are significantly structurally weaker than a monolithic hull form of the same weight (cube-square law again). As has been said before "...it all depends..."

Even better if it is set in concrete......

Not to pick at old wounds, but I think real history, not limited studies, should play a role in design. The thread below hashed a lot of this out 18 years ago....
Canting Keel Monos vs Multihulls https://www.boatdesign.net/threads/canting-keel-monos-vs-multihulls.13511/

 

Are you talking about multihulls ?

 

It is a mono.

 

Monos are the different stories . Doubting powercat heavy seas ability.

 

What are you trying to say?

Powercats are very capable of being in heavy seas, but the one above less so.

Many cats are far safer than monohulls, but they max out at upside down or falling apart, as others mentioned.

The thing about catamarans is they are super stable. They are great options for rough seas and wide work spaces while offering economical operating costs.

 

Igor wrote about mono and I miss that thread title is multi vs mono. I focus my mind on multi stability only. Sorry.
That shark cat design : looks quite similar to the one above ,except heavy cranes causing higher center of gravity.
Anyway that shipyard showing many self -rotation ability but not cats .
Late Donald Crowhurst proposal was inflatable baloon fixed high on the mast , similar to my " invention" .Self righting bags are sometimes placed on ribs.
So the final conclusion is as you wrote above. Except special solutions that can be eventually apllied to self-rotating ability.